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Can anyone think of an explanation?
Sounds like an angry ghost to me. Try a seance to see if it'll tell you how to put it to rest.
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forget the term
Chatter.
My brother's CX bike chatters like mad if you brake gently, but it's fine if you stand it on its nose. I expect you have the same issue with wet braking, except that you can't brake harder because you don't have the reserves of friction due to the water. Sometimes the chatter can be traced to too much free play in the brake centre pivot (or C-arm pivot if dual-pivot) or free play in the headset. Other times you can have everything perfect and it's just the combination of rider weight and fork stiffness (or lack thereof, with 1" steerers) which just refuses to play nicely.
While the chatter doesn't have exactly the same root as it does on a CX bike, I wonder whether the brake track on an old CXP30 without UB Control is sufficiently far off parallel that the effective change in fork length as the fork bends backwards could create the same brake servo effect by an inclined plane action. It would certainly be worth slinging a modern machined front wheel in there as a control.
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bothwell
gbj_tester
@TM
I was given a bike recently and it was in a sorry state. I've ended up replacing everything apart from the cranks and the headset in the process of building it as a geared beater. It's an old 531 road bike and on inspection, doesn't seem to have any signs of a front end impact.
I've set up a front Shimano 600/ultegra front brake, with new pads, new cable (used compressionless outer) hooked up to an old mtb lever (there is also a back brake but no issues there)
The front wheel is a mavic cxp 30 which had never been braked on previously, it is not machines but it has a flat for a brake track. Headset has no play, forks look very tidy, no signs of damage.
In dry performance is amazing, really powerful, great modulation.
As soon as the rim gets a tiny bit wet, when applying the front brake it isn't very effective for a second and then shudders/judders really badly,( like what cantis can do, forget the term) fork flexing back and forth, pretty terrifying.
Can anyone think of an explanation?
I've set up/ridden a heap of steel bikes wth caliper brakes but never had an issue like this before.
I'm wondering if the rim needs cleaning, or if it just needs to bed in, but would appreciate any other suggestions .