(First) suspension design - feedback please

[Kemper]

Hello there everybody!

As some may already know, our team (K-Alpha - KAIST South Korea) is starting this year. And, among other stuffs, I am member of the suspension design team. Think about two freshmen that never had vehicle dynamics before reading (ok, consulting) Milliken, Olley and other famous books on suspension/chassis all at once and trying to make something that seems good. Despite the title, this is not our first suspension design, but is, in fact, the first that we have the guts to post here to be criticized by the ones who been in our place before.

First of all, we are not sure about anything we did. Our design process consisted first on finding rules that limit things (the "bus chassis template", for example), the size of things that limit things (tires, wheels, brakes, wheel hub, bearings blah blah blah) and other designs. Then, concepts that we heard from someone or read somewhere (like the roll center position, upper/lower a-arm ratio...) and then put everything together on solidworks in a 2D fashion, tilt the chassis 3 deg to the left, check the camber, 3 deg to the right, check the camber, mirror the "car" and make an analysis on the roll center movement while tilted. If everything seems "right", then proceed for the side view analysis (for anti-dive / anti-squat stuffs). Funny thing is that we though we had everything right for 3 times or so, but when we made the suspension design again, the former design looked shitty. That's why we finally decided to post something here.

Some of our design data:
Wheelbase - 1550mm
Front track - 1200mm (tire center line to tire center line)
Rear track - 1390mm (tire center line to tire center line)
Wheel size: 13"
Tire out radius: 21"
Suspension travel: +-1"
Ground clearance: 1.25"

Initial Camber
Front: 1.57"
Rear: 1.5"

We want to test the suspension model on Adams, but as we know nothing about the software (although we got the software license sponsored), it might take some time. Any one in the mood for a "quick and dirty guide"? =)

Enough talk, pictures follows:
(by the way, the "chassis section" is just a bunch of lines between the a-arm mounting points, no link with how our chassis will look like)
http://farm4.static.flickr.com/3154/2840209258_b85766684f_o.jpg
http://farm4.static.flickr.com/3094/2840208440_2cd1996236_o.jpg
http://farm4.static.flickr.com/3041/2839372667_bb248410dc_o.jpg
http://farm4.static.flickr.com/3054/2839370893_3f911348ef_o.jpg
http://farm4.static.flickr.com/3210/2840202192_a725499cb4_o.jpg

Now, rock-throwing time! lol

[Matt N]

I have designed one suspension and these are things I would consider given what you told us here. Also, don't read into my notes as if I have the answer for the "ideal" value - I don't.

You might consider using more than +- 1 inch of suspension travel - think about jacking, running on uneven lots/big bumps.

It looks like you are using centerpoint steering (your steering axis goes through the center of the tire in front view). You may not like the steering feedback this gives. Research it.

If your front track is really narrower than the rear, you will probably find that the drivers hit cones with the inside rear tire. There are some good posts on this forum on this subject which you might want to search for.

3 degrees of roll is an awful lot. Hopefully you are using this analysis to highlight the trends of what happens at the extremes of movement. Most FSAE teams end up with cars that roll under 2 degrees at full cornering.

Matt
WWU FSAE

[J.R.]

If you don't have Tyre data, suspension is mostly guesswork. Looks pretty good except the rear track being larger, that will also reduce your handling ability, in addition to hitting cones. If packaging is a concern, increase front track.

Good job keeping the tyres from toeing out on the outside, a problem that we had this year. With your static camber, getting 1.57 deg might be next to impossible for manufacturing. Don't forget to make sure that the camber is adjustable since the actual results may differ due to compliances etc. Theory works great to start, but your racing a car at comp., not the model.

You might want to do a basic load transfer analysis on your system now that you've got the basics, and try to figure out what loads the wheels will see and adjust your camber values from there.

Keep up the good work!

[flavorPacket]

rear track is WAY too wide. You will not be able to make it around tight corners.

front balljoint is too close to wheel center. will really hurt your compliance.

[murpia]

For a 'first post' that's pretty impressive and a credit to you...

