Chain Rides between Chainrings on Downshift

[cbjesseeNH]

Sometimes, when shifting down from the big ring to the small ring, my chain drops between the rings and free-spins until I wiggle the shifter up and back down to re-drop the chain on the small ring. I'm running 50/34 Rotor rings on identical cranksets on both my road and tri-bike, and it only happens on my tri-bike.

How might I fix that? It's not good to loose power on a TT when this happens...

 

[alfeng]

Are you using the proper front derailleur for the particular chain?

That is, what specific chain/-type ("type" as in "speed") & which front derailleur are you using?

I presume you are using bar ends ... are they Shimano?

 

[davereo]

Do you have any sharp bends in your cable housings. Sounds to me that your cable may be sticking and the spring in your deraileur is not quite strong enough to ovecome the binding.

 

[jpr95]

Have you cleaned and oiled all the pivot points on the front derailleur recently?

 

[cbjesseeNH]

The bike is a stock Cervelo P2 with Rotor chainrings as the only change in drive parts. I just had it overhauled the 2nd time in 3yr with new cables and housings. It has always had this problem, so I'm wondering if the Rotor rings might just be made well for 10 speed Shimano.

 

[oldbobcat]

After ensuring that the front derailleur is positioned correctly--outer cage parallel with chainrings and at the proper height above the large ring--try backing off the inner limit screw a a little so the cage can pull the chain down with a little more enthusiasm. On round double rings the proper height is 1-2 mm above the tall teeth of the big ring. I don't know if Rotor specifies this differently. Derailleur cage height can be problematic with rings that are in any way elliptical.

Often there's a fine line between dropping the chain off the small ring and getting it all the way over. Finding it involves a bit of trial and error and sometimes installing a chain catcher.

 

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[alfeng]

Originally Posted by cbjesseeNH .

The bike is a stock Cervelo P2 with Rotor chainrings as the only change in drive parts. I just had it overhauled the 2nd time in 3yr with new cables and housings. It has always had this problem, so I'm wondering if the Rotor rings might just be made well for 10 speed Shimano.

Does the problem occur when the chain is on every cog OR only the smallest two cogs?

 

[cbjesseeNH]

Originally Posted by alfeng .


Does the problem occur when the chain is on every cog OR only the smallest two cogs?

Now that you mention it, I think it happens in the smaller cogs. Simply shifting it back to the big ring doesn't help and I have to shift to middle cogs before I can get it to drop cleanly to the small ring. Does this prescribe any particular gearing adjustment?

 

[alfeng]

Originally Posted by cbjesseeNH .

Now that you mention it, I think it happens in the smaller cogs. Simply shifting it back to the big ring doesn't help and I have to shift to middle cogs before I can get it to drop cleanly to the small ring. Does this prescribe any particular gearing adjustment?

As described, the shifting problem is probably, but possibly not, due to your TT bike's slightly shorter chainstays ...

Beyond not shifting to the inner chainring while your chain is still on the smaller cogs, the more obvious possible resolutions might be:

1. to change to a Shimano front derailleur designed for a Triple crankset ... AND, there are no guarantees that a 6603 or 6703 (or, whatever Shimano's current numbering system is AND presuming you are running a 10-speed drivetrain -- with a Shimano drivetrain, ALWAYS match the front derailleur to the chain, BTW) will eliminate the problem; but, I think it may ... I reckon that the extra lip on the bottom edge of the outer plate may (again, no guarantees!) be enough to push the chain over far enough to ensure a clean shift.
2. or, a 36t inner chainring might solve the problem ... adjust the size of the largest Cog as necessary to maintain the gearing

IMO, the option of not downshifting when the chain is still on the smaller cogs is the easiest solution ...

It should be easy enough to "train" your mind to remember to downshift off of the smaller cogs just prior to moving the chain to the inner chainring ...

I think that you can do it with one hand (either) if you want ...

• using one hand could certainly get you in the habit of double-shifting ...
• depending on how far the bar ends are separated, you would either pull up with your left finger tips just before pushing down on the left lever with the back of your hand OR pull up with the heel-or-back of your right hand just before pushing down on the left lever with your right hand ...
• of course, using both hands to execute the two shifts will be mechanically easier

 

[cbjesseeNH]

Thanks for your recommendations - I'll add them to my list of options to consider. I also found on the Cervelo forum comments about adding a shim washer to move the FD a bit more rearward, as well as using a Rotor SL 101-Ti chain, which might be hair wider than the Shimano 6700 10-speed chain.

The Rotor support people said they have no solid 50T chain-ring that fits on a 110BCD compact crank, which is odd, as I have one. I'll send them a picture, as they requested. Maybe it's 3yr old, but I bought it direct from them...