ReverendDexter
ReverendDexter SuperDork
1/18/11 2:21 p.m.

So, reading GRM, you guys are always stuffing the widest tires you can on whatever rims you have.

However, that goes against my understanding of how to properly fit a performance tire, where you want to go relatively narrow (100-110% of wheel width) to keep your sidewalls stiff and your transitioning better. At the 25, I asked the tire shop guy what sort of fitment was ideal, and he was saying that the tire should NEVER be wider than the wheel, and was mounting tires that were more like 95% of wheel width.

So why the discrepancy? Is this an endurance road race vs autocross thing?

For brakes, I see lots of people constantly upsizing calipers and rotors and piston count. But my understanding is that the limiting factors are tires and heat. With my craptastic stock foxbody brakes, I can lock up 255-width RA1s on an autocross course, and the only time I've EVER experienced fade was going westward on highway 36 out to the coast, on the last corner of a 3 mile, 11% grade switchback section (which follows a 2-mile 10% grade section).

Now, I do run LMP DOT4 fluid and SS lines, but I'm on the crappiest pads you can buy (I just needed something to put these calipers on, and haven't gone back to put better pads on yet).

So what gives? Is my traction just so crappy everywhere I go that my brakes are enough? That can't be right.

foxtrapper
foxtrapper SuperDork
1/18/11 2:43 p.m.

Lets start with the brakes first. If yours are doing the job, you don't need to upgrade. You're right, bigger rotors don't make you stop faster. Same fancy pad compounds or mega sized calipers. But they can help with things like modulation (holding the brakes on the edge of lockup, with control), fade, pedal effort, feel, etc. If you're car is fine on those points already, an upgrade is a waste of money. At least for now. Become a better driver and that may change.

Tire sizing is as much art and driver skill as it is science. If it was all a pure science, the formulas would have been mastered, and there would be one tire on all race cars. But just as some folk like a car that's more tail happy than others, some like a stiffer sidewall than others.

For a while there the rage at autocrosses was to have the largest tire possible crammed on the rim. Inflate the heck out of it to make the sidewall somewhat stiff, and go off convinced this was giving you the edge. I did notice that the really fast drivers didn't do this. Mostly it was the noisy drivers. By and large, I agree with your tire shop.

ReverendDexter
ReverendDexter SuperDork
1/18/11 3:38 p.m.
foxtrapper wrote: Become a better driver and that may change.

Curious on this. I'm always looking to improve my driving; what is it that I am or am not doing that would increase my "need" for brakes? Smoother application and weight transfer to better utilize available grip? Better lines increasing my braking zone entry speed?

red5_02
red5_02 Reader
1/18/11 5:45 p.m.

In a way Reverend, yes, as all those things will make you faster. And the faster you go the more friction you'll need to slow yourself down.

nderwater
nderwater HalfDork
1/18/11 6:38 p.m.
ReverendDexter wrote: I asked the tire shop guy what sort of fitment was ideal, and he was saying that the tire should NEVER be wider than the wheel, and was mounting tires that were more like 95% of wheel width.

Let me guess, did this clown also happen to sell wheels or refinish wheels with curb rash?

foxtrapper
foxtrapper SuperDork
1/19/11 5:34 a.m.
ReverendDexter wrote:
foxtrapper wrote: Become a better driver and that may change.
Curious on this. I'm always looking to improve my driving; what is it that I am or am not doing that would increase my "need" for brakes? Smoother application and weight transfer to better utilize available grip? Better lines increasing my braking zone entry speed?

As you aproach a turn you want to brake at the very last possible moment, as hard as is achievable, and only enough to get you into the turn at the highest possible speed.

What most of us actually do is start braking way to early, not hit the brakes as hard as we could, and ride the brakes way too long into the turn.

That or we spike the brakes far too hard, sometimes late, sometimes early, skidding.

When you get to the point as a driver where you can tell absolutely that you are not maximizing your braking potential because of mechanical deficiencies with the car, and can say what the defficiency is, then it's time to upgrade. Until then, it's driving skill that is the dominant limiting factor.

Take me for example. I'd love to claim great racing driver status. But my lap times on an autocross course can easily vary by 5 seconds, and sometimes more. That's not the car, that's me. Upgrades to the car would get lost in the noise of my poor driving. The best place to invest for faster laps would be in the my driving skills, not my car.

92CelicaHalfTrac
92CelicaHalfTrac SuperDork
1/19/11 6:55 a.m.

The tire:wheel ratio isn't set in stone. The type of tire you're fitting has much to do with it.

As for brakes, if i were getting fade at the end of a 3 mile run like you were describing, that wouldn't be enough for me, and i'd be wanting an upgrade. Any decently working stock brake system will lock up pretty much any tire in my experiences. It's a question of how many times it can do it.

That said, I haven't done more than stainless steel lines, pads, good fluid, and brembo blanks to any of my cars, and they simply just don't need more.

I'm running 245/45-16 on 16x8 wheels. (Also keep in mind that one 245/45-16 won't have the same width as another.)

ReverendDexter
ReverendDexter SuperDork
1/19/11 9:28 a.m.
nderwater wrote:
ReverendDexter wrote: I asked the tire shop guy what sort of fitment was ideal, and he was saying that the tire should NEVER be wider than the wheel, and was mounting tires that were more like 95% of wheel width.
Let me guess, did this clown also happen to sell wheels or refinish wheels with curb rash?

Not at all. Mind you, this was the tireshop at the racetrack, and they don't even sell anything with a high enough treadwear to be in a AX street tire class.

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