Some data that would help us critique the design a bit more would be some graphs:

Camber gain vs. wheel bump position
'Antis' vs. wheel bump position

I agree with the comments that rear track > front track could be an issue. Consider making the outer width of the car equal front and rear. If you use equal sized tyres then the tracks will be equal. If the rear tyres are wider then in fact you will need a wider front track.

Regards, Ian

[Aaron@Kaist]

anybody wondering how the eff Kemper is a Korean name?? hahahahahhaa our formula team has one american team manager, two brazilian guys on the suspension, a chinese guy on the engine, and a few koreans doing god knows what hahahahahahhaha Kemper you are awesome

[exFSAE]

Good, lot of work done, but as has been said for us or a design judge to really "get it" on the fly, having some suspension curves would be good.

I'd be interested in what the loads, cambers, and steered angles are for all the tires during

(a) Pure braking
(b) Trailbraking / entry
(c) Mid corner (including driver steered angle)
(d) Corner exit
(e) Straight-line WOT

Also, what steering torque you expect at max lat g's at least. Steering feedback is good.. but driving like you're in a wrestling match is not.

[Kemper]

Thanks for all the feedback! You have no idea how nice is to hear that we are going in the right way!

But not everything is perfect, and we know that... I guess we are in the "guesswork suspension" now, considering that we cannot interprete very well tire data. I know how important for the design of a good suspension is to make it based on the tire data but we needed to do something with our limited knowledge and this came out. But, just to ask, how bad can a good suspension desing become without considering the tire data? We know it will affect a lot, but what we need to know is if the difference is between a 1st place and a 4th place kind of variation or a difference between a car that finishes a race or one that kills a driver? (a bit too extreme, but...)

About the track size, we had this "huge" track on the rear because we already had the drivetrain components and, to don't waste money we did the smallest track possible with long halfshafts. But as different tracks might not be a good idea, we did other two drafts:
Draft 2 - Narrow tracks
The idea is to spend another 400bucks on 2 new shorter driveshafts and save some weight making a smaller car (and something tells me that it becomes nimbler too...). The roll centers move something less than 20mm from -2 to 2 deg roll on both front and rear suspension (not considering tire deformation). The range of our suspension can be 1.25bump - 1.5 rebound. but we will need a clearance of 1.3 inches.

Draft 3 - Wide Tracks
Saves money, and maybe gives us some stability on lateral acceleration issues. Is there any other benefit of wider tracks? The roll centers are moving less than 60mm from -2 to 2 deg, bump/rebound range the same of draft 2.

Please don't see us as lazy people, but can you give us some light on how to make the graphics? We did a "roll angle/camber" graph for an older design but as we did it on excel (read: tilt .3 deg on solid works, take note of camber, repeat), it was too painful and somewhat hard to believe it is the right approach.

Ah, about our steering linkages: We haven't choose the position of it yet (side view), although the front view we set in this way:
steering linkage should have the same lengh of the lower A-arm (front view) and be parallel to the lower A-arm in order to make no interference in the suspension behavior (we called it "parallelogram geometry"). Does it work the way we thought? Or making it "front-view-wise" coincident and with same lenght of the lower A-arm is more reasonable to avoid bump steer? (I'm not very sure about what I just wrote, but anyway... lol)

The anti's on the first post, we did a rough calculation on where each suspension SVSA IC should be and tried to make something that looked reasonable. Does it? Also, we were struggling on figuring out how wide should an A-arm be (side view). Any magic formula/rule of thumb to calculate it or is the aesthetical pleasant choice?

Enough talk, here it comes what we have of data, made in 1 day (and we also have classes). I am sorry that we have no graphs, but again, is because we have no idea about how to make them accurately...


http://farm4.static.flickr.com/3251/2843363878_f2007e7e92_o.jpg

http://farm4.static.flickr.com/3204/2842529049_d6b821b3e0_o.jpg

http://farm4.static.flickr.com/3145/2842529223_eb129b710b_o.jpg

http://farm4.static.flickr.com/3145/2842529541_528ff6e39b_o.jpg

http://farm4.static.flickr.com/3093/2843364738_a37f7fbfc0_o.jpg

http://farm4.static.flickr.com/3197/2843364994_460cb35a9d_o.jpg

http://farm4.static.flickr.com/3130/2843365142_f4e742fe05_o.jpg

http://farm4.static.flickr.com/3193/2842530201_d269883fab_o.jpg

Another questions:

"getting 1.57 deg might be next to impossible for manufacturing" - why? (I really don't know)
Can't we just set up the position of the ball joints in a fashion that it will be initially with 1.57 or is it due to welding problems, machining problems, material flexing and all that it becomes impossible?

