Hi - dialing in new TT bike - 2006 Cervelo P2SL, bone stock except seat substitution - drivedrain is Dura-Ace FD/RD, Ultegra 11-23 10 speed cassette and chain, FSA Mega Exo 39/53 cranks. Built up nicely by a very reputable shop - I had to buy at some distance from my home as no local Cervelo dealers. Anyway, the problem:
In the 39 / 11 and 39/12 combinations, the chain skips on the rear derailleur. It seems to pass through the derailleur at a sharp enough angle that it can't simultaneously be aligned on both the upper and lower pulley. The derailleur is otherwise well-adjusted (including the B screw) and shifts nicely. Bike brand new and deraillur hanger is not bent, etc. It's not a stiff link, etc. With the external bearing BB/crank, the chainline (I'm assuming, correct me if I'm wrong) is fixed, so there's nothing I can adjust there. I know that these are cross chain gear combinations that I won't be using intentionally on the road, but I was wondering if anyone had any insight into what I could be missing. Is this just somehting that's inevitable with the geonetric limitations of extremely short chainstays ? TIA for replies.
Don't use those gears, it puts the chain at too extreme of an angle. It's nothing wierd at all, find another suitable combo with your 53. Same with big to big, it's also to be avoided. Nothing new at all, it's been the bane of derailleur setups since the Byzantine era.
I know not to cross chain - I was asking about the structural/mechanical issues. How about the chain possible being too long ? It's borderline by the 90 degree rule for pulley alignment in the 53/11.
OK, didn't mean to sound pedantic, but it read as cross chaining. I do chain length the old fashioned way, and it should be good way to check: wrap around big/big without going through derailleur, use the length that would connect this plus a link. You indeed sound a bit long.
Hi - dialing in new TT bike - 2006 Cervelo P2SL, bone stock except seat substitution - drivedrain is Dura-Ace FD/RD, Ultegra 11-23 10 speed cassette and chain, FSA Mega Exo 39/53 cranks. Built up nicely by a very reputable shop - I had to buy at some distance from my home as no local Cervelo dealers. Anyway, the problem:
In the 39 / 11 and 39/12 combinations, the chain skips on the rear derailleur. It seems to pass through the derailleur at a sharp enough angle that it can't simultaneously be aligned on both the upper and lower pulley. The derailleur is otherwise well-adjusted (including the B screw) and shifts nicely. Bike brand new and deraillur hanger is not bent, etc. It's not a stiff link, etc. With the external bearing BB/crank, the chainline (I'm assuming, correct me if I'm wrong) is fixed, so there's nothing I can adjust there. I know that these are cross chain gear combinations that I won't be using intentionally on the road, but I was wondering if anyone had any insight into what I could be missing. Is this just somehting that's inevitable with the geonetric limitations of extremely short chainstays
No, it's just that there are reasons you aren't supposed to do that. And, you've discovered one. If you persist you will discover the others. That's a very nice bike you've got there, in the course of riding and racing it'll be subjected to some amount of abuse. No reason to add to that, needlessly.
I know not to cross chain - I was asking about the structural/mechanical issues. How about the chain possible being too long ? It's borderline by the 90 degree rule for pulley alignment in the 53/11.
Just a note: The short chainstays on the Cervelo will make the bike more sensitive to cross-chaining (more precisely, cross-chaining will result in more extreme chain angles).
Quick check for too-long chain: Put the chain on the big-big combination, and fold the chain by hand to simulate the chain length of one less inch of chain. If this doesn't make the chain unacceptably tight, you can pull out a link.
Hi - dialing in new TT bike - 2006 Cervelo P2SL, bone stock except seat substitution - drivedrain is Dura-Ace FD/RD, Ultegra 11-23 10 speed cassette and chain, FSA Mega Exo 39/53 cranks. Built up nicely by a very reputable shop - I had to buy at some distance from my home as no local Cervelo dealers. Anyway, the problem:
In the 39 / 11 and 39/12 combinations, the chain skips on the rear derailleur. It seems to pass through the derailleur at a sharp enough angle that it can't simultaneously be aligned on both the upper and lower pulley. The derailleur is otherwise well-adjusted (including the B screw) and shifts nicely. Bike brand new and deraillur hanger is not bent, etc. It's not a stiff link, etc. With the external bearing BB/crank, the chainline (I'm assuming, correct me if I'm wrong) is fixed, so there's nothing I can adjust there. I know that these are cross chain gear combinations that I won't be using intentionally on the road, but I was wondering if anyone had any insight into what I could be missing. Is this just somehting that's inevitable with the geonetric limitations of extremely short chainstays ? TIA for replies.
Lou D'Amelio
I would say so, along with cogs that are really close together. The angle is causing the chain to 'tickle' the cog next to it, not surprized...just don't go there.
