I bought this 3" x 21" dunlop tire. It's 26.625" diameter.
http://www.ebay.com/itm/Dunlop-D404...Parts_Accessories&hash=item5652576c2a&vxp=mtr
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 tire won't fit my 28" Rockshox fork. Sooo, I'm getting a complete front end from a 1987 Suzuki RM125, including 21" wheel and hydraulic disc brake. The trick is to fit the Suzuki fork onto my bike. Then the Dunlop should mount on the RM125 wheel and bolt onto theRM125 fork, like it's meant to be.
and this tire is for the rear:
http://www.ebay.com/itm/2-75-21-Shi...Parts_Accessories&hash=item5657285693&vxp=mtr
It's a 2.75" X 21" tire. Sadly, it's only available in offroad, trails, enduro or dual-purpose. The last choice looks like knobby mountain bike tires, but for motorcycles.
Using reference information, rear tire side clearances on my rear dropout should increase by 1/4" from the Dunlop's dimensions. The tightest left-side chainstay clearance would be ~1/8".
Looking better, might not need to clearance the rear dropout with a 2.75" x 21" m/c tire.