Wavetrac Differential

[Tyler Jones]

Hello,

My name is Tyler Jones, I'm the Project Director/Controls Team Lead for the Iowa State University FSAE team.

Our 2014-2015 car runs a Torsen T1 differential with a custom gear housing. We are having problems with the car unloading the inside rear tire in tight corners which is an issue since the Torsen differential essentially becomes an open differential when a wheel is lifted. We are considering switching to the Wavetrac differential to mitigate this issue. In order to pursue this further I need to obtain some models or drawings of the Wavetrac's internals to begin designing a custom gear housing as soon as possible, but after contacting them directly Wavetrac is reluctant to give out any drawings showing the internal workings of their differentials. Hopefully a team that currently uses the Wavetrac differential or has used one in the past may be able to share any models or drawings they have of the internals?

Additionally I had some questions about the FSAE stub shafts that RCV Performance sells for the Wavetrac differential. I determined that the spline on the male stub shaft has 33 teeth vs 27 teeth for the female spline in the Honda Civic model Wavetrac suggested for FSAE use. Can anyone confirm which model the RCV stub shafts are compatible with?

Link for the stub shafts:
http://www.rcvperformance.com/product-details.aspx?sku=D5253
http://www.rcvperformance.com/product-details.aspx?sku=D6768

Link to Wavetrac:
http://www.wavetrac.net/

Attached are the drawings of the models Wavetrac says they have sold to other FSAE teams.

683684

Thanks!

[Drew Price]

It looks to me from their video that it's a helical ATB like a Quaife / Torsen, but with a ramp clutch like a Gripper / Guard.

Cool idea, it looks pretty compact. Was the tech rep not able to tell you which diffs their little stub tulip flanges worked with? Might be best to get the info right from the source.

Does something preclude you from tweaking your roll stiffness distribution enough to keep the inside rear on the ground?

[Tyler Jones]

It looks to me from their video that it's a helical ATB like a Quaife / Torsen, but with a ramp clutch like a Gripper / Guard.

Cool idea, it looks pretty compact. Was the tech rep not able to tell you which diffs their little stub tulip flanges worked with? Might be best to get the info right from the source.

Does something preclude you from tweaking your roll stiffness distribution enough to keep the inside rear on the ground?

I'm waiting on a response from RCV about the stub shafts and the Wavetrac rep couldn't help any more than providing me with those drawings.

Our suspension guys are trying to tune the car to keep that inside rear planted but it has been a recurring issue with the past two or three cars. Either way I think the Wavetrac is a better design than the Torsen anyway, so if I can find the resources I would like to switch even if we solved the lifting issue.

[CWA]

Hi Tyler,

There will be plenty of people here able to give suggestions on the diff queries you have I'm sure. As Drew is, I am curious to know a bit about your base chassis and what options you have available to you that may potentially help with this issue.

Can you give some info on your suspension setup. What steering axis angles do you have (caster, KPI) and what steering axis ground level offsets do you have (scrub radius, caster trail)? Do you have any scope for adjustment?

What is your Front/rear static weight distribution? What is your estimated CG height? What are your front and rear wheel rates? What are your front and rear 'roll centre' heights? Do you have anti-roll bars fitted, front and/or rear? If so, what is their contribution to wheel rate? Do you have any adjustability for any of these parameters?

Have you had the car on scales to check for cross-weight imbalance? Does your suspension actually move as it should; or do you have lots of friction in it?

[Tyler Jones]

Hi Tyler,

There will be plenty of people here able to give suggestions on the diff queries you have I'm sure. As Drew is, I am curious to know a bit about your base chassis and what options you have available to you that may potentially help with this issue.

Can you give some info on your suspension setup. What steering axis angles do you have (caster, KPI) and what steering axis ground level offsets do you have (scrub radius, caster trail)? Do you have any scope for adjustment?

What is your Front/rear static weight distribution? What is your estimated CG height? What are your front and rear wheel rates? What are your front and rear 'roll centre' heights? Do you have anti-roll bars fitted, front and/or rear? If so, what is their contribution to wheel rate? Do you have any adjustability for any of these parameters?

Have you had the car on scales to check for cross-weight imbalance? Does your suspension actually move as it should; or do you have lots of friction in it?

You will have to wait for the suspension guys to get back to me, most of that's a little beyond my depth of knowledge. I can tell you that the weight distribution is approximately 49% front (was designed for 45% and was measured at 45% at Formula North but managed to change at FSAE Lincoln, not sure if ride heights changed or what) and that we have tested most configurations for ARBs and are currently between no ARB front and rear or a soft ARB front and no ARB rear.

I will get back to you as soon as I have the other technical info.

[craigorydean]

Hello Tyler,

My name Is Craig Kellermann. I am team member from the University of Texas-San Antonio. By any chance have you looked at what offering could be had else where on the applications of differentials.(Taylor, Drelxer, Front diff. from atv, spool, ext). What I stated maybe options to look at depending on what you actually see as you problem.

I personally would really look at your whole drivetrain and see what is your goal of this sub-system and see if changing to that differential is meeting your goals. First, you may have to sit down and really see what problem you are trying to solve as a whole. If the only problem is that you want to "Close" the Differential a little bit. Then so be it but i think you have a list of problems like "Closing" the Diff ,having a solid model, spending $X,XXX dollars, weighing XX lbs, and having some bias ability during cornering. Once you have those thing figured out maybe a new solution will present its self.

After this past year I have nothing but great things to say about taylor racing products as they were a key reason why we were on of only 17 or 18 teams that finished enduro at lincoln.I know our team currently has a model for a Taylor Racing MKII diff.(This is the Diff we use), if you decide to go with that option I would be more than glad to get the solidworks file to you.

[Tyler Jones]

Hello Tyler,

My name Is Craig Kellermann. I am team member from the University of Texas-San Antonio. By any chance have you looked at what offering could be had else where on the applications of differentials.(Taylor, Drelxer, Front diff. from atv, spool, ext). What I stated maybe options to look at depending on what you actually see as you problem.

I personally would really look at your whole drivetrain and see what is your goal of this sub-system and see if changing to that differential is meeting your goals. First, you may have to sit down and really see what problem you are trying to solve as a whole. If the only problem is that you want to "Close" the Differential a little bit. Then so be it but i think you have a list of problems like "Closing" the Diff ,having a solid model, spending $X,XXX dollars, weighing XX lbs, and having some bias ability during cornering. Once you have those thing figured out maybe a new solution will present its self.

After this past year I have nothing but great things to say about taylor racing products as they were a key reason why we were on of only 17 or 18 teams that finished enduro at lincoln.I know our team currently has a model for a Taylor Racing MKII diff.(This is the Diff we use), if you decide to go with that option I would be more than glad to get the solidworks file to you.

I'm aware of most of the options available for FSAE differentials. From what I have found we can choose between:
A spool (causes understeer)
Detroit Locker (causes understeer)
A Cam and Pawl a.k.a. front diff from atv (also causes understeer but doesn't work with a lifted tire)
Torsen (<$1000, bias adjustable, doesn't work with a lifted tire)
Quaife (<$1000, bias adjustable, doesn't work with a lifted tire)
Wavetrac (<$1000, bias adjustable, works with a lifted tire)
Taylor (>$2000, bias adjustable, based on Quaife so doesn't work with a lifted tire?)
Salisbury-style differential (expensive, bias adjustable, can be made to work with a lifted tire)
Drexler (>$2000, bias adjustable, can be made to work with a lifted tire)





Goals in order of importance:

price <$1000
bias adjustibility
works when one wheel is unloaded
weight < ~10 lbs (not a huge driving factor here)



The Wavetrac is cheap (~$600), the bias can be adjusted by changing the material of the friction plates, it works when one wheel is unloaded, and with a custom housing the weight increase over the Torsen would be negligible. The only drawbacks that I can see are the lack of the drawings necessary, the time invested in designing a new housing, and having to partially disassemble the housing every time you want to adjust the torque bias.

