Help Solve a Mysterious Clunk in my drive chain/pedals!

[dmohalloran]

I've got a clunk noise which is baffling me. I wonder if anyone has any ideas?
The clunk only ever happens under seemingly specific conditions.
(1) I'm in the fourth cog at the rear (running 8 cogs at rear) - Not noticed in any other cog
(2) In small ring at front but more pronounced when in big ring.
(3)The clunk is felt in my shoes/pedals when at 5 o clock and 6 o clock in the revolution,
(4) It sounds like a clunk

When I put the bike on the stand and pedal in the above configuration, I cannot see anything which would indicate this.
All the gears are indexed and shifting is good, I've checked the pedal's removed and greased and reinstalled and lubricated them, also lubricated and tightened the cleats.
There is no lateral play in crank arms. I don't have a torque wrench but the bolts seem tight.

When searching the internet, can't seem to get any ideas. The Bottom Bracket might be suggested but surely it would be in any gearing, The crank arms might be suggested but there is no lateral play in them. It does not appear to be the chain or derailer as it is not consistent in other gearings.

My main concern is it is a sign of something bigger, but currently it is a niggle. I have cleaned and done everything in my mechanical skills to check it out. I'll hit the LBS if nobody has any suggestions. I'd prefer to fix myself as I can't be without out my bike as it is my transport to and from work.

I've got a cannondale synapse claris 8 2014 model with shimano m540 pedals if this makes any difference.

Any ideas would be most appreciated. Thanks

 

[steve50]

Chain skipping a tooth under load perhaps, you state your ride is a 2014 model, how many miles have you done? I'm thinking maybe chain stretched????

 

[Globalti]

Does the clunk only really happen when the chain is on the fourth cog of the cassette? If so, I would take off the cassette and check for signs that the cog is fretting and eroding the splines on the cassette body. It's quite common. Try tightening the lockring much tighter, the old English engineers' torque measurement is: "eff-off tight".

OTOH if the problem was with the cassette, your clunk would not be happening at the same position of the cranks. If that's the case I'd be taking off the cranks and looking for signs of movement on the interface with the BB axle. Is it square taper? You say the bolts "seem tight" but they do work loose over time on new bikes and need re-tightening a couple of times in the first few miles. To do this properly you need a torque wrench. Did you tighten them a couple of times after first acquiring the bike?

 

[dmohalloran]

Hi,

thanks for the replies.

Steve50 - I had the chain replaced 700 miles ago. In total the bike has done 1400 in total since new. It could be chain stretch but the LBS replaced cassette and chain and cables in April.

Globalti - I went around the bolts two days ago and they were hand tight, I've not get a torque wrench but could borrow one and give them a check. Again its been 700 miles (April) since the last tightening by the LBS.