agmetal

I recently did a 650B conversion on one of my bikes, and I'm a little bit frustrated with the brakes, and I'm curious if anyone else has had a similar experience.

I went from Tektro R539s with 700C Sun CR18s to R559s with 650B Velocity A23s, using the same Kool Stop salmon pads. After some initial concerns about the front brake not being as responsive as it was before, I de-glazed the pads with a file this afternoon, and that didn't seem to make any difference (at least in the front - the rear now locks right up, and I don't think it was before). I also switched the pads between front and rear, and that didn't seem to make a difference either.

I'm pretty confident that the front brake was more responsive before - even using the R559s with the CR18s and the same levers on a previous build (and also with Mavic A119s on another bike). I like the confidence that comes with being able to brake late and stop quickly from the hoods position, and I don't currently have that.

Has anyone else experienced this? Anything else I should try? I don't particularly want to go back to the 700C wheels if I don't have to - the 28mm tires are a little narrow for my taste.

dweenk

I did the same thing on a Raleigh Olympian a few years back.

I did switch to upright bars and cheap VO City brake levers. I'm using the pads that came with the brakes. The rims are CR18's, and I set the front brake tighter than the rear.

I feel confident in the braking; if I wanted any improvement, I would sitch the pads to Kool Stop.

agmetal

I already have Kool Stops on this bike, though. I'm kinda tempted to try standard pads to see if it makes any difference.

It has occurred to me - on the other bikes where I've used these calipers before, the pads on the front brake have been close to the middle of the slot, but on this one, they're all the way at the bottom. Could that make the difference?

Andy_K 

The braking response definitely goes down as you move the pads down the slot. Lower pads have proportionately less mechanical advantage. But the R559s have a longer lever arm to adjust for the longer brake reach. They won't be as powerful as the same brakes would be with the pads at the top of the slot but they should compare to R539s with the pads at the bottom of the slot. In any event, the slot placement is a relatively small effect. The mechanical advantage mostly comes from the distance from the pad to the pivot point, which is larger than the range of pad adjustment.

FWIW, I've had R559s with 650B A23 rims working very well. What levers are you using?

Ghrumpy

It's not the pads or the rims' braking surfaces. It's primarily a leverage issue. You've significantly reduced the mechanical advantage of the brake calipers by using longer arms to reach your smaller rim. Your new rims require 19mm more reach than your old ones. It's just a guess but I'd say that's probably a 25% reduction in leverage from the brakes for your 622 wheels. The caliper arms on the R559s are probably a little longer above the pivot point than the R539s but I'd doubt they are longer by enough to make up for the increase in reach, especially with the pads at the bottom of the slot.

agmetal

Shimano ST-6500s

Andy_K 

I would expect those to work.


I can't completely disagree with Ghrumpy's reasoning. I just did some quick measurements, and I have the R559 lever arms as being about 10% longer than the R539s, whereas the reach has increased by around 33%. So you've lost a bit of mechanical advantage. It should still be plenty to achieve the maximum braking your tire traction will allow. You'll just need to squeeze harder than you used to.

agmetal

It occurred to me that a friend has a bike equipped with 700C A23s, so he brought his bike over and we tried his wheel on my bike. The brake pads went from the bottom of the slot to the top, and definitely improved the brake response - from needing to really squeeze to stop quickly, to being able to endo the bike. Another friend suggested that the difference in rim diameter could also make a difference, in terms of where the braking force is being applied relative to the center.

Maybe I'll go back to 700C...it looks like it might actually be possible to squeeze a 32mm tire onto the rear, possibly requiring dimpling the chainstays which I was planning to do for the 650Bs anyway, since clearance is so tight. I'll test that sometime this week, maybe tomorrow, to see if it's reasonable.

Road Fan

I think Ghrphy has a really good idea. I’d double check every aspect of rear brake caliper and cable installation, rooting out squish, flexing, compression, friction, and free play.

agmetal

The rear brake is fine, but the front requires a lot more force at the brake lever than I like.

