Clunking sound

[kal el]

Hi
I had my mountain bike in for some repairs on Tuesday. It needed a new set of rear break pads, a new rear break cable and they told me that my rear wheel needed to be tightened. I rode it back home after the repairs and it seamed ok. I rode it back from work last night and I heard a clunking sound coming from the rear wheel. It only does it when I'm in the saddle, at first I thought it was the peddles but it doesn't do it when I'm out of the saddle. any idea why this is happening?

 

[sight-pin]

They may of moved your seat post to put the bike on the work stand, is your seat post clamp tightened up?

 

[mjr]

Any idea what they meant by "tightened"? Bearings? Axle? Spokes? Did they tighten it or just say it needed doing?

With the bike leaning on a wall or its stand, if you push the wheel rim at the top, can it move much, relative to the frame? If so, the bearings are slack - there's all sorts of possible reasons for that, so the bike model name or a picture of where the rear axle connects to the bike would probably help people to suggest what you can do. If not, try lifting the back wheel off the ground by holding the seatpost and see if you can make the back wheel clunk, possibly pushing a pedal with your hand, to try to figure out where it's clunking and if it's really the rear wheel or if it could be nearer the pedals... then describe it here and someone may have more idea what could be wrong.

 

[kal el]

The mechanic tightened up my rear wheel, my seat is clamped and locked. I road to work this morning and there was no clunking sound however I rode back this evening and it started clunking after about 15 minutes, it seems to be coming from the drive shaft or whatever its called, the bit where the peddles turn.

 

[slowmotion]

The mechanic tightened up my rear wheel, my seat is clamped and locked. I road to work this morning and there was no clunking sound however I rode back this evening and it started clunking after about 15 minutes, it seems to be coming from the drive shaft or whatever its called, the bit where the peddles turn.

It could be that one of the crankarms is loose on the shaft that goes through the bottom bracket. It happened to me once. @ianrauk diagnosed the problem and fixed it with a twist of a large hex key.

 

[alicat]

Go back to the shop and tell them what's happened.

 

[kal el]

So I took my bike back to the shop last Tuesday. I showed them a video that I filmed so that they could hear the clunking sound. they said it was the bottom bracket. I got a call yesterday ( Saturday 14th Oct ) saying that they had greased the bottom bracket and that it was noise free. I rode it to work this morning and guess what, after 20 minutes of riding the clunking came back. I wonder if it is the cassette? the screws seam pretty tight. its really doing my head in now.

 

[Tom B]

Lift the bike off the ground at the rear and rock the wheel size to side can you feel any play? Does the wheel spin freely?

Does it do it on free wheel or pedaling, all the time pedaling or just after starting pedaling following some freewheeling?

Is there extra or different "slack" when picking up the pedals after freewheeling?

I find most unexpected unobvious clunks pops and bangs from the rear come from unhappy freewheel units. I find they have a tendency to be noisy for a while then go sounding fine before leaving you with a collection of 20odd neutrals at a point equidistant from any source of aid.

 

[Ajax Bay]

I heard a clunking sound coming from the rear wheel

Rear wheel

The mechanic tightened up my rear wheel, [Comment: what does this mean? What did they tighten?]. . . this evening . . it started clunking after about 15 minutes, it seems to be coming from the drive shaft or whatever its called, the bit where the peddles turn.

Bottom bracket

they said it was the bottom bracket.

And they greased it. It was OK to start with; then it started clunking again.

after 20 minutes of riding the clunking came back. I wonder if it is the cassette? the screws seam pretty tight.

Why do you think it might be the cassette? I do not recognise the 'screws' to which you refer. The cassette is secured by a lockring.
OP - I recommend you do what @mjr suggested and come back to us.

unhappy freewheel units

OP hasn't got a freewheel, he's got a freehub and cassette. Or has he? But you won't have an "unhappy freewheel" and "20odd neutrals"; 14 maybe, but no more.

 

[mjr]

It could be that one of the crankarms is loose on the shaft that goes through the bottom bracket. It happened to me once. @ianrauk diagnosed the problem and fixed it with a twist of a large hex key.

It's sounding most like that to me too (the shop will probably have removed and reattached them as part of greasing the bottom bracket, so it could be silent a while before it worked loose) and if it's a square-taper bottom bracket (and greasing suggests that plus non-cartridge bearings) then one crank might be rounded off and keep working loose a few miles after it's been tightened, but it's difficult to say much with so few details.

The tell-tale for a loose-and-rounded crank arm would be that the clunk happens roughly the same point on the pedal stroke every revolution, plus leaning the bike against a post, holding one crank in each hand (one arm under the downtube) and waggling them gently will result in movement you can just about feel. Happily, it's usually the left crank arm that is damaged and a new one of those is under £10 (plus fitting - if you don't have a torque wrench capable of 40Nm on the bolt, it's worth paying a pro to fit). If you're unlucky, the £20 bottom bracket will have failed. If you're really unlucky, the right crank is farked and that's more expensive.