Road Bike / TT Chainset - Shifting Problems
Scenario: I am the wife's bike builder for her triathlon escapades. For a triathlon this weekend, she wants to use a road bike, rather than a TT bike, due to the technicality (not gradients) of the course. I have fitted, to a Moda Stretto, Dura Ace 7800 double shifters, front derailleur, chainset (the chainset has TT chainrings - 55/42) and Dura Ace 7900 11-23 cassette. The rear derailleur is an Ultegra 6700.
The problem I have is that on the workstand, the setup works like a sewing machine - absolutely perfect shifting up and down throughout the range; however, when my wife rode the bike, she complained that it wasn't shifting up from the 42 chainring onto the 55 chainring properly. I had a quick look myself, and diagnosed that with any load/tension on the setup (actually riding the bike!), shifting from the 42 ring up to the 55 ring on the front chainset was, when in any of the highest 4 gears on the rear cassette (the 4 smallest rings), 'unclean' at best. Shifting up and down on the front chainset, with the rear cassette in any of the lowest 6 gears (largest cogs), was absolutely faultless. To cap it all, when the bike is back on the workstand, there are absolutely no problems with gear changes from the 42 chainring to the 55 chainring with any cog at all selected on the rear cassette.
Any thoughts, guys?
Cheers,
Mark
The problem I have is that on the workstand, the setup works like a sewing machine - absolutely perfect shifting up and down throughout the range; however, when my wife rode the bike, she complained that it wasn't shifting up from the 42 chainring onto the 55 chainring properly. I had a quick look myself, and diagnosed that with any load/tension on the setup (actually riding the bike!), shifting from the 42 ring up to the 55 ring on the front chainset was, when in any of the highest 4 gears on the rear cassette (the 4 smallest rings), 'unclean' at best. Shifting up and down on the front chainset, with the rear cassette in any of the lowest 6 gears (largest cogs), was absolutely faultless. To cap it all, when the bike is back on the workstand, there are absolutely no problems with gear changes from the 42 chainring to the 55 chainring with any cog at all selected on the rear cassette.
Any thoughts, guys?
Cheers,
Mark
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Comments
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Fascinating. Hard to see how sitting on and pedalling the thing could change the way the front mech operates.
Is it possible the chain is too short? Is the front mech properly aligned parallel to the chainrings and at the correct height? Is the cage bent at all? Does the bike have internal cabling that could be misbehaving in some way?0 -
Hi - thanks for the reply.
Chain okay - brand new Dura Ace 7801. Short enough from smaller front chainring to smallest rear cog, and long enough from larger front chainring to largest rear cog.
Front mech aligned correctly - parallel to chainrings and at correct vertical height.
Cage, although secondhand, appears to be the correct shape.
With the particular road bike frame in question, it's just a normal FD setup - no internal cabling.
It's all a bit academic now - with time short (and the participant athlete panicking!) I put it all back together in its normal Ultegra 6700 triple road bike form.
The only thing I can think of is that, because it worked on the 6 inner rear cogs, but not correctly on the 4 outer rear cogs, it might be something to do with the chainline. If I need to do it again, I'll play with some chainset washers, putting the front chainset further 'outboard' and see if that makes any difference.
Cheers,
Mark0