Feel free to bring up any negatives to the Wavetrac I haven't thought of, but from what I can see it it meets all my goals.

[Tyler Jones]

Hi Tyler,

There will be plenty of people here able to give suggestions on the diff queries you have I'm sure. As Drew is, I am curious to know a bit about your base chassis and what options you have available to you that may potentially help with this issue.

Can you give some info on your suspension setup. What steering axis angles do you have (caster, KPI) and what steering axis ground level offsets do you have (scrub radius, caster trail)? Do you have any scope for adjustment?

What is your Front/rear static weight distribution? What is your estimated CG height? What are your front and rear wheel rates? What are your front and rear 'roll centre' heights? Do you have anti-roll bars fitted, front and/or rear? If so, what is their contribution to wheel rate? Do you have any adjustability for any of these parameters?

Have you had the car on scales to check for cross-weight imbalance? Does your suspension actually move as it should; or do you have lots of friction in it?


You will have to wait for the suspension guys to get back to me, most of that's a little beyond my depth of knowledge. I can tell you that the weight distribution is approximately 49% front (was designed for 45% and was measured at 45% at Formula North but managed to change at FSAE Lincoln, not sure if ride heights changed or what) and that we have tested most configurations for ARBs and are currently between no ARB front and rear or a soft ARB front and no ARB rear.

I will get back to you as soon as I have the other technical info.

Okay, here is the response from one of my suspension guys:

"Man that is a lot of questions, I don't have access to all those numbers at the moment and I might hesitate a little bit to put that much info on the forum (probably not a big deal, but he's asking for almost our whole design haha). I can answer all of those though, and it sounds like he is wondering whether the front wheel motion in steering is jacking the inside rear too much, and also if our bar setup is lifting it. I wouldn't suspect too much jacking from the front (although I wouldn't totally rule it out) since we have 3 deg of caster and about 0.8 inches of scrub radius, which doesn't displace the wheel downward very much (a lot of caster would be more like 5-7 deg). Wheel rates are about 112 lbs/in on all corners, and since we've tried almost all the bar combinations I don't think he can help us too much otherwise. It's got low friction and it's good on the scales, so we can pretty safely rule those things out."

[CWA]

Goals in order of importance:

I would have said that your priorities would at least be more like:

n: Works when one wheel is unloaded
n+1: Price
etc..

You already have a diff which doesn't work when one wheel is unloaded (and to you right now, technically it is free). The diff has been deemed unacceptable because it does not perform. So I assume, based on this reasoning, you would not deem another diff that also does not work when one wheel is unloaded as acceptable either, regardless of how much it costs.

So you won't be buying a another, different diff that also doesn't work when it is unloaded. This means you can cross a load of options off that list straight away. In fact, I don't see how "Works when one wheel is unloaded" is not your first priority, because it is your only prerequisite.

But ignoring my pedantry, it may be possible that you have some chassis tuning options available to you that actually negate the need to replace your existing diff completely. Once your suspension guy can post up some info, this can be determined. It might save you some time and money.

[EDIT - I've just seen your reply r.e. suspension settings - I'll take a read now..]

[Tyler Jones]

I would have said that your priorities would at least be more like:

n: Works when one wheel is unloaded
n+1: Price
etc..

You already have a diff which doesn't work when one wheel is unloaded (and to you right now, technically it is free). The diff has been deemed unacceptable because it does not perform. So I assume, based on this reasoning, you would not deem another diff that also does not work when one wheel is unloaded as acceptable either, regardless of how much it costs.

So you won't be buying a another, different diff that also doesn't work when it is unloaded. This means you can cross a load of options off that list straight away. In fact, I don't see how "Works when one wheel is unloaded" is not your first priority, because it is your only prerequisite.

But ignoring my pedantry, it may be possible that you have some chassis tuning options available to you that actually negate the need to replace your existing diff completely. Once your suspension guy can post up some info, this can be determined. It might save you some time and money.

[EDIT - I've just seen your reply r.e. suspension settings - I'll take a read now..]

That is the reason I came into this thread with the Wavetrac in mind. The only other option that met that prerequisite was a clutch-style lsd and they are too expensive for our budget. As for using the "free" differential we already have, we like to keep our cars together each year so the 2015-2016 car will need a new differential either way. As for chassis tuning, our suspension team leads seem to think we are out of options since we have already tried all arb options and 3 or so different spring setups. There is more to look into there, but I think the Wavetrac would be beneficial even if we tuned to keep that wheel planted in most situations. However, as you said, it would save us time and money which are both pretty big concerns for our team. If you have suggestions to keep that wheel planted I will definitely relay that back to my suspension guys.

[CWA]

"Man that is a lot of questions, I don't have access to all those numbers at the moment and I might hesitate a little bit to put that much info on the forum (probably not a big deal, but he's asking for almost our whole design haha). I can answer all of those though, and it sounds like he is wondering whether the front wheel motion in steering is jacking the inside rear too much, and also if our bar setup is lifting it. I wouldn't suspect too much jacking from the front (although I wouldn't totally rule it out) since we have 3 deg of caster and about 0.8 inches of scrub radius, which doesn't displace the wheel downward very much (a lot of caster would be more like 5-7 deg). Wheel rates are about 112 lbs/in on all corners, and since we've tried almost all the bar combinations I don't think he can help us too much otherwise. It's got low friction and it's good on the scales, so we can pretty safely rule those things out."

First, there is no magic formula that your suspension guy might have happened upon, so don't be so worried in sharing your setup details. Even if your setup did give the best handling car in existence (which by the sounds of it, no offence; it does not), true value is found in the understanding of the relevant mechanisms and knowing exactly why the setup gives good performance. It is entirely possible that the setup that might be golden on one car would be considerably less effective on another car. But knowing how to adjust the setup slightly depending on your situation is where the real skill lies. Point being, your setup details are pretty arbitrary - you should have no qualms with sharing this info.

Second, one way or another your car is lifting a rear inside wheel. This is truth, this is reality, so you say. Don't forget that THIS is the root cause of your issue, and replacing the diff that doesn't fully lock is effectively a band-aid fix. In my opinion, until you've exhausted all of your options in chassis tuning, you should not be spending more money on hardware. Even if you have tuning limitations on your own chassis, I'd say you have a duty to figure out the mechanism causing this behaviour so that you can: a) justify to your budget providers why exactly you need a new diff, and b) pass on this knowledge to next year's team as a warning. If you guys are designing the next car, this is an even bigger reason to want to figure this out. Don't forget, plenty of teams do not lift their I/S/R wheels round corners, and so manage to make do with lesser diffs. You should strive to find out why that is, even if it turns out you can do nothing about it.

Now, I am 90% sure that real answers to all of the questions I asked will expose the cause of this behaviour (by 'real answers' I mean actual, measured values, rather than nominal design-intent values. The car certainly won't have turned out exactly as the ideal spreadsheet solution showed you it should - friction for one can have significant effects). There may be some obscure mechanism causing your I/S/R wheel to lift that isn't related to control arm jacking ("RCHs"), steering system jacking, or roll moment / lateral load transfer / static weight distribution, but I will be surprised if this is so. As said, there are plenty of cars out there that do not jack their inside wheel; it should not be difficult to determine why your car is different, it abides by the same rules of physics after all.

See if you can post back with the vales you haven't answered: Static Weight Distribution, estimates of F&R Roll Centre Heights and CG height. Also do you know what your chassis torsional stiffness might be? For starters, if this is too low, it will explain why bar tuning has not been as effective as you were perhaps expecting. Do you really have just 20N/mm wheel rates front and rear? To confirm this, please don't consider me patronising in checking; what are your spring rates and what motion ratio do you have? How much 'stiffer' does your range of bar combinations supposedly make your wheel rate in roll (a number would be useful, for you and us both)? Also, more out of curiosity than much else, how much does your car actually weigh?

