What is effect of difference in wheel track width on handling?

[JoeMag]

If the track width of front and rear are different, what is the effect on cornering? For instance, is it better to have front and rear wheels with the same track width or rear wheels furthur out than fronts?

[gtu935]

Hmmm. Good question. I'm suprised no engineer types have jumped on this one. I will tell you this. After modifying my 930 and turning into a hybrid 935. I was using 12 inch wide fron tyres with 15 inch rears stuffed under 13 inch rear quarters. I suffered with extremly bad understeer for years. Just recently we down sixed the front tires to 10 inch wide giving the front track a 68 inch stance. We also increased the rear track 4 inches to a full 80 inches. The car handles much better now. I don't think this answered your question at all but, I thought it might be helpful.

[jluetjen]

Here's my $0.02...

It effects handling in a number of ways. Assuming that all other things remain the same...

1) A wider track will make the springs feel weaker since you will be using a longer lever on them. So a wider front track will make the front suspension feel softer, promoting a reduction in understeer.

2) A wider track on one end of the car rather then the other will affect the way that load is transfered when cornering. When Porsche developed the 917/10 out of the 917K, they had a number of handling issues, one of which was power oversteer (going from 600 to 1000 HP -- duh!!! ). When they developed the 917/30, they actually reduced the rear track some. So the changes looked like this...

Track Front/Rear (rear as a % of Front)
917K 1564 mm / 1584 mm ( 101.3%)
917/10 1620 /1638 ( 101.1%)
917/30 1670 / 1564 ( 93.7%)

The result was that when cornering, the load would be transfered to the outside front wheel sooner then the outside rear wheel since the front wheels had a wider track and thus were traversing a longer distance for a given amount of body roll. This will tend to reduce oversteer.

3) A wider track will lower the roll center at that end of the car, thus making it softer in roll. This is because the car's CG will not change, and thus the vertical distance between the CG and the roll center at that end of the car will increase, kind of like using a torque-wrench with a longer handle that pivots at the roll center. So a wider track at the front will make the front softer in roll which will once again tend to reduce understeer.

4) A wider track will tend to reduce the load transfer to the outside when cornering, which in general will increase cornering power since you'll be able to get more traction from the inside tires. So once again, increasing the track at the front will tend to reduce understeer.

So in general, making the track wider will increase cornering power -- but -- it is important to maintain an optimum ratio between the front and rear tracks as a means to managing the load transfer from the front to the back, and thus the resulting balance between the two ends of the car.

Now, widening the track using spacers also does a number of other things to the geometry...
1) It increases the front scrub radius, which can increase the steering effort and affect the feel and kickback.

2) It can increase the load on the bearings since the load will be cantelivered further out then the suspension designer may have intended.

3) Without working out the geometry, I'd also be concerned about causing the suspension to pick up a tendency to change track under bump/rebound which can cause all sorts of sudden strangeness in the handling as the suspension loading is changed.

[J P Stein]

Originally posted by jluetjen

1) It increases the front scrub radius, which can increase the steering effort and affect the feel and kickback.
[/B]

Having ventured into the 2.5-2.75 positive scrub radius territory, I can confirm the above. Were these the only issues, I could have lived with it. As steering lock is dialed in the centerline of the wheel gets out ahead of the center line of the strut. At low speed, with a lot of lock, I could feel the front tires "shuffle".

I do auotcross where lots of steering lock is common. This translated to severe understeer at speed... as the lock was dialed out, it transitioned to oversteer. Depending of the corner radius, you got one or the other....sometimes both. Driving the schizophrantic
SOB was making me the crazy too. I drove it that way for a season, then gave up on the 10 inch wide front wheels. I couldn't figure out a way to cure the problem.....short of a total redesign of the 914/911 front suspension.

The cure was 8 inch wide front wheels, getting the scrub radius to a manageable 1.5 (or so) scrub radius. Then it oversteered everywhere.....that I could dial out.
I lost only .5 inch of tire width and was able to get sufficient heat into the tires to make them work.

On a track car, this probably wouldn't have been a huge problem. The tightest turns on a track are about the same as the widest turns on a typical AX course, but I can see where mega wide (12 inch) front wheels would add another 1 inch of scrub radius and thus get you in trouble

[Brant]

another ancedotal to confirm JP's findings
a local race shop out here uses front 1inch wheel spacers to dial in or dial out understeer.

I wanted to increase my front track but was told the push would be bad from this experienced shop (using my numbers, rear track, spring rates, and tires) so did not actually try it.


I've already got enough push

brant

[VFR750]

If I increase the rear track with spacers, will that reduce oversteer?

[JoeMag]

John -- Thanks a lot for all that info... Maybe it's just best to take a look at the factory race car I'll be mimicing...

Anyone know what the front and rear track are from a 3.8RSR?

