Bicycle Mechanics - FD Adjustment

I have a Ultegra set up, when I am on the big chainring and on the second smallest cog & smallest cog there is a rub on the FD when sprinting. I tried to adjust the FD limits but this throw the chain off during shifting. Is the cranks causing this? or is there any other ways of eliminating this rub? When I put the bike on a stand and turn the cranks by hand there is no rub.

Is the derailleur perfectly straight and paralell with the chainrings? Are you running it 1-2mm above the large chainring when you pull the cage out over the big ring? If those are both ok, bend the cage a little with some duckbill pliers... grasp the length of the outside of the cage and just pull out slightly 1-2x and usually it takes care of this.

I have a Ultegra set up, when I am on the big chainring and on the second smallest cog & smallest cog there is a rub on the FD when sprinting. I tried to adjust the FD limits but this throw the chain off during shifting. Is the cranks causing this? or is there any other ways of eliminating this rub? When I put the bike on a stand and turn the cranks by hand there is no rub.

Most likely the cause is bottom bracket frame flex. Back in the old days, almost all bikes did that.

Before you start bending on your derailleur cage, I'd first make sure that everything else is set up correctly. I think that the mistake most guys make is trying to start the front derailleur process somewhere in the middle. My recomendation is to disconnect the shift cable and start with the position of the derailleur on the seat tube. After you're sure that everything else is right, a little gentle tweeking of the shape of the derailleur cage might be just the ticket.

I agree with Retro, it's due to BB flex. For racers, I've always set the outside limit a bit further out than a recreational cyclist.

Regardless, in an all out spring, you WILL get some chain rubbing. I've never been able to eliminate it completely. Minimize it? Yes! Eliminate it? NO!

L8R

thanks for all the replys. I think it is most likely the frame/BB flexing as there is quite a gap 2-3mm between the chain and FD. I guess I just to put up with it.