"Some data that would help us critique the design a bit more would be some graphs:

Camber gain vs. wheel bump position
'Antis' vs. wheel bump position"

The camber gain graph I can figure it out, but can you give us a hint on how to show the behavior of the anti's on wheel bump?


And, by the way, Koji (our SolidWorks guy) has a lot of credit here too, the reason that I am posting is, while I do the "meaningless" work of putting the data on the forum, he is doing the front wheel hub FEA analysis and other stuffs. The foreign division of K-Alpha is working hard (and this includes you too, the management is going great - at least on what I see and hear =] )

3 days "sleeping" on the design room and counting...

[Drew Price]

Originally posted by Kemper:
"....maybe gives us some stability on lateral acceleration issues. Is there any other benefit of wider tracks?


Less lateral load transfer during cornering, at the expense of 'nimblicity.'


Originally posted by Kemper:
"getting 1.57 deg might be next to impossible for manufacturing" - why?


You're basically onto it. Manufacturing tolerance stacking, therman expansion and stresses, how flat and level your setup floor is, etc. You can probably get the 1.57*, since you will want your suspension adjustable (for camber and toe at an absolute minimum) to account for these things, as well as to make testing and tuning adjustments. Getting 1.57* during your alignment procedure will depend on how accurately and repeatably you can measure the camber angle in the first place. With a nice digital level and a perfectly flat surface plate, maybe. With a string, a ruler and a plumb bob, probably not. Besides, how are you going to tell if your wheel rim halves are flexing 0.03*, that will throw you off. Usually camber is specced to within few tenths accuracy.

Your ideas on the steering pivots are pretty correct, I think you have it.

For a-arm 'splay', or how wide the points are in side view, usually it's a strength calculation, you want them light, but you also want them not to buckle, you have to balance those.

Best,
Drew

[Wes Johnson]

You may want to consider your camber versus steering curves. Kingpin inclination seems a bit on the high side to me, but you will have to factor in your caster numbers and find out what you want your camber to do with respect to steering.

I could see how one would like to run 0 front view kingpin offset, but steering induced diagonal load transfer isn't necessarily a bad thing, and perhaps even less of a bad thing when you are compromising your camber angle to get to 0.

WJ

[bahous]

i'm not part of the team that started this post, but it doesn't stop me from asking you guys a question:

how much kingpin and caster angles would you want for the rear suspension?

What I'm sure of is that the instantaneous center (IC) should be further than the front suspension in order to induce less camber gain when accelerating to have the biggest contact patch between the tire and the road.

But since the rear wheels are not steering, what conclusion can you take regarding the kingpin and the caster angles?

JP Bahous

[Bill_Murray]

You might want to put a little thought into what wheels, brake, and uprights you plan to use. If you are planning to buy wheels, the offsets they come in are limited. From your pictures it looks like you haven't left much space between the outboard suspension points and "mock" wheel center. I little bit of time considering the outboard packaging during this phase of design can save hours of cursing later.

While on the packaging topic, same idea applies to the A-arm clearance on the wheels. Packaging your ball joints too close to the wheels may leave very narrow A-arm options or limit you steering ability.

Insuring a bit of space and margin for error in these areas before you become too set on geometry points will save time later.

[Kemper]

We have the brakes, rotors, wheel flanges and etc. modelled. The wheels are our main problem. We were going to buy one custom from Kodiak, but they stop answering us (don't know what happened). Anyway, our configurations are not the most bizarre (in my conception): 13"x6" +1" offset (towards outside) on the front. We modelled the one we would ask from Kodiak in order to check whether the upright and joints would be touching. Our biggest issue is being with the steering system, that we are not finding a good place to fit that doesn't hit the rims or is in the middle of the template...

About the kingpin and caster angles, we are running on 6.5deg kingpin on the front, 8deg on the back, caster of 4deg on front wheels and no clue what caster to use on back (zero maybe?). The suspension is behaving nice without tire models (at least on a freshman's concept of nice...). I will update the "graphs" soon

About the IC (I'm supposing you are not talking about the SVSA one -for anti-squat-) on the rear, ours is on a place that doesn't force the A-arms mounting points on the rear subframe assembly, while keeps the camber change "stable". We haven't test the IC higher, but it might conflict with our drivetrain as well (i don't want extra load on the driveshaft).

[Steve O]

bahous:

I am a little confused as to why you would have kingpin on the back? The primary function of your kingpin incline on the front is steering geometry and more notably steering response/feel. To simplify it a lot, to much kingpin and you will be physically lifting your car through your steering wheel which = heavy steer and tired driver. If you don't have enough you will be fighting to get your wheel to turn back to center. The right amount is a personal preference, do you want only some center seeking feel or do you want your wheel to go back to center when you let go of it and punch the gas with the trade off of course of heavier steering. Kingpin will affect certain other aspects of your steering suspension and geometry but I fail to see why you would need KPI on your rear wheels unless you had RWS. Ideally you want your contact patch from your tire inline with your upright mounting points, it will be the best transfer of road to suspension; placing KPI in the rear only keeps you from doing that. You may want caster/camber individually adjusted on the rear for one reason or another but not a KPI.

Steve

[Pennyman]

Originally posted by Steve O:
bahous:

I am a little confused as to why you would have kingpin on the back? The primary function of your kingpin incline on the front is steering geometry and more notably steering response/feel. To simplify it a lot, to much kingpin and you will be physically lifting your car through your steering wheel which = heavy steer and tired driver. If you don't have enough you will be fighting to get your wheel to turn back to center. The right amount is a personal preference, do you want only some center seeking feel or do you want your wheel to go back to center when you let go of it and punch the gas with the trade off of course of heavier steering. Kingpin will affect certain other aspects of your steering suspension and geometry but I fail to see why you would need KPI on your rear wheels unless you had RWS. Ideally you want your contact patch from your tire inline with your upright mounting points, it will be the best transfer of road to suspension; placing KPI in the rear only keeps you from doing that. You may want caster/camber individually adjusted on the rear for one reason or another but not a KPI.

Steve

Why would having your tire contact patch inline with your pivot points create "better" load transfer? This simply doesn't make sense to me.

You would agree that KPI in the front reduces the scrub radius(among other things), reducing the amount of steering kickback under braking, so obviously the reason why you want KPI in the rear is to reduce longitudinal forces reacted through the suspension under hard acceleration. It has to do with compliance too. The farther the contact patch from the KPI intersection with the ground, the more the tire will want to tear the upright and toe links right off the chassis when you accelerate.

[BilletB]

Here are a couple of my quick observations...

1) You are gaining positive camber in roll, why? I see the 'important' tire (outside tire) going to more positive camber at 2* and 3* of roll. The inside tire gains more negative camber than your outside tire. Again, why? Am I missing something? Do you know what camber angles you are shooting for during steady state cornering?

2) As said, the large rear track width. I wouldn't mind a car having a large track width, but I only see trouble stemming from a rear track width larger than the front.

3) Roll center.. Eh.. I say mostly ignore them. Look at IC's. It looks to me as though your IC's are pretty far off the ground. That can be ok, but have you considered their jacking effects on the car in cornering?

4) Side view anti's. Why so much anti- geometry? IE what is your justification? I don't like them, doesn't mean they're always bad/wrong, but I think they need heavy justification.

Just my observations and some points I hope can generate some thought. Seeing as this is your teams first attempt I say you're on the right track to learning a lot and being successful. Keep it up!

[J. Vinella]

Originally posted by BennyHL:
Here are a couple of my quick observations...

1) You are gaining positive camber in roll, why? I see the 'important' tire (outside tire) going to more positive camber at 2* and 3* of roll. The inside tire gains more negative camber than your outside tire. Again, why? Am I missing something? Do you know what camber angles you are shooting for during steady state cornering?

2) As said, the large rear track width. I wouldn't mind a car having a large track width, but I only see trouble stemming from a rear track width larger than the front.

3) Roll center.. Eh.. I say mostly ignore them. Look at IC's.



1) If your outside tire gains negative camber in roll then your FVSAs (Front Virtual Swing Arm) are less than half track. Great for outside camber in roll but we also brake and accelerate too. This will kill your cambers in pitch and heave. You will find that most teams have FVSA more than half track. Look at UC - Boulder's suspension from a few years ago. I think they came close to half track. Jersey Tom might comment on the resulting consequences. Oh and both tires are "IMPORTANT"!

2) Large is relative, but you must also be able to navigate the course. A compromise between "load sensitivity" vs racing line must not be underestimated.

3) Sigh...Benny think about that one. http://fsae.com/groupee_common/emoticons/icon_wink.gif

UW Formula SAE #118 @ Michigan, missed #120 by that much.

[BilletB]

Yep, you've got it... I went fishing a 'lil off the deep end on #1. However, teams have run half track FVSA's and been successful. I guess my point is to think about which part of dynamic handling to place the most emphasis on. I have my opinions and others have theirs. As a new team I think it's extremely important to grasp this part of the competition early. It's not an F1 car, it's not a circle track car, etc. etc. It is an FSAE car!

Large is always relative. But the relativity of rear track greater than front track brings about a big issue for an FSAE car.

As for roll centers, I won't get started. IC's are the best representation of suspension geometry in the 2D world, imo. Roll centers are nothing but a use of IC's to simplify something which isn't so simple. =] It's a 'tool' to have, but not an important one.

[df_fsmb]

Hi guys!

I will not start a new thread, as this one is appropriate for my question.

We are a new team, designing a car for 2011 comp. We are not students of automotive engineering, but general mechanical eng., so we don't have vehicle dynamics courses.

We are certainly learning about vehicle dynamics through Milliken books etc. and someday hopefully we will make a full mathematical model for vehicle dynamics. But not yet for next year's car so we need a simple approach to suspension design.

I would just like to have your opinions on that approach. Is it a solid start or a complete waste of time?

Here it goes.

We will limit our interest on tyre loads and camber changes during a steady-state cornering.

1.) Model an initial suspension geometry and attach it to an infinitelly rigid chassis. (using Abaqus, for example). That naturally includes picking the initial wheelbase, tracks and CG position.

2.) To simulate a corner, load the model with a horizontal force in the CG.

3.) Observe tyre loads and camber angle and then calculate the lateral force (from TTC data, for a chosen slip angle) to see if the car would stand a cornering force from step 2. If it does/doesn't load it more/less to determine what cornering force the car would be able to reach with a given suspension geometry, spring rates (and slip angle).

4.) Iterate the suspension geometry (or wheelbase, track...,) and do it again...

Of course we can than also observe roll angles and so on, but it is mainly to get some idea of what effect do changes to suspension geometry have on load transfer and lateral tyre force, of course.

[BrandenC]

Originally posted by df_fsmb:
Hi guys!

I will not start a new thread, as this one is appropriate for my question.

We are a new team, designing a car for 2011 comp. We are not students of automotive engineering, but general mechanical eng., so we don't have vehicle dynamics courses.

We are certainly learning about vehicle dynamics through Milliken books etc. and someday hopefully we will make a full mathematical model for vehicle dynamics. But not yet for next year's car so we need a simple approach to suspension design.

I would just like to have your opinions on that approach. Is it a solid start or a complete waste of time?

Here it goes.

We will limit our interest on tyre loads and camber changes during a steady-state cornering.

1.) Model an initial suspension geometry and attach it to an infinitelly rigid chassis. (using Abaqus, for example). That naturally includes picking the initial wheelbase, tracks and CG position.

2.) To simulate a corner, load the model with a horizontal force in the CG.

3.) Observe tyre loads and camber angle and then calculate the lateral force (from TTC data, for a chosen slip angle) to see if the car would stand a cornering force from step 2. If it does/doesn't load it more/less to determine what cornering force the car would be able to reach with a given suspension geometry, spring rates (and slip angle).

4.) Iterate the suspension geometry (or wheelbase, track...,) and do it again...

Of course we can than also observe roll angles and so on, but it is mainly to get some idea of what effect do changes to suspension geometry have on load transfer and lateral tyre force, of course.

It looks like you guys are on the right track. Don't forget to look at straight line acceleration and deceleration as well as cornering. You'll find what works well for one case might not be so good for the other, how you decide on a compromise is the tough part. Also, don't overlook compliance in the suspension system. Each suspension member is effectively a spring, all together it might be helpful to look at the suspension as a series of springs (which usually work against what you are trying to do camber wise).

[Bobby Doyle]

Originally posted by df_fsmb:
Hi guys!

I will not start a new thread, as this one is appropriate for my question.

We are a new team, designing a car for 2011 comp. We are not students of automotive engineering, but general mechanical eng., so we don't have vehicle dynamics courses.

We are certainly learning about vehicle dynamics through Milliken books etc. and someday hopefully we will make a full mathematical model for vehicle dynamics. But not yet for next year's car so we need a simple approach to suspension design.

I would just like to have your opinions on that approach. Is it a solid start or a complete waste of time?

Here it goes.

We will limit our interest on tyre loads and camber changes during a steady-state cornering.

1.) Model an initial suspension geometry and attach it to an infinitelly rigid chassis. (using Abaqus, for example). That naturally includes picking the initial wheelbase, tracks and CG position.

2.) To simulate a corner, load the model with a horizontal force in the CG.

3.) Observe tyre loads and camber angle and then calculate the lateral force (from TTC data, for a chosen slip angle) to see if the car would stand a cornering force from step 2. If it does/doesn't load it more/less to determine what cornering force the car would be able to reach with a given suspension geometry, spring rates (and slip angle).

4.) Iterate the suspension geometry (or wheelbase, track...,) and do it again...

Of course we can than also observe roll angles and so on, but it is mainly to get some idea of what effect do changes to suspension geometry have on load transfer and lateral tyre force, of course.

I would say the majority of FSAE students are in your boat - mechanical engineering majors with no access to formal vehicle dynamics courses. Don't feel alone.

Much of what you are attempting can be done using hand calculations built into an Excel worksheet. An FE model is unnecessary and would prove quite cumbersome in achieving the results you listed. Build analytical tools in Excel that utilize your tire data. Then design your kinematics around the results you gain from those tools. Don't forget about the fidelity of your analysis, from the tire data itself to the assumptions you make in your calculations.

Good luck.

[Neil_Roberts]

Chapter 14 of Think Fast is meant to guide students facing Kemper's dilemma: good engineering tools, great motivation, but no motorsports engineers available to provide suspension design targets to shoot for. Here are some of the geometry goals that I recommended in Think Fast. For the complete list, and the reasons for why I recommend them, you will need your own copy.

• Front and rear wishbone lengths as long as can be packaged
• Front and rear static ride heights, cambers, and toe settings that are representative of real setups
• Front force based roll center height within 0.3” of the ground plane, either above or below, and stable relative to the chassis within 0.01” through the full suspension stroke
• Rear force based roll center height adjustable between 1.1” and 2.6” above the ground plane, and stable relative to the chassis within 0.01” through the full suspension stroke
• Front static virtual swingarm length between 100” and 150”
• Rear static virtual swingarm length between 80” and 120”
• Front and rear bump steer within 0.1° of constant toe through the full suspension stroke
• Minimal anti-anything, minimum variation of the anti-whatever that there is through the full suspension stroke
• Minimal rising or falling rate
• Front and rear motion ratios that use up all of the damper stroke through the design suspension stroke

There is a link to thinkfastbook.com in the book list thread, and a thread called "New Book" by mmcdermott in this forum.

[BrandenC]

Originally posted by Neil_Roberts:
Chapter 14 of Think Fast is meant to guide students facing Kemper's dilemma: good engineering tools, great motivation, but no motorsports engineers available to provide suspension design targets to shoot for. Here are some of the geometry goals that I recommended in Think Fast. For the complete list, and the reasons for why I recommend them, you will need your own copy.

• Front and rear wishbone lengths as long as can be packaged
• Front and rear static ride heights, cambers, and toe settings that are representative of real setups
• Front force based roll center height within 0.3” of the ground plane, either above or below, and stable relative to the chassis within 0.01” through the full suspension stroke
• Rear force based roll center height adjustable between 1.1” and 2.6” above the ground plane, and stable relative to the chassis within 0.01” through the full suspension stroke
• Front static virtual swingarm length between 100” and 150”
• Rear static virtual swingarm length between 80” and 120”
• Front and rear bump steer within 0.1° of constant toe through the full suspension stroke
• Minimal anti-anything, minimum variation of the anti-whatever that there is through the full suspension stroke
• Minimal rising or falling rate
• Front and rear motion ratios that use up all of the damper stroke through the design suspension stroke

There is a link to thinkfastbook.com in the book list thread, and a thread called "New Book" by mmcdermott in this forum.

Are these kinematic goals aimed at designing a traditional race car, or for an FSAE car? What works well for sweepers might not be so good for boxes.

[df_fsmb]

Bobby Doyle and BrandenC, thanks for your answers. We did it both ways and both tools are giving us useful information, we believe.

Now I have another question, basically regarding the rules.

You need to have "at least 25mm of usable travel in jounce and 25mm of usable travel in rebound with driver seated".

What is 'usable' travel?

As I see it, this means that if you have, say, 600N of static load (with driver seated) on your front wheel, your maximum front ride rate can be 24N/mm. If you have lower rate, then the spring deflects for less than 25 mm in static situation and so you do not have enough 'usable' rebound travel.

Am I correct or am I missing something?

[BrandenC]

Originally posted by df_fsmb:
Bobby Doyle and BrandenC, thanks for your answers. We did it both ways and both tools are giving us useful information, we believe.

Now I have another question, basically regarding the rules.

You need to have "at least 25mm of usable travel in jounce and 25mm of usable travel in rebound with driver seated".

What is 'usable' travel?

As I see it, this means that if you have, say, 600N of static load (with driver seated) on your front wheel, your maximum front ride rate can be 24N/mm. If you have lower rate, then the spring deflects for less than 25 mm in static situation and so you do not have enough 'usable' rebound travel.

Am I correct or am I missing something?

That rule (B6.1.1) has always seemed extremely vague to me. Now that ride height can be under 25.4mm it seems even less meaningful. I've understood it to be that the suspension arms must be physically capable of moving enough that the wheel has at least 50.8mm of travel (half in jounce). Whether the suspension will actually actuate that much during a typical lap is beside the point. A rules inquiry would be the only way to know for sure though.

[Adambomb]

Originally posted by df_fsmb:
Bobby Doyle and BrandenC, thanks for your answers. We did it both ways and both tools are giving us useful information, we believe.

Now I have another question, basically regarding the rules.

You need to have "at least 25mm of usable travel in jounce and 25mm of usable travel in rebound with driver seated".

What is 'usable' travel?

As I see it, this means that if you have, say, 600N of static load (with driver seated) on your front wheel, your maximum front ride rate can be 24N/mm. If you have lower rate, then the spring deflects for less than 25 mm in static situation and so you do not have enough 'usable' rebound travel.

Am I correct or am I missing something?

Technically it's possible if you have "negative preload," ie when the car is in full droop the top of the spring doesn't touch the perches. Not that I would ever want to do that. I don't recall them being too picky on that rule, mostly that you had suspension that would move 2 inches without self-destructing. But again that is not an official answer.

[df_fsmb]

Thanks!

The intent of the rule is clear, as they want to assure that every team demonstates the ability to design a proper suspension (so that you don't come with spring/damper but 0 travel).

But, I think we will just go for the maximum stifness that still allows 26mm of droop.