JPIII -- What does 2.5 to 2.75 positive scrub radius mean?

[J P Stein]

Originally posted by JoeMag
[

JPIII -- What does 2.5 to 2.75 positive scrub radius mean? [/B]

Here are my thoughts....and they are mine

I bit of visulization is required as I don't have a sketch handy.

A straight line verticaly thru the centerline of the strut.
A straight vertical line thru the center line of the wheel.
If these 2 lines intersect at the earth & the outside of the tire (car sitting at rest), this is 0 scrub radius.
The steering rotates around the centerline of the strut.
Any offset inboard(-) or outboard (+) describes an arc....radius....as steering lock is dialed in.

A bit of positive SR is a good thing as it provides steering wheel feedback. 0 SR makes the steering feel dead, but makes the steering effort light. I've never thought about the effects of negative SR


Large positive SR offsets bring the "scrub" part into the equation. At large steering lock angles, the tire ceases to roll properly and starts to scrub noticeably. The centerline of the wheel gets out ahead of the pivot point of the steering. Steering effort is high as you are fighting the mechanical advantage of the longer lever arm. As I see it, the large positive off set lessens the amount of steering angle required to bring on the onset of severe scrub.

Ya'll have to remember, these are my conclusions from delving into why my car started pushing when I added wide wheels & tires up front.

[jluetjen]

Originally posted by VFR750
If I increase the rear track with spacers, will that reduce oversteer?

That depends on everything else, nothing on the car is in isolation. In general -- I don't think so. Your signature doesn't say enough about your car though.

1) What sort of wheel and tire sizes are you using at each end of your car?
2) What's your car's ride height? Were any geometry changes made to achieve this ride height?
3) What are your alignment settings?
4) What sort of tire pressures are you using?
5) When does the car oversteer?
-a) Corner entry (turn-in to prior to the apex)?
-b) Mid-corner ( When balancing the throttle around the apex)?
-c) Corner exit (When hard on the gas from the apex to the track-out point)?
-d) When transitioning from one to the other?
-e) Rough surfaces or smooth?

[jluetjen]

I've got a picture in my head of how the things change, but I've got to play around with some suspension software some before I can put it into an image to paste here.

[fatbillybob]

For those who have played with such changes....
Regarding oversteer or understeer how many millimeters of change to a spacer can you feel at the seat of the pants? 25mm? 15mm? Or do you feel more of the effects of the scrub radius? Regarding oversteer or understeer How many millimeters of change to the ride height of a car at one end (thus changing the chassis rake) can you feel?

[J P Stein]

Bear with me a bit. My experiences are with my car and the hoops I've jumped thru with it.

First off, I've never ran with more than a half inch rake. It didn't feel any different that 1/4 inch, so I'm not much help there.

Some explainations are in order for spacers/wheel width/scrub radius issues.
My 914 was designed with 5.5 inch wide wheel, 25ish inch diameter tires.
At the height of my SR problems, I was running 10 inch wide wheels & 22 inch diameter tires....both of which have an effect on scrub radius. The 4 1/2 inch wider wheel moved the center line of the wheel outboard 2 1/4 inches (wheel backspace being approximately the same). The shorter tire would add a bit ....WAG, 1/8 inch. Another WAG would be 1/2 designed in scrub radius. Add that up, & you're looking at a big number.

Understeer is not seat of the pants, but seat of the... hands. Pilots would call it a "stick shaker" as the steering wheel starts shaking at the onset of under steer.
With the longer lever arm provided by that much SR, you can *really* feel it.

Anyhew, going to an 8 inch wheel moved the center line
wheel back inboard 1 inch & I was in business.
As I said before, with the bigger radius corners the car would oversteer.....simple to dial out with anti roll bars & springs, but the tight stuff was killin' me with understeer.

As a side note, I stiffened the front anti-roll bar by 2 1/2 (that's a bunch) inches and softened the rear springs by 25 lbs to get close to neutral....which shows how much oversteer was really there.

My best guess would be that you wouldn't feel much difference with a 1 inch (25mm) spacer up front other than steering effort......tho at the edge of traction the increased scrub may become a monkey wrench in your traction circle.

[VFR750]

Actually my question was based not on a current problem, but a situation I will be creating soon. I am getting 14mm spacers to used with a set of 16x8 Phone Dials from a 944 Turbo (23.3mm offset) The spacer will bring them close to where a typical 16x8 Fuchs (11.6 mm offset) would be on my car.

When the PD aren't mounted, I can run my 16x7 Fuchs pushed out another 14mm (apperance only)

Questions should really be, What is the effect of widening the rear track for a given car set up?

[Wil Ferch]

Short answer ( of course.... with caveats just because it IS short)...

All else equal..... ( repeat..all else equal, like effective spring rate)...

widening front track will tend toward more oversteer ( i.e. it dials-out understeer).....widening rear track will increase understeer.

