My cannondale caad 9 with ultegra components just started to develope a problem. I know that you should not cross chain but when you do and pedal backwords the chain drops down the rear cassette and jams. I know the standard answer is dont cross chain the bike but it never did this before so I was wondering what to look for. One other detail is that I just switched to a new chain. It didnt make any diffrence.

Probably you just need to adjust low limit screw on the rear derailleur?

No, that didnt work. The rear derauler shifts up through the gears just fine and sits directly under the big sproket on the rear cassette. This only jams pedaling backwards which from what I understand the rear derauler doesnt act as a guide

Drtumolo: It is possible that as your cogs have worn in it shifts easier when you back pedal. Running cross-chained will exacerbate this tendency as well as causing excessive noise and wear on your chain and gearing. There are comparable gear ratios available without cross-chaining; why persist in doing it? No good reason to back pedal, either, except to position your pedals for mounting the bike and moving off.

Backpedaling is a good way to check for tight links, but other than that - what's the point?

derailleur systems are designed to tun in one direction only. For them to work the guide (ie derailleur) has to be before the sprocket, otherwise the sprocket is trying to shift it in the derailleur. If you can't visualize this imagine trying to deflect or block a pass while standing beyond the receiver.

The reason your system auto shifts when backpedaling in crossed combinations or from the innermost cassette sprocket is that the unguided chain is trying to find a straight line path and so will try to change sprockets to one closer to the side it's coming from. Sometimes new systems are more forgiving, but with chain and sprocket wear they might get less so. OTOH, I've seen new drive trains that were extremely unforgiving of backpedaling while crossed mellow as the sprockets wore in.

There is no adjustment or modification that can change this because the chain is running unguided in reverse, but if you get used to how it shifts, you can drop the RD one or two shifts so it matches the sprocket the chain has selected, thus avoiding the jam.

Thanks for all the info. I do know that it is not good to cross chain the bike and also know that backpedaling is not productive. I just noticed that it changed since it never did this before. I can live with it just wondering what changed.

Might simply be that the old chain was too dirty to be flexable and the new chain is too new to be flexable. In either case it's a moot point.

Brad