Driveline 'clunk' help...am I missing the obvious?????

[malvern newbie]

Been out nearly every night on the LTS, on a 1 hour circuit.....on the last 3 nights I have had included, as my fitness improves, a stiff 200 mtr climb up a fair gradient.....anyway, this has shown up an annoying clunk which only occurs when really putting pressure on in a climb, it happens as the left hand pedal is approaching about 7-8 o'clock on the down-stoke....initially granny ring/ low gear, I have tried other combos and it is deffo load related and occurs every revolution of the crank, but the bike is totally silent under normal / light load and climbs...I checked the crank (un72) and it had a little play, but have replaced it tonight with a new one, with crank arms torqued to 45 Nm's, and it is still doing it :evil:

Full circa 1997 XT groupset /compact hyper crankset, ig 90 chain etc.....all look good, chain has no apparent tight spots but is a bit worn, as are the rings, but nothing in the way of burrs/ heavy marks etc. :?:

Mech hangs true with no obvious faults......the cassette has maybe 1/2mm radial rock, but nothing more........HELP!!!!!!!!!!!!!!!!

 

[mechagouki]

I'd guess your driveside crank is hitting the chainstay.

 

[jimo746]

When the noise occurs are you sat down? Or stood up pedalling?

 

[spesh]

i know exactly what this is as i had the same with some 3pc cranks on my cruiser
after replacing the bottom bracket it still made the same clunk at the same position even after torquing up the cranks
it was>
.
.
.
.
.
a pedal spindle loose in the crank arm! wtf it was more than finger tight but when you put pressure on it the pedal clicked in the thread on the arm luckily it didnt damage the arm but once done up with a 15mm spanner it stopped clunking
try yours hopefully it is what ive said

 

[JustKev]

I'd guess your driveside crank is hitting the chainstay.

That was my 1st thought too

 

[hamster]

As it's a full susser I suspect that you have a little play in the suspension bushes somewhere.

 

[malvern newbie]

DUH!......I can't believe I missed the problem which was so obvious!.....The bike is freshly built, with new linkage bearings, and I checked it over after a couple of rides. all threads were locktited during the build and all torqued with either a low range or normal norbar as applicable. I have totalled up the time I have been out on it, and it has had 15 hours plus of fairly hard work, all offroad since the check.

The problem was a loose main lower pivot causing the lower chainstay set up to move laterally under heavy pressure, and the noise was indeed the RHS crank arm tapping this part....I was engrossed with the left hand side for some reason during the climbs, and just did not twig that at the same position when the noise occurred, the RHS arm would be in line with the frame!!!!

Upside........she has a new bottom bracket and chain now

I suppose it is obvious that all the parts are going to be a bit more time consuming maintenance wise than a chromo rigid, so will be paying more attention from now on........many thanks for the replies all.