Dan Wendling

What factors set having too much tire width or too large an difference between front and rear widths for a road course car?

Wider cant always be better!

What indicators does the driver have when tracking the car to help optimize the tire selection?

sothpaw2

Just a humble opinion here, but I thought that width was limited by availability, cost, and more importantly, what can reasonably be engineered to work with the vehicle and have it run reliably. I would think you go wider until you have a neutral handling car assuming that is your preferred driving style (for most I think it is).

davidfarmer

ultimate grip is not deteremined by width, contact patches just don't go up very much. Plus some of the fastest cars on track now are TTA guys, running 275 or even 255mm tires.

I think wider tires hold up better, manage heat better etc. So a car with higher power/torque needs a larger rear tire to manage the extra heat generate by acceleration, not cornering.

Therefor I feel you can go just as fast on a 275 as you can on a 335, just for a shorter period of time. Also, in a Vette, running a tire 20mm wider or so helps keep the rear from losing grip prematurely as it heats.

Just my opinion, I was forced to run wider tires on the front recently (my wide tires were shorter than the narrow ones) and the car was perfectly balanced.

Everett Ogilvie

There is a thread on contact patch theory going on in the C6Z section, with some good stuff if you weed through the BS. Interestingly, I was reviewing Herb Adams' old classic Chassis Engineering a few weeks ago before that thread was started and when I re-read the part about contact patch area not changing size with larger tires I was confused again briefly, but of course the basic is that for equal tire pressure the patch shape changes with different size tires, but not the patch area (wider tires make the patch wider, but shorter longitudinally). Lowering tire pressure can change the patch area within a small window. I definitely agree about larger tires ability to handle more heat, longer. It is an interesting topic for sure.

ltborg

It also depends on the rim you are using. If you have a set rim width, going too wide causes tire mechanics problems, basically because the sidewalls are angled too much. You can end up pulling the shoulders of the tire off the road surface and not properly loading the sidewalls of the tire.

I seem to remember seeing an article in Racecar Engineering (or maybe Sports Car) that did a test on the opposite example. For a given tire size, nearly all tires will have more grip and better response with a wider rim. Obviously this has a point of extremes too but it was an interesting read.

RDnomorecobra

another factor ... wider tires are heavier, typically, given same diameter

John B

My past experience is going to have me voting for the largest balanced tire & wheel package you can fit. My C4 is 335 18s all the way around. My C5 315/18 front 345/18 rear. Each step bigger on both cars required adjusting suspension. Each step larger = faster lap times. Sure if you put these sizes on a Miata or F500 it would go slower. With less power & weight.

Sidney004

Same here. My car always got better with more rubber(currently running a 305 square setup). As for the contact patch, I have got to dig up a technical article by Michelin; even though the contact patch on a 275 is the same as a 335, its the shape of the larger contact patch that yields the grip advantage when cornering.

RDnomorecobra

I assume both of you are referring to autocross, not road race/HPDE?

Dan Wendling

I am hoping to see more responses that consider the impact on the handling characteristics of the car.

Things like:

Effect of wider rears on corner exits.
Interaction with anti-sway bars,

Sidney004

Not me.

RDnomorecobra

i guess it was john B saying he runs 335's on the front. beefy.

froggy47

Most take tire temps (pyrometer).

Slalom4me

The topics you are interested and much more on the subject of tire
performance is discussed in
Regarding tire width, a brief exerpt of what Mr Haney has to say:
.

Dan Wendling

Thanks for the technical reference. Just what I wanted, cant wait to read.

Westwopper

I agree. If you over tire a car it will slow due to mass.... There is a happy balance. Per Froggy.... If you are not validating with a pyrometer, you may put to much tire and not be able to get them up to temp./using them efficiently.

Large rears are used some times for the durability of the casing, especially in pure race tires like the ones on Newlanders car. The larger the air chamber the more load the tire can typiclly handle.

bosco022

I agree with this.

Our GTA type stock cars run some terrific lap times using a 10" wide tire on car that make about 500 FWHP. The 27x10x15 Hoosier 3035
gives these car incredible side bite and then the less mass and rolling resistance makes for a very fast straight line tire. I think that GT-1 type tires on these cars would be overkill. Might be a bit faster across the apex and under braking, but slow you down on acceleration. Plus they will put a much bigger hole in your wallet. 1600 a set vs 600 a set.

When we get these cars up around 600 HP (SPO types) they get a lot more difficult to drive and you really have to roll smoothly into the gas off the corner.

yooper

BrianCunningham

the issue is getting heat into the tires

Corvette are relatively heavy cars.

a A-mod can run whatever size they want.
They don't run 335's because they can't get the heat into them.

Poor-sha

It's interesting to see that the LMP cars are going with bigger fronts now to match the rears. This all started with the Acura/HPD car but I know that Audi and Puegot have done that as well now. I seem to recall that when HPD first did this they had issues finding a power steering system that could deal with that big a tire.

I guess that is a long winded way of saying that it sure seems like bigger is better within constraints of the rules.