I race a Cannondale tandem with Ultegra components and a triple 30/42/52 combo. In two of our practice sprints today - while winding it up out of the saddle, the chain came off the big ring to the outside. We kept control both times but this does affect our confodence when we need to go really hard. It seemed to follow a change made out of the saddle but not immediately - maybe a coincidence. When checking gears on a stand - all is perfect and the front mech will not throw the chain. Any thoughts would be really welcome - also any tips on sprinting in general on a tandem.
First check the 52 to make sure it is not bent. Otherwise, the chain must be getting pushed off the chainring by the front derailuer I would think. Can you hear the chain rubbing the front derailuer before it comes off the chainring? - may have to ask stoker. Is the front derailuer angled at all toward the outside?
You could try raising the front derailuer up on the seat tube a little to get a wider part of the cage around the chain when in the big ring, but then it might not shift as cleanly or shift off the big ring.
Do you swap over from training wheels to racing wheels before races? That will alter the chainline enough to cause problems. Best to ride racing wheels a day before the race and get your adjustments down. I think the current Ultegra chainrings would be pretty hard to bend, but it never hurts to look. Let a mechanic take a look. I don't remember ever being at a race where there weren't several shop mechanics. But I haven't raced my tandem yet.
While the tandem gearing checks out 'perfectly' on the tuning stand, once you put your bodies weight on it, it changes the load dynamics.
Try not to overpower each other when standing; hard to believe the alu C'dale frame would really flex that hard that it would derail chain.
It seemed to follow a change made out of the saddle but not immediately - maybe a coincidence.
Can you elaborate on "the change", e.g., was this a rear derailleur shift?
In general, if you threw the rear chain off the big ring only one of a few things could have happened:
Work Around: If the chain derails followed a shift to your smallest cog, make sure your FD cage is parallel with the big ring and the outer plates is within a millimeter of the chain in the big/small combination in the stand. Yes, it will probably rub once you're actually riding but if you find you're not dropping the chain you can move the stop screw out a 1/4 turn at a time until the rub is gone and/or until it starts to drop again. Chain rub in the big/small may be the lessor of two evils.
Root Cause: Some thoughts on what could be going on here.
1. The bottom bracket is deflecting under load: make sure it's tight.
2. Your big ring and/or chain is worn-out and the chain is slipping off. Consider getting a new chain / chain ring if it's got a lot of miles.
3. The crank and chain ring are deflecting under load. Spin higher revs or get stiffer cranks / crank system.
4. The rear triangle is deflecting under load: spin higher revs.
As for sprinting on a tandem, it's all about working well together and keeping the revs up. Climbing hills out of the saddle is great training for sprinting since the movements are the same and, again, higher revs will always yield better performance and reduce bike problems in both situations.
Thanks very much for the sound advice. Tandem Geek - the change was from the 12 to the 11 as we picked up speed and found we needed a bigger gear. The drivetrain is in good condition so I do not think it is that. We are both 70kg and pretty strong riders - pilot is a very accomplished elite sprinter - so I guess i cannot rule out some sort of flex. I do find that if we are a little gentler and wind it up smoothly then we seem to have less problems here.