robispec, why did you choose to run 225 wide tires on a 17x8 and not a 17x9 rim?

They're lighter :)

At least, that was our reason.

Interesting thread.

I came from the world of autocross where our (my brother and I) main car needed the widest possible tire. Now that I'll be doing HPDE's and the like, I naturally assumed that this need of wide tires would translate to big tracks. Obviously it doesn't and its better for me as skinnier tires tend to be more affordable than wider tires.

I've seen a LOT of autocross guys assume that everything from AutoX carries over to the track, because the general attitude at AutoX is that if you can AutoX, you can do anything.

Now, granted, I've AutoX'd exactly once, but from that experience, I did gather that the only thing that transfers over is your comfort level with the car. Both setups and skillsets are somewhat mutually exclusive.

I've taken some national level autox guys around a track and they freak out at the speeds, while I've ridden with them at an AutoX and was blown away by how quickly an AutoX car transitions.

Very true. Although I would like to say that I know that autox experience doesnt necessarily translate to big track skills, hence why I signed up for the beginner groups in my first two HPDE's.

With autox, I noticed that people tend to be more abrupt and less smooth, but that's because the driver needs the car to rotate and transition quicker. I've been watching big track videos and I'm amazed at the smoothness of the inputs.

i run 235/40/17 on 17x8.5's for autox and the track, might start looking into wider wheels and tires now.

So technically speaking, at the same inflation PSI, tire size only changes the shape of the contact patch right? because physically at 35 psi you need x amount of surface area to react the weight of the car.

So is it the reduction in tire deflection that reduces heat generation when going to a wider tire, or are people reducing inflation pressure to get the same amount of deflection but more usable surface area? Since getting that larger surface area on a smaller tire could result in uneven wear and strange feel because the tire is underinflated?

Cheers
Nathan

There is a saying: A "Good" autocrosser can make a "Decent" roadracer
However, A "Good" Roadracer doesn't make a "Decent" Autocrosser.. Basically the principles of Autocross translates easier going into Roadracing...and Not the other way around.. :burnrubber:
I personally do HPDE's as well.. but I know from experience there are different aspects to each.. and have to respect the differences. If not you end up in the fence..

Bill

The engineering side of you is coming out again.

The interaction between cornering force causing deflection, sidewall resistance, how the tire pressure supports the sidewall, etc all play together.

I can go on for hours....


In short, stretched = better response, higher risk of debeading, and possibly larger contact patch (with proper setup)

I guess mostly what I'm asking, is if you have a 245 and a 215 on the same car and same PSI, the amount of force being put into each square inch of tire (statically at least) that is contacting the ground is the same. So does the bigger tire get less heat because it needs to change shape less to keep that amount of surface area (Wider, narrower contact patch), does it naturally deflect less because it's wider (more stable shape?), or is there something else going on?

If you were to take a "stamp" of the tire's contact patch, would the actual surface area indeed be the same if both were at 35 psi, or does the construction of the tire and sidewall stiffness come into play as well, making a different size tire with the same weight on it and the same inflation pressure actually have a different contact patch surface area?

Nathan

Sidewall deflection will be dependent on the height of the sidewall, and how it's angled depending on the tire and rim width

Tread deflection may be lessened; it depends on the tread pattern of the two different sizes. The blocks aren't identical on all sizes.

The larger tire has a larger mass to sink away heat with.

Contact patch will vary with alignment (and the above mentioned tire width/rim width)...

Andy Hollis, a multiple national champion autocrosser has found that 225 RS3's and Rivals on 15x9's were the fastest setup on his One Lap CRX and his STC Civic on track.

Also, RPF1's have a 0.3 lb of a difference in weight for 17x8 vs 17x9.

Hmm this thread has me reconsidering what width I should go with. I have 17x9 RPF1s on order, and was going to get 255s for track, autox, and occasional DD/spirited driving. Now I'm not sure.. recommendations? I thought 225s would be too much of a stretch for a 9" wide wheel? I'm not looking to be competitive or anything, just to add as much grip as I can from a wheel/tire upgrade.

Maximum grip? 255.

You may have fitment/rubbing issues depending on your setup though.