Front wheel jjjjjjjjjuuuuuuddddeeerrrr when braking?

thebouldwhacker

I picked up a bike the other day to use a tourer. As the title suggests when I apply the front brake the front fork goes mental, so much so that I just cant use it... Everything seems tight, no obvious play in the headset, brake blocks look fine and are flush.
Any suggestions?
Thanks
TBW

blorg

This is pretty normal with cantilever brakes. Brake pad toe-in can help, as can using shorter (road) pads. But to an extent you have to get used to it. I have had it (to varying degrees) on every tourer and cross bike I have owned.

Lumen

I think it depends where the hanger is.

Headset = bad.
Fork = good.

doozerie

In my experience, if the brakes are set up correctly there should be no noticeable shudder when you brake. The cause could be any of a number of things but it sounds like you've ruled out some of the more common ones. Other possibilities include that the cantilevers are rocking a lot on their mounts (shouldn't happen with decent cantilevers, even well used ones), that the brake blocks themselves are poor quality, that the rim has a buckle or dent(s), or that the brake blocks are grabbing on the rim at some points possibly due to some contamination on the rim or the blocks or the brake pads being slightly off the edge of the rim.

It's also worth checking whether the two brake blocks are in line with each other when viewed from the front, and that they are hitting the rim at the same angle.

doozerie

@Lumen, For what it's worth, my hanger is on the fork and it works fine.

07Lapierre

Oil or grease on the wheel rim? (change the brake blocks and clean the wheel rim with degreaser)

kumate_champ07

a buckle in the front wheel?

thebouldwhacker

Just back now, thanks for all the posts, I will recheck everything again tomorrow as suggested below but just to answer the suggestions...

Blorg: I think you mentioned that elsewhere too, I'll keep the road blocks in mind and give it a go if nothing else works, they are pretty serious blocks and I would like to keep them if possible. I cant see it being a design flaw though, surly having a braking system that works is a basic tick on the check list for a bike worth just under €1k new?

Lumen: the brakes are on the fork and seem solid without any play.

Doozerie: there's no noticeable movement, as in when I'm stationary and lock the front brake there is no rocking out of anything, if I hold the back ones too the bike is glued to the ground without any movement at all.... the blocks seem to be the original ones and are fairly unworn, plenty of rubber left in the grove... they both seem to hit the rim square on and there is no warp or buckle.

07Lapierre: I cleaned the bike yesterday (it took about 4 1/2 hours as she was pretty filthy) I washed the blocks and scrubbed the rims (shudder was there before cleaning too but took her on her first spin today and really noticed it) again the blocks are in pretty good nick, same as are the rear ones which have no issue at all.

kumate_champ07: nope, true and without any wobble.

The only thing I can think of is that its a freak coincidence that the friction between pad and rim is sending a vibration frequency at a level which matches the fork (which is carbon btw) causing the amplification of the vibration out of the forks. A bit like how a bow and string works in tandem with the body on a violin.... or am I in the wrong forum for such vibration frequency theories.

07Lapierre

Change the brake blocks.

thebouldwhacker

You reckon as Blorg suggested or try long pads again?

doozerie

thebouldwhacker wrote: »

the blocks seem to be the original ones and are fairly unworn, plenty of rubber left in the grove... they both seem to hit the rim square on and there is no warp or buckle.

The sometimes frustrating thing about cantilever brakes is that they can be manipulated in multiple planes. This has the benefit of allowing you lots of scope to get them adjusted "just right" but it sometimes feels like you need several extra pairs of hands to get there. Try to adjust them as follows:

* each brake pad is in line with the sidewall of the rim. Sounds like that is already the case already.

* the front end of each pad should touch the rim first with a very slight gap still between the rear end of the pad and the rim. The rotation of the wheel will pull the entire pad flat against the rim. The easiest way to achieve this setting is to wrap a rubber band around the rear end of each brake pad, push the pad flush against the rim, in so far as it can get flush with the band in the way, and tighten it in position. Then remove the rubber band, obviously.

* the other main adjustment is how parallel the brake block "arms" are with the ground, when viewed from the front, when the pads touch the rim. I usually adjust the brake pads so that the arms are pretty much exactly parallel with the ground as the pads engage.

If all else fails it could simply be that the brake pads are not suited to your rim but assuming the rear brake pads and rim are the same then you'd assume you'd have issues there as well - you could swap the front and rear blocks, as a test, maybe.

Another faint possibility is that the wheel itself is loose so worth checking that too by seeing if you can rock the wheel by pushing/pulling the rim from side to side.

Edit: This is better than my description above as it has helpful pictures and is probably more coherent too.

Also, forgot to mention checking that both brake pads connect with the rim at the same time.

thebouldwhacker

That's a pretty comprehensive reply and is worth a shot. Failing that new blocks it is.

Thanks for all the replies, nothing worse than a new bike acting up. I'll attack tomorrow morning.

Thanks again
TBW

kona

The minute I saw the title I thought carbon fork. There is something odd about vibrations and carbon, Ive no idea how to solve it as I have no idea what causes it. The only thing I can suggest is give it a full service and go from there.Usually dissassembly and reassembly can fix weird issues.

thebouldwhacker

Wouldn't doubt we, our awesome buddies over in the good'ol U.S. of A. are pretty much going with yer suggestions re toe in and pad placement, entire thread here if ye wanna read.

Kinda hard to explain...The pads are typically much longer than road pads, so there's more total surface at play. if they are parallel to the rim, the entire surface gets applied right away, and as the rider pulls harder on the brakes, it applies the entire surface. With it toed in, it applied less surface initially, and then -- under hard braking -- only the very front of the pads are really digging in, with the back of the pads applying less pressure.

It allows more modulation, and seems to avoid the binding / releasing effect. It's much worse with some bikes than others, and some are more sensitive to the settings. I've learned to adjust the brakes myslef and actually really like them.

BTW, I met an older mechanic who said they all used to be like this when the brakes were more common. It's just that the art of adjusting them right has become a bit of a lost art.

I've heard the Tricross is worse with this -- likely the frequency of fork vibration. That's probably why they applied the factory fix.]

I'm off to adjust and will let ye know,
Thanks again
TBW

thebouldwhacker

Job done!

I loosened the pad, put a plastic insert (a bit thicker than a match) in behind the rear of the bloc gripped the brakes and tightened into position, I just did a few circles outside and their spot on, not a shake out of her.

Thanks very much for all the replies, boards comes to the rescue again, happy days.

TBW

studiorat

thebouldwhacker wrote: »

The only thing I can think of is that its a freak coincidence that the friction between pad and rim is sending a vibration frequency at a level which matches the fork (which is carbon btw) causing the amplification of the vibration out of the forks. A bit like how a bow and string works in tandem with the body on a violin.... or am I in the wrong forum for such vibration frequency theories.

Not far off. This is where the fork mounted hanger helps. When the fork flexes under breaking the cable stretches and it can match the resonant frequency of the forks. Highly unlikely IMO.

Just to add another thing to try to the others. Adjusting the QR tightness may also help.