Originally Posted by
OneIsAllYouNeed
This should go down in history as the most helpful response on any bike forum. Thanks very much.
Other replies in this thread are appreciated, too.
For completeness, the candidate conversion is my 1997 Trek T200 tandem. It had a major overhaul around 2011 with a new Co-Motion Speedster disc brake fork, BB7 road brake, and 203mm rotor. The fork crown barely clears a 700x37mm tire vertically, but the fork legs are fairly far apart (roughly 64mm), making it a good candidate for a 27.5x2.1 tire. The rear has cantilever bosses for 700c rims and a brake bridge that is tapped M6 thru. It was the tapped hole that got me thinking about retro brake options. We have a 700x37 tire stuffed under a 45mm fender currently. I just measured the seatstay and chainstay gaps, and it looks like 50mm 650b or 44mm 700c tires will be the widest possible without risk of rubbing.
So the Weinmann 800, with 78mm pivot spacing cleared a 56mm tire with about 3mm all around. If the Raids are similar, but 3mm narrower, they'd probably fit my theoretical 50mm tire. It does sound like the 800s will be a sure fit.
Hey, happy to help. FWIW, from the La Rueda Tropical page it looks like the Raids have the same pivot spacing as the Weinmann 800/900s (75mm nominal), so I'd think that Mafac should clear a 50mm tire w/ room to spare - or for fenders. I've never used Raids, so still worth a direct measurement, but pics of them on 650b conversions do show that they offer good clearance. They're a common choice for conversions - though a 38-42mm tire is a more typical target. Compass offers a 650b tire in 48mm width, so you have at least one option around that size. There are some 27.5 x 2.0" tires available, too.
The canti bosses at the rear might interfere with the centerpull caliper/shoes, but I suppose you could just mount the brake to the underside of the brake bridge/seat stays.
I should have added this detail before: the measurements above were taken with the brake caliper aligned ~radially with the wheel. The front and rear calipers could/will be at some angle wrt the wheel's radius and that could affect clearance at the caliper arms if the tire size is already encroaching upon the upper limit (if you'll be using a 50mm tire versus the 56+ shown above this shouldn't be a problem at all). Front angle wrt wheel radius would depend on fork geometry (but it sounds like that's irrelevant as you're using a disk up front), rear depending on caliper offset from the seat stays and where the wheel center is sitting in the dropouts. By eyeballing things, it looks like depending on fork offset the front caliper could also be more radially aligned with the wheel than the rear.
Clamping the 800 caliper/pads to the rim and pivoting about the center of contact there is a good amount of angular freedom of movement before the caliper bumps into the tire - more than enough to account for the caliper offset from the seat stays, etc.
Let me know if you have trouble finding a Raid caliper, I can probably let the 800 go.