Only on descents - I'm stumped (and kinda scared)

[lph]

Just thinking out loud here:

if it "feels" like your brakes hanging up, then something is giving friction. The vibrating could be the handling of the bike itself, some bikes do that at speed I've heard, but with screeching and shaking and a feeling of resistance somewhere it's not just "chatter".

It sounds maybe like a broken or cracked ball bearing, that only sometimes gets caught in a position where it starts complaining. Maybe under lower speeds and less pressure it never gets stuck in that position but keeps rotating. I don't know what a freewheel looks like inside, but I think there are ball bearings there too, and it could be a hub of course.

I've opened up and overhauled the ball bearings on my mtb hubs many times but on my road bike I just bought new machined bearings (wrong term, maybe, sealed bearings?) and my lbs thumped out the old ones and put them in. Done in a few minutes.

Good luck figuring it out.

PS. Hey, I just read a good tip on the RoadBikeRider Newsletter - swap the part you're suspicious of and see if the noise goes away. Try swapping out one of the wheels and free-wheeling down the same hill maybe?

[maillotpois]

before you even got to the coastinf test my forst thpught was freehub. when mine died it sounded like someone was killing a pony. take it to thelbs - and let us know what it was.

[OakLeaf]

So.

Did I mention we have a dearth of competent professionals in my area?

So I took it to the shop and described the symptoms. Owner/mechanic absolutely denied that the freehub could be the cause. Said that when the freehub fails, it just doesn't "catch." You pedal and the wheel doesn't turn. Me: "That would be when a pawl fails. What about when a bearing fails?" Him: "That would be different, yeah" [in a tone of voice that implied it still wouldn't cause the symptoms I described]. Couldn't tell me what it would do if the bearings failed, if not what I was experiencing. Took it on a short test ride on flat ground that I told him ahead of time wouldn't reproduce the symptoms. Him: "I can pedal pretty fast." Yeah, but how fast and how long can you coast on flat ground after you stop pedaling? He also couldn't think of any other way to diagnose freehub bearing failure. So ...

The more I thought about it and went over everything in my head, the more I was sure it couldn't be anything but the freehub. So I just told him to order the part and replace it, and that I'd cover overnight shipping since I needed the bike for the tri this weekend.

Got it back, rode it tonight. Made sure to coast down all the big hills. No problem.

I had thought that the play in the old freehub wasn't excessive, but there's like NONE now. So that would've been diagnostic ... I also realized that the chain had been binding a little bit, for some time now, when I pedaled backwards, which I'd chalked up to maybe I'd tweaked the derailleur hanger or cage a little bit, but it's not doing that any more either. So that would've been diagnostic, too.

Oh, and when I picked the bike up, the counter guy told me that the owner/mechanic had never replaced a Mavic freehub before, and was watching and pausing a YouTube video while he was working on mine.

But anyway, it appears to be fixed. I'm really glad you chimed in, mp, or I might not have had the confidence in my diagnosis to insist on it.