I have worked on many World Challenge GT/Touring, and some ALMS machines. The tinnerman washers were used by most of the top teams, and really are effective and the best solution that I have seen to mount a splitter. This way you keep a flat bottom, no chances of tearing off mounts, and you get the benefit of a washer helping to distribute the load evenly on the mounts.
Let me know if you have any questions. I have worked on a couple splitter designs for GT3 Cup cars, and I have a full solid model of one for my S2000, done in PRO-E. Just dont have the time/money/tools needed to build it
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