IGD

I have a 2006 Colnago C50 with Shimano DA front and rear derailleurs. I am running 12 - 25 cassette and a FSA 50 / 36 crankset. On the road and on the work stand the front shifts perfectly between both chain rings. on a fluid trainer and rollers I cannot shift from 36 chain ring to 50 chain ring. Can anyone out there explain why? Thanks.

Mach42

Try this. Keep your cadence up and reduce pressure on the pedals while you shift. Just like on the road. In fact, it seems more sensitive to low rpm shifting than while I'm outside. I've seen the same thing with my trainer. Even with what seems like low pressure on the pedals, if I'm pedalling below 70 rpm or so, it won't shift. It'll just try to grab, let go, repeat ad nauseum.

twinquad

The front derailleur acts on the top part of the chain, which is under tension. Too much tension, and it can't do what it's supposed to do. On the work stand, of course, there's no tension, and on the road, you unconsciously let up on the tension long enough for the shift to complete. You can do this because the bike's momentum lets you maintain your speed during the shift.

On a trainer, however, when you let up on the pedals to ease the tension, the wheel slows down very quickly because the resistance overcomes the inertia of the wheel and the trainer's flywheel. Thus either the chain is under too much tension to shift, or it's moving too slowly to shift.

You can do a few things. One is to just use one chainring, and set the trainer to a resistance level that gives you an adequate range using only the rear derailleur. Second, you can set the resistance to a low level, which may allow you to shift on the front. Third, as suggested above, you can pedal fast just before the shift so that when you let up on the pedals, the wheel is still moving fast enough to allow you to shift.

A more involved solution would be to increase the inertia in your system, either by using a heavy rear wheel, or getting a trainer with a heavier flywheel.

Good luck.

IGD

Mach42 / twinquad - Thank you both for your help. Spinning faster in a lower gear and trying to ease up a little while changing from small front to large front is the trick. Chain tension seems to be the factor. Thanks again.