granto

Only when I'm on my highest front cog, my rear tension pulley (the lowest one) seems to get jerked by the chain at random (seems I don't even have to be peddling) and makes a horrible noise like the whole drivetrain is about to shear off. I guessed it was poor lubrication in the tension pulley but lubing its pivot point didn't seem to help much. Other suggestions?

DannoXYZ 

If you're not pedaling and the chain's not moving through the derailleur, where's the popping motion coming from?

Al Criner

Are you sure your freehub is OK? Can you replicate this problem on the repair stand?

granto

not sure.. maybe it does require pedaling, like if I pedal backwards through my freewheel, normally the chain would just be going through the lower pulley, but every once in a while it yanks it forward... Let me take a ride and be sure of that

LarDasse74

I agree with DannoXYZ - if you are not pedlling and the derailleur is doing a dance, the problem is comeing from somewhere else.

If while you are coasting your derailleur jerks forward occaisionally it is most likely a problem with the ratchet mechanism in the freewheel/freehub. THis is somethign you shoudl fix soon as freewheels and freehubs generally get worse if left alone, not better. And you will be left either not being able to coast (loss of freewheel function) or not be able to pedal (loss of forward motion).

granto

Have not tried on a repair stand, because I don't have one I'll throw it up at the local co-op and tell you what I see

granto

(because I know nothing about freewheels), why would it only happen when I'm in the highest front gear?

FBinNY 

I suspect that your problem isn't with the pulley itself but in the chain, or the point of chain/pulley engagement.

Like Danno, I assume that despite the "seems that I don't have to be pedaling" it only happens when the chain is moving. If it happens while coasting it's another ball o' wax altogether.

It's easy to check the chain first. Backpedal while holding a bit of pressure against the lower pulley until the chain just begins to sag. Watch the chain as it comes off the pulley for any links that don't straighten immediately, and feel for any tugging on the cage. Either indicate stiff links, which can usually be solved with chain lube.

If the chain is OK, use a stand or have someone hold the rear wheel off the ground. Put the bike in the problem combination, pedal forward and watch carefully as the chain engages the lower pulley. You're want to see if it's trying to come off the side of the pulley. If so, that's your problem, and the solution may lie in checking the RD and hanger alignment.

Note, some RD pulleys are more prone to the problem than others, so if all checks out but it still does this when the chain feeds from an angle (outer chainring, middle and inner cassette sprockets) you might want to find a lower pulley with deeper teeth (more like a sprocket) as these do a better job keeping the chain in the groove.

LarDasse74

Sorry I missed that detail in the OP. It could also be, as FBinNY says, an issue with derailleur alignment. Using the big chainring up front might be putting the chain at an angle the derialleur cannnot accomdate. But you need to see if this is happening when you coast or just when the chain is moving.

granto

OK, well the chain is very new, but here's my observation when backpeddling- If I do it slowly enough, the cog on the lower pulley does not spin until I pull it far enough and it finally spins. So it seems like I may just do a slight backpedal while riding and this happens. But I lubed the heck out of that cog to no avail. I could try loosening it, or... replacing it?

DannoXYZ 

Does the lower-pulley spin freely by hand? There is typically a stationary bushing on the inside that the pulley spins around. Some pulleys have ceramic bushings that can be cracked by overtightening the bolt, leading to a binding pulley. Or if the bushing is too narrow (like it was swapped from another pulley), then the washers on the side would bind up the pulley.

granto

It spins, but not for long. It's an old bike, but it's been OK for months so far. I have made no adjustments to the pulley (and haven't wrecked the bike)... it just appeared. Maybe just worn out bushings?

granto

OK, looks like it's probably the hub. While backpedelling, the top of the chain goes slack and the rear pulley is pulled forward, and the tension is held in by the freewheel and not the pulley. Guess it's time for me to learn about freewheels

granto

OK one more question, I forgot that I just installed a new threaded freewheel on this bike a few months ago. (Still not understanding how the freewheel/hub works), could the backpedal resistance come from the hub itself? In other words I thought it was a mechanism in the freewheel, and not the hub, that allowed it to "click" when coasting or pedaling backwards....

granto

Alright now that it's clear that I was waaaay off, here was the real problem: there was a chunk of metal on the face of one of my chainring's teeth, which was catching on the chain, and the lower pulley was sticking enough to be pulling forward during the process (the lube finally took to the pivot point and it no longer pulls forward when backpedaling). Filed it off, back to normal. Thanks for everyone's help.