jarelj

I have a new (2 weeks old) Trek 7.6FX with linear brakes and Bontrager SSR wheels. The front end chatters HORRIBLY under moderate to hard braking, I can literally watch the front wheel and forks shaking as I brake. I looked around for advice in prior threads, and experimented with toeing in the front brake pads, which just resulted in horrible brake squealing plus the same chatter. It has symmetrical brake pads on it, and I've seen contrasting information on whether symmetrical pads should be toed in or not. Any advice on what else to look at would be appreciated. I've already crashed once due to this chatter, was braking going around a corner and lost the front end on damp pavement, first time I've ever crashed a bicycle on the street. I've pretty much given up using the front brake for now, it's just not safe like this. My old Trek 7500 was nice and smooth on braking, no chatter at all, I'd like to have the new one feel the same way. I'm planning to take it back to LBS this weekend.

DOS

Could be your headset needs tightening. A loose headset would cause the fork shaking you describe when brakes are applied. Regardless, new bike, take it back to where you bought it and have them fix the problem.

Al1943

If it's new take it back to where you bought it and make the dealer fix it and don't let up until you're really happy with it.

Al

DanBraden

We bought my dad the 7.5 and the headset was loose when he got it, Fortunately I asked him to check to make sure all parts were tightened (especially the headset). Turns out it was slightly loose, and so were the cranks. Since my dad hasn't had a bike in years it was good experience, a kind of RE-introduction into bicycle maintenance.

Mr. Underbridge

Should be easy to discern headset looseness from brake death shudder. What's the frequency of the back/forth oscillation? If it's about once per wheel rotation, it's headset (Also, if you can apply the front brake fully when stopped, push back and forth on the bars, and feel a knocking sensation, it's the headset). However, if the frequency is a lot higher, and feels like a resonance effect, then it's death shudder.

Death shudder isn't all that well understood, and preventing it is generally a matter of trial and error. My bike does it with the new brakes I bought (good brakes, too). It just happens with certain combinations of frame/fork/brakes.

In order, I would try these:

*CHECK QR TENSION (!!!)
*change toe in/out.
*change pad angle on the "roll" axis (if that makes sense).
*tighten brake pad retention bolts
*tighten brake arms into the cantilever bosses - you might need to add a washer or replace the bolt if it's too long
*Clean rims and pads
*Alter headset tension a bit (even if it's not technically loose - just don't make it too loose or too tight)
*Sand the pads to get rid of glaze (usually temporary relief)
*Replace brake pads with different type (kool-stop salmons, in particular)
*Change brake arms to something lower-profile that might not bend as much.

LarDasse74

This sounds like a bad scene, daddy-o. Chattering brakes are common on cantilever-equipped bikes, but not linear pull brake equipped bikes. In fact, replacing chattering cantilever brakes with linear pull is an almost sure-fire way to solve the problem...

I think the shop that sold it to you should fix it. See if you can talk to the tech dude and ask if he has seen it before and what he can do to solve it.

Good luck!

jarelj

Well, I checked the headset and I can't feel it moving but I hear a faint "click" from it when I push/pull at a stop with the front brake applied, so sounds like maybe it's slightly loose. I'll mention that to the tech at LBS this weekend.

DanBraden

you don't need to take it in to a bike shop to tighten the headset, it's super easy if you have some allen wrenches. Check out this video on youtube that show step by step threadless headset adjustment/ tightening.

How to adjust a Threadless Headset

jarelj

Update on this: I removed/reinstalled the brakes and pads, adjusted them, and it became a little better but still chatters the fork when I brake hard. It's scary to use the front brake at all if I'm not totally upright, I can't use it when leaned over at all. Took the bike back to LBS, tech rode it around the parking lot to see what I was talking about regarding the fork chatter when using the front brake. His diagnosis was "put 1000 miles on it and the sidewalls on the rims should have any high spots worn down by then and it will smooth out." Doesn't sound plausible to me, shouldn't the rims be machined properly in the first place without any "high spots"?

AEO

even if machined, there are some imperfections at the rim joint sometimes.

machined sidewalls are something recent. before there really wasn't any machining done to the brake surface.

might want to look into a brake booster which strengthens the fork so your brakes don't overpower your fork into a chatter.

Mr. Underbridge

That's probably bullsh*t. Rims will sometimes have high spots, generally restricted to the site of the joint. However, the type of shudder you're probably experiencing is NOT due to high spots on the rim. You can tell based on the frequency of oscillation - if it's more than once per wheel rotation, then it's not because of a high spot. Also, you can generally tell by feel if it's a resonance gain effect, or if it's just a high spot grabbing your brake. If it's really strong, it's probably resonance.

In fact, if it is in fact a resonance effect (as I suspect), then irregularities in the wheel would actually disrupt the resonance effect.

I might check the brake stud bolts and the brake pad anchor bolts to make sure everybody's snug, that the headset's properly tight, etc.

I have a bike that has brake resonance, and here's what I do every once in a while: Sand the pads, sand the rims lightly, toe the pads in. That helps for a while, but it's not a long-term solution. Other than that, others have suggested a brake booster, that might help. Good luck.

AEO

also rare, but possible: check that the fork crown race is installed in all the way to the bottom and is not half way in.