Lynx516

I have built my first TT bike a Cervelo P2sl and I am having a problem I have never encountered on my previous 4 road bike builds.

When I shift up on my front chain ring and I am in a middle gear on the rear cassette I chain will derail off the front. If I bring it in a bit by tightening the upper limit screw in any more than it is then it doesn't shift at all. So I am stuck between not shifting at all and unreliably shifting straight off the chain ring.

The front derailleur is parallel to the chainring and at the correct height so I am baffled. Has anyone seen anything similar or got any advice?

Amoxicillin

might be the cable tension for the lower position. start over, reset the screws to somewhat middle position, then try pre-loading the cable a little bit and then re-adjust.

Lynx516

Its on friction shifters so cable tension should not be an issue.

James

strock 

Hmmm . . . Sounds like you're doing everything right. If you haven't spent a huge amount of time tinkering with the derailleur height and/or the upper limit screw, I'd probably give those another shot. FD adjustment can be really finicky sometimes. One other thing you might tinker with is the derailleur angle. As you know, the outer cage plate is supposed to be parallel to the chainrings, but sometimes the plates aren't entirely straight and a little deviation from the general rule is helpful. If I've got this right (and I might not), angling the front of the cage ever so slightly to the outside might allow a quicker upshift while helping to prevent an overshift. Unfortunately, this adjustment will have the opposite effect when you downshift. But if it's only a very minor adjustment, you might get away with it. I'm afraid that exhausts my limited store of ideas. Best of luck to you!

Edit: I'm betting you've already checked this, but just in case you haven't -- have you got the chainline right? Since the derailment is happening when you're in the middle cog on the rear cassette, this seems unlikely to be the issue, but I'm grasping at straws, now.

mbusha

Don't know if this will be helpful, but I once had an identical problem once when mixing chainring styles (I think it was a 39T 6600 chainring with either a 52 or 53T Biopace chainring). I could get it to shift reasonably well after a lot of tinkering with the FD angle. However, I'm guessing that the real problems was that ramps for directing the chain up onto the big ring weren't designed for this combination, since changing the big chainring to a 6600 solved the problem perfectly.

Homebrew01

Derailleur adjustment (especially rear) should have nothing to do with this problem unless the FD is not moving out far enough, and the chain is rubbing the cage. If the cage is not rubbing, then you could set the FD any crazy way and the chain should stay on the big ring.
It could be a frame alignment problem, such as crooked BB shell or rear triangle, or something screwy with the crankset.