small chainring jumping

[willcycle]

My '05 Burley Tosa tandem's drive chain skips under load when in the 30 tooth chainring regardless of the rear sprocket I am in (i.e. 32, 28, or 24). I have about 1000 miles on the bike and just replaced the chain and cassette. While it was doing this I found the left stoker bottom bracket cup was loose which I subsequently tightened back up. The bike still skips and there is no visible damage to the 30 tooth ring. Can there be something wrong with the ring even though it is not obvious? What else could the problem be? (The bike was never dropped).

Thanks,
Will

[TandemGeek]

Can you be more specific about the skipping? Is the chain not engaging, it is "slipping", are you getting chainsuck, is the bike just trying to autoshift to the middle ring, or do you just have a lot of noise coming out of the drivetrain?

Lacking any other details and assuming the chainring is fine, my wild-ass-guess (WAG) is to suggest that if the noise "could" be coming from the rear derailleur you may have an interference problem with the idler pulley on the rear derailleur. For most tandems running long-cage rederailleurs, the B-screw adjustment needs to be almost all the way in and the chain length needs to be pretty close to spot-on otherwise the rear derailleur's guide wheels can end up meshing with the chain and the cassette sprockets which creates a lot of chain chatter. Again, this is just a WAG.

[willcycle]

I suppose you could best describe what I am feeling as a slip. I would begin climbing a hill in the 30 tooth chainring and all will be well for a while. The grade may increase slightly and my stoker and I now apply more force to the pedals and I would feel a slip. If I continue with the pressure after another ten feet it would slip again. It feels like what happens if you have a new chain on a worn cassette except that both are new and it ocurred before replacing them. I have a new chainring on order (even though my existing one seems fine) to try in an effort to fix the problem as it is disconcerting to say the least.

[willcycle]

As an after thought, I don't know how long I was riding the bike with the left stoker bottom bracket cup partially unthreaded from the frame (it backed out on its own). Is it possible this caused some kind of odd wear on the small chainring even though it is not obvious?

[ricardo kuhn]

this is a even crazier theory....

maybe the Cassete palws are not engaging all the way, I use to have a XT hub on my tandem that did something similar before one day stop working after the interface with the Hub when FUBAR on a climp...

make sense, only happend on high torque enviroments, and the granny ring is for sure the torquiest,,well is just a Guess

Hopefully i just the chainring, i will take it apart and at least spinned or even better flip it..not to mension correct (File) any possible deviation

Oh another Sur American trick comes to mind
since the chainring have two "High tension" points,(were the crank place the most amount of pressure at certain teeth) the rest of them kind of hold the chain in line (like the shimano prinsiple with the cut outs toiimprove shifting)

anyway the Trick only works in "Old school" ROUND chainrings, Not Biopace and not the ones with ramps and cuts on the chainring ,
the Idea is to rotate the chainring ONE hole at the time(72 deggres) that way the chainrings wear a lot more even.(the smaller the chainring the more useful the trick becomes)

again this is useful in places were chainrings are dificult to find, but I quess also apllys if you are on a really long tour and you don't want to carry spares..

I hope the little tricks helps and the small chainring stops jumping..

[TandemGeek]

I suppose you could best describe what I am feeling as a slip. I would begin climbing a hill in the 30 tooth chainring and all will be well for a while. The grade may increase slightly and my stoker and I now apply more force to the pedals and I would feel a slip. If I continue with the pressure after another ten feet it would slip again. It feels like what happens if you have a new chain on a worn cassette except that both are new and it ocurred before replacing them. I have a new chainring on order (even though my existing one seems fine) to try in an effort to fix the problem as it is disconcerting to say the least.

I've had slipping before but it was on a worn-out middle chainring and by worn-out, 10k miles not 1k and it was an abrubt event. Assuming the granny ring is asymetrical, did you try flipping the chainring over to see if the problem affects both sides of the teeth? If not, it might be worth a try. I'm also assuming that you don't have any front derailleur interference problems. Assuming everything about the chainrings, front derailleur, rear derailleur, and chain are addressed there is one other consideration: the rear hub's cassette carrier. I have had two or three hubs where, for various reasons, the pawls would slip under heavy loads which caused the same very disconcerting drivetrain slip.

Given that this is a new tandem and that you are presumably the original owner, it should be covered under warranty; have you talked to your dealer and/or Burley about the problem? Your dealer would have done most of the assembly and should have caught the loose bottom bracket. If nothing else, I would think/hope they would be covering any costs for replacing possibly defective components or might be able to suggest a solution if there is a systemic problem lurking out there that they may already be aware of.

[SDS]

I am inclined to yield to the superior experience of others in this matter, but an alternative possibility I think should be considered until ruled out is a tight link or twist somewhere in the chain.

You can check for a tight link by running the chain through the drivetrain backwards and looking for nonconformation of a link of the chain to the curve of the cassette cogs, or more easily seen, the jockey pulleys. Movement of the jockey pulleys or the cage allows you to determine the location of the tight link.

Sometimes chains pick up helical twists (they're bent) during hard shifts. After that they may skip under load, and you have to untwist them, best done in my experience with channellock pliers and a screwdriver blade pushed through the center of the link. This is something best not done to excess. You can look for the twist by running the chain through the drivertrain and looking to see whether all the sideplates of the chain are in the same plane.

My preference would be to take the bike back to the dealer and to describe the behavior and ask them to fix it. My guess is that the problem is at the back end and not the front end, but a general inspection including checking to make sure the chainring bolts are tight is certainly appropriate.

[willcycle]

I replaced the small chainring and that solved the problem. Since I had the original ring off I examined it carefully and saw a couple of teeth with slight indentations in front which I suppose would explain the jumping. I wonder if this could have been caused by standing while climbing in the small ring since a reduced number of teeth handle the load?

[LuisB]

Will,

We too took delivery of a 2005 Tosa and on our second ride (just 50 miles), the chain broke which caused a slipping on the rear cog. Did yours come with a Sunrace chain and cassette? I'm thinking of replacing the cassette now because I'm thinking the realibility is suspect.

LuisB

[turtlendog]

Will,

We too took delivery of a 2005 Tosa and on our second ride (just 50 miles), the chain broke which caused a slipping on the rear cog. Did yours come with a Sunrace chain and cassette? I'm thinking of replacing the cassette now because I'm thinking the realibility is suspect.

LuisB

If if makes you feel any better, we broke an LX drive chan on our tandem at ~1500 miles. The break, however, was at the service link (Shimano pin) and I suspect something went wrong with the join.

The symptoms were a bad case of autoshift BTW (one side broke, but the other was still hanging on).

[willcycle]

LuisB,

My tandem came with the lousy Sunrace chain and cassette. When the chain broke a cog on the cassette also bent. I replaced both and will stay clear of anything made by that company.

Will