Yet Another Cycling Forum
General Category => The Knowledge => Topic started by: andyoxon on 04 November, 2019, 04:32:55 pm
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Last few rides I've notice my front cantis seem to be juddering - rear are fine. 'Juddering' was particularly bad on downhill stoney tracks. Both front and rear have 'salmon' blocks; front cantis are relatively new. Canti bosses are non-removeable on 531 frame. There is some play in the front cantis and if I hold front brakes only and try to move the bike - it does rock; if I hold front & rear brakes tight bike doesn't rock. Wondering if the headset many be slightly loose. Thoughts?
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This is often the result of flex in the steerer. As the fork flexes back, the cantilever brake cable tightens. When the fork springs forward, the brake pressure loosens off. Some folk use fork crown cable hangers (instead of headset cable hangers) to avoid the issue.
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I had massive juddering when I got my cross bike earlier this year. Check the headset (mine was slightly loose), check the toe on the brakes, and clean the brake blocks and the rims. If it still judders then you might need to take further action, but that lot fixed it for me...
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things to check
- toe in
- position of brake block centre vs canti boss (too far back is good for power, bad for judder)
- brake block type (thin/hard = more judder)
- rim cleanliness
- rim deformation
- boss/pivot wear (do the arms pivot on the bosses directly or is there an integral pivot bushing in the brake arm?)
- headset free play
- cracks in steerer
Fixes for judder include
- better toe-in
- squashier brake blocks
- an uphanger
- different brake blocks (shorter and/or without an offset mounting)
- a few turns of PTFE tape on the boss in a direct pivot brake (*a)
- a few turns of PTFE tape on the boss in a brake with integral pivot bushings (*b)
- clean/dent free rims
(*a) this works by mitigating play between the brake arm and the brake boss. The PTFE tape doesn't last forever but it does last months, usually, sometimes longer.
(*b) this works (to my surprise and provided the exact right amount of tape is used) by stopping the inner part of the integral bushing moving around. One thing you shouldn't do is overtighten the bolt with this type of brake; you can easily cause the boss to fail, because the boss sees a tension load (from the bolt) with this type of brake which it does not see otherwise.
cheers
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Fork crown mounted hanger definitely sorted it for me.
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Thanks all, for the comments/tips. Don't think it's the fork's natural flex - pretty sure it's not been an issue before, so something seems to have changed. May even stick the old cantis back on to test, after I've checked some of the potential easier fixes.
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All other things having been fixed, as above, note that with cantis and V brakes, you do not want "toe in". You want slight "toe out" or, at worst, a neutral setting to reduce juddering.
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I managed to discern, by grappling with the frame & forks, some level of play in the headset. So tightened this up, trying to get it right, but quite difficult with play in the cantis too. Anyway Sunday's ride was fine, so it may well have been the combined headset and canti play that was leading to problems... Will keep an eye on it... :)
edit.
Thanks for the offer Oxford Guy, seems OK so far.
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I managed to discern, by grappling with the frame & forks, some level of play in the headset. So tightened this up, trying to get it right, but quite difficult with play in the cantis too. Anyway Sunday's ride was fine, so it may well have been the combined headset and canti play that was leading to problems... Will keep an eye on it... :)
Andy - if you still experience some juddering, I have a spare uphanger (silver alloy) that you can have, which might help, originally from SJS, but I didn't need it in the end.