Member Question: Adjusting the Rear Derailleur

When I switch one wheelset for another, the rear derailleur needs to be adjusted because the rear wheel on each holds the cassette in a different position. Where do I start?

Member question from Fgray

I have no cycling background and am not mechanically inclined. When I switch one wheelset for another, the rear derailleur needs to be adjusted because the rear wheel on each holds the cassette in a different position. It's a three dimensional adjustment, and is somewhat elusive to me.

Where do I start? Which of the three screws does what? Do all three need to be adjusted, or is it a two dimensional task? Is there a way to shim the cassette on one wheel to mimic the other so less adjustment in necessary?

Answer

The problem you are experiencing with different wheels is quite common. You are correct to think that a shim will make it line up the same way. However, this is easier said than done. The shims that you need aren't made in the precise sizes required for fine-tuning a cassette like you desire, so the likelihood to successfully use a shim is remote. I’d recommend visiting your local bike shop if you want to attempt this route.

Adjusting the limit screws

The alternative solution is to learn how to dial in the shifting for each wheel. In many cases, you may not need to adjust the limit screws and only need worry about the barrel adjuster. On the rear derailleur, the "H" limit screw prevents the chain from falling between the frame and the small cog on the cassette and the "L" limit screw prevents the chain from falling between the large cog on the cassette and the spokes. As their name implies, the limit screws limit the travel of the derailleur. Turn the limit screw clockwise to make the derailleur travel less in the appropriate direction or counter-clockwise to allow it to travel more. If the limit screw is too tight the chain will not be able to go into the smallest or largest cogs, and if it is too loose the chain will fall off the gears altogether.

Adjusting the barrel adjuster

The barrel adjuster can be used to improve shifting, and is often the only adjustment you’ll need when changing wheels. As you are looking at the bike from the rear, you can turn the barrel adjuster either clockwise or counter-clockwise. If your shifting is sluggish when you shift to an easier gear (from smaller to larger cog in the cassette), then turn the barrel adjuster counter-clockwise about 1/4 to 1/2 turn and try the shifting again, repeating until it works properly. If the shifting is sluggish when you shift to a harder gear (from a larger to a smaller cog in the cassette), then turn it clockwise and repeat until it works.

 
There are a few other things to keep in mind as you make these adjustments. Ensure your rear wheel is properly installed in the rear dropouts before making any shifting or braking adjustments. The cassette needs to be tight or your adjustments will be in vain. Your rear derailleur cable system must be freely moving because a sticky cable will cause serious shifting problems regardless of any adjustments.