Bicycle Mechanics - Why is my chain slipping?

I've got a 2-year old bike with Shimano Ultegra components. The bike has about 8100 miles on it.

About 3 months ago, the chain started skipping between gears, sometimes three gears in the reverse derailleur at once. My chain keeps slipping off of the chainrings.

Since then, I have had a complete overhaul of these components, and my LBS has been unable to help completely cure the unusual shifting. I've also replaced the rear cassette, the chain, and the middle chainring.

The LBS guy said there may be other things to try, including:
[I] replace the derailleur cables.
[2] convert the triple to a double

I'm at a loss. Any ideas on what may be causing this? I'm about to kill myelf on this thing. I can't get out of the saddle without thinking this is the time I'll be slamming into the top tube in a way nobody wants to think about.

Thanks in advance for any advice.

Try replacing the pulleys on your rear derailleur, they may not have enough of hteir teeth left to keep it on the selected cog.
Ride Smoother
Pat

My guess is that it has something to do with your shifter.

It is probably not holding the cable well enough.

Does it always jump from a big sprocket/chainring to a smaller sprocket/chainring?

Chainstay flex can sometimes cause phantom shifting, but this sounds a bit extreme for that. Is it possible that the LBS only told you that they replaced the cassette, but actually didn't? It sure sounds like a worn-out cogset to me.

Replies to Pat, Mike, and D*Alex....

First off, thanks everyone for your replies!

Pat: Odd you should mention pulleys. When I recently pulled them off to clean them, it dawned on me that once I had them off they were different. The Ultegra came with two different kinds of bearings. One was a standard sealed bearing, the other was a plastic or ceramic bearing. The guys at my LBS checked it out and said I had returned them to the proper positions, but I'm not so sure. I've searched the Shimano www site but couldn't find out why there is a difference or which positions they should be in.

Mike: I'm not sure how to definitively confirm that a shifter is the problem here. Any advice? FWIW, I think the slips have always been small to larger cog in the rear but am not 100% sure, more like 99 & 44/100ths.

D*Alex: Re your first point, would you expect chainstay flex to be a problem with a CF frame? (The bike is a Trek 5200. FWIW, the bike has a bit more than 8,000 miles & a 200-lb rider, who wouldn't mind getting to 170 again, but that's another thread) And yes, the LBS really did change me to a brand new cassette. I watched the installation myself after inspecting the new cassette they took out of a display case. That's a good point, though, because I know of at least one shop that cheats on parts, but that was on the east coast when I lived in DE.

Thanks again,

Trekaholic

It would be difficult to test the shifter. Maybe if you put the shifter into the most troublesome position and manually pulled on the cable to see if it moved or slipped.

Did you take the shifter apart to see if there is anything visibly apparent?

I do know that with the old friction shifters, the problem you described was very common. The friction cups would slip, the lever would sneakily creep forward, the cable would slack, and the derailure would move out to the lower cog. Of course, that was easy to adjust with the turn of the tensioning lever.

I'm not sure what kind of shifters you have, but shifters still work on a similar idea, though now ratcheted instead of simple friction.

If the problem happens like every other turn of the crank, check for a bad chain link - one that is frozen. A bad chain link can cause a similar problem as you describe, but the problem is very frequent - very madening.

I haven't taken the shifter apart. FWIW, it's a 2001 Ultegra - full group.

I'll look into the other items you've listed.

Thanks,

Trekaholic

The pulley that has some built in "slop" or "play", side to side, goes on top.

Also; It has occurred to me that your free hub mechanism, ( the ratchet system) may be worn out, it can be easily replaced.

Ride slipless
Pat

Hey Trek:
If you haven't done it already replace the rear cable. If theres no improvement try going to your LBS and have them check the rear deraillure alignment.
Good Luck....TT:rolleyes: