11-29-2009, 11:43 PM
I just replaced my very-worn-but-still-functioning 12-26 tooth cassette and chain with a brand new SRAM PG-950 11-28 tooth cassette and PC-991 chain. Everything works perfectly except it skips on the smallest sprocket (with the biggest chain ring) even under the lightest load (just turning the cranks with my hand while on a stand). Note that it does not "autoshift" to another sprocket -- it just jumps a tooth approximately once per revolution of the sprocket and remains in the same gear.
I checked and I'm certain that the pulley is properly aligned with the 11 tooth sprocket. So, for example, the limiting screw isn't preventing it from fully engaging. I've adjusted the cable tension barrel and B tension (99% sure I got those right). The chain length is correct (pulley centers at right angle with ground). As an experiment I tried removing links to make the chain extra short, but that only made a very slight improvement. I measured the chain to make extra sure it really is new. When I put the bike on a stand and turn the cranks slowly, I can see the teeth at the bottom having trouble engaging -- the chain rides up near the tops of the teeth for a moment before finally slipping all the way down into the proper position. What would cause this?
I checked and I'm certain that the pulley is properly aligned with the 11 tooth sprocket. So, for example, the limiting screw isn't preventing it from fully engaging. I've adjusted the cable tension barrel and B tension (99% sure I got those right). The chain length is correct (pulley centers at right angle with ground). As an experiment I tried removing links to make the chain extra short, but that only made a very slight improvement. I measured the chain to make extra sure it really is new. When I put the bike on a stand and turn the cranks slowly, I can see the teeth at the bottom having trouble engaging -- the chain rides up near the tops of the teeth for a moment before finally slipping all the way down into the proper position. What would cause this?