New Forgelines, MPSC's on them. Is the width OK? Sidewall sure slants, looks like the tire's stretching out to fit.

Track tires, don't want to take any chances. Didn't look like this on my stocks. These are stock front and265/35 18 rear.

Front is 9", rear is 10.5", according to the invoice.

Attached Images

	tire1.jpg (151.4 KB, 57 views)
	tire2.jpg (167.0 KB, 57 views)

I came across this in the following thread, maybe I'm better off with the wheel wider?

Are Wider Wheels Better.....


Where the wheel comes into play:

The easiest way to prevent a tire from deforming (besides buying a more aggressive tire) is to give the sidwall a little leverage. This is done by increasing the wheel width. By making the rim WIDER than the tread, the lateral pressure on the tire can be more directly transmitted to the wheel. Since the wheel is rigid, it will not deform, and will allow the sidewall to brace itself against the wheel.

Every tire has a spread of rim widths that it is designed to fit over. Let's say a size 225/45/17 tire will fit on a 17 inch rim from 7 to 8.5 inches wide. While the wheel is designed to fit on a 7" wide rim, the actual tread of the tire would likely be as wider (possibly much wider) than the rim itself. Because the sidewall is ballooning outwards beyond the wheel, it has little leverage to resist side-to-side movements. When the sidewall cannot resist these side-to-side movements, the tread begins to roll over onto the sidewall, and the entire sidewall deforms. In extreme cases, the tire could loose its bead and put the driver in danger.

Most experts will recommend that for aggressive driving, even the minimum wheel width listed by the manufacturer for a tire is still too small. It is possible for a skinnier tire to perform better on a skinny wheel than a wide tire on a wide wheel. Rather than getting 225/45/17 tires and putting them on 7" wide rims, try a 215/50/17 on a 7.5" wheel. You may find that combination more responsive, and as a bunus the slightly smaller tread may fit under the fenders more easily. More extreme, try fitting those 225/45/17 tires on an 8" wide rim, and roll, bend, or cut the fenders!

Why is this important? Because sidewall deformation robs traction, overheats tires, and causes them to wear unevenly! In some cases, lateral stability might be so good that the tire pressure need not be as high. If we can lower the tire pressure without worrying about the sidewall deforming, then we can increase the contact patch!

Of course, if cornering is not important- then neither is sidewall strength. But you wouldn't deprive your Mazda6 of twisty roads, would you?

One thing to correct is the side wall deformation is actually desired in standing start straight line acceleration hence why the drag cars have those huge tires. It slingshots you our of the hole which allows you to gas it before the light goes green.

On a road track this is a different story as it is not a standing start.

The optimum rim width for the front MPSC of 235/40-18 is 9.5" and for the rear 265/35-18 is 9.5" but, Michelin's specs on the tirerack site show they will work on rims that are 8"-9.5" on the front, and 9"-10.5" on the rear...

So, yes, they are bowed-out because you're right on the edge of rim size for these. Next time you get track tires, you might want to go with 285s in the rear, but MPSCs don't come in 245/18 for the front, and a setup of 235/285 (front/rear) which will give you a lot of understeer.

If you're going to run sport cups, you might want to send the wheels back and get a 8.8"/9.5" combo which will work better with the sizes that sport cups come in.

brad

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21-year PCA Member
PCA DE Instructor

Thanks Beez-

I've got pics emailed to my shop, I'll see what they say. I just want to be sure the setup is safe.

I think Forgeline was trying to do me a favor in maxing out the rear width for esthetics.

I happen to have a set of 285's (I think, maybe they were 295's) from my Dad's C2S setup last year. Called Jim at Tirerack frantically when I found out they were too wide for my stock wheels and he set me up with the right size.

I'll look into other tires. maybe Hoosier has something that will work.