At any sane (or approaching insane) speeds on the street, the effects are minimal.....will be felt more-so on the track.

Also...when talking understeer/oversteer..a lot depends on whether you're talking low speed ( high steering angle) effects or high speed ( low steering angle) turns. If all you do is autocrosses..and you dial out understeer, don't be surprised that your formerlyy neutral beauty at high speed turns ...might now have "pucker-factor inducing" oversteer in high-speed sweepers.

- Wil

[VFR750]

Wil,

Thanks for the short answer. I thought that was the effect.

I am mostly interested in the high speed effects, as those consequences are far more damaging. In AX the car understeers quite a bit. Haven't gotten to adjustable sway bars yet, so I have not started to do any dialing in for specific conditions.

[fatbillybob]

Originally posted by Wil Ferch
Short answer ( of course.... with caveats just because it IS short)...

...when talking understeer/oversteer..a lot depends on whether you're talking low speed ( high steering angle) effects or high speed ( low steering angle) turns. If all you do is autocrosses..and you dial out understeer, don't be surprised that your formerlyy neutral beauty at high speed turns ...might now have "pucker-factor inducing" oversteer in high-speed sweepers.

- Wil

This is what happens to me. This dynamic really screws with my ability to tune the car. Can you tell me why this happens? It is really scary to oversteer at high speeds. If you go out on a low speed skidpad and set the car up sometimes its not what you get at full speed. I guess that's were you start tuning a mechanically gripping oversteer car with understeer using aero aids but lets not talk about that and stick to mechanical grip.

[jluetjen]

Originally posted by Wil Ferch

widening front track will tend toward more oversteer ( i.e. it dials-out understeer).....widening rear track will increase understeer.

I don't agree with this conclusion Wil, but I do agree with the rest of the stuff that you said. Here's why. I ran a few numbers on a mathematical model* that I have that estimates a narrow-body 911 (58 inch front track, 55 inch rear track), with 175 lb front springs and 400 lb rear springs, and a .75 inch sway bar at either end. In a steady state 1-G turn, the car will have the following loads on the tires.

Front........Rear (outside/inside)
322/78....523/197

If I do nothing more then change the rear track to 59 inches (adding 2 inch spacers to either side), the results would be as follows:
311/89....521/199

or a change of...
-11/11......-2/2

If I were to change the track to 62 inches in the front and 55 in the rear (add 2 inche spacers to the front), the results will be as follows:
325/75....514/206

or a change of...
..3/-3.....-9/9

Conclusion: Widening the track at back end of the 911 (which has a narrower track to start), will predominantly transfer load from the outside front tire to the inside front tire, thus most likely reducing understeer/increasing oversteer.

Widening the front track (the wider tracked end of the car) will predominantly increase the load on the inside rear tire, but the transfer will be relatively less effective then changing the rear track. I would expect this to reduce oversteer/increase understeer.

* Note, my model doesn't explicitly take into account suspension geometry, so things like handling changes due to camber change and scrub radias are not accounted for. I am only estimating the load load transfer based on the (estimated) CG, estimated RC's, spring and sway bar rates. While hardly definitive, and not tested explicitly me yet, it does give an idea in a theoretical sense of the affect of changing the track at one end of the car.

[tab5303]

Wider track on the semi trailing geometry doesn't lower the roll center like somebody said, it actually raises it.

So a wider rear track will give a very slightly softer spring rate (not noticeable at all) and a higher roll centre.- probably not noticeable either. So far as body roll goes, the two effects will probably cancel each other out. (the higher roll centre gives less body roll)

Further to that, a wider track will not make a noticeable difference to a car's levels of tyre grip either.
Even if you could completely eradicate weight transfer you would only increase grip by a maximum factor of around 0.15 over whatever attainable G you had to start with.

Another example considering the weight transfer myth:
Widening a 911 by 12 inches at both ends would only improve grip by something like a factor of 0.02
So if your car was originally capable of 1G, it would now have 1.02G.
But I suppose it looks great with the very wide track though, and it's less likely to tip over if you race with slicks ;-)

[Mahler9th]

Interesting thread... it seems at the root was basic info on track width. I always recommend that my students start with the Puhn book. I read parts of it every winter off-season (have for over 10 years). There are other books as well. But I always recomment this book and the Going Faster video as basic starting points.

As for a 3.8 rsr set up... depends on which car. As I recall, there were 964 cars with 3.8's and 993 cars with 3.8's. Those two platforms have very different suspensions.

My own car is a '75 911 underneath, but looks like a 993 race car. It has a very simple suspension without a lot of fancy parts, and runs bias ply slicks. Very simple, and straightforward... not nearly as complex as 964 or 993 cars.

So I would suggest an approach to set up that fits the platform, the tires you are going to run, et cetera.

- MM