Rear derailleur won't shift up from smallest cog to next smallest cog but then shifts ok

Question

I have a 1x10 Deore setup and recently I have been having shifting problems when I am moving up from the smallest cog to the next cog. It won't shift to the 2nd cog but if I shift twice up to the 3rd cog it will skip the 2nd cog and then shift smoothly up the rest of the gears and then on down from the largest cog to the smallest it will shift into each cog smoothly. Indexing didn't help so, since the drivetrain has about 3000 miles I figured it was time to change the shift cable and chain but that didn't help either. I then changed the derailleur hanger - no improvement, so I replaced the derailleur - no improvement. My cables are internal so I checked to make sure that there is nothing going on inside the frame but the brake and shifter cables are not binding and I also cleaned all friction points. The cassette looks fine - . So what is left to adjust/replace?

Answer 1

The teeth on the cogs have burrs so given your information i would guess it is just a worn 2. cog even though I agree it does not look too bad. If you use this cog a lot it is further indication that this might be the cause. If all the other shifts work fine indexing should be ok. I don't quite understand why you replaced all those other parts before trying to change the cassette. 3000 miles is nothing for a deraileur or shifting cables, but more than enough to significantly wear (or wear out) chains and cassettes.

To check for wear you can put the chain on the suspected cog. Backpedal to release any tension. Then while keeping the bike still, turn the cranks forward and check and see what happens when the chain engages the teeth on the cog as you apply torque to the crank. If the chain rises vertically on the teeth the cog is probably worn. You should then compare with a less/little used cog and see the difference. (the more worn cog will have more vertical rise) If a big difference is observed you almost certainly have a worn out cog.

(I would maybe also try to adjust the B-tension and see if that can help)

Answer 2

A few things you might check. First the B gap--the distance between the tips of the rear derailleur's upper jockey wheel and the tips of the largest cassette cog--should be 5-6mm. Manipulate the B-screw to achieve this distance (clockwise increases the gap, CCW decreases the gap). Obviously the derailleur and chain needs to be in the low gear position and chain on the smallest front chainwheel if more than one. It may also be worth to check the jockey wheels beginning position at the small cog. Without the influence of any cable tension, the jockey wheel's teeth tips should lie in line with the outside plane of the small cog.

Next, be sure the small cog is set into the proper spot on the freehub body. By this I mean that it can sometimes be difficult for the small cog to engage the splines of the freehub body until pressure of the lock ring is involved. This can cause the small cog to be misaligned on the body. It doesn't look like this is the case but maybe worth checking. The cassette cogs have a set alignment relative to each other and is the reason there is one wide spline so the cogs can only go on one way.

Finally, there might indeed be excess wear to the second cog's shift ramp. Some detail is lost from editing your picture, but I've marked the shift ramp areas in the photo below. Note the second cogs ramps have some burrs and may be worn abnormally thin compared to the next cog. This may be the heart of the shift problem.

Answer 3

The H screw needs to be set before anything else. Indexing comes after the H screw is set.

1. H Screw .2 indexing 3. L Screw.

H screw adjustment with chain on the biggest cog at front and smallest cog on cassette. Indexing middle cog and small cog on cassette. L screw adjustment to make the jump from second big cog on cassette to first big cog on cassette, adjusting it so derailur doesn't go into spokes.

If you want full steps let me know.