[0-60Motorsports]

aaannnddddd he's back, great info Ian! Thanks.

[Mr. Shadow]

I'd run a 285 in the rear, but PS2's don't come in 285/35 according to tirerack.

[t3stman]

Thanks for the info! This should be a sticky

[SilberFuchs]

Originally Posted by Obioban

Argh, that article does have me bothered. The problem I'm having is that the data in there doesn't jibe with what I've seen in reality.
-lateral Gs don't drop with wider tires without a decrease in tire pressure
-drag racers run super low pressures on the rear
-you can easily and noticeably shift the balance of a car by tweaking tire pressures.

.... but I am considering rewording that section now. I certainly don't want to mislead people and post something that isn't true....




Edit: This is where that test gets really werid. They actually get less deflection and lower tire pressure than high. Something must be amiss...

Ian, I am certainly no expert on tires and pressures, but I'm not quite sure your statement about different tire widths at the same tire pressure leading to the same grip is quite correct (UNLESS a higher tire pressure is required for a wider tire bearing the same load to get the same full thread contact patch). My understanding (albeit limited) about tire pressures is that it's about optimizing even and full contact patch for a given axle load, thus leading to maximal thread contact patch and even tire wear. For example, since the M3 has a near perfect 50/50 weight ratio, the front and rear tire pressures should be about the same, unless one wants to dial more or less understeer/oversteer by messing with inflation pressure. Also, the tire pressure is dependent on the driving conditions -- in other words, while a certain pressure may be ideal for mostly straight line driving (highway, slow city, etc.), it may be too low for an autocross event or high-speed cornering at the track which require higher pressures due to high lateral weight shifts (thus, the chalk test to get the optimal pressure) so as not to ride on the shoulders of the tires.
-

[Obioban]

I have eliminated the part of the post about contact patch/tire width. I don't know enough to know which side is correct, and I don't want to propagate false information... so it's deleted.

[jph28]

Originally Posted by Obioban

I have eliminated the part of the post about contact patch/tire width. I don't know enough to know which side is correct, and I don't want to propagate false information... so it's deleted.

I am not even close to an expert in this stuff, but I do know that the standard "FrictionForce = mu*NormalForce" (where mu is the coefficient of friction) equation is quite invalid with tires. Specifically, this equation (Amonton's 2nd Law) is not valid for elastic, deformable materials because they generally have nonlinear mu. Plus, there are issues of tire heat management that will generally benefit wider (street) tires.

There is an interesting thread here about this topic (I found this in a Google search just now, I'm not a forum member )

This thread is and will be an excellent resource for the forum for sure, but the low-level details of the physics of transient/dynamic tire traction are probably (hopefully for me, since I don't quite grasp them!) beyond the scope Thanks for posting this, hopefully we'll all learn something and the number of "tire threads" will go down!

One thing I will add about this topic is that a good, high-performance tire (i.e., high coefficient of friction) will out-grip and wider, lower performance tire just about every time in dry conditions. The first consideration in trying to gain traction should be finding a high-quality performance tire, not just getting the widest "budget" tire that will fit under the fenders...

[eas]

Such good information, thanks Obioban!

[Draddy]

great information.

this is why myself and many others i'm sure, visit this site. In reality, this doesn't mean anything for most of us who don't push to the extremes, but it's just really cool to know about, and a great discussion (above with the contact patch situation).


P.S. I have wider tires just because it looks cool.

[M_Threezy]

Ian,

This is phenomenal. Thank you for the great information!

[DuncanS]

245/35/19, circumference: 80.9"
and
275/30/19, circumference: 80.1"

285/30/19, circumference: 80.8"

Would a 285/30/19 be a better match to a 245/35/19?