Your Rear Derailleