forward vs rear tack width.

[Infinity]

Is it necessary to have the front track wider than the rear track? What shall be the outcome of increasing the rear track width ? Because,as far as I believe ,rear track plays least role in the steer part at least

[Brian Evans]

No.
Increased rear grip.
Maybe.

The traditional reason for the front track to be greater than the rear track was that the rear tires were wider than the front tires. If you designed to the same over-all width, the wider rear tires resulted is a smaller track dimension.

On front wheel drive cars it's not uncommon for the front track to be narrower, with wider front tires, for the same reason.

You'll need to set your track dimensions based on your design criteria and goals.

Brian

[Erich Ohlde]

not sure where I read this but (possibly a carrol smith book). The reason the rear track width isn't wider than that of the front is for stability, e.g. imagine steering hard on a tricycle. Also, the rear track follows a slightly tighter radius when going around a corner, this can cause problems such as clipping cones when the front/rear tracks are the same.

[J. Vinella]

I cannot believe that hitting the cone on the inside rear has not been brought up. Those 2 seconds can kill an auto cross lap. Just one thing to keep in mind.

EDIT: HAHA missed the last part of your post jayhawk...my bad indeed.

[Erich Ohlde]

lol

[jowen]

A difference in trackwidth will affect both inelastic and elastic loadtranfer. Springrate and antiroll bars affect elastic loadtransfer and rollcenterheight decide the proportion of the inelastic loadtransfer. I guess you can play with this to get some desired transient characteristics. Correct me if I'm wrong...

[J. Vinella]

You are correct but looking at the tire data and their load sensitivities, would a really wide rear track gain you much? Everything on the car is a compromise. Especially things in suspension.

[VFR750R]

At steady state maximum cornering I challenge someone to show me that the rear still tracks inside the front. In this case the outer tires, at least, are at max slip angle just like the front and are in fact yawed out.

A truely over the top case is Nascar when a driver gets close to the inside white line. The fronts can be touching the line but the rears are 6-8" off the line. Some of what allows this is aerodynamics but if you take the aero away the rears would still not track inside the fronts.

Now in transitions and slaloms chicanes ect. where the rear hasn't reached max slip angle yet or is returning to center is a different story.

Also it has an effect on the diffs ability to transfer power effectively. Cars with spools are usually built with a measurably smaller track on the rear which lessens the speed difference on the inside and outside tires during cornering, and the corresponding push.

[NetKev92]

Good points and interesting observations on where the rear goes 'engine and turbo guy'. I would propose though that you may have generalized a few observations too far the other way. If you reduce things back to a slow wagon going around a corner, clearly the rear cuts to the inside. By observation, you point out that some racecars actually do the reverse with the rear running around the outside. If you read up on my current ride, an old '92 Miata, you'll find that the engineers at Mazda intentionally tuned the compliance of the rear wishbone mounts to provide a certain amount of steering input from the rears as a result of side load on the wishbones in a corner. I suspect that the folks in NASCAR could be similarly savvy. I'm not certain that many of the FSAE teams are likewise as skilled at manipulating compliance to passively achieve steering input to control where the rears walk. That's not intended as a knock. I think most of us could agree though that most teams have bigger problems than the perfect steering balance and tracking. Keeping an amateur-built car running for 60 minutes can be a challenge in itself.

I think Brian said it best at the top. Packaging with wide rears drives the whole situation. I can imagine some odd confrontations in passing if a car was appreciably wider at the tail than at the front. The only reason to widen the rear a great deal would be to increase the size of a flat bottom or tunnels between the wheels for a downforce improvement. Given the low CG and relatively wide stance of modern racecars, additional platform stability would come at the cost of increased unsprung weight. With the stiff springing of a downforce car, you also have a huge deal of roll stiffness before installing sways for front to rear roll stiffness trim. Maybe without downforce there's a stronger argument.

For taking the hit in drag though by increasing frontal area for rear downforce, why not take the free width and widen the front likewise to improve downforce at the nose too? In most cases it seems downforce at the nose to maintain balance is more difficult to come by than downforce generated at the rear with a diffuser behind the rear axles or a wing on top. By the same argument, if you put the width in rear for roll stability, why not go for as much as you can up front too?

-Not an expert, but I do see some sense in the traditional layout.

[B Hise]

VFR750R is dead on.

At a certain point along the lateral acceleration line (0 to max), vehicle sideslip changes from positive to negative. Meaning the rears go from tracking along the inside to the outside. This zero point is dependant on the setup of the car but it always happens. Even in very transient courses like the ones we see at FSAE, this phenomenon is still observable.

We chose track based on getting the car to hanlde the way we want it to. You can train good drivers not to nip cones. That said our rear track is thinner than the front...

Bryan

[Infinity]

Thanks guys for all these comments but what i really was interested in was that "If it is unavoidable to change the track width at rear to lesser than that of front then can its side effects be improved by any other methods?" Cause it seems almost unavoidable to change my rear due to constraints of the differential and axles.

[jdstuff]

Originally posted by Infinity:
Thanks guys for all these comments but what i really was interested in was that "If it is unavoidable to change the track width at rear to lesser than that of front then can its side effects be improved by any other methods?" Cause it seems almost unavoidable to change my rear due to constraints of the differential and axles.

Exactly what offset between the front/rear tracks are you trying to achieve? Give the average wheelbase of an FSAE car, we're not talking 12" here (or even 6" for that matter)!

I find it hard to believe that your rear packaging is so tight that you cannot move each wheel inboard ~1". Also, what is stopping you from making the front wider?

[B Hise]

Infinity,

Are you worried that your car will always understeer unless you make a track change?

There are many, many ways to achieve handling balance past track change. For the purpose of FSAE competition, you may learn more by getting the car built and tuning it for a certain handling response rather than messing around with changing the tracks in the design phase.


-Bryan

[NetKev92]

Adjusting front and rear sway strength can do a lot to shift your car's balance without messing with the big things.

[VFR750R]

NetKev92, you bring up an interesting point on rear steering compliance or in the extreme case rear steer. Several companies have created rear steer cars (3000GT, Skyline, maybe others). These systems usually were designed at low to moderate speeds to get the rear tires to thier slip angle quicker to lessen yaw acceleration lessening grip in transitions. For instance you take a 3000GT and apply a quick steering input the front of the car almost immediately turns but it takes some time for the rear to build up slip angle to get to max grip. By that time you may be done turning or worst case the car has accelerated towards the outside of the turn in the rear due to the delta in force between centrifugal motion and the tires force. That means when the rears get to max slip angle they have to stop this inertia, going past the slip angle for max grip. Almost like the tire is an undamped spring in lateral load (it is). By rear steering the car to max slip angle with the front, the rears response is much better. Probably not noticeable with newer short sidewall radials, but would make a huge change on a heavy car with bias tires which take large amounts of slip angle to find max grip. Would make it much more difficult to initiate a 'drift' in a car like this.

I know, doesn't have much to do with the topic.

[Charlie]

Originally posted by NetKev92:
Good points and interesting observations on where the rear goes 'engine and turbo guy'. I would propose though that you may have generalized a few observations too far the other way. If you reduce things back to a slow wagon going around a corner, clearly the rear cuts to the inside.

Wagons go where they are drug. No vehicle dynamics involved.

VFR750R is correct, in that with equal tires at steady state cornering the rears are not going to be any closer to the corner than the fronts.

It doesn't have anything to do with some kind of funny compliance-based rear steer. Just the basics.

[rjwoods77]

Hey Charlie,

Not to post steal but the link to that pic of the blue anodized upper balljoint holder that used the grooves for camber adjustment isnt valid anymore. You got that laying around?

[Tim.Wright]

Something else to think about;
Since weight transfer is inversely proportional to track width, a wider front track will see less weight transfer than at the back (assuming a cg at half the wheel base), therefore better grip due to tyre load sensitivity (which I'm not sure is much of an issue with light cars).

This would be ideal for the turn in phase, which in my opinion, is the most important part of the turn.

Like I said, just an idea. I doubt that there is one single reason why the front track is usually larger. Will be interested to see what other responses we get on this one.

Tim Wright
Curtin Motorsport

...no I'm from the other university in Western Australia

[C.Zinke]

of course, a wider track reduces the weight transfer. But this can be completely compensated be the anti roll bars.

[Ashley Denmead]

Originally posted by C.Zinke:
of course, a wider track reduces the weight transfer. But this can be completely compensated be the anti roll bars.

not quite right.....maybe some more reading is in order http://fsae.com/groupee_common/emoticons/icon_wink.gif

there are 2 types of weight transfer!

[C.Zinke]

except that there is some more reading in order, we are right both. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Brian Evans]

What about the logic (or lack thereof) of gaining back back balance using ARB's to kill the grip of the end that actually works to match the end that isn't working?

Brian

[B Hise]

Brian,

I take that by "killing" the grip you mean stiffening the bar. I wouldnt say that by stiffening an end you "kill" the grip. Not all the time anyway. Stiffer is not always worse, sometimes its better.

Example:
Problem: car understeers
Cause: front is rolling too much
Effects: excessive geometric/inelsatic weight transfer, positive (or not enough negative) camber on outside tire and inside tire.
Possible Solution: Stiffen front ARB

Each setup change has its pro and cos. The front ARB change will add some warp stiffness and change the roll damping and we could go on and on with tradeoffs. Not to mention the balance of these tradeoffs lend heavily to the cars and the tires design. The point is that you have to figure whats really causing your handling problem and find the best soultion for it. Rules of thumb don't really help much.

Bryan

[GSpeedR]

What about hitting the outside cone during corner exit? A narrow rear track physically allows the CG more room as the car is tracking out. You have cones on both sides of the car.

[B Hise]

I would rather design the car to be fast and run drivers that dont hit things. There are too many important things to consider before hitting cones when making these decisions.

[rkellz007]

I have been silent, but now I must speak. First a disclaimer: if I happen to eviscerate anyone in the process of composing this post the offended person(s) should accept my apologies in advance. We are all here to learn and sometimes the best discussions are stimulated by the stupidest questions.

C. Zinke....WTF?!?? Increased lateral load transfer from narrow track can be "completely compensated"� by the ARBs??? Ashley was correct, you need to visit the library. Total lateral load transfer from inside wheel pair to outside wheel pair in a corner has nothing to do with the presence/absence/stiffness of ARBs. Nothing. Nada. Zip. ARBs only allow you do dictate the relative amount of lateral load transfer to/from the front and rear wheels.

EXAMPLE: Let's just say that your car at max lateral G is going to transfer 200lb from inside to out because of the CG height, weight, lateral acceleration, and track width, and that your car has the same track width and suspension front and rear 50/50% front/rear balance, is not accelerating longitudinally, etc.. 100lb will transfer from the inside front to the inside rear and 100lb will transfer from the inside rear to outside rear. Now, put a really stiff ARB on the front without changing anything at the back and maybe you get 175lb to move from the inside front to the outside front, 18lb to move from inside rear to outside rear and 7lb to move from inside rear to outside front. 200lb has still moved from in->outside, but where it came from and where it went has changed. Dampers, springs, roll-center heights, etc.. don't change the amount that will transfer, they do change the timing and split.

Viffer: A car that is perfectly neutral steer throughout the turn will have the front and rear wheels follow paths of the same curvature. BUT, the paths will not be the same. The rear path will be offset (ie have a different center) from the front path. WHY??? Car has same FR/RR track: in a straight line the rear wheel exactly follows the path of the front. Imagine that both tires were dipped in paint and so are laying down a straight stripe. Turn the front wheel and paint a red dot where the stripe for the fronts departs straight for a curve. Will the rear tire leave the straight line exactly at the red dot you painted??? Nope. It will leave the straight line before the red dot, how far depends on a couple of things like wheelbase: longer WB the rears depart straight a further distance before red dot.

SO, how do we know that the fronts and rears in this ˜perfectly neutral steer' car follow the same radius path? Imagine that the corner is a constant radius 180deg bend. Before the bend the front and rear tires are painting the same line, after the corner they are also painting the same line. Since both the entry and exit lines are tangent to the circle the paths have to be the same radius. The difference is where the centers of the two curves are. For the rears it is some distance ahead of the center of the front path. This distance is, er...thinking, pretty much the same as the distance between the red dot and where the rears depart straight ahead.

If you want to see this in action go stand on a street corner and watch a city bus turn. The front tires are running over cars in the far lane while the rears are smooshing the happy and unsuspecting pedestrians standing on the corner.

SO what is the deal with the NASCAR example? The car is not ˜perfectly neutral steer'. It is setup for a slight steady state (SS) oversteer (OS) balance, which is probably faster. This means that the rear tires will be running at a greater slip angle than the fronts in the SS. Greater slip angle implies larger radius path since the direction of the slip vector is tangent to the path. In layman's terms yes it is possible to set the car up so that the rear tires paint a larger arc in SS, more rear bar etc...

"At steady state maximum cornering I challenge someone to show me that the rear still tracks inside the front. In this case the outer tires, at least, are at max slip angle just like the front and are in fact yawed out."�

HERE IS WHERE I TAKE YOUR CHALLENGE. Max slip angle (SA)...If only you knew what that meant. The max slip angle for a tire is 90degrees. Its not very useful, but it is possible. I don't think you mean that Jonny-NASCAR is running with a 90deg slip angle at both the front and rear tires. Slip angle is a function of a couple of things, we will leave camber out and just look at it in terms of normal force and lateral force. If you put 250lb normal force on a tire and ask it to generate its maximum lateral force this condition will be achieved at some distinct slip angle, lets say 7 degrees. If you now increase the normal force to 350lb and ask the tire to make its maximum lateral force it will generate more than before and do so at a greater slip angle, say 10deg. Put the same tires on both ends of a car that is perfectly symmetrical EXCEPT for the stiffness of the front and rear ARBs. Lets say you give the car a much stiffer front ARB than the rear such that the normal force in SS cornering is 350lb on the front outside and 250lb on the rear outside. Now you have a front tire running at 10deg SA and a rear at 7deg SA. Guess what...the fronts will track a larger radius arc in SS. If you did this to a Cup Car the driver would be able to do exactly the opposite of what you describe: put the rear tires on the stripe and have the fronts up on the banking. The driver would hate it, it would not be fast, and the tires would not be happy but it would be possible.
So what does any of this have to do with us??? Well, if the outside rear is travelling on an arc inside of what the CG is travelling then applying throttle will induce an Under-steer (US) moment. If the rears travel outside of the CG then its going to be an US moment on throttle. I will leave it to the reader to figure out how it works with trail braking. Also, like the bus the US, Neutral, slight OS cars will cream cones (pedestrians) in tight corners if a driver chooses a line that brushes said cones with the front tire.

[GSpeedR]

Originally posted by B Hise:
I would rather design the car to be fast and run drivers that dont hit things. There are too many important things to consider before hitting cones when making these decisions.

Keeping the car on the racetrack seems like it should be pretty high on your list, dude.

Edit: the original poster asked for a reason why a narrow(er) rear track could be beneficial and I gave one. People were shooting down the "hit the inside cone" theory and I'm merely stating that isn't the only part of the track you should worry about. No need to post "everything's a compromise" everytime, the discussion won't go anywhere.

[B Hise]

edit (thanks Kyle)

a well designed and tested car with a rear track wider than the front with a good driver will not hit cones or go off and be competitive. The same is true for a car with a narrower rear track.

Using your head is key in these situations.

[Kyle Walther]

did you mean uncompetitive, or will be competitive ? becuase the way i read it is, a car with a wider rear track is not competitive, but won't always hit cones.

[Chris Allbee]

Kyle, you are WRONG!!!!

[NetKev92]

Originally posted by Charlie:

Wagons go where they are drug. No vehicle dynamics involved.

VFR750R is correct, in that with equal tires at steady state cornering the rears are not going to be any closer to the corner than the fronts.

It doesn't have anything to do with some kind of funny compliance-based rear steer. Just the basics.

Is there any further justification or resource to justify this position? After waiting a couple days and continuing to read the posts here, I am unconvinced. I still believe that there are too many assumptions at work here. The significant post on page two seconds this opinion.

The wagon is not a worthless comparison. It demonstrates that there are tire slip angle conditions that do lead to the rear wheels tracking inside the fronts. This I would argue is basic.

[rkellz007]

Heck, you dont need a wagon or a bus to prove it. Just watch an FSAE car in the first half of a hairpin. Although "your mileage will vary" odds are pretty good that the fronts are following a path farther from the inside cones.

Also, B Hise said:

"At a certain point along the lateral acceleration line (0 to max), vehicle sideslip changes from positive to negative. Meaning the rears go from tracking along the inside to the outside. This zero point is dependant on the setup of the car but it always happens."

Er, oh reallly??? ALWAYS? Maybe not even most of the time.

[VFR750R]

A wagon is under the assumption of zero slip angle. An fsae car is not.

Everyone agrees that the higher your lateral loading the more tire slip, right?

Simple case. Car traveling in straight line. A team member comes up to the car and pushes on the rear perpendicular to car motion. That lateral load will bring on a slip condition and the rear will no longer be tracking with the fronts. You can even try that with your 'wagon'. In fact to keep the car going straight and not do circles you will have to countersteer with the fronts. End result, you can't even go in a straight line, the car will walk away from the straignt line the same as the slip angle.

Hairpins aside, if your rears are tracking inside your fronts, you aren't cornering hard enough.

[rkellz007]

viffer:

As for your 'Simple Case' yes I totally agree...

But the example has almost nothing to do with the way a race car (or any other four wheeled vehicle) responds to the inertial forced generated by cornering.

The inertial forces act through the CG of the car. To make your example relavant you need teh team member to push on the CG of the car. If your car is tuned for neutral steer (NS) it will move to some straight path angled to the original path.

IF on the other hand the car were tuned for US then the car wil start to follow some arc off in the direction the team member is pushing.

IF the car is tuned for OS the expiriment causes the the car to start turning an arc inside the original path: toward the side the person is pushing on.

It seems like you have a lot of experience with cars tuned/designed for OS. As for the assumption that the wagon wheel operates at 0 slip angle, I am not too sure about that.

Anyhow, the results are that for the NS and US cases the rears are following a path inside the fronts. For NS the paths are the same radius, different center. For US the path radii and centers are both different: path of fronts is bigger radius. For OS the path radii and centers will be different front and rear, if the rear SA is big enough then the rears will track outside the fronts: a la dirt track.

Ryan

[rkellz007]

viffer:

One more thing: "Hairpins aside, if your rears are tracking inside your fronts, you aren't cornering hard enough" is not therefore a true statement.

It would be correct if it read: "Hairpins aside, if your rears are tracking inside your fronts, YOUR CAR IS NOT SETUP FOR SIGNIGFICANT OS"

[GSpeedR]

Originally posted by rkellz007:
C. Zinke....WTF?!?? Increased lateral load transfer from narrow track can be "completely compensated"� by the ARBs??? Ashley was correct, you need to visit the library. Total lateral load transfer from inside wheel pair to outside wheel pair in a corner has nothing to do with the presence/absence/stiffness of ARBs. Nothing. Nada. Zip. ARBs only allow you do dictate the relative amount of lateral load transfer to/from the front and rear wheels.

No, not really. Weight transfer, acceleration and tire force are nested/coupled functions of each other.