Dfrost 

I switch back and forth on the Marinoni between 700C wheels (H+Sons tb14 with Compass 32’s) using Ultegra short reach dual pivots (pads at slot bottoms); and 650B wheels (Pacenti Brevets with Compass 38’s) using Tektro R556 (predecessor to R559) dual pivots (pads near slot bottoms). Both setups use Kool Stop salmon pads. Levers are Ergo 8-speed brifters. And BTW, the Miyata 912 has R600 medium reach dual pivot rear and Gran Compe 610 braze-on centerpull front brakes, 700Cx28 Compass tires, Ergo levers and KS salmon pads on both ends.

Braking with all those setups is quite similar and is powerful enough to meet my expectations, there are plenty of steep (10% or more) hills on my common routes, and I don’t mind letting my bikes go on those descents. I had worried about braking degradation with the 650B setup, but don’t notice much, if any. As @Andy_K measured, the long reach Tektros compensate for their longer pivot-to-pad dimension by increasing the pivot-to-clamp dimension, and the caliper arms seem stiff enough. Maybe I’m just less demanding of my brakes?

If the rear works OK, but the front requires noticeably more force, then something else seems amiss.

JohnDThompson 

I just did a 650B conversion on my wife's bike, replacing the original Universal 68s with long-reach Tektro dual-pivot calipers (don't recall the model number). Using Kool-Stop "Continental" pads, there's plenty of braking power.

nlerner

So the rear brake is more powerful than before the conversion while the front brake is not? I'd suspect the problem is in the levers.

ThermionicScott 

I must admit that the braking response on my 650B conversion isn't as good as short-reach dual-pivot calipers on 700C wheels. But I chalk that up to the flexy Weinmann centerpulls as much as the increased reach. (I bought a pair of Tektro 559s to see if they would work better, but they couldn't coexist with my fenders.) I just resign myself to being in the drops whenever I might need to have full braking power, and everything has worked out (so far.)

fietsbob

Measured distance .. fulcrum to work / pivot to pad, of course if it's longer , the MA will be reduced..

Leverage is a ratio of differing lengths..

DIMcyclist

Fwiw, I've done three 650b conversions, but all of them from smaller 26" MTB (559mm) to larger 584mm rims, mostly to avoid this very situation.

My '84 Univega 'Franken-Rover' was unusual in that it was designed for standard 559mm rims- but with caliper brakes (it also had a bit more BB drop than you'd expect for an MTB frame). I used Tektro R556s with Kool-Stop 'salmon' pads & had no issues with braking or stopping, even in very wet weather. The rims in this case were Velocity Synergy (machined sidewalls); the levers were Shimano 105/1055 SLR on Soma Sparrow mustache bars.

Ultimately I decided that using a 650b frame was better to begin with, rather than converting a frame designed for a different wheel size.

mountaindave 

That is odd. I've done at four conversions from 700c to 650b using two Tektro R559's, a cheapo Chinese knockoff, and Mafac Raids. All have sufficient stopping power and are drop bar bikes. Two have aero levers, one has standard Suntour Superbe levers and the third has Dia Compe Guidonnet levers. The Guidonnets are not as good, but I believe that is a function of the available leverage, not something inherent in the bike as I my friend has similar braking on his standard reach brakes with the same levers.

Hoopdriver

I did such a conversion a few years back, using R559s. Going in, I was a bit worried that the long reach would make braking marginal; however I was pretty much amazed at how well they worked. I'm using Velo Orange Diagonale rims with the standard Tectro pads. Shimano 105 levers.

noglider 

Yes, I think something is wrong, though I'm not sure what it is. Long reach dual pivot brakes work great. I've even used a pair of no-names, and they were just as good as the name brands.

agmetal

I'm gonna test-fit 700x32s this weekend, and then try to take it for another ride with the 650Bs on it to see if more use helps. If I can get the 32s on it, though, I might just stick with that. My goal was wider tires than the 28s I had on it, not specifically to go 650B. Not sure why I never bothered to try 32s before though...

jPrichard10 

I had this experience with DC 750 centerpulls, but that's to be expected. Switched to Mafac RAIDs and braking improved significantly because of the better geometry of the caliper.