From digest.v7.n967 Sat Jan 17 19:06:18 1998
From: Todd Green <tag_at_cs.utah.edu>
Date: Sat, 17 Jan 1998 13:36:33 -0700
Subject: ASC+T and mismatched tire sizes and re: Tires on M3: R1 v. MXX3

>So, do I get the additional handling/performance of the R1 on the
>street, or do I bite the $1300 bullet and buy the MXX3? The R1s would

Personally I'd keep street tires for the street and track tires for the track. In my (one day :) experience the R1's were considerably rougher riding than my street (snow) tires. Besides you can push the MXX3's to very unsafe speeds as it is on the street. Why would you need the added grip of R1's?

Anyway, Carl Buckland was gracious enough to loan me a pair of his 17x8 RC's with 255/40 R1's mounted for a day to make sure they didn't rub the rear fenders on my stock '97 M3 suspension. They only rubbed once, very briefly, during the entire day, and only on Carl's side when I was giving him a ride back from the Intermountain Vintage Racing annual meeting. I'll let the readership draw their own conclusions (sorry Carl, coudln't resist ;) Seriously though it was during a pretty evasive maneuver due to the highway dept putting an unlit barrel in the middle of the road for no apparent reason. Inside clearance seemed to be fine. They came no where near the muffler.

>Also, I know I've asked before, but I still haven't gotten a definitive
>answer, yet: Will 235/45 front and 255/40 rear clear the fenders? And,

A lot of this will depend on your wheels, your particular car, 95 v 97, amount of camber etc. Since I'm spec'ing out RC's and they seem to be a popular track wheel, here is a chart I made detailing the differences trying to see if I could fit the 17x9's

Offset Rim Width 1/2 Offset->Outside Offset->Inside Stock

    17x7.5  41mm    190.5mm    95.3mm  54.3mm         136.3mm
    18x8.5  41mm    215.9mm   108.0mm  67.0mm         149.0mm

BBC RC's

    17x8    38mm    203.2mm    101.6mm 63.6mm          139.6mm
    17x9    32mm    228.6mm    114.3mm 82.3mm          146.3mm

For the metrically impaired

    17x8    1.50"    8.00"      4.00"   2.50"           5.49"
    17x9    1.25"    9.00"      4.50"   3.24"           5.76"
    delta   0.25"    1.00"      0.50"   0.75"           0.25"

BBS 8 vs 9:
This means that the rim sticks out 3/4" further and in 1/4" further, but the t{i,y}re itself is centred 1/4" further out. Hence the track is 1/2" wider. This is entirely due to the change in offset.

BBS 8" v stock tire center is ~1/12" out more BBS 9" v stock tire center is ~1/3" out more

Thanks to Andrew West for pointing out the error in my original table, and the 8 vs 9 commentary.

If you go with 255's in the front you will need spacers. I'll tell you exactly how wide as soon as I get my wheels mounted next week, and test Carl's 15mm.

>if so, will they upset the ASC-T system?

Contrary to Ben Liaw's experience, though in no way meaning to debunk his reports, I had no problem with the ASC kicking in at 75-80mph. I had 225/45 Pirelli 210 Winter Assy. on the front and the 255/40 R1's on the back.

Todd

• -- '97 M3, not quite yet frothing BMW CCA #<is probably bigger than yours>