I always mount the caliper behind the fork legs because the bikes I've worked on have been designed that way.

I covered over 40,000 miles on my DR600 fitted with an FZR600 f/wheel, FZR1000 320mm disc & a Nissin 4-pot caliper from a GSX-R. There may well be a tiny degree of flex when using spacers but none that I've noticed. My pads wear evenly & don't jam in the caliper. I've nevernoticed any vibration. I also fitted ISR calipers to my GSX-R & 1100EFE calipers to a GSX750EF for a friend.

One thing to note that I learned from my DR experience: Where possible, use the matching caliper for the disc you're using. On my DR set up, I have to grind a slight bevel on the bottom of the pad material so that it clears the disc mounting buttons. At least check in advance that that the pad clears the buttons when aligned with the top edge of the disc, not a huge hurdle but a potentail problem you can eliminate at the design stage. I used a GSX-R caliper because that's what I had & I did not want to use the Sumitomo caliper as fitted to FZR's as they have no dust seals fitted.

Machining from a large billet is the way to go if you have the facilities (& skill - big power tools take no prisoners, as we say here in Oxford.....) It may be an idea to make a mock up first, using spacers etc to ensure alighment etc before proceeding with the final version?

Good luck

Steve

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