Bicycle Mechanics - Front der causing chain to drop off big ring, help please.

I've recently inherited a Cannondale XS800 with Shimano 105 STI shifters and Ultegra crank (double). Since the 105 shifters does not distinguish between double or triple, I can sometimes cause the front der to shift beyond the big ring causing the chain to drop off onto the crank.

My main problem is that the "high" adj screw has already bottomed out on the front der but it still doesn't keep the front der to move beyond the big ring. I have went to Parktool's website, followed Shimano's instructions, etc., all to no avail. Everything is lined up as instructed (as far as I can tell). Does anyone have any idea how to fix this? I can't find any way of moving the front der more inward (towards the frame) such that the high adj screw will do what it is suppose to do.

Any help or idea would be greatly appreciated! Thanks in advance.

This kind of thing can be a real buggaroo.

OK, first, make absolutely sure that you are tightening the correct limit screw for the outside limit. Forget about what you have read. Experiment and see if the other limit screw has any effect on the outer limit position.

Assuming that you have the correct limit screw...

The next thing to do is to make sure that the derailure cage is lined up straight with the sprocket. Sometimes they are not lined up straight, and the derailure cage pushes the chain at an angle - and off the chainring.

Getting the derailure cage lined up correct is a sensitive adjustment. Believe it or not, you can "tap" it into position by taking a hammer and lightly tapping the bolt that tightens the front derailure to the post tube. You will see the forward/backward position of the derailure cage move as you tap the bolt and essentially twist the derailure mounting around the post tube.

Good luck.

What happens if you add a bit of slack to the cable so it can't pull the cage all the way to the big ring?

Hmmm - Ultegra crank and 105 shifters. I'm thinking it probably didn't come from the factory that way.

The first thing that I would do would be to try all of mike's fixes. If, after doing all of that it still drops the chain, I'd stick a 2mm spacer under the right side of the bottom bracket to move the whole crankset over that way a tiny bit.

Thanks everyone for all your input.

I spent a couple of hours playing around with your suggestions, and in the end managed to keep the front der from pushing the chain off the big ring. As to why the High limit screw doesn't do what it's suppose to do is still a mystery to me.

Thanks for all your help. Happy Riding.