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Old 07-20-11, 01:04 AM   #1
cyclezealot
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An irregular clunk/jolt originating from my crank..

My race bike is a Klein Quantum.. As I turn the pedals , I infrequently hear a noise , sort of like a clunk and the pedal will seemingly advance a cog or two.. This happens irregularly . Maybe a couple times a ride. This might indicate what.? It's crank bearings have been changed once. Maybe like five years ago.. I ride my race bike far less than I do my commuter bikes. So in those five years, I have not put all that much mileage on it.
I told my shop's mechanic about the noise.. What I hear him say, wait until it gets worse. You advice. Thanks..
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Old 07-20-11, 01:08 AM   #2
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ps.. As I use my hands to apply pressure on the pedals, I sense no lateral free play . They seem solid . Would not bad bearings result in free play in the pedals.?
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Old 07-20-11, 01:14 AM   #3
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It's your cassette body - the ratchet is failing.

What's your rear hub?
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Old 07-20-11, 05:15 AM   #4
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Kimmo.. As I hope I made clear , the 'jolt' is in my crankset.. My rear wheel set is by Rolf Vector Comp ( original) and my crankset is Shimano Ultegra. ( Cartridge bearings.?) Would everyone recommend I just let it go until it fails.. That course of action might get me waylaid in the middle of no where.?
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Old 07-20-11, 08:07 AM   #5
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First let's eliminate a common cause of occasional clunking, which may or may not apply. Sometimes the rear wheel will overrun the freewheel a bit for a second. Not enough to truly freewheel, but by a few degrees, then when the pedals catch up the freewheel reengages causing the clunk. This usually happens as the crank comes through the vertical position and the pedal pressure drops. You can duplicate the sound at home, by turning the crank back a few degrees then forward hard.

If it isn't that, and occurs randomly it could be a stiff link, or section of chain, causing it to climb the tops of the teeth and slip back one tooth. The clunk is when it settles. Check the chain by back pedaling slowly while pushing the RD cage forward to slacken the lower loop. Watch of any links that don't straighten immediately as they come off the lower pulley. Solution, lube the chain.

If it happens under load, especially on an older bike, it could be a worn (stretched) chain or worn rings. This allows the chain to climb to the tops of the sprocket and slip back one position. Start by checkingthe chain for stretch using a 12" ruler, and seeing if 24 links measures more than the 12" it should. Up to 1/16" is OK, beyond 1/8" that's your problem, and between those it should be OK but replace the chain before it destroys your sprockets.

You can also check for sprocket wear by lifting the chain away at the middle of the wrap. You shouldn't be able to see more than 3-4mm of daylight under the chain. If you see more than 6mm or so, it might be slipping.
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Old 07-20-11, 08:20 AM   #6
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Thanks for the top notch information all. From FB's response, I do know that the chain is stretched.. My chain gauge reads almost .75 wear .. And some of the teeth on my chainrings are in bad shape.. We will soon replace the entire rear cluster and one chainring in the front.. Guess it makes sense to wait and see if the problem disappears after those changes are made..
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Old 07-20-11, 08:26 AM   #7
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Ditto on the chain.

If you have a stand, put the bike up, shift to the smallest cog and chainring then slowly and gently turn the cranks using as little torque as you can. Watch the rear derailleur cage. If there are stiff links as they pass through it you will see it wag as they pass through it.
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Old 07-20-11, 08:33 AM   #8
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About a stiff link.. I've had those problems many times before. From my experience, when a link sticks, it sticks far more often than what I am currently experiencing.. I might go out on a 30 mile ride and I doubt it happens more than twice.; but when it happens, its like a thrusting sensation I can feel through my shoes..
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