Sometimes the chain can seem to shift to larger or smaller sprocket.
Or, the chain cannot fit along the sprocket.
WHY? Maybe your RD needs a little treatment.
Lets try this for your bike. For illustration purposes, I use the SRAM RD, the same can be applied to SHIMANO’s.
All you need is find the H and L on your RD.
1. Put the chain on the largest gear on your crank and smallest sprocket on the rear. In this position we will set the high tension of your RD.
2. Put your screwdriver on your H bolt, turn the bolt ¼ clockwise or counter clockwise. Make sure your RD is aligned with the smallest sprocket.
3. Now change to the smallest gear on your crank and biggest sprocket on the rear. In this posistion we will set the low tension of your RD.
4. Same as Step 2, you can adjust it ¼ clockwise or counter clockwise. And try to align the RD with the sprocket.
5. Then, adjust the B screw. The B screw is to control the derailleur body angle. Shift to largest sprockets and you’ll see the pulley of your RD.
If the pulley is rubbing the cog, thighten the B screw to increase the spring tension.
If there is a large gap between upper pulley and cog, loosen the screw.
To find a good setting, loosen the B screw until the upper pulley begins to rub, then tighten the screw to get clearance.