Trying to put the front 355 wheels on all 4 corners of my pre-abs 328. The offset on the stock wheels is 15.15 front 7" wheel, 15.45 rear 8" wheel. The offset on the 7.5" 355 wheel is 46.15. The wheels do fit on the front, but seem to be inset further than the stockers. Rear needs a pretty big spacer -- but I cannot figure out the size. 31 mm isn't right because of the difference in the width of the wheel it seems. Help? Do I need to measure anything to help work this out? Pic attached, but the car's still jacked up at that point. Image Unavailable, Please Login
Offset is measured to the wheel centerline so you would need to take the wheel width difference into account at the rear -- i.e., to have the outboard edge of a 7.5" wheel with a 46.15 in the same place relative to the coachwork as the stock 328 wheel would require: rear spacer = (46.15 mm - 15.45 mm) + (8" - 7.5")/2 = 37.05 mm thick rear spacer However, this is only for the wheel edge -- the sidewall on an 18" tire (with the same outer diameter as the stock 16" tire) may not "bulge out" past the wheel edge as far as the stock 16" tire, so you may need to fudge up the spacer a little more to put the outboard tire sidewalls in the same places relative to the coachwork. At the front, you really want to keep the centerline of the tire and wheel in the same place (because of steering geometry issues) so with the ~31mm spacer (which is correct to keep the tire centerline in the same place), the outboard edge of 7.5" wheels should actually be a little further out than the outboard edge of the stock 7" front wheel -- although the same "tire bulge" issue applies so this would counter act this effect. If you can measure the distance from the mounting plane of the wheel to the most outboard surface of the tire for each (dimension C in the jpeg), you could do some more exact calculation. Image Unavailable, Please Login
Thanks so much for that, it almost makes sense to me On that 'C' measurement, is this taken with the rim off the car or with the weight on it? I can't see how to put the weight on the rim without having the spacer for which one is measuring...
No need to have the tire under load to measure "C" (even though it will bulge out a little more at the ground when it is under load) -- just lay the outboard tire sidewall down on a flat floor and measure up with a ruler, or tape measure, to the mounting surface. One point I'd make though, is that I've always found the "visual heft" of the uber-low sidewall tires to seem less so you could put the outer tire sidewalls in the exact same location relative to the coachwork and still might "look" different than stock.
OK 'C' measurement: front old: 3 7/8 inches front new: 2 19/36 inches rear old: 4 3/8 inches rear new: 2 22/32 The rear tire is darn near flat vs the rim. The front is actually slightly negative, in that the center of the rim protrudes a small bit from the tire.
Your numbers illustrate the (big) difference in the "bulging" of the tire sidewall for the 16" vs 18" tires with the same outer diameter. Rear To put the outboard tire sidewall in the same place relative to the coachwork: 4 3/8" - 2 11/16" = use a 42.9 mm spacer There is some risk to this as you are increasing the overall rear track dimension -- so the bending moment on the axle shaft is higher. If they are aluminum, did you consider/investigate if you could get the 10" wide 355 rears cut and rewelded to an 8~8.5" width so they could fit? Front As I said earlier, I think you should stay with the 31mm spacer for steering geometry reasons; consequently, even though the new wheel is 0.5" wider, and the wheel centerlines would be in the exact same place, the new outer tire sidewall actually winds up being ~1.5mm more inboard than the old outer tire sidewall. Either live with this, or maybe you could fudge up the spacer thickness a couple of mm without a lot of adverse steering effect. Of course, please make your own decisions.