Yes as your suspension guy recognised, I was trying to determine whether steer jacking might be the main contributor to your issue. You've suggested your caster angle and scrub radius are within a reasonable range, but don't forget that these aren't the only contributors to steer jacking. Your KPI and trail contribute to this too - what are these values? Also, the stiffness of your tyres / suspension / bars / chassis contribute to this mechanism - an infinitely stiff car will theoretically lift its inside wheel with far less caster and scrub than you have mentioned. Your quoted wheel rates do not seem too stiff if they are accurate, but you need to be sure that you don't have too much friction that acts to ruin these rates - the friction will act as a false suspension stiffness, at least in that it will serve to increase the effects of steer jacking. Also, your 'scrub radius' in CAD is a hypothetically ideal value, which technically isn't even a kinematic dimension (it should be based on the contact patch centre of pressure, not just the centre line of your wheel). As can be imagined, in real life, exactly where this centre of pressure is in relation to your steering axis ground level intersect depends for starters on the width of your wheel, it's camber angle with the road (with steer and roll), and tyre sidewall stiffness's.

So can you also post up: static front camber, camber gain of your front suspension, wheel widths, tyre type and sidewall sizes? Do you have your own scales? If so, you might as well get the car on them, record and then post here: a) your static corner loads with driver and steering straight ahead, and b) your static corner loads with driver and max steering angle in any direction (might as well do both directions to assess asymmetry). I believe this would be a great starting point.

[craigorydean]

I'm aware of most of the options available for FSAE differentials. From what I have found we can choose between:
A spool (causes understeer)
Detroit Locker (causes understeer)
A Cam and Pawl a.k.a. front diff from atv (also causes understeer but doesn't work with a lifted tire)
Torsen (<$1000, bias adjustable, doesn't work with a lifted tire)
Quaife (<$1000, bias adjustable, doesn't work with a lifted tire)
Wavetrac (<$1000, bias adjustable, works with a lifted tire)
Taylor (>$2000, bias adjustable, based on Quaife so doesn't work with a lifted tire?)
Salisbury-style differential (expensive, bias adjustable, can be made to work with a lifted tire)
Drexler (>$2000, bias adjustable, can be made to work with a lifted tire)





Goals in order of importance:

price <$1000
bias adjustibility
works when one wheel is unloaded
weight < ~10 lbs (not a huge driving factor here)



The Wavetrac is cheap (~$600), the bias can be adjusted by changing the material of the friction plates, it works when one wheel is unloaded, and with a custom housing the weight increase over the Torsen would be negligible. The only drawbacks that I can see are the lack of the drawings necessary, the time invested in designing a new housing, and having to partially disassemble the housing every time you want to adjust the torque bias.

Feel free to bring up any negatives to the Wavetrac I haven't thought of, but from what I can see it it meets all my goals.

Personally from my stand point the fact that you have to make a custom housing and do not have access to a drawing are very big negatives, but that is from the perspective from where I stand on this issue. Yours may well be very different and that is the beauty of FSAE.

I would love to see how you weight those thing of importance and maybe develop a Pugh matrix of the different options.

Also Why would you want to completely un-load any of your tire during the events, i dont think i would want to do that on my car.(maybe i am not thinking clearly on this) If this is something that is happening in your car and you are developing jacking effects in the rear. Then a spool would be a great option but your suspension would have to further develop this plan of action, but it doesnt do anything with you second goal. It could super exceedon the other 3 of the goals though. Or your suspension guy may want to get rid of the jacking and the need for the new style diff will go away.

[Tyler Jones]

First, there is no magic formula that your suspension guy might have happened upon, so don't be so worried in sharing your setup details. Even if your setup did give the best handling car in existence (which by the sounds of it, no offence; it does not), true value is found in the understanding of the relevant mechanisms and knowing exactly why the setup gives good performance. It is entirely possible that the setup that might be golden on one car would be considerably less effective on another car. But knowing how to adjust the setup slightly depending on your situation is where the real skill lies. Point being, your setup details are pretty arbitrary - you should have no qualms with sharing this info.

Second, one way or another your car is lifting a rear inside wheel. This is truth, this is reality, so you say. Don't forget that THIS is the root cause of your issue, and replacing the diff that doesn't fully lock is effectively a band-aid fix. In my opinion, until you've exhausted all of your options in chassis tuning, you should not be spending more money on hardware. Even if you have tuning limitations on your own chassis, I'd say you have a duty to figure out the mechanism causing this behaviour so that you can: a) justify to your budget providers why exactly you need a new diff, and b) pass on this knowledge to next year's team as a warning. If you guys are designing the next car, this is an even bigger reason to want to figure this out. Don't forget, plenty of teams do not lift their I/S/R wheels round corners, and so manage to make do with lesser diffs. You should strive to find out why that is, even if it turns out you can do nothing about it.

Now, I am 90% sure that real answers to all of the questions I asked will expose the cause of this behaviour (by 'real answers' I mean actual, measured values, rather than nominal design-intent values. The car certainly won't have turned out exactly as the ideal spreadsheet solution showed you it should - friction for one can have significant effects). There may be some obscure mechanism causing your I/S/R wheel to lift that isn't related to control arm jacking ("RCHs"), steering system jacking, or roll moment / lateral load transfer / static weight distribution, but I will be surprised if this is so. As said, there are plenty of cars out there that do not jack their inside wheel; it should not be difficult to determine why your car is different, it abides by the same rules of physics after all.

See if you can post back with the vales you haven't answered: Static Weight Distribution, estimates of F&R Roll Centre Heights and CG height. Also do you know what your chassis torsional stiffness might be? For starters, if this is too low, it will explain why bar tuning has not been as effective as you were perhaps expecting. Do you really have just 20N/mm wheel rates front and rear? To confirm this, please don't consider me patronising in checking; what are your spring rates and what motion ratio do you have? How much 'stiffer' does your range of bar combinations supposedly make your wheel rate in roll (a number would be useful, for you and us both)? Also, more out of curiosity than much else, how much does your car actually weigh?

Yes as your suspension guy recognised, I was trying to determine whether steer jacking might be the main contributor to your issue. You've suggested your caster angle and scrub radius are within a reasonable range, but don't forget that these aren't the only contributors to steer jacking. Your KPI and trail contribute to this too - what are these values? Also, the stiffness of your tyres / suspension / bars / chassis contribute to this mechanism - an infinitely stiff car will theoretically lift its inside wheel with far less caster and scrub than you have mentioned. Your quoted wheel rates do not seem too stiff if they are accurate, but you need to be sure that you don't have too much friction that acts to ruin these rates - the friction will act as a false suspension stiffness, at least in that it will serve to increase the effects of steer jacking. Also, your 'scrub radius' in CAD is a hypothetically ideal value, which technically isn't even a kinematic dimension (it should be based on the contact patch centre of pressure, not just the centre line of your wheel). As can be imagined, in real life, exactly where this centre of pressure is in relation to your steering axis ground level intersect depends for starters on the width of your wheel, it's camber angle with the road (with steer and roll), and tyre sidewall stiffness's.

So can you also post up: static front camber, camber gain of your front suspension, wheel widths, tyre type and sidewall sizes? Do you have your own scales? If so, you might as well get the car on them, record and then post here: a) your static corner loads with driver and steering straight ahead, and b) your static corner loads with driver and max steering angle in any direction (might as well do both directions to assess asymmetry). I believe this would be a great starting point.

We have scales, I can try to get the static corner loads some time this week. The suspension guys are working on providing the other info.


Personally from my stand point the fact that you have to make a custom housing and do not have access to a drawing are very big negatives, but that is from the perspective from where I stand on this issue. Yours may well be very different and that is the beauty of FSAE.

I would love to see how you weight those thing of importance and maybe develop a Pugh matrix of the different options.

Also Why would you want to completely un-load any of your tire during the events, i dont think i would want to do that on my car.(maybe i am not thinking clearly on this) If this is something that is happening in your car and you are developing jacking effects in the rear. Then a spool would be a great option but your suspension would have to further develop this plan of action, but it doesnt do anything with you second goal. It could super exceedon the other 3 of the goals though. Or your suspension guy may want to get rid of the jacking and the need for the new style diff will go away.

