Rear end clanking when hitting bumps
 

Whenever I hit even a small road bump, I'll hear a loud, metallic CLANK that appears to be coming from the rear end. In fact, it make almost the same noise as if I suddenly engage the freehub, which makes me wonder if it might be related to the freehub. It only happens when I'm on the bike riding, if I just raise the rear end off the ground and and drop it I can't hear it. <-- Wrong, still makes noise if I'm not on the bike.

I just rebuilt the rear hub (older shimano) and it is well adjusted, no play. No play in headset, front hub or BB either. Any ideas?

Is this a road bike or MTB? Suspension?

Anything loose on the bike? Fenders?

Assuming it is a metal frame, also carefully check for cracks.

Chain stay slapped by the chain?

Brad

Could be chain slap, or it could be that you pause then engage the pedals in the process of going over bumps. Both are common, and it calls for diagnosis by ear, and tracking what's happening.

Sorry for lack of details on the bike. It's late 80s (guessing) Norco Bigfoot MTB that I picked up for 20$ in very rough shape and built back up. So, steel frame (tange 1000, straight gauge crmo), I cannot see any cracks.

It's set up ghetto-single speed right now, with a 48T biopace ring in the middle position and the 17T cog on the cassette getting an almost-perfect chainline. The tension is sufficent, no chain slap.

No fenders or other accessories on the bike, in fact as of right now there isn't even a rear brake or seatpost/saddle :P

Possible I guess. But it does happen if I'm just coasting downhill and hitting bumps, or even larger cracks in the pavement.

Sounds like the wheel might be loose in the dropouts?

I hate diagnosing noises while depending on other's observations.

If it's easy to remove this chain (ie. has a reusable connecting link) try removing the chain and coasting down a bumpy hill. Whether the sound stays the same or changes will rule some stuff in and other stuff out, helping to narrow the focus.

Otherwise, you have to gown the list of everything that might be loose, including anything bolted together.

Nope. Horizontal dropouts and single speed setup. If it was loose, it'd be out.

Yeah, not easy I know. This one's got me pretty stumped, hence why I'm asking. I'll try coasting chainless tomorrow, I have a kmc quicklink, good idea, thanks.

Loose hub bearings or broken axle?

Ben

So I tried coasting without a chain: still noisy. Also, I provided an erroneous observation in my first post: the noise is still present if I am not on the bike, just less noticeable.

I think I've narrowed it down to the cause, though not the solution: the noise appears to come from the cassette rattling around on the freehub radially. To clarify, the lockring is tight, there is not axial play. Yet, grabbing the large cog with my fingers I can easily move it around a couple mm radially. A bit less noticeable with the smaller cogs.

Cassette is shot? Tighten the lockring even more? Live with it?

Now I'm confused. You say its a single speed setup, but you have a cassette? It sounds like you left out the spacer behind the cassette.

Yeah, I'm running ss with a cassette until my lbs gets the ss spacer kit they've ordered for me.

As for spacers behind cassette, I'll dig around in my parts bin but I don't remember there being anything. What am I looking for?

You probably need the shimano 1mm spacer that looks like this.
http://bikeforums.net/attachment.php...hmentid=451939

It could be that your cassette body is loose on the hub. However we have no idea what type of hub you have. If it is a Shimano hub, the solution would be to remove the axle and to tighten the hollow bolt that holds the freehub body onto the hub using a 10 mm hex key

Shimano FH-HG20. I'll check for the freehub tightness later and report. As for that spacer, it wasn't there, but this bike was weird enough that someone else might have forgotten to put it back on. In any case, I shouldn't need it once I get the spacer kit.

Quick update, I didn't get around to checking the freehub body tightness, I need to undo the axle and bearings for that, I'll around to it when I set up the SS spacer kit.

However, I did give the cassette lockring a few more clicks, and it quiet things downs a lot. I should buy a torque wrench one of these days.

If your freehub body is loose, you could destroy the hub by riding it that way. Failure to check things properly could cost you the price of a complete new wheel

Yep. Freehub body torque spec is ~30 ft-lbs, and the motion of the wheel will unscrew the fixing bolt if it's not torqued down enough.

Remove the kickstand.

Haha, no kickstand :)

So I took apart the hub. The freehub body retaining screw was still tight-ish, but I unscrewed it and torqued it back down as tight as I could with a 6" wrench (I know, I should get a torque wrench).

With that out of the way, I found out my rear axle and skewer are bent :( And they were brand new a week ago! Ridden something like 30 km. Off to the lbs to get new ones, but what could have caused this?

If the freehub body was not tight enough, it could sag and bend the axle. You might not need to change the skewer, it doesn't support any weight

I haven't seen the bike, so can't speak with authority, but I suspect that you're on more on the long list of victims of running a single speed set up with a tight chain. Whenever I hear a single speed owner claim that he made sure that "the tension is sufficient" I'm willing to bet a few beers that it's excessive. Single speed chains cannot have tension, they must be slack at all times. Not very slack, but slack none the less. The rule of thumb is about 1/8-1/4" sag in the lower loop.

A tight chain is the surest, fastest way to bend the axle as the rigid chain pulls the sprocket forward.

Yeah, that's likely actually. I followed sheldon's recommendation to set it "tight as it can be without binding" but I might have gone a bit over the limit. Lesson learned.

I hope you've misread Sheldon's recommendation, because, if not it's plain wrong, and it may be part of the reason so many newer SS, IGH, and fixed gear riders are having so many problems.

The right advice is the same as it's been for a century. Chain drives (without tension idlers) must have a slack loop. That applies to ALL chain drive systems including bicycles, machinery and trucks. There doesn't need to be much slack, just enough slack to accommodate any eccentricity in the sprockets and ensue that the chain is never tight.

Chain drives have two loops, the power loop which is under the tension needed to transfer torque from one sprocket to the other, and the return loop whose sole purpose is to make the system endless. Tensioning the return loop (lower on a bicycle) only serves to increase the loads on all the components, and cause premature wear or failure. Here's a link to images of chain drive trucks. Note the visible sag in the lower loops.