Bicycle Mechanics - Need Help, Please -- "Dropping" Chain

The bike is a 2002 Cervelo P2K (Ultegra). No problems stock. I bought a pair of Zipp 404's, with a 12-25 casette (stock casette is 11-23). When I put the Zipp's on, re-adjust the hi and lo limits and tension on the rear deraileur, everything works fine -- in the garage and on the flats.

The problem is during heavy loads, i.e. climbing, during which the chain drops off (to the inside) of the front (small) chainring. Further, it only happens when in the two biggest cogs. Front deraileur is not causing the problem, I've triple checked. I've had two separate bike shop mechanics check and tweak it, and it works fine on the rack and in the parking lot, but get on a hill and chain drops. I've been able to replicate the problem on the trainer, but unfortunately, the LBS's are closed today Sunday. Any suggestions on what to check? Really would like to take advantage of the nice weather to get a ride in.

Many thanks in advance for any direction you can provide!

Could the chainline be off a bunch? The centerline of the cassette shouldn't be too far out of line with the space between the two front chainrings (on a double) or the middle chainring (on a triple). If the chain is coming at the chainring from too much of an angle, it may be prone to falling off.

Another thing to look for: chainring teeth that have nicks or gouges. Sometimes these throw the chain. You might take the chainrings off, clean them if necessary, and closely inspect the teeth. If you find any burrs or gouges, you can usually fix them with a file. Good luck!

When you changed the cogs, did you also replace the chain? You went up 2 teeth on the cassette-that means your chain needs to be 1 full link longer. That might explain why the chain wants to bail out-it's under too much tension.

Originally posted by D*Alex
You went up 2 teeth on the cassette-that means your chain needs to be 1 full link longer. Not necessarily true as it depends o how the chain was sized to begin with.

Originally posted by D*Alex
You went up 2 teeth on the cassette-that means your chain needs to be 1 full link longer. Not necessarily true as it depends on how the chain was sized to begin with.

Thanks for the replies. I checked the 'straightness' of the chain while on the small ring / big cog. Looks a little off, in the direction that would cause the problem I'm having.

One quesiton...should there be a spacer on the inside of the cassette (between the biggest cog and the frame)? I installed the cassette as it came out of the box, there was no inner spacer.

As far as the chain length, I wasn't aware that it needed to be lengthened, I thought you could release some of the tension at the deraileur barrel nut to compensate for the bigger cog. I'll have to call the LBS on Monday and find out why they didn't mention I needed a longer chain.

Originally posted by Greg_LV
...When I put the Zipp's on, re-adjust the hi and lo limits and tension on the rear deraileur, everything works fine -- in the garage and on the flats.

How far do you have to readjust your limit screws? If both wheels have 9-speed gears, only a minuscule tuning should be required. As others have noted, on a 7- or 9-speed system, the center cog should line up between the two chainrings.

A long shot: if one of your rear wheels is slightly mis-dished, and you are mounting the rear axle at a slight angle to compensate (this assumes horizontal, rather than vertical, dropouts), you could be throwing off your chainline enough to cause problems, particularly if there is too much space between your high gear cog and your dropout or between your chainrings and your stays. With today's flexible chains, you will not notice a problem until you apply a load.

Another possible cause, if pushing big gears up hills, is chainring flex. Check to see if the rings appear to wobblw when climbing.

Are you very violent with your bike when you climb? What are the circumstances surrounding your chain falling off?

Originally posted by Greg_LV


As far as the chain length, I wasn't aware that it needed to be lengthened, I thought you could release some of the tension at the deraileur barrel nut to compensate for the bigger cog. I'll have to call the LBS on Monday and find out why they didn't mention I needed a longer chain. Man,buy a book!The deralier barrel is for derailer cable tension and controlls shifting.Try the big ring/big cog combo with the bike in a stand or support to check for chain length.

Originally posted by Greg_LV
I thought you could release some of the tension at the deraileur barrel nut to compensate for the bigger cog.

bzzzzzt

Wrong answer, thanks for playing.

There was a lengthy thread about this several weeks ago. Try a search, or check Sheldon Brown's site. The quick check is that you should be able to shift to your large-large combination. The other thing to check when you change cassettes is the, whats-it-called, b-spacing adjustment? It's a little screw that adjusts the spacing between the guide pulley and the cassette. (Which won't help your chain-drop problem, just thought I'd mention it.)

Does it matter what rear cog you're in, or does the problem occur in several? It's more prone to overshifting on the small chainring when you're on one of the outer cassette cogs (because of the chainline). At the risk of stating the obvious, front der adjustment can be pretty picky... maybe your adjustment is still allowing just a tiny bit of overshift?

I suffered through a period of dropping chains. It occurred under load from big to small ring (all Ultegra).

After going over der adjustments repeatedly, it became evident that the problem was due to an underlubed and dirty chain. A good cleaning and lubrication remedied the problem.

Chain suck is generally thought of as an MTB problem, but if you don't take care of your road bike chain, you will suffer similar maladies.

Anyway. . . just a suggestion.