Originally Posted by R A W L S
Will running wider tires on the 10" rim make a measurable difference, or do I need to run the wider tires in conjunction with the wider 10.5" wheel to see a difference?
The wider the wheel is the more sidewall support you provide the tire. Each wheel width can accept a range of tire sizes but as you get to the limit in either direction (narrow or wide) there are things to consider. I'll just go over putting wide tires onto the wheels and ignore stretched rubber as there is no E9X that would need to under-size their tires on these wheels.
As you pass the optimal tire width for a wheel and continue to increase width, you start to bulge the sidewall. Under turning the sidewall of progressively wider tires will each flex/roll over more. This flex slows down the turning response of the car (what people generally define as being sharp when working well). Too much rubber will give a sloppy feel, with poor feedback. You can get the same feel from luxury oriented tires with soft sidewalls that roll over easily under turning. When you go too wide, you also start to pull tread up off the ground near the sidewalls. So in effect you are making the tire narrower by putting it on a narrow wheel. The opposite effect is true for stretching a tire. When a tire manufacturer lists section widths, they'll note what wheel width was used. A wider wheel would increase section width/usable tread by a fraction of an inch. Because of this, a 285+ will effectively be wider then on a 10" wheel, give you more grip and stability.
The 10" wheels are optimal with a 275. 285 is still perfectly fine on a 10" wheel. even a 295 would be alright, but would begin to show signs of bulging. There are so few 10.5" wheels on the market, that most club racers have been running 285's on 10" wheels for years. If they had the choice or the room, they would run them on a 10.5" wheel. For that reason we're seeing a huge amount of interest for the 10.5" size from club racers running 285's or wider.
If you already own 10" wheels, then there's no problem running a 285 or even a 295. The slight decrease in performance is probably not worth getting new wheels. If you're getting a new set, then 10.5" wheels are the right choice for anything wider then 275. You could even run a 305 or 315 on the 10.5" wheel but that's at the limits of those tires, and the sloppiness of the overly wide tire would not out perform narrower, better fitting sizes.
Originally Posted by dnvrdrvr
Gearhead999s, thanks for the feedback. Thinking about the tracks I'm running on, I'd say I'm in 2nd once/twice per track. Mostly 3rd, get into 4th on the straights. Based on what you've said, I'm thinking square might still be okay for me.
paintpro21, What about the 10F/10.5R combo vs. square? I had 285 RE-11s (on the OEM 19x9.5 wheel) on my rear last year, moving away from street tires this year and am loathe to give up any rear width... $$ savings by rotating back to front is less important, more important to me is the performance & predictability in handling.
a 10/10.5 or 9.5/10.5 stagger will perform very similarly in comparison to a 10" square setup, as it will mostly be tire size dependent. Both those staggered setups would use the same rear tire, and the only question would be are you going with a 265 or 275 front tire. The idea as mentioned before is the staggered setup would avoid the sensation over oversteer by reducing front grip. I'm not a huge fan of reducing the performance of one part, to change the feel of something. I would personally just drive a car differently that has a square setup.
If you're set on running a 295 rear then you have to run a staggered setup. You can't get a 295 to fit up front so square is out of the question. To determine your front wheel width, just see what front tire sizes are available in the RE-11. If it's a 265 then you'd be fine with the 9.5's. If it's a 275 then get the 10's. It's as simple as that.
Originally Posted by Richbot
What about the Kumho XS or Falkens in 315/30-18 for a rear tire? I know a few people have used similar widths and offsets with 315 street tires on the rear, but who knows what kind of alignment or fender massaging is necessary.
The BFG drag radials are available in that size too. Just an idea, might be a good combo with the Apex 10.5" wheel if you're trying to hook up 500+whp in some of these supercharged applications.
You are correct. a 315/30/18 could be mounted. For 500+ whp guys that could be the right choice if their primary goal is straight line power. Should a 315 be mounted on an 11.5-12" wheel? Yes. Does the M3 have room for that? No. So compromises have to be made. Doing so won't be terrible, it just isn't performing 100% optimally. That's what makes it hard to give fitment advice, as "better performance" doesn't mean the same thing for everyone. Time savings and setup for track, autocross and drag are different, and the costs of each minor improvement can really start to add up.
The most recent car running 315/30's on a 10.5" wheel clearly had rear inner fender lips rolled flat, and they dialed in a lot of negative camber. That much rear negative camber will have major inner tire wear issues on the street and the track. You're also not using the full width of the tire, so you could start to ask yourself if a narrower tire with less camber would yield the same grip or better.