A big reason I'm a proponent of the Wavetrac is that the work required to get it on the car is the exact same as what we have been doing for the past several years with the Torsen. Yes, not having the drawings means it is basically impossible to switch to this differential, so if I can't obtain them in a timely manner we will probably stick with the Torsen.

The jacking issue is definitely something to attempt to resolve. I would not want to intentionally design jacking into the system to use a spool as I think that would compromise the overall performance of the car compared to one with a working differential and some small jacking issues in very tight corners. The 2014-2015 car with its current issues did end up placing 3rd in Endurance and 3rd overall at Formula North and was going to place very well at FSAE Lincoln until we were black flagged during endurance (lost the endcap of our muffler, dang new sound rules). This is partially why I think simply switching to a differential that can lock with an unloaded wheel is a good idea (if it ain't broke, don't fix it? I guess that depends on what you view as broken, which for most is probably the chassis setup).

I'm grateful for everyone's advice, even if it looks like I'm dead-set on the Wavetrac. If I can't get the necessary technical info for the Wavetrac we will be sticking with the Torsen anyways and will be forced into trying to solve this unloaded wheel situation via chassis tuning, like it or not.

[CWA]

If you're getting third place at comps, the issue can't be too bad. That fact does help put things in perspective. Do you have any vids of this behaviour on track?

What is your maximum achieveable steady-state lat acc? (Do you have instrumentation to check this?)? Is the' inside rear wheel lifting / limited amount of drive torque' issue what dictates this limit? So during a steady-state skidpad test, rather than ploughing or spinning at the limit (ie rather than saturating an axle of its capacity to give out any more lateral force), do you just reach a given speed and maintain it, with your inside wheel spinning up if you give it more gas? Meaning you can never get your outer wheel to spin up / the car to spin out?

If this is not the case, make sure you are clear about what makes you think that this wheel lifting / wheel spinning is even an issue. Your car is obviously quite quick at the moment. You could find a way to send more drive torque to the outer wheel than you are already doing - whether you do this with your existing diff by solving the jacking, or whether you find a better locker. But perhaps your outer wheel has been close to saturation anyway (and is also balanced well with your front) hence your competitive results. In this case, adding more locking torque will actually allow the driver to saturate the outer rear tyre with drive torque (rather than spinning up the inside as you are currently doing), which will end up making a worse contribution to consistently lower lap times than before (car is more of a handful, harder to handle).

This is of course pure speculation / mental masturbation on my account, but food for thought at least, something to be aware of when you begin making changes.

[Z]

Tyler,

It seems you are using the standard "because racecar" approach to solving your problem. Namely, chase the most complicated and expensive solution you can find. (And if you think spending yet another ~$600, AND making a new diff-case, is NOT expensive, then please send the money to me! :))

As noted by others above, there are many simpler and cheaper fixes to your problem. Please re-read CWA's posts, and also get your suspension guys to read them several times. There are many clues there that can help your Team build a faster car.

Regarding your list of options/goals for selecting a diff, I think you are somewhat missing the point. It is important to realise that you need the "whole package" (= diff + suspension + everything-else) to work well, and that means you cannot just choose a diff based on a simplified list of "diff characteristics" taken in isolation.

Because cost is so important (to me!) I would immediately narrow my diff choice down to these two cheapest options.

1. A "spool" diff.
This is about the simplest machined part you can make for the car (= lathe + drill-press?). (Or you can turn your current Torsen into a spool by squeezing some extra washers in there, or filling with epoxy, or weld...) Your current car seems to be already "set-up" for a spool, so no extra work there. And, most importantly, there are countless cars, both past and present, that have won FSAE comps with a spool. The "spool causes understeer" is one of those lame arguments that might be true of a spool in general, but has turned out to be irrelevant with the car set-up used on many winning FSAE cars.

2. An "open" diff.
Your Torsen is close to this, though cheaper ones are available from any car wrecking yard. The set-up changes required on your car, to suit a fully open-diff, are quite easy. From reading your above posts I suspect your suspension guys are, ... ummm, a little lost. [Well done Z! The politeness lessons are working.] Short list: Fit stiffer front springs, and/or softer rears. Or a VERY stiff front-ARB. Make sure frame is torsionally stiff enough for this to work!!! Make sure front dampers reach droop limit before rears. Reduce excessive "steer-wedging" from steering geometry. Beware of VERY short end-view Rear-Virtual-Swing-Arm-Lengths. And some others ... but the above should cover it. Personally, this is my preference because I like the idea of having all four wheels always planted.

The choice is yours, of course, but throwing money around does not equal good engineering. Besides, I am sure you will eventually meet a girl who will teach you how to really squander your hard-earned dollars! :)

Z

(PS. "A good engineer is someone who can do with one dollar, what any fool can do with ten!")

[BillCobb]

You can turn an open diff into a limited slip diff with some really bad bearings and some worn out side gears. It only differentates when you are at lots of load trasfer and most often its when you are at full throttle.

Push some brake fluid into the spinning wheel using a rotary valve in the steering system.

Keep the rubber on the road.

[Jay Lawrence]

Would also recommend a spool. It's cheaper and lighter (lower polar moment) and there's less stuff to break/look after. If your car is already lifting the inside and you can't tune it out, adding a spool will not cause understeer (vs. a car that has the inside planted).

[Tyler Jones]

Tyler,

It seems you are using the standard "because racecar" approach to solving your problem. Namely, chase the most complicated and expensive solution you can find. (And if you think spending yet another ~$600, AND making a new diff-case, is NOT expensive, then please send the money to me! :))

As noted by others above, there are many simpler and cheaper fixes to your problem. Please re-read CWA's posts, and also get your suspension guys to read them several times. There are many clues there that can help your Team build a faster car.

Regarding your list of options/goals for selecting a diff, I think you are somewhat missing the point. It is important to realise that you need the "whole package" (= diff + suspension + everything-else) to work well, and that means you cannot just choose a diff based on a simplified list of "diff characteristics" taken in isolation.

Because cost is so important (to me!) I would immediately narrow my diff choice down to these two cheapest options.

1. A "spool" diff.
This is about the simplest machined part you can make for the car (= lathe + drill-press?). (Or you can turn your current Torsen into a spool by squeezing some extra washers in there, or filling with epoxy, or weld...) Your current car seems to be already "set-up" for a spool, so no extra work there. And, most importantly, there are countless cars, both past and present, that have won FSAE comps with a spool. The "spool causes understeer" is one of those lame arguments that might be true of a spool in general, but has turned out to be irrelevant with the car set-up used on many winning FSAE cars.

2. An "open" diff.
Your Torsen is close to this, though cheaper ones are available from any car wrecking yard. The set-up changes required on your car, to suit a fully open-diff, are quite easy. From reading your above posts I suspect your suspension guys are, ... ummm, a little lost. [Well done Z! The politeness lessons are working.] Short list: Fit stiffer front springs, and/or softer rears. Or a VERY stiff front-ARB. Make sure frame is torsionally stiff enough for this to work!!! Make sure front dampers reach droop limit before rears. Reduce excessive "steer-wedging" from steering geometry. Beware of VERY short end-view Rear-Virtual-Swing-Arm-Lengths. And some others ... but the above should cover it. Personally, this is my preference because I like the idea of having all four wheels always planted.

The choice is yours, of course, but throwing money around does not equal good engineering. Besides, I am sure you will eventually meet a girl who will teach you how to really squander your hard-earned dollars! :)

Z

(PS. "A good engineer is someone who can do with one dollar, what any fool can do with ten!")

Z,

The Wavetrac is $600, compared to $400 for a Torsen (which we gut and use a custom housing with anyways), or ~$300 for an ATV front differential. In my mind the price difference is almost negligible over our current Torsen since we would be spending $400 out of that $600 either way. I personally programmed and ran the CNC turning to make the custom Torsen housing using donated 7075, so other than time invested the housing was free. Yes, time is a very valuable resource for FSAE but in most cases I would rather invest my time over straight cash in the car.

