Tire sizes for chassis balance

This question would be for any forum member who has significant experience designing or setting up racecar chassis.

Assuming a car with a 45% front, 55% rear weight distribution. Rear wheel drive and high powered with no aero aids (a GT40: 2,400 lbs with a 400 to 450 HP engine).

If you were given 20” of tire width per side that you were free to distribute as you chose front to rear, IE: 10” & 10” or 8” & 12”, etc. What would be the optimum distribution for best lap times around a fairly normal road-racing course?

To begin with, for steady state cornering, would you distribute tire width in proportion to weight? In the case of the example, 9” front & 11” rear. (I am assuming that you would get best utilization of your tires by keeping the forces per square inch constant on both tires. This in turn presumes similar tire pressures front and rear). My thinking here, is that you can use sway bars to transfer load from front to back or vise versa, but the actual lateral force seen by the tire is dependant on static weight distribution (modified by any acceleration or deceleration) and is not shifted by sway bars. Am I on the right track here?

A further presumption is that tires are designed to work best with a certain level of distortion of the round shape. IE: a 26” dia. tire has a front to back contact patch length that is optimum for that tire and making it longer, by reducing pressure, or shorter by increasing pressure results in less total grip. Is this assumption accurate? (I realize that there is also the matter that if pressure gets very far from optimum, you have lateral distortion and no longer get even pressure per square inch resulting in uneven temperature across the tire face, which, of course, reduces the tires’ performance).

Is this accurate to this point? Is there anything else, as far as tire size goes, that needs to be considered for steady state cornering?

Once we have optimized the car for steady state cornering, to get best overall lap times, would you then move some tire width forward to favor braking, or (more probably) rearward to favor acceleration, or would these be of approximately equal importance and therefore cancel each other out, leaving us with our original distribution? Is there anything else I should be looking at as part of this?

Thanks for the help here. I find myself getting a little beyond my own experience and beyond the books I have been able to find, and therefore being forced to make some assumptions. I want to be sure the assumptions are accurate and that I am not missing anything else.

Kevin

Kevin,

I don't think there is a simple equation/ answer to this question because of the huge number of variables involved. Logging data on YOUR car is the only way to go. F1 teams have whole tire companies to answer these questions and look what happened at the US grand prix.
There will also be no single perfect set-up as variables are constantly changing. It is best to struggle with understanding the variables and how to adjust to them.
Track time is the only way to refine your data.
My .02c is that it is easier to tune with the Hoosier Bias Ply Vintage Street TD than high performance radials as the limit of the tire is more progressive than Radials.

Regards
Andy

Kevin, I think Andy's right in what he says and you've basically got it figured. With regard to tyre pressures, you would set them up to give as even a temperature across the tyre as you could get, and then possibly use small variations in pressure to fine tune, not enough to change the footprint significantly but enough to slightly alter the slip angles that the tyres run at, thus altering the underster/oversteer attitude, although normally this would be done mainly on rollbar and shock settings and possibly roll centres (ride heights)and spring rates. Specifically in regard to rim widths 9's and 11's would probably work well but if you had a higher output motor and wanted to stay within your 20" parameter then you would probably want to look at 8's and 12's. Around those sizes but slightly outside your parameters I would tend to go 9's and 12's. If you look at the different sizes run on the original cars as they were developed you would have a pretty good indication of what works in this application. No need to replicate design and development work as it's already been done by Ford!! Just look at the specs of the originals!! /ubbthreads/images/graemlins/grin.gif

BTW, welcome back! I used to read your posts on engines etc with interest then you disapeared for a year or so. I see built2rev is also back after an extended absence. I hope this means further interesting contributions./ubbthreads/images/graemlins/cool.gif

Regards

Andy,

Good point, I agree that bias ply tires are more forgiving and would be a good starting point.

Russ,

Thanks for the kind words.

Is Adam still posting on the engine forum? Now that was someone with some interesting information to share!

As for the tires, I suspect you are right about not reinventing the wheel, but there are some things that bother me about history.

During the racing life of the cars (I will use the Gulf cars for example since they had a long competitive life) the front tires stayed about the same size (on the Mk1s) while the rear tires grew about 30% larger. My concern is that this was in the infancy of tire development. It may be that given more time the rears would have gotten even larger (take Can-Am for example, the rear tires continued to get much wider in that series).

There is also the question of whether what worked with early bias belt tires is directly applicable to modern radials.

And, there is the fact that the suspensions on the original GT40s were nowhere near as developed as modern suspensions and may have limited the effective width tire the car could use due to camber issues.

In any case, thanks for your thoughts,

Kevin

Hi Kevin,

You raise a lot of good points. There's a lot of evolution in street tires since the sixties. There have also been new directions in suspension design, push rod inboard coil over shocks and the like.

Seems to me most of whats new speaks to todays racing reality, ground effects, under car venturi tunnels, trying to find the last ounce of unsprung weight to move. I don't see much that actually improves on the original GT40 suspension design in terms of maintaining the contact patch.

It would be very interesting to see how an original GT40 does on todays racing rubber in comparison to comtemporary GT racecars in a non downforce shoot-out (slalom, lower speed corners not low g). I suspect the old girl would hold her own

The GT40 was essentially a two seat formula one car with a roof. Clasical F1 suspension design. The tires got wider as the power came up and the drivers wanted to keep the rear tucked in.

Some of the Can-Am rubber just got to be enormous, looks great on a McLaren Mk8f and they had just a few more ponies to lay down too...

Kevin,
I am no expert but I will give you some info as to the setup for my MK4. It may or may not make sense to you but it is the fast way around the track. First, the basic suspension geometry of a GT40 was used on Riley and Scott chassis's until the middle of the 90's. So it is not as antiquated as you may think. Secondly, the chassis needs to be setup to do what you want it to do. Driving it on the street is not at all similar to racing. If it is not squared and bump steered the tires won't make the difference. Once the chassis is done, I believe the wider the better on the tires. I would run wider in the front if I could. Most of the current race chassis's use very similar sized tires for the front and rear.
My car is setup with around 1 degree negative camber. The tires run on the inside edge which burns off faster than the outside but it is significantly better in the corners. Tire temps run around 20-30 degrees hotter on the inner edge and taper evenly across to the outer edge. We also run different tire pressures from corner to corner.
There are very good people out there that setup race cars. Pick their brain for an hour and don't skimp when it comes to chassis setup.

J What,

Thanks for the info.

You bring up one of the things that started me wondering about this. Cars like the Ferrari Enzo use a very staggered sizing 245mm front and 345mm rear, almost 50% wider.

Yet the Audi A8 and other LMP1 cars run only about a 1" size differential, 13" front and 14" rear wheels if I remember right. I haven't read the LMP1 rules so I don't know if there is something that favors very wide front tires (I know the rear width is restricted). This may be explained when you consider that the LMP1 cars run both a rear wing and a rear diffuser. Does the massive downforce give so much more rear grip that they need very large front tires to equal it since there is not so much downforce up front?

It seems that the more I learn, the more I realize how little I know.

Kevin

Hi Kevin

I think one of the main reasons you'll see a large front to rear size disparity between road going exotics and high downforce racing cars is that, with a very wide front tyre on a road car, you may have too much contact patch area for the available load on the tyre in the wet. Early onset terminal aqua-planing is no good for selling Enzos.

With the race car you've got very different tyres and you're varying the suspension setup and downforce (if you have it) for the conditions, so you don't care what the factory said was appropriate for road use in the wet.

But, for as much grip as you can get with the front of a mid-engined car you probably want some additional grip available at the rear, so that you're slightly understeering the limit rather than neutral. This way the rears have some capacity left to get the power down when you hit the apex even as the nose continues to drift out. Throttle steer and driver prefence would therefore also have a part to play in selecting optimum front to rear widths.

If you want to go further I would add centre of gravity height and how it moves given your wheel rates and cross weights. If you go this far you will probably want to factor in your track to wheelbase ratio, and a myriad of other things. I've got a headache now. Please don't mention front to rear tyre sidewall height ratios.

All I know is that I don't know. I'm going to start with road tyres of 235 on the front and 335s on the rear (based generally on predicted load vs grip) and test until I'm comfortable.

Richard-

Are you running on 15's or 17's? If they're 15's, what is the tire you have found?

Chris

Hi Chris

I'm not much use to you, as I'm running 18"s!

While I plan to start testing with a 235mm front (eventually), I wouldn't be surprised if a 225 is actually better for my application. A 215 might be ok, but I'd be surprised if it was better.

I'm not up to date on 15" tyre sizes, but I'd imagine the Pirellis for the Countach are still available. I can't remember which front tyre width it used, but the back are 335/35x15 I think.

Doh! I guess I got a little presumptuous.

I have been searching and following the exploits of other forum members as well and from what I have read (and I saw it on the internet so it HAS to be true!!) Pirelli has stopped making the 335/35-15 for the Countach. There were several posts on some forum that alluded to owners being out of luck for original rubber.

For me, I really want to go with 15's on WIDE rears (like 14') but it just doesn't seem like a reliable source of decent rubber of street duty is available.

CK

You could try looking for Avon race tires, the make them for up to 15-18" width (16.0 X 27.0 - 15), they aer not DOT and they are very expensive