Originally Posted by Big Windy
tom, i think what swamp is saying (and I agree with him) is, where do you get 20mm from? how do you know that this is the threshold? is there actual evidence that shows that certain added stresses exist beyond 20mm?
the point is that a lot of people (including myself) will say that you are good with less than 10mm, or 15mm, or even 20mm, but that is not from actual experience or evidence, rather from common practice. 20mm could be a totally arbitrary number for all we know...it would be nice to have some real data, but i think there are simply too many variables.
I never said this was a rule for everyone to follow, it's simply something we don't recommend. The additional shank length is only one of the factors.
Past experience can be seen on e46 M3s, which are on their 8th year of seeing US roads with no ill effects of spacers within these thresholds. If referring to e9X M3s, there's not enough time to tell, but I doubt it would be drifting far from past models. Spacers are even a common mod of Ferraris that see track duty to increase track width.
Staying under 20mm is perfectly fine, I would simply avoid going further than that if at all possible.