I do agree that we should try locking the Torsen and testing with it as a spool, but I don't know of any methods to achieve that which aren't destructive for the internals (its not cheap to destroy the gears in pursuit of something which may turn out not to be beneficial). To manufacture our own spool would require some method of hobbing or wire EDMing the correct spline for our stub shafts, which we don't have the resources for, meaning it is definitely not cheap. We could buy some stubshafts that have a bolt pattern instead of a spline, but the only ones I can find are $185.00 per stubshaft. Perhaps we could manufacture our own but that's more time invested in something the car isn't tuned for at the moment.
I would also like to see a list of recent results for cars using spools as I haven't seen a single car in the top ten at any North American competition using spools to my knowledge.

My suspension guys are the ones that initially suggested switching to a locking differential because they are afraid that standard options for tuning the chassis to keep that wheel down would cause the car to understeer significantly. They have already tried most configurations for arbs, including stiff arb front and no arb rear. The car currently has stiffer springs front than rear, though there are options to try different springrates there. I can't answer for droop limits, steer-wedging, etc but I will have my suspension guys look into it.

Don't get us wrong Z, we are a very fiscally conservative team. Most of our design is centered around keeping the car cheap (usage of a lot of laser cut steel, aircraft fabric body, lots of 3D printed components). We took 2nd in cost at Formula North with a very accurate cost report (I know this doesn't exactly equate to real world cost but it at least correlates to a cheap car).

More quotes from my suspension guys:

"I think our actual weight dist. was 49%, and one of the aero guys said the cg raises about 1.5 in with wings (i'd like to measure that to see for sure). From what I've been learning, I'm pretty suspicious of our roll centers and how they work with the rest of the car, and also how the travel available is mostly for compression. Also we haven't explored 100% of the bar options, there are some left which could likely fix the problem by adding a lot of roll stiffness, but the car would probably understeer a ton (since we'd be adding most of that to the front)." -Suspension guy 1

"Static weight distribution: 45% front actual
CG height: 10.2 measured without wings, 10.5" with wings (estimated, This is what I think it was, can probably get a better estimate later)
Roll Center: 3.0" front 3.8" rear
Chassis Torsional stiffness: Not sure off my head. (Somewhere around 1800lbs/degree simulated)
Wheel Rate: 20n/mm seems low, maybe a conversion error? Check the tech sheet we turned in for lincoln, that will give you a closer estimate. Springs are 225s.
Ignore the ARBs and do not consider them into wheel rates.
Car weight is 405lbs?

Steering Geometry: Look at the design tech sheet. Those are all "designed" numbers,
Static Front Camber: -2*
Wheels: 6in width

The suspension does not have bushings and does not carry much friction at all. Essentially is negligible for most calculations we do. (Not always negligible, but its low enough that I'm ignoring it for now and focusing elsewhere)

We have scales and the car is on them frequently for testing. I don't have a data sheet with me to record actual corner weights.

I think some of the focus shifted on to figuring out the suspension issue. Which while I appreciate it, is not exactly why we need a different differential. I do not think it is a "band-aid fix". While I see the point that the suspension is causing an issue, I don't see how a differential that will handicap the car in odd situations is acceptable. A different differential isn't the solution to this problem, its a solution to a few different problems. One of which, is the diff is currently untuneable.

I have a few hunches to our wheel lifting issues. One of which is an overall roll gradient vs wheel travel issue. The car was allowed to roll quite a bit in the last cars, and I think it rolls more than calculated. Which is easy to prove in this car, but I haven't had a chance to yet." - Suspension guy 2

I guess I will have to track down the rest of those numbers myself when I find the time.

Thanks for the advice, keep it coming!

[Tyler Jones]

If you're getting third place at comps, the issue can't be too bad. That fact does help put things in perspective. Do you have any vids of this behaviour on track?

What is your maximum achieveable steady-state lat acc? (Do you have instrumentation to check this?)? Is the' inside rear wheel lifting / limited amount of drive torque' issue what dictates this limit? So during a steady-state skidpad test, rather than ploughing or spinning at the limit (ie rather than saturating an axle of its capacity to give out any more lateral force), do you just reach a given speed and maintain it, with your inside wheel spinning up if you give it more gas? Meaning you can never get your outer wheel to spin up / the car to spin out?

If this is not the case, make sure you are clear about what makes you think that this wheel lifting / wheel spinning is even an issue. Your car is obviously quite quick at the moment. You could find a way to send more drive torque to the outer wheel than you are already doing - whether you do this with your existing diff by solving the jacking, or whether you find a better locker. But perhaps your outer wheel has been close to saturation anyway (and is also balanced well with your front) hence your competitive results. In this case, adding more locking torque will actually allow the driver to saturate the outer rear tyre with drive torque (rather than spinning up the inside as you are currently doing), which will end up making a worse contribution to consistently lower lap times than before (car is more of a handful, harder to handle).

This is of course pure speculation / mental masturbation on my account, but food for thought at least, something to be aware of when you begin making changes.

I don't remember our exact lateral g's but our best skidpad time was 5.381s. From our data acquisition we have seen up to 2.1g's during Endurance. I drove the skidpad event at Formula North and from what I could tell (I'm by no means a professional driver) the car wanted to over-rotate at the limit. The car doesn't appear to be jacking at all during steady-state. The time we noticed the problem most is the hairpin at the end of the slalom at the FSAE Lincoln Endurance course.

[Tim.Wright]

As CWA alluded to, you are trying to substitute your diff to solve a suspension problem.

First things first, what is your calculated/simulated LLTD? What is your calculated/simulated US (using what ever metric suits you, i.e. Static margin, stability factor, UG)?

This is the place to start. Only once we have this information can we start a discussion. If your suspension guys can't tell you these numbers, then I wouldn' be paying any attention to their predictions of too much understeer. Especially not given your subjective feedback points in the direction of instability/oversteer.

By the way, your roll centres appear to be nearing the height where the outer wheels will "lock" its travel near limit lat acc. Not necessarily a bad thing, but it causes very asymmetric roll movements which raises your CG when you corner. Can make the LLTD/balance tricky to predict using the normal simple calcs too. Though I'm sure your suspension guys are aware of this right? Are you looking at your damper positions when you are testing?

[JT A.]

They have already tried most configurations for arbs, including stiff arb front and no arb rear.



This is what sticks out to me. If they have tried big changes to front & rear roll stiffness (I don't know what values you consider "stiff" and "soft") but the chassis doesn't respond to it, it's an indicator that your chassis' torsional stiffness is too low. Could be a global stiffness issue or it could be a local stiffness problem right at the bellcranks/bellcrank posts. You said 1800 ft*lb/deg was your torsional stiffness from FEA. If that includes the whole chassis & suspension assembly (hub-to-hub stiffness in other words) then it is probably acceptable, but if that's "frame only" then I'd consider it a bit more soft than desirable. If your analysis was over-constrained in any way to inflate your results, or any errors in welding/assembly could mean you're even further away from a good target.

I'd recommend doing a physical test of your hub-to-hub torsional stiffness. Or an alternative that may be easier to set up, you can measure your wheel rates on scales - if they're significantly softer than what your wheel rate calculations would predict, you've most likely got problems with installation stiffness of your belcranks.

[CWA]

I think some of the focus shifted on to figuring out the suspension issue. Which while I appreciate it, is not exactly why we need a different differential. I do not think it is a "band-aid fix". While I see the point that the suspension is causing an issue, I don't see how a differential that will handicap the car in odd situations is acceptable. A different differential isn't the solution to this problem, its a solution to a few different problems. One of which, is the diff is currently untuneable.

Well I don't see how you can justify spending x-hundred of your sponsor's hard earned dollars when you: a) haven't even attempted to determine and address the root cause of the issue, and b) can't quantify (or even make estimates of) the gains fixing this 'issue' will bring you.

