Join Date: May 2012
Location: Denver, CO
Vehicle: 2003 SLK 230
Other Toys: '91 E Stock MR2 and '84 FSP Celica! And my poor '72 Capri that I need to finish.
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I'm no expert, but I do my own alignments and I change wheels a lot. Summers, winters, race... yeah. I get tired of changing wheels all the time. Of course, not so much on the SLK, and I did just learn how important the lenghth of those bolts is when I put on our winter set up. So, my comments are more general observations on the issue than car specific. Though it's worth noting- sounds like our winter set up is like yours - no stagger to get skinnier winter tires in the rear.
My Celica gets the most attention just because I'm trying to make it go through parking lots around cones really fast. Street wheels for that car are 14x7 -9. Most of the time, that's what's on the car when I do an alignment. The race wheels are 15x10 -13. That's a pretty good difference. But think about it- camber, caster, toe- I don't see any way any of those can be changed by changing the wheel. Shoot, some people do alignments on hub stands - no wheels on the car at all.
Will the wheels change some things? Sure, in my case, going to the wide wheels makes a big difference in scrub radius, (about 1.3" if the on line calculators are right) but there's nothing I can do about it. Having said that, I've never noticed any unusual handling as a result. A change that significant is probably not a good idea for a street car since it will load the wheel bearings differently (not a big deal on an autocross car - the wheel bearings are almost *always* loaded a LOT). But I can't see how any of the settings that can be adjusted will change by changing wheels, and the kind of difference you're talking about is likely to be much smaller than that and likely to have even less consequence since you're talking about the rear.
I've actually taken measurements on my E Stock MR2 with the 15" summer tires and the 14" race tires. It's set up with -2.8 camber, +5 caster and about 1/8" toe out. Swap to the race tires and it's still -2.8, +5 and 1/8" out.
As I said, I'm no expert, and what I have to learn probably out-weighs what I know by a good bit. That's just my thoughts on it. I'd be interested in hearing from anyone who sees something here that I'm missing.
(edit) Oh, just one more thing. The guy I bought the winter wheels and tires from seemed to know his stuff. He used them on his SLK for one year. He said that the non-stagger is what MB recomend for winter tires. I haven't confirmed that anywhere but wouldn't be surprised. Every time I've looked, manufacturers recomend a little skinnier tire for winter.