Example: Stiffen front swaybar. Front tire force changes, let's say it reduces (it could increase who knows). If the rear tire forces hasn't increased enough to bring the total tire force back to where it was your lateral acceleration will decrease. You have less lateral acceleration and thus you have less total weight transfer. One of the more important concepts Mr. Rouelle stressed. But hey at least you weren't a dick to C.Zinke.

[VFR750R]

rkellz007, agreed on all accounts. You explain it better then I. When I wrote that I failed to think about OS vs US. I was mearly defending the point of racecars at full slip vs a damn wagon.

A 50/50 weighted wagon will track nuetral at the limits of grip, but at less then that...

[C.Zinke]

You missunderstood me. I only said that you can compensate the weight transfer at one axle. So if i'm reducing the front track width i can adjust the ARB to get the same load transfer at the front . Of course this will result in higher weight transfer a the rear. But this wasn't my intention to say a so i did not write about it.

To get a summary i repeat:

If you keep the average track width, the width of each axle is not important for static cornering an the weight transfer. It is just an parameter like the ARB stiffness an can be compensated.

[Kyle Walther]

the way i see it is that your rear tires will always track underneath the fronts until they develop enough slip angle no matter your setup. so during the slalom case where your slip angle is constantly being forced to redevelop you stand a greater risk of dragging the rear wheels underneath the fronts and clipping cones. So, a smaller rear track would always aid in avoiding these cones. or has this point already been made.

[Matt Gignac]

My take on it is the following:

You generally want your rear natural frequency to be higher than the front, so that if you go over a bump, your front and rear bump responses are out of phase, so that you get back to a level chassis sooner after that bump (since the rear hits the bump a wheelbase/velocity seconds later than the front)

If you had equal front and rear tracks, and assuming no ARBs, same motion ratio front and rear, this would result in more roll resistance at the rear, which I don't think is something we need at all. Therefore, we reduce rear track, to get our lateral load transfer distribution somewhere we want it. We then add ARBs front and rear to keep our roll motion small.

Also, we generally design our cars to have an ever so slight understeer gradiant, which would mean the rear tracks inside a touch. In our case, the cone argument may be a good point.

Matt Gignac
McGill Racing Team

[kwancho]

Most cars are also rear-heavy, so having more roll resistance in the rear isn't necessarily a bad thing. You also don't really want to add much rear ARB as it'll take weight off the inside rear wheel, which hurts traction.

[Goran Malmberg]

Originally posted by Timo:
Something else to think about;
Since weight transfer is inversely proportional to track width, a wider front track will see less weight transfer than at the back (assuming a cg at half the wheel base), therefore better grip due to tyre load sensitivity (which I'm not sure is much of an issue with light cars).

Is that really so? If we shorten the rear track to the extent to become a tree wheeler, the rear wheel will take no weight transfer and the front wheel all of it.
The chassis is supposed to be rock solid, so a wider track should describe a longer tangent movement. But as the front, or rear, track is made wider, the total weight transfer will get less.
Am i thinking right....
Goran Malmberg

[Buckingham]

With regard to the 'hit the cone' comments. Doesn't the fact that you CAN hit a cone with the inside rear mean that a narrower rear track would make you less likely to do so?

[kwancho]

Originally posted by VFR750R:
At steady state maximum cornering I challenge someone to show me that the rear still tracks inside the front. In this case the outer tires, at least, are at max slip angle just like the front and are in fact yawed out.

A truely over the top case is Nascar when a driver gets close to the inside white line. The fronts can be touching the line but the rears are 6-8" off the line. Some of what allows this is aerodynamics but if you take the aero away the rears would still not track inside the fronts.

I ended up poking around this question with my vehicle dynamics professor, and I think the fact that the front tires are closer to the inside line than the rear tires only means that the car body has adopted a negative sideslip angle.

Assuming a constant radius turn and a neutral-steering car and that the fronts are at their maximum slip angle (positive), as speed increases the rear slip angles go from positive and transition to negative (more yaw!) to a point where the overall body sideslip angle can also be negative.