Selecting a tire size (how wide is too wide?)

I learned my lesson in the non-power brake thread; I will divulge the specific question, but I still want to discuss it in the more general case.

The question I'm trying to answer is how much tire is overboard?

I was told years ago that it was easy to over-tire a Street Prepared BMW 2002. Light car, not much power, and that too much tire tended to not actually work that well. I didn't get a lot of specifics.

Given what I've seen going fast, I think at least for autocross that may not have been such a concern:

In any case, I'm now trying to choose the tires around which to build a car. Specifically, the Vespa 400 that ditchdigger and I are working on. Hoping for about 1100lbs and 150hp.

In hoping to find one tire suitable for autocross and a small amount of street use, I want to turn up my nose at these 165/55-12 Yokohama A048Rs because it just doesn't seem like enough tire for such an absurd little beast. I wanted something more, like a 225/45-13, or some of the 20.0x7.5-13 FSAE Hoosiers (not sure what we'd run on the street)...

The question is, with such little weight and power (even though it's a fair bit of power for that weight), am I being silly?

How do you determine how much tire is enough, and beyond what point are you:
Just carting around extra weight?
Not going to be able to use the tire hard enough to get it warmed up?
*Other concern I hadn't thought of

I'd say you've gone too wide when it can't be brought up to temperature.

However, with really soft compounds you could go crazy-wide to keep the temperatures high enough, but then you run into aero drag at speed, high cost, short life, and higher unsprung weight. And then there may be those pesky rules. It's all about compromise...

I know very little about it, but all the fast FSP cars are running the 275/35 15 Hoosiers. A friend of mine ran a Corolla GTS. He started off with the smaller 13s, but went a good bit faster on the 275 15s. I'm planning to run the 275s on the Celica.

My Firefly turbo is about 1450 lbs, and roughly 125 HP. Although I haven't tried everything, it worked best for me on 175 50 13 rubber. I tried 7 x 20 slicks, and 185 60 13 RA 1's and the BFG 175's made it easier to drive and faster.

I run 205 RA1s on my Locost. That's 1250 lbs and about 175 crank hp. Works well but you do have to push a bit harder on a warmup lap than a Miata. Brandon has the same size Nittos on his sub-1000 lb, 150-ish hp Locost.

Good stuff!

Keith, any thoughts with respect to autocross and a lack of warmup laps? I suppose the same thing applies, and you make darn sure to run two drivers...

It's awkward needing such a tiny tire diameter. It'd be awesome to be under 20".

For autox, IMO, there is no such thing as too wide from a grip perspective. Warm up Shmarm up. A soft compound tire will work from just above ambient temp.

That said, when you start making the car wider, you start increasing the distance that the car needs to travel to get through various features.

Autocross around here is pretty mellow - the Locosts rule the roost quite comfortably. The car works just fine with that setup, although I know I could buy faster times by going to an autocross compound.

Our track days are what might be called an "endurocross". 0.9 mile kart track. You get get an out lap, three timed laps and a cooldown. The Locosts have to run harder on the out lap to get the tires warmed up.

In reply to DILYSI Dave:

I'd agree with this based on my experience with the Wreck Racing Miata (runs 255/40R13 on a 13x10 wheel) and the CS RX-8 that I race (275/35 or 285/30 on an 18x8 wheel). Everything I've raced in autox prior to that I just used what was cheap, so it was narrow. We switched from a 245mm to the 275mm on the RX8 early last season and started picking up several tenths even though Hoosier recommends a 225mm tire for that size wheel. EDIT: and the 285 is a little faster than that.

The one thing about this is when you have a wheel width limit, like in CS, you do eventually reach a point where the tire is just too wide for the wheel and you start loosing time due to poor response.

Also note that this was all with the Hoosier A6, which is happy at extremely low temperatures, so warmup isn't an issue.

Keith wrote: Our track days are what might be called an "endurocross". 0.9 mile kart track. You get get an out lap, three timed laps and a cooldown. The Locosts have to run harder on the out lap to get the tires warmed up.

Sounds like fun

JohnyHachi6 wrote: The one thing about this is when you have a wheel width limit, like in CS, you do eventually reach a point where the tire is just too wide for the wheel and you start loosing time due to poor response.

It seems one of the things to do in DS these days is to stuff a 275 A6 on a stock-width 15" ITR front wheel. While it apparently makes the car feel like crap, it is ultimately grippier/faster. I never would have believed it possible until I saw a pic.

for what it's worth R888's in 225-45-13 were almost 2 sec faster at CMP than 205-50-15's on my CRX ( = low hp .. relatively light wt.... w/me onboard = 2315#)

Groovy. Sounds like I'm not crazy for wanting to wedge a bunch of tire under it.

I think we're going to go with the Hoosier 225/45-13s. Another GRMer has some used R6s to measure and build around, and we'll get some A6s when we're closer to running for real.

Thanks for the input!