STU wheel and tires

[Tom Hunsinger]

I got an e-mail from someone asking about wheels and tires in STU:

"I read the SCCA Solo II rules and it appears that any tire size up to
245 is legal on 4wd cars in STU. Does this mean that different tire
sizes may be used front and rear if they are 245 or less. Also, can
different wheel sizes be used front and rear?"

I am posting it not only for the yes or no answers but also would like to know if anyone has any experience or suggestions on using different sizes front and rear on AWD. Thanks!

[aurorasti]

I don't see why you couldn't, but I also don't see why you would. I do know (I think) who sent you that e-mail. I think it would be too hard on the diff's. But, it's not my car, so go for it.

[Captain_Solo]

Unless the car is one of those funky engineering marvels that comes from a certain Stuttgart company, AWD cars tend to not like size mismatches front to rear. Especially subaru. That company warned me about three times not to do this before I even drove my car off the lot. Like Brian said, it's not my car...

I'd humbly suggest that your friend put the same size tires on all four corners and go with the largest section width the rules allow, in this case 245. Any chassis imbalances can then be dialed out by adjusting swaybars, dampers, tire pressures, alignment, spring rates, etc...any of which can be legally adjusted or substituted(within reason) in STU...

BTW: You can certainly use tires that are narrower...

[Clay Turner]

I've read this discussion before, but my memory is fuzzy this morning: I believe that different-sized tires will destroy limited-slip differentials. These LSDs are designed to slip while delivering power on a percentage basis to inside and outside wheels in a turn; when you mount different-sized tires to rims, that LSD is working all the time, instead of just in the turn. It won't take that kind of abuse very long.

Don't AWD cars have a center diff that operates the same way between the front and rear axles? Ergo, creating the same effect?

Fuzzy in Colorado Springs,

Clay

[russjnco]

You are pretty much right on the money Clay. I can't speak for the Subarus but the setup is fairly similar. On the Evo there is a center diff that runs off the gearbox and then there is the front and rear diffs. I think Mark Daddio tried a wider front tire setup on an '03 just because of the open diff but I don't think it worked out too well for him.

[MrViggy]

Yep, the Subi's have center LSD's. The STi's actually have adjustable center diffs (IIRC 50-80% of the power can be directed to the back constantly). The WRX's have a 50/50 split in power for MT, and a 60/40 split in power for AT (for the MY02's; dunno about you other guys).

The manual (mine anyhow) says different sized tires are a big NO-NO! As a matter of fact, on the AT cars, if you put the donut on, you have to put a fuse into the fuse block to disable the center diff.

Viggy

[aurorasti]

The driver controlled split is fron 50-50 in lock to 35-65 full open. I think the manual says that the tires need to be within 3/32 of wear. I can't remember off hand, but it was something like that.

[MrViggy]

The STi has a 'full open' mode? I was not aware of that... Hmmm, how can I convince the wife that trading my car in for an STi Limited is a good thing?

Viggy

[aurorasti]

You have it your question...limited. You would be getting only 1 of 800. 400 of one color. Convince her it's about value in the long run. Besides, it's kinda sleeper and doesn't grab alot of attention from Johny law.