I'm considering a 650b conversion that will require 80mm reach brakes. The frame can clear 56mm tires. It looks like Dia-Compe 750 might be the ticket. Does anyone know how wide a tire can fit in those or similar centerpulls?

The number in brake names Usually refers to the drop. I think I remember (this is shaky, not Gospel) that 750 is the max drop, but even if it's the middle, you'll be at the limit. Width won't be an issue, since the arm span is comparable to seat stay width.

Also keep in mind that because of the relative positions of the crown and rear bridges to the wheel radii, front brakes are usually closer and rear farther than the reach measured at the mounting bolt.

Perhaps 42mm, but that would be pushing it. 50mm clearance total between the calipers of my Gran Compe 610. Normally you would to go to cantilevers for 2"+ tires, but perhaps there are some big-mouth caliper brakes out there that will work. Maybe check out Tektro 559?

Your memory is correct, so don't worry about shaky. 750 is the max drop. These are long reach brakes.

Be aware that there is a tendency these days to call what used to be medium or standard (47-57) reach brakes "long reach", and to call long reach (55-73) brakes "extra long reach". This old LBS mechanic finds that annoying . At any rate, it's always a good idea to measure and go by the numbers, as the old nomenclature has become non-standardized.

I think you want Weinmann 800, not 750 wherever the reach is 80mm. Might be able to do a 750 up front depending.

I'll see about measuring tire clearance on an 800 and 750.

[MENTION=75765]OneIsAllYouNeed[/MENTION]
Here you go:



750 = Negative, tire rubs. Guess you won't have much clearance if any depending on your specific tire.



Weinmann 800 w/ pads at bottom of slots = 2-3mm per side (measured by passing allen wrench through gap):





Weinmann 900 w/ pads adjusted to ~80mm reach = 3-4mm per side (measured by passing allen wrench through gap):





In case the question comes up, I used the 800 and 900 on a past 650b conversion and braking power was good enough.

It looks like Mafac Raids might work and might offer additional clearance - plus those would probably be easier to track down than Weinmann 800/900 (note the wider fulcrum/pivot spacing over Dia Compe/Weinmann in the link below, though that doesn't necessarily guarantee additional clearance). I don't have a set of Raids here to check. Maybe Paul Racers if you file the slot a bit, though if you need a true full 80mm reach there might not be enough mtl. at the bottom of the slot leftover after filing.

Edit2: Nix the wider pivot spacing for Raids, the 800 and 900 measure at ~76mm c-t-c spacing between pivots, it'd still be interesting to see how Raids compare. The 750 spacing between pivots measures at 65mm as linked below.

MAFAC, Dia-Compe & Paul Centerpull Brake Specs : La Rueda Tropical

Just for the record I have 48mm tires (and fenders!) squeezed under Weinmann 750s. Getting the wheel in and out is a little tricky, but that doesn't bother me much.

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.

In another forum I saw a discussion of dropped or offset brake shoes, too.

^True, and there are also drop bolts. Pivot spacing is critical when the tire starts to fill the caliper. Drop bolts/shoes might also put the fatter part of the tire in the narrower upper portion of the caliper, which might not matter depending on tire width/height.

The modern options for centerpulls appear to mostly be targeted at the BMX crowd... and they've been renamed to U-brakes. I think the Paul Racer brake may be the only one worthy of crossover between C&V and BMX, but I digress.
For this conversion, a "990 plate" is the critical missing link. It's a pair of brake bosses that bolt to a caliper brake hole. The most readily available options is the Diatech (Diacomp) Boomerang BR-990. I bought a very, very similar one made by S&M. Since I couldn't find the info elsewhere, the post spacing is 85mm and the drop is 36-45mm on the 990 plate. Most available U-brakes mount to the same "990" brake bosses and have a reach/drop of 28-50mm. I'll probably use an Odyssey Springfield or Evo 2 brake for this project.

On my 650B conversion, I'm using Weinmann 750 centerpulls with brake levers that have a built-in quick release. The QR doesn't need to be built into the levers, but having one somewhere in the system makes getting the tires in and out much nicer.

IF your center pull was in Fact a U Brake , then a Much wider tire capacity is available..

Campagnolo Euclid U Brake

Going by a similar setup I have on one bike with Diacompe 710's, I get 48 without fenders and 42 with to be conservative, I would figure 46 and 40.