Your chassis setup causes the 'unloading of your rear inside wheel'. You described the 'unloading of your rear inside wheel' as an issue and the main reason for wanting to buy a new diff in your very first post. We are trying to help you cure this issue, but you seem to have no interest in this, and have it in your mind that you just have to have a new diff. If you are not interested in curing the issue, and are actually only interested in having a tuneable diff you can mess about with, or you just want to piss away money, don't try and gain support for this approach under the pretense of trying to objectively solve a problem.


I have a few hunches to our wheel lifting issues. One of which is an overall roll gradient vs wheel travel issue. The car was allowed to roll quite a bit in the last cars, and I think it rolls more than calculated. Which is easy to prove in this car, but I haven't had a chance to yet." - Suspension guy 2

Curious reasoning. I don't see how excess body roll in isolation can be the root cause of lifting a wheel. My VW Polo rolls a shed-load but doesn't jack a wheel. Therefore roll gradient alone is not the metric that will explain this behaviour.


We have scales and the car is on them frequently for testing. I don't have a data sheet with me to record actual corner weights.

????? Why post this?? Don't you have budget for pen and paper??


I guess I will have to track down the rest of those numbers myself when I find the time.

????


Steering Geometry: Look at the design tech sheet. Those are all "designed" numbers

?????


Ignore the ARBs and do not consider them into wheel rates.

Just how exactly should we be quantifiably considering the varying stiffnesses of ARB's, then???


Wheel Rate: 20n/mm seems low, maybe a conversion error? Check the tech sheet we turned in for lincoln, that will give you a closer estimate.

"Wheel rates are about 112 lbs/in on all corners" is what you posted on Page 1. Why don't you check the 'conversion error'??


CG height: 10.2 measured without wings, 10.5" with wings
said the cg raises about 1.5 in with wings

There is inconsistency between what your 2 suspension guys are saying. Why aren't they aligned?? Are they not on the same team?


"I think our actual weight dist. was 49%, -Suspension guy 1
Static weight distribution: 45% front actual - Suspension guy 2

I appreciate the diligence in relaying the info as you receive it Tyler, but come on! Weight distribution is what it is, it is not open to interpretation, this value shouldn't vary depending on source! Measure it!
_____________________

Some constructive notes:


Also we haven't explored 100% of the bar options, there are some left which could likely fix the problem by adding a lot of roll stiffness, but the car would probably understeer a ton (since we'd be adding most of that to the front).

The theory implies that more front bar/springs than rear gives more understeer. The theory more specifically says that a lateral load transfer distribution is what adjusts your balance. Lots of practical issues can interfere here, so that adjusting springs and bar do not actually change your LLTD as you might expect. These practical issues can be extremely significant, to the point you may not believe. Understanding what the theory should tell you is one thing, but don't be blind to what you are actually seeing, even if it disagrees with your current level of understanding of the theory.

As many have already stated, compliance if a practical consideration that can make basic spring/bar LLTD predictions redundant. If your chassis is too soft, stiffening front springs / bars won't actually adjust your lateral load transfer distribution. Have you verified that adding lots of front spring and front bar actually gives your car excessive terminal understeer? Try it. If it doesn't, your chassis is too soft, and all those LLTD predictions you are taking from Milliken equations under the assumption that your chassis is rigid are meaningless and far from the truth.


The suspension does not have bushings and does not carry much friction at all. Essentially is negligible for most calculations we do

More friction on one axle than the other will also take your car far away from the LLTD predictions you have based solely on spring/bar stiffness. I am curious to know how you are so confident that you have negligible levels of friction. I don't just mean control arm friction when unloaded, I mean net system friction when loaded, including your damper seals. Total system levels can add up to be quite significant. Even if you can see your suspension actuating, your friction levels may be considerably higher on one axle than another, contributing to an unexpected balance difference.


From what I've been learning, I'm pretty suspicious of our roll centers and how they work with the rest of the car

Yes. This is as Tim said, and is a possible cause. Why not explore it.


how the travel available is mostly for compression.

As Z suggested to you, be careful you are not hitting rebound stops (over-extending your damper). Once you reach the level of lat acc that causes on one of your inside dampers to full extend, you will see full, immediate load transfer across this axle. If this is happening on the rear axle of your car, not only will it completely reverse the base chassis 'understeer' balance you believe you have, but it could also very well contribute to lifting a wheel..


I don't remember our exact lateral g's but our best skidpad time was 5.381s. From our data acquisition we have seen up to 2.1g's during Endurance. I drove the skidpad event at Formula North and from what I could tell (I'm by no means a professional driver) the car wanted to over-rotate at the limit. The car doesn't appear to be jacking at all during steady-state. The time we noticed the problem most is the hairpin at the end of the slalom at the FSAE Lincoln Endurance course.

If you reach 2.1 g then your issue is not big. What kind of % reduction in lap time do you really believe curing this issue will bring to the team? Will that % be worth the amount of money you seem set on spending?

If you depart with oversteer on the skidpad, adding more torque to your outside wheel will not increase cornering capacity, so again a new diff will not benefit you here. Also, check that the terminal oversteer you appear to be seeing actually aligns with your predictions for steady-state behaviour. Why don't you have limit understeer? Is what you see versus what you expect acceptable?

Hairpins are usually the most steady-state of all the FSAE endurance event corners. What exactly do you think it is about the hairpin that is different to the skidpad that might help to explain why you see this inside wheel spin at the hairpin but not at the skidpad? Does the jacking only occur during turn-in at the hairpin? If so, why are you worried about getting torque down here, again, consider how you will actually be faster by removing this symptom.

[Z]

Tyler,

Based on your last few posts I reckon I now know your root problem(s).

A car a few years ago at FSAE-Oz had something similar. It was a DISASTER! From memory, it finished top five. Don't take this the wrong way, but poorly built and poorly set-up cars can still do well in FSAE competitions. You are only competing against other students. You only have to drive 20 miles at an average speed of ~35 mph to finish top of the ladder...

Anyway, some of the above clues stick out to me like a sore thumb. I would be more certain if I saw the car in the flesh. Even some photos would make it easier to know for sure.

But (!), the above mechanical details are not your MAIN problem. Your biggest problems include these:

* Your Suspension-Guys are incapable of instantly quoting the most important numbers in their area of responsibility. This is BAD. (What happens when they have to remember birthdays, and wedding-anniversaries, etc.!)

* Too many theoretical numbers, too few measured numbers. Given the typically overconstrained FEA analyses of frames (as mentioned by CWA [Edit: Oops ...by JT A.]), I take your quoted frame torsional stiffness as MEANINGLESS. (BTW, once you have made the few simple bits that are needed, a hub-to-hub torsional test should take about ten minutes. Every initiate Team-member should be required to do such a test, along with all the other important measurements, before they are accepted as official Team-members.)

* You are chasing a complicated solution of a "tuneable" diff, even though your Team is incapable of tuning what they already have in front of them! A tuneable diff will simply add one more riddle to your already long list of unsolved puzzles. (Yep, I know of one mid-field Oz-Team that bought a Drexler a few years back (at ~Aus$4K!) because of "spinning inside-rear". They are still mid-field, the inside-rear still spins, and the Drexler has never been opened ... because no one knows how to adjust it!)

Decide whether to go with an open or locked (=spool) diff, then adjust the rest of the car to suit.

Oh, and it is perfectly acceptable to "look over the shoulders" of the winning Teams. This is called "researching the prior-art". For example, my records show that 1st*, 2nd, 3rd, 4th, and 5th at last year's FSAE-Oz-14 all ran spools. Now, what's the name of that Team that recently won the big US Michigan-15 comp...? :)

(*Edit: Oops! NOT 1st (Monash had a Drexler), but the others all had spools. And so does that Team that keeps winning all the Northern hemisphere comps...)

Z

[Tyler Jones]

Well I don't see how you can justify spending x-hundred of your sponsor's hard earned dollars when you: a) haven't even attempted to determine and address the root cause of the issue, and b) can't quantify (or even make estimates of) the gains fixing this 'issue' will bring you.

