I measured my Diamondback cruiser frame to see if my Dunlop motorcycle tire would fit in the rear dropout. It's gonna be EXTREMELY tight in the rear frame!! Measurements were taken without the rim, but the tire is so stiff that the numbers should still be very accurate. I'm using measurements from my 2.35" x 26" Schwalbe rear tire as reference points on the frame.
The Dunlop 404 80/90-21 is the front tire for Harley Davidsons. It is 5/8" taller than my 26" tire and 3" wide. There is 5/16" clearance to the top fender bracket/cross brace, and 11/16" clearance to the front of the chainstay. Left side chainstay clearance is tight @ minus 1/16"+. Right side chain stay clears by 1/16"+". Left side seat stay clears by 1/8"; right side seat stay by 1/16"+.
Looks like the left-side chainstay and the right-side seat stay will need "clearancing". Since issues are on both sides of the dropout, the wheel cannot be "dished" to solve the problem.
Well, I won't be certain until I mount the motorcycle rim onto the bike's rear wheel hub and try to bolt it on the bike. I believe this is the smallest motorcycle tire available in 21" rim size (26.625" tire diameter).
This Dunlop won't fit my 28" Rockshox fork. Sooo, I'm getting a 1987 Suzuki RM80 disc brake 17" fork and an RM125 21" wheel. The trick is to fit the Suzuki fork onto my bike. Then the next trick is to fit the RM125 wheel and caliper onto the RM80 fork. Correctly-sized wheel bearings SHOULD match the RM125 hub with the RM80 axle. This is the EASY way of installing a 21" m/c tire. If rotor and caliper don't line up, then I'll re-spoke the RM80's (17") wheel hub onto the RM125's (21") rim. More expensive to do it this way, so it's the last resort.
First I need to fit the RM80 fork onto my bike, which has 1" steerer tube. If cannot, then I'll use a K10 fork and its hub brake, and re-spoke it for the 21" m/c wheel.