Jerking chain

[pablo.amiba]

Hello,

my bike is riding fine but when I push it too much (like trying to gain speed going uphill) then the chain starts jerking (goes from smooth movement to a sudden jump forward... similar to when the chain moves to an adjacent sprocket and then shift back) ... noise comes from the back wheel

By the way, the chain is new and I just overhaul my weeks and derailleur...

I'm confused... any ideas?

Do you think the freewheel can be worn?... really don't want to take it apart if I don't have to...

thanks

Pablo

[Alex Ramon]

If the chain is new and the freewheel is original, you'll likely need a new freewheel. Chains and freewheels/cassettes wear together and generally need replacing at the same time.

[fuzzrock]

(11-22-2009, 03:12 PM)Alex Ramon Wrote:  If the chain is new and the freewheel is original, you'll likely need a new freewheel. Chains and freewheels/cassettes wear together and generally need replacing at the same time.

Dumb question - but how does the freewheel cause the bike to lurch forward?

I also experience this uphill problem -- but I thought it was skipping sprocket gears in the back - bike is very used so I replaced the chain, cassette, real-derailleur, and shifting cable. 3 qualified store repair departments (including Trek) looked at it and haven't figured it out. None mentioned the freewheel. I also suspected the Shimano xt m765 dual-shifter as well -perhaps it's shifting out of gear(?). Also, once in a while the shift cable pops out of it's holders on the frame...

[Joe_W]

Alex was talking about the rear sprockets, they are cassettes (that are put on a freehub) or freewheels that are screwed onto the threaded hub.

So you replaced chain and cassette. The rear derailleur was most probably not the culprit... I suspect the chain rings to be worn. It is difficult to say where the noise really comes from while riding... does this happen on every chain ring in the front?

[fuzzrock]

Thanks Joe.
It occurs on the small and middle chain rings for sure - I don't use the big chain ring going uphill so I'd have to attempt that. I just discussed this with another Trek service guy and he suspected the chain ring as well after briefly inspecting my rear wheel (off the bike) - spinning the cassette and seeing that it stopped in the opposite direction and had no wobble or play.

He suggested swapping out the rear wheel with a friend's bike to isolate the problem. If it's still there then it's most likely the chain ring, otherwise the freehub.

What I dont get and am trying to imagine is how a worn chain ring would behave like this. Is
the chain actually slipping or skipping teeth, without falling off?

[Joe_W]

Yeah, when the teeth are worn the chain does only engage some of them and stronger forces cannot be transferred. The chain will then disengage and slip and reengage on the next set of teeth. It is worse for rear sprockets since the diameter is smaller, but it happens on the chain rings too, the mechanism is exactly the same http://www.sheldonbrown.com/chains.html at 2/3 of the page. It happened to me on my old Giant last year, so I still remember it. It might be cheaper to get a new crank set rather than buying individual chain rings, so compare prices.

[fuzzrock]

I just bought new small and middle chainring and compared them to old set - they don't look that old. I am actually suspecting my Shimano M765 shifters now - I don't think they are shifting evenly between rear cassette sprockets - can actually feel some stiffness/binding when in the shifting lever - watching the rear derailleur do its thing while shifting up and down it seems inconsistent and leaves the chain in-between gears - even after fine-adjustment with the knobs - can never get the upshifting to match the downshifting in accuracy - I am ordering new rear shifter to see if this does the trick - I already have new chain, xtr derailleur, cassette, shift cables, middle and small chainring. That leaves the shifter and the rear-wheel/freehub. I'm betting on the shifter.