cellery

Hi guys,

I have been riding a 2010 Trek 2.1 (double) for several years on upgraded Bicycle wheel warehouse Blackset Race rims. Aside from that, seat and pedals everything else is basically stock.

2010 2.1 - Bike Archive - Trek Bicycle

I recently went through my third casette replacement, having used 105 10sp casettes 12-25, ultegra 6700 11-28 and now back to 105 in the 12-27 tooth variety. I always use a 105 or ultegra chain with a KMC missing link for 9/10sp chains and yes it is turned the proper direction.

When sizing a chain I follow the usual advice to bypass the rear mech, making sure it clears the big-big combo with an overlap of a couple chain links.

After having done so, then tuning the cable so that everything is shifting nice and smooth - I noticed something the other day. I know this is a cross-chaining scenario and I know it is not normal to backpedal while riding, but this has never happened to me on previous casette/chain iterations: when I ride in the big-big combo and try to backpedal, even slowly, the chain is coming off and getting hung up on the top part of the casette, like it doesn't want to move backward through the derailler's top run.

If I go up one smaller rear cog, I can back pedal fine, and the small front-big rear combo also backpedals fine. Just big-big. It only concerns me out of the fact that maybe one day I'll be out riding in a group, forget the bike has this issue and then throw my chain in a random bout of backpedaling. Note that the chain rides perfectly fine in big-big when pedaling forward; so it's not something that is likely to be a safety issue while riding - I am mostly curious about why this would happen. The only thing I can think of is that maybe I am one link too short; but I have measured chains the same way for years without this issue coming up before. Any ideas?

CACycling

The RD is designed to guide the chain coming from the bottom of the chainring to the proper place on the bottom of the cassette. There is nothing there to guide a chain from the top of the chainring to the top of the cassette. The worse the crosschaining, the more pronounced the effect. This is not a flaw, it is a problem that only arises when the components are used in a way they are not designed to be used.

Stay away from severe crosschaining and, if you don't, at least don't pedal backwards too.

cellery

Thanks for the reply. I do understand the cross chaining concept and do avoid it - but as do as we do on large/fast group rides - some small amount of cross chaining will be inevitable. This only piqued my interest because backpedaling had never been an issue in that particular combo until the latest casette/chain iteration - so I am trying to figure out what changed and why.

AnkleWork

Yes, no doubt, the chain is one or two full links too short.

cellery

I have heard down the hall that adding a 2nd connecting link, such as a missing link would be OK in order to lengthen the chain by one link. Is this the case or would I be tempting disaster by having two of them on the same chain?

AnkleWork

I think you'd have to add a connector plus an inner half link.

cellery

That makes sense - I have shortened many chains but never added to them. Thanks for the advice.

FBinNY 

The fall off when backpedaling big/big has nothing to do with chain length. just the fact that you can shift to big/big without wrecking the drive train is absolute proof that the chain is long enough.

The reason this happens has to do with chain line, and chain and sprocket architecture. Compare the teeth on chainrings and rear sprockets. Chainrings have pointed teeth so they can slip into and engage a chain coming from various angles.

Rear sprockets, OTOH, are shaped with square tooth profiles. There's no need to pick up a chain coming from an angle because the RD feeds it on straight on. (when pedaling forward). In fact the thinking on cassette tooth profiles is quite the opposite. They're designed to snag and lift the chain when it comes from an angle so you get better shifting.

So when you backpedal crossed, you're feeding a chain onto a sprocket tooth shaped to lift it and derail it easily. Once it derails it will drop to a smaller sprocket which also happens to be better aligned, which is why they rarely move over 2 sprockets.

If you prefer a nice mental image, imagine you're the captain of a boat trying to navigate into a slip in a crosswind. Would you prefer to have boat with a typical pointed bow, or a nice square ended barge?