Bobbing rear deraileur

[hwttdz]

Ok, I successfully changed my chain, and I'm relatively certain that the chain is the correct length. I put on a new cassette at the same time, as well as new deraileur pulleys (one of the old ones was broken). In examining my chain I found that if I spin the chain, either direction, the rear deraileur bobs back and forth. I can't think this is a good thing. So is there something amiss? Oh the motion is regular, i.e. at at constant frequency about 2-3 times a second. And related to how fast I spin the chain, faster chain speed -> faster motion. Doesn't do it freewheeling, don't know if that helps. Any ideas?

 

[gene d]

I turned the knob at the back of the derailleur and that seemed to fix the problem.

 

[drewjc]

very good description there gene d!!! i assume you are talking about the cable tension adjuster knob? and how did this fix the problem?

I think that hwttdz's problem may not be a problem at all. It is most likely a slight imperfection in the shape of the chainrings and/or the sprockets on the cassette. If you have egg shaped gears then the chain will pull tighter at certain stages throughout the pedal stroke and thus the derailleur will tension itself differently. If you have ever ridden a bike with Bio-Pace chainrings you would notice this action a great deal more. If you dont notice any change in the sensation while riding i dare say that it is nothing to worry about. You will often notice a similar "problem" on track bikes as an out of shape track cog is often mis-diagnosed as a tight spot in one's chain.

NB. when i say egg shaped gears i was exagerating to make a point...the gears wouldnt really be egg shaped, but maybe slightly eliptical. I noticed that both of my bikes with derailleur gears do this to some degree, one of which is almost brand new, so dont worry too much.

 

[hwttdz]

I am not inclined to agree as the frequency is not periodic with the chain rings. If the chain rings were non circular then the same spot on the chain ring would produce the same swing every time, correct? There's a ticking sound as well but I can't describe it now as I'm not at home and have no bicycle. I'm also going to ride by a shop this afternoon and have them take a glance.

 

[DaveN]

Originally posted by hwttdz
I am not inclined to agree as the frequency is not periodic with the chain rings. If the chain rings were non circular then the same spot on the chain ring would produce the same swing every time, correct? There's a ticking sound as well but I can't describe it now as I'm not at home and have no bicycle. I'm also going to ride by a shop this afternoon and have them take a glance.

Check that all the pins on the chain are in correctly, particularly where you joined the new chain. The pin could be catching the RD slightly and just needs to be correctly positioned.

Dave N

 

[tyler_derden]

Biopace chain rings?

TD