finky

iTrader: (2)

I have an 86 GXL that has been significantly reduced in weight to FP rules (around 2200#). I still have mostly stock setup suspension for the moment. I am trying to see what works well for people. I don't know what size to shoot for. I am trying to figure out if 15x10 would be counterproductive on a car w/out PS. I know there are a lot of variables but just looking for ideas.

25BP

I have heard of 10" wheels fitting without spacers or moving the struts inboard. It will depend on the size tires you are running as well.

I run 12" up front and I needed to move the strut inboard to clear the tires. 12" are the widest you can run in Prepared, and there is a weight penalty for it, but most people will run the extra weight for the extra contact patch.
I can tell you trying to keep up with a nonPS car with large slicks on a autox course is hard to do. I ran without PS but put it back on. The PS rack has a faster ratio so it makes sense to do anyway regardless of wheel size.

Hope it helps, best of luck.

Travis R

A15x10 would work, but Hoosier doesn't make a tire in that size. Go to www.hoosiertire.com to see what's available (under road race tires, then specs). IIRC the closes thing is a 15x9.5. The benefit is a much lighter tire and wheel package, but if some of your competitors decide to move up to 12" wheels, then you really are down quite a bit in the rubber department.
16x10 is the most common in FP, even though the available tire has a pretty short side wall. That's what I'll be running.
Good luck

in2twins

I will confirm it shortly once the car is sitting on the ground, but I believe a 23.5x11.5x16 will fit on a 16x11 with about a 5.5" backspace. This is with stock strut mounting position. I will eventually be running a 25x12x16 up front, but I need another set of 12" wheels. Carl Byck

RotaryAXer

iTrader: (4)

Mr. Finky,

In some cases wider isn't always better. I don't know if I can explain this well or not but the more surface area you have supporting a certain amount of weight the less pressure you have on that surface. So if you have a really wide tire supporting say 550 lbs then the pressure on the contact patch of that tire would be smaller than if you you were on a narrower tire. Most tires will have a point where there isn't enough pressure to create good adhesion between the rubber and the road.

Take this or leave it. You want wide tires but until you start introducing aero load, really wide tires may not be necessary.

If you need a first hand account of this my good friend and co-driver is also co-driving and SM Focus that they started with 275 width tires and didn't get the grip needed they have now gone to 245 fronts and 225 rears. The car now has much more grip.

RotaryAXer

iTrader: (4)

FWIW

The Hoosier P/N 43361 22.5 X 9.5-15 C3000

Is recommended to be mounted on a 10" wide wheel.

TNH

[QUOTE]Originally posted by RotaryAXer
Mr. Finky,

In some cases wider isn't always better. I don't know if I can explain this well or not but the more surface area you have supporting a certain amount of weight the less pressure you have on that surface. So if you have a really wide tire supporting say 550 lbs then the pressure on the contact patch of that tire would be smaller than if you you were on a narrower tire. Most tires will have a point where there isn't enough pressure to create good adhesion between the rubber and the road.

Take this or leave it. You want wide tires but until you start introducing aero load, really wide tires may not be necessary.

If you need a first hand account of this my good friend and co-driver is also co-driving and SM Focus that they started with 275 width tires and didn't get the grip needed they have now gone to 245 fronts and 225 rears. The car now has much more grip. [/QUOTE

I am sure you are right in theory, but we are not near that limit with 12" slicks in an autocross compound. In my case I can run any compound I like, but that said, a Goodyear 25x13x16 R430 Bias ply heats up to full operating temp on a 2500lb car in about two laps at Infineon(about 4 minutes). What sort of tire temps were you seeing, how much does the Focus weigh, what tires were you running? I would think a hoosier AX tire would be near operating temp just rolling across ~110* pavement... Carl

Travis R

I saw that. But it looks like it's only offered in the R25 and R35 compound. I didn't know if that would be too soft. I guess it depends on the ambient temp where you run. I think they might get overheated and not work as well in the Texas summer (~140F surface temps).
The next one down #43385 23.0 X 9.5-15 is offered in the R45 compound but is supposed to be mounted on a 6" - 7" wheel... obviously cantelevered.
There is also #43390 24.0 X 11.0-15 that is meant to be mounted on a 10" rim, but that is going to make a pretty big sidewall @ ~4.5", and is probably going to rub (since it's section width is really 12").
I'm just throwing out ideas here. Do people run R35's?

RotaryAXer

iTrader: (4)

Not sure of the tire temps but I would bet they were not even over 110* (I wasn't around when the pyrometer readings were done), with a nice even distribution (a bit "hotter" on the inside). In Michigan you take any heat you can and hope the sun is bright. Sometimes we are lucky to see a surface temp of 80* in the middle of July.

The Focus is right around 2500 lbs. The tires are the A3SO3 DOT Hoosier Radials. The new rear tires are the A3SO4 tires.

Ideally the tires would be warmer. But with the typical heat style autocross there is plenty of down time between the 40-70 second runs (6-8 total with 2 drivers). If running on the track then the situation is a bit different.

Jeff (Finky) I would maybe just get some cheap DOT-R compounds on the car (on the stock wheels!) and get it sorted out a bit. Then maybe make the step up to real race wheels and tires. You are obviously not afraid to hack apart the car to make it work but it would suck to get these really wide tires on the car before you have coil-overs all around and any other mods done and come to find out they won't fit once you do these things.