[Ed – The information contained in this post is solid, though digging deeper into the problem I eventually came to find that the main culprit to my tale of woe was a bad chainring.]

So, about my Trek’s massive shifting problem and it’s skipping the chain off the small ring into the bottom bracket… I was partially vindicated over the weekend when I had a chance to question the owner of our shop about what I’d done to cure said skipping of the chain into the bottom bracket.

First, a recap. My Trek has been skipping the chain off the front ring under full power (call it 600 to 900 watts) ever since I updated the components from a 9 speed triple to a 10 speed double – I just didn’t know what was happening until recently because I rarely lay down that kind of power in the little ring. On a recent vacation I made all of the connections to what was going on, only because the barrel adjuster had come out of adjustment by two full turns with the bike transported on the back of my wife’s SUV. The chain would drop every time I climbed a hill out of the saddle. With the rear derailleur adjusted properly, it’s considerably more difficult to drop the chain… but not “more difficult” enough.

After adjusting the rear derailleur to perfection, I had to do some research into what else could be wrong. The easy answer was set screws on the front and rear derailleurs. Those are set perfectly on the Trek. The next answer is “chain line”, and that’s a little harder to deal with because of all of the changes I made to the bike. Put simply, the double crankset needs to move the chain rings closer to the frame. This can only be done by choosing different bottom bracket bearings, but if the current bottom bracket fits my Shimano double crankset perfectly so I don’t need shims or wavy washers. Oh, how I hate the wavy washers!

Now, if I loosened the barrel adjuster, thus moving the system outbound, the almost imperceptible clicking of the chain would go away but the bike wouldn’t shift properly up the cassette (from small to big cogs). This is how I was able to determine the chainrings wanted to go in, toward the bike. That wasn’t going to happen, though… so common sense (but not proper bike mechanicry) dictated, add a shim to the cassette. I started with a .5mm shim. That helped a lot, but I was still getting an minute hop when I back pedaled slowly… I gave it a full mm shim… and silence. Not only was the drivetrain quieter than it had ever been (both with the 9 speed and 10 speed drivetrains), it stopped skipping off the front under load in all but the smallest three gears on the cassette – gears I’d never use to climb out of the saddle in anyway – and I ran out of room at the rear dropout, anyway. I might be able to fit another half-millimeter in there, but I’m calling it good enough as is.

Well, Saturday I was able to run all of this by the owner of the local bike shop. The man built frames for a living and apprenticed under frame building nobility in England back before I was born. If ever there was a man to bounce my thinking and actions off of, that’s the guy.

His answer was exactly as I’d hoped; we’d prefer to move the chainset in by choosing a different bottom bracket, but if we can’t do that, shimming the cassette out, while not ideal, is the next best thing.