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BIG tires need mods

I recently wrote about "Neutral cars, big tires," and refered to some 
mods that bigger tires would require.  I forgot to talk about the 
mods.

I currently use 235/45/17 R1's for the track and for autocross.  They 
are the only size race tires that will fit my M3, which is lowered 
1.5 inches back, and 2+ inches front, with H&R coilovers.

I have never liked the size, as it is larger in diameter than the  stock 
tires (235/40/17), and therefore changes the affective  rear end gear
ratio, making the gearing taller.  I think that the 3.15 rear end is 
already too tall, and don't want to exacerbate the condition.  
Ideally, I want a tire with a larger footprint, but with the same 
diameter as stock.  In addition, I plan to change my rear gearing to 
3.38 or 3.46.  

BFG says that it will think about making a 235/40/17, and/or a 
245/40/17 within the next 18 months.  In the meantime, Hoosier makes 
a 245/40/17 right now, available in road race and autocross 
compounds.  I mounted up a Hoosier  to an 8" rim, and it fit great on 
the rim.  It would not, however, fit on my car.  The front tire 
rubbed on the lower spring perch of the coilover.  I could cure that 
by dialing out the negative camber , but that would defeat the whole 
purpose.  Instead, I will add a 20mm spacer, available from Roland at 
H&R, which will make the tire fit fine, and even give the bonus of 
added track width.  The fender clearance is not a problem.

The rear wheel fits ok on the inside, I think, but it rubs on the 
fender.  I will need to flair the fender.  I could do it by rolling 
the inside of the fender lip with a pipe, or a baseball bat, but I 
think I had better stay away from the heavy machinery, and let a body 
shop do it.  I am not sure that even with the rolled lip it will fit, 
but we will see.  BTW, the Hoosier 245/40 has a tread width of about 
9 1/4 ".  The road race compound lasts MUCH longer than the autocross 
compound, but is not as sticky at lower temperatures.  At hight temps 
(above 150 degrees), they are said to have the same grip.

I will report on the results.

Regards to all,

Carl

By the way, a STOCK ride height M3 will accomodate tires as large as 
255/40/17 on the rear, and 245/45/17 on the front.  This is, I 
believe, the setup that Bob and Patty Tunnell run on their stock M3, 
that they run successfully in A stock.  They are running stock 
camber, which is closer to 0+ or -.

Carl Buckland
1000 Boston Bldg
Nine Exchange Place
Salt Lake City, Utah  84111
801-531-6686
Fax 531-6690
E Mail [email protected]