Your chassis setup causes the 'unloading of your rear inside wheel'. You described the 'unloading of your rear inside wheel' as an issue and the main reason for wanting to buy a new diff in your very first post. We are trying to help you cure this issue, but you seem to have no interest in this, and have it in your mind that you just have to have a new diff. If you are not interested in curing the issue, and are actually only interested in having a tuneable diff you can mess about with, or you just want to piss away money, don't try and gain support for this approach under the pretense of trying to objectively solve a problem.



The initial purpose of the thread was just to obtain some technical information about the Wavetrac, not to ask for suspension advice. This is why I'm appearing biased, I was already decided on switching to the Wavetrac if I could obtain the necessary drawings and info. I'm also not very knowledgeable about vehicle dynamics, so I can't comment much myself. I will try to get my suspension guys to respond more directly to your questions, as they are the ones against keeping our current differential.




????? Why post this?? Don't you have budget for pen and paper??



We are not all in direct proximity to the car at all times, I will take the time to try to get corner weights for the suggested scenarios this weekend.




????



My suspension guys apparently don't have the time to get some of the numbers you want so I will have to try and find some of them myself, most should be on spreadsheets on our network harddrive.




Just how exactly should we be quantifiably considering the varying stiffnesses of ARB's, then???



I'm not sure they fully understand how ARB's contribute to overall wheel rates.




"Wheel rates are about 112 lbs/in on all corners" is what you posted on Page 1. Why don't you check the 'conversion error'??



According to that number, 20n/mm is accurate. Not sure where he is coming from.




There is inconsistency between what your 2 suspension guys are saying. Why aren't they aligned?? Are they not on the same team?



They don't work directly together as much as I think they should. This is more a team organization issue which I will try to resolve this coming competition year.




I appreciate the diligence in relaying the info as you receive it Tyler, but come on! Weight distribution is what it is, it is not open to interpretation, this value shouldn't vary depending on source! Measure it!



I stated before that before Formula North the car weighed in at 45% front and at Lincoln the car weighed in at 49% front. Ride heights and aero were adjusted between those two competitions so that may be contributing to the difference. Again, they don't work directly together as often as they should. The whole team has some communication issues I'm trying to resolve.



_____________________

Some constructive notes:



The theory implies that more front bar/springs than rear gives more understeer. The theory more specifically says that a lateral load transfer distribution is what adjusts your balance. Lots of practical issues can interfere here, so that adjusting springs and bar do not actually change your LLTD as you might expect. These practical issues can be extremely significant, to the point you may not believe. Understanding what the theory should tell you is one thing, but don't be blind to what you are actually seeing, even if it disagrees with your current level of understanding of the theory.

As many have already stated, compliance if a practical consideration that can make basic spring/bar LLTD predictions redundant. If your chassis is too soft, stiffening front springs / bars won't actually adjust your lateral load transfer distribution. Have you verified that adding lots of front spring and front bar actually gives your car excessive terminal understeer? Try it. If it doesn't, your chassis is too soft, and all those LLTD predictions you are taking from Milliken equations under the assumption that your chassis is rigid are meaningless and far from the truth.


We have experimented with somewhere around 150-225lb/in but have springs up to 400lb/in. I will try to verify when we get a chance to test, which may take at least a week before we have the opportunity. I would hope our chassis is reasonably stiff as it weighs ~83lbs with powdercoat and the whole car weighs 405lbs without driver, but I understand that may not be the case.




More friction on one axle than the other will also take your car far away from the LLTD predictions you have based solely on spring/bar stiffness. I am curious to know how you are so confident that you have negligible levels of friction. I don't just mean control arm friction when unloaded, I mean net system friction when loaded, including your damper seals. Total system levels can add up to be quite significant. Even if you can see your suspension actuating, your friction levels may be considerably higher on one axle than another, contributing to an unexpected balance difference.



Do you have a suggestion on how we can verify levels of friction from one axle to the other? The control arms pivot about sphericals and the rockers/bellcranks pivot about needle bearings so they shouldn't hold much friction. I don't see this being a major contributor to the overall issue, but feel free to correct me.



Yes. This is as Tim said, and is a possible cause. Why not explore it.


What should I tell my suspension guys to look into specifically? Are the roll centers too low or too high? Again, I'm the wrong person to be answering these questions. Is there a good resource to familiarize myself with vehicle dynamics on a basic level? (I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)



As Z suggested to you, be careful you are not hitting rebound stops (over-extending your damper). Once you reach the level of lat acc that causes on one of your inside dampers to full extend, you will see full, immediate load transfer across this axle. If this is happening on the rear axle of your car, not only will it completely reverse the base chassis 'understeer' balance you believe you have, but it could also very well contribute to lifting a wheel..


How do we resolve this, if it is happening?




If you reach 2.1 g then your issue is not big. What kind of % reduction in lap time do you really believe curing this issue will bring to the team? Will that % be worth the amount of money you seem set on spending?

If you depart with oversteer on the skidpad, adding more torque to your outside wheel will not increase cornering capacity, so again a new diff will not benefit you here. Also, check that the terminal oversteer you appear to be seeing actually aligns with your predictions for steady-state behaviour. Why don't you have limit understeer? Is what you see versus what you expect acceptable?

Hairpins are usually the most steady-state of all the FSAE endurance event corners. What exactly do you think it is about the hairpin that is different to the skidpad that might help to explain why you see this inside wheel spin at the hairpin but not at the skidpad? Does the jacking only occur during turn-in at the hairpin? If so, why are you worried about getting torque down here, again, consider how you will actually be faster by removing this symptom.

Personally I think we perform very well as is, it is my suspension guys that wanted to get a new differential in the first place. The Wavetrac is the most economical differential that solves the issue of an unloaded wheel. I'm more or less trying to facilitate the switch should the rest of the team decide that it is worth the extra $200 and time spent. As is, myself and the Technical Director think we should stick with the Torsen unless I can get those Wavetrac internals drawings before the school year begins. I'm still not certain why everyone in this thread is so against the switch for financial reasons when $200 is next to nothing compared to the ~$20,000 we raise each year to spend on the car. I actually reduced the cost of the 2014-2015 car by $600 compared to the 2013-2014 car by switching away from a Tilton bias bar to one constructed with McMaster hardware, so you could almost say I covered the cost of a different differential already.

I guess I didn't consider that a hairpin is steady-state. I feel as though the extra speed carried followed by hard braking before turn in might contribute to more load transfer/roll but as I said before, I'm not a professional driver nor am I an expert on Vehicle Dynamics. The jacking is only really noticeable in the hairpin, but when it is noticeable it is really noticeable. The inside rear wheel actually completely lifts by 1-2" off the ground. This makes me think that the issue might be related to over-extending the damper as you said. Let me know what you think.

[CWA]

Well it is good that you can accept that your team can be better aligned. I won't harp on about this any more, there is only so much you can do and I appreciate what a challenge things like this can be. I know I'm no good at these kinds of things. There is a reason that there are so many team / project management roles in industry.


The initial purpose of the thread was just to obtain some technical information about the Wavetrac, not to ask for suspension advice

Well, perhaps. But by including your reasoning behind your decision to change the differential in your initial post, you have faced some scrutiny. This scrutiny, whilst unexpected, and probably not what you wanted, is exactly what you will be given by the design judges at competition, is it not? You'll be asked to justify many decisions you have made regarding hardware purchase / calibration in exactly the same way. If you don't like it / can't handle it now, you will surely struggle in the design event. You should know as well as I do that "we had some spare cash left over so decided to blow it on a new diff without knowing if it will actually make us faster or not" will not cut it in front of a design judge. Even if you do have the cash lying around, this is not what FSAE engineering, nor engineering in the real world is about.

Your team should embrace this problem as a chance to develop their knowledge and understanding. Instead, your suspension group at least, seems rather defensive and unwilling to act on an issue which should certainly be of great concern to them. Also, I can tell Tyler that you are curious as to whether you can make a simple tweak that will negate the need for a new diff. If this is something you want to know, you'll have to ensure your suspension team collaborate in providing this basic vehicle setup info. If this is something you don't want to know, and you just want to buy the diff anyway, then ok, but on a personal level there is nothing more I can contribute. And I'm sure a lot of other members will frown if this is the process you decide. But this is completely up to you.


I'm not sure they fully understand how ARB's contribute to overall wheel rates

If not this way, how else are your suspension guys numerically predicting an ARB's contribution to LLTD? They must have some idea, or how else would they be able to state that the largest ARB they have will give 'too much understeer'? How do they know that doubling the largest bar they have won't still deliver understeer that is within an acceptable range? If you have not been quantifying how each ARB contributes to 'roll stiffness' or 'wheel rate' at each axle (these metrics effectively describe the same thing), then you can't possibly predict your balance as you have claimed.


I stated before that before Formula North the car weighed in at 45% front and at Lincoln the car weighed in at 49% front. Ride heights and aero were adjusted between those two competitions so that may be contributing to the difference

OK. As a matter of interest, does your car lift it's wheel on a hairpin with and without the wings? Also, for the record, it's not too important in this case, but know that ride height changes (or 'rake' / static pitch angle changes) do not alter your static front / rear weight distribution. At least not by any percentage the useful side of a decimal point, given the pitch angles feasible to a race car with 2" of suspension travel. This is a filthy myth that somehow seems to be quite rife on the racing scene.


Do you have a suggestion on how we can verify levels of friction from one axle to the other? The control arms pivot about sphericals and the rockers/bellcranks pivot about needle bearings so they shouldn't hold much friction. I don't see this being a major contributor to the overall issue, but feel free to correct me

I know from experience that friction levels can be great in spherical bearings and dampers. Our damper rod end seals were horrendously stiff (bad product purchase, cheap brand), and our brand new control arm spherical bearings were staked into their housings too tightly, resulting in lots of friction.

To quantify the friction levels on your car; a) lift car up so that all four corners droop when unloaded, b) lay the car back down on the ground until springs support it, c) roll it a couple of metres to relieve the tyres of any jacking force, d) measure ride height / damper travel (this is 'position A'), e) have a person stand on each end of the car at the same time so that the suspension is in bump, f) have these people stand back off the car, so that the springs try to raise the car back up to ride height, g) again roll the car a couple of metres to relieve the tyres, h) re-measure the ride height / damper travel, call these new readings 'position B' because they will sure be different values to position A.

If you know your actual wheel rates, you should now be able to mathematically derive some friction levels for each axle. Then when you start to compare these friction force values to your known normal load variations caused by dynamic load transfer, you can start to consider whether this friction is 'too much', or not. Also consider the the difference in friction levels between axles, this delta is what will contribute to unexpected balance changes away from target.


What should I tell my suspension guys to look into specifically? Are the roll centers too low or too high? Again, I'm the wrong person to be answering these questions. Is there a good resource to familiarize myself with vehicle dynamics on a basic level? (I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)

Milliken is a great starting point, hopefully your suspension guys are living out of RCVD right now. If you can't stomach Milliken, definitely don't try Pacejka or Matchinsky.

RC's too high, as I think Tim might have said. There are some great threads in this forum on control arm jacking, the most useful posts I've found on the subject are by Z. Try and find these, they helped me understand jacking with more clarity than any book I've read. Z also explains the limitations of the RC metric too, which should also be heeded when considering control arm geometry.


How do we resolve this, if it is happening?

Have a look good hard look at your system, you should be able to creatively figure this out on your own (or rather, tell your suspension guys to). If you are lucky, you may have adjustable spring seats, allowing you to retain your spring and wheel rates but change the resting place of the damper rod relative to the body for a given ride height. If this is not an option, things may be more awkward for you.


I guess I didn't consider that a hairpin is steady-state. I feel as though the extra speed carried followed by hard braking before turn in might contribute to more load transfer/roll but as I said before, I'm not a professional driver nor am I an expert on Vehicle Dynamics. The jacking is only really noticeable in the hairpin, but when it is noticeable it is really noticeable. The inside rear wheel actually completely lifts by 1-2" off the ground. This makes me think that the issue might be related to over-extending the damper as you said. Let me know what you think

No you are correct, the entry and exit of a hairpin are transient. To be honest, I think I was a bit liberal in deeming an FSAE hairpin as steady-state, as it will be barely be, I think I gave the wrong impression here. Here are three bunches of some further, loose thoughts from me. But for them to develop any further, and lead into a potential solution, investment from your team is required:

1. If the wheel is only jacked at the hairpin during braking / turn in, you shouldn't be trying to apply drive torque here, so why are you concerned by it? As soon as you release the brakes / longitudinal load transfer reduces to zero / reverses direction, the jacking should cease / wheel should grip the road again. Is this the case? If so, again, think about how much you will benefit by addressing the issue here.
- If the jacking only occurs during turn in, but you still want to address it, it could be that your inner rear damper is becoming over-extended due to combined cornering and braking load transfer. Or it could be that your rear damper rebound rates are significantly higher than your bump rates, and the jacking is a consequence of the transient body roll velocity during turn in. There are some tests you can perform to rule each of these mechanisms out.

2. Perhaps the wheel remains jacked for as long as you hold the steering wheel at roughly the same angle (the tightest it ever needs to be = to negotiate the hairpin) for the duration of the turn, regardless of whether you are braking or applying drive torque. Should this be the case, this does indicate that steering system jacking could be your strongest contributor. Post up those corner scale measurements / your KPI and trail values asap! If the hairpin turn radius is less than the skidpad turn radii (I can't remember the event formats off the top of my head), this may explain why your wheel remains jacked during the hairpin but not during skidpad.

3. If the jacking behaviour is only ever seen on one corner of one track, consider, is the track skewed or uneven at this corner where you see this behaviour? If you were to set up some cones and repeat the braking / hairpin manoeuvre on a known flat test surface, does your wheel still jack? Even if you deduce that the jacking is caused by an uneven road surface, there is still an aspect of your car's setup that is facilitating this behaviour - I'm sure no other cars jack their wheel on the same corner.

[DougMilliken]

...(I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)
We never expected anyone to read it cover-to-cover, I'm always amazed (and pleased) when someone says they have.

Off topic:
This reminds me, there was a funny picture of a guy on his back with RCVD opened over him -- as if he fell asleep and it fell on him (or maybe it was real and not a pose?) I didn't snag a copy when I first saw it, and haven't seen it since. Anyone know where this was posted? Link please!

[Steve Krug]

Reason from the fundamentals:

F=ma, a =v^2/R, T =Iα. Newton's laws with undergrad mechanics of materials can get you a long way in FSAE if they are understood well across the vehicle. I.e. it is worth asking, what supplies the F, a, and T and how much of these do you want in what directions and magnitudes? How do you maximize the F, a and T? Do you even want to maximize them? What combinations of these do you want over the course of a lap? You've got this 550lb mass navigating mostly in 2-D, what needs to happen to this force-moment model to reduce the time it takes for this mass to get from the start to finish line? How does suspension influence this? What does the differential do to influence the forces, accelerations, and yaw moment? Do you even care? Will the differential solve the problem you're trying to solve?

My best advice would be to find a way to make learning engineering not seem like a chore. Be inspired to read RCVD, go to/pay for OptimumG seminar, Carroll smith books, etc.. (An FSAE car with good aesthetics help this and can be a good motivator, also a great way to recruit new members!)

[FunkyChicken]

We tell our guys to start with Tune to Win before going into RCVD. Tune to Win when read in its entirety gives a great foundation for the kinematics of a vehicle and how the systems work together. I use RCVD as a textbook of sorts. The important thing I think your suspension guys need to see is that the entire car works as a system, and there's probably not one quick, easy fix to the problem you're seeing. Something I try to drill into my members heads is that the big picture matters more than the details. It could be that the entire system of your chassis is causing the problem, it could be a setup adjustment that was overlooked. You'll only find out through extensive testing.