There are lots of instructions and videos about this subject but I personally believe most of them are incorrect …not the best advice. Many years ago I read an article written by a bicycle mechanic with a lot of experience. With my own trike I have found his suggested method by far the best. My own personal experience of paying a local bike shop to adjust my derailleur only to have to readjust it myself soonafter as the adjustment the mechanic made did not last very long … and hour or two and maybe a day at best. But if I use the method this man taught the adjustment lasts for a very long time. I don’t waste my money anymore at the local bike shop (LBS). In fact, I don’t want them touching it. Here is how to adjust a rear derailleur. Don’t assume anything as far as it currently being correct.
Start by removing the adjusting cable from the rear derailleur. In doing so you know that the cable has no influence on the current adjustment. With the cable unattached you can then adjust the rear derailleur correctly.
A rear derailleur works by moving the chain from the smallest sprocket onto larger sprockets until it moves onto the largest sprocket or cog in the rear. So with that understanding adjustments can be made. The chain needs to be moved from one side to the other side keeping it aligned with the sprockets as it moves across under them. This alignment can be accomplished thru the barrel adjuster. Keep in mind that the cable is disconnected at this point so you can’t make this adjustment now.
The chain needs to start in the lowest gear (smallest sprocket) position. The derailleur has a spring built into it which pulls the mechanism over to that position. That is its natural or default position. No cable is needed to do this. The cable is needed only to pull the mechanism over to the other side.
There are adjusting screws to “limit” how far the mechanism can move to either side. The derailleur sprockets need to be lined up right below the sprocket or cog. It is important that it is adjusted correctly for best performance. Definitely you don’t want the chain to go too far to the inside to where it comes off of the sprocket and onto the spokes getting stuck there. The “L” lower adjusting screws handles this.
With the cable still disconnected you can manually move the derailleur mechanism across to the other side. It should stop under the largest sprocket or cog. It it does not stop where it should you use the “H” high limit screw to align it. If the chain comes off of the largest sprocket on the outside the high limit screw needs to be adjusted so it is not allowing the chain to move out that far. You should stand, sit, squat behind the drive train so you can see what is happening as you turn these limit screws. I am using Parktool’s video here to illustrate how to adjust the L and H limit screws. Keep in mind that this mechanic is not removing the shifting cable in his illustration … something I take issue with.
There is one other adjustment screw on the rear derailleur. It is called the “B” gap adjustment. It controls the position of the rear derailleur as far as the gap or space that is between the chain and derailleur. This video does a pretty good job explaining how to adjust the B screw.
Once you get all of these adjustments made it is time to reconnect the shifting cable. This should take care of the major adjustments and only a minor adjustment of the barrel screw may be needed at this point to smooth shifting. Hopefully you will already be dialed in and won’t have to mess with anything.
