Re: Tire size and traction (archive)

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: In a post below, you stated that 18" wheels/tires are at a disadvatage to 17" in straight line acceleration because of a decrease in sidewall flex. It sounds as if any handling advantage a "plus size" wheel/tire combination would have is offset by a disadvantage in straight line acceleration. (BTW, I'm not sure how to calculate it but, how much shorter is the sidewall on an 18" 255/40 than on a 17" 245/40?)
: Craig

That was me (Marc "S") that posted about sidewalls and traction, though Marc B still had a good response for you :).

Here are my thoughts.. If you plan to keep your M3 totally stock, then I don't think down-sizing to a smaller rim diameter is a good idea, given what the M3 is all about. Bigger diameter rims look better to most people, and they are more desirable when it comes to resale value. To a small extent you're also likely to get better handling in turns. The only negative to 18"+ rims other than straightline traction is that you are more likely to have a stiff ride and feel the bumps in the road more. (Less cush from the shorter sidewalls).

To compute sidewall, simply utilize the profile ("series") as a ratio and multiply by the width (which is in millimeters). So, a 255/40 would be .40 times 255 = 102 mm sidewall. On a side note, if you wanted to compute the diameter of a set of tires/rims, double the sidewall and add the diameter of the rim, and now you have the diameter of the tire. Diameter is helpful for figuring out if a proposed combination will change ride height, how much the tire/rim fills the wheelwell, and gearing. A smaller diameter will effectively give you lower gearing and improve acceleration!

To buy a second set of rims that have a smaller diameter would make sense only if you are really into racing in the straight line. For me, I bought new rims/tires for my car about a year ago. It was kinda frustrating because my car comes stock with 16" rims, and it seemed like such a shame to spend all the $$ on aftermarket rims and not buy 17" rims which would look much nicer (or even 18"). Ultimately I had to put my hard-earned money into a new set of 16" rims that were simply wider and lighter than the stock ones. For *me* it was a great choice, I gained more traction and do better than other cars similar to mine at the race track that have bigger rims. A couple tenths gained from traction is a big deal to me - considering my best time in the 1/4 mile is an 11.81, losing 1 or 2 tenths in my 60-foot time when launching would mean I would have been hitting 12's in the 1/4 mile.

If you plan to make some light power mods to your M3 and straightline traction matters, potentially you could simply max out the width for 18" rims for more traction, I'm sure you can go much wider than what the factory does..

Here's one trick that isn't well known... When it comes to picking the width of the tire for a given rim width, ideally the two widths are pretty close to matched. From the factory, tire width is usually bigger than rim width typically by 1 inch. Using that as a basic point of reference, all else equal, going with a wider tire (bunching the tire up on the rim more) will increase sidewall flex, and going with a narrower tire (stretching the tire across the rim more) will decrease sidewall flex. (And, as you now know, more sidewall flex = more traction for going straight). So, if you are not modifying the power of your M3, or very lightly modifying it, and you are a little torn between going with the 17"'s for more straightline.. You could potentially have close to the best of both worlds if you shoehorn a significantly wider tire onto wide 18" rims. The cheapest option for you is to buy a good performance tire for the rears that is wider than the factory tire but still fits on the factory rim. The next (significant!) step up in costs is buying a whole new set of rims where you could start with an even wider 18" rim and buy an even wider 18" tire.

Ok, as a final thought on improving straightline traction (it's on the minor/nitpicky side but what the hell), when picking a tire, you might look at how the tread in the center of the tire appears. It's the centermost part that has the biggest determination on straight-line traction, and the edges of the tire have the least. I bought Yokohama AVS-S1 performance tires which are very grippy in general but in addition have a wide "slick" in the middle of the tire - a completely unbroken "slick-like" block of tread.

I'm trying to avoid recommending 17" rims/tires just because my sense of what M3's and their drivers are "all about" wouldn't mesh as well with going from 18" to 17", unless the person was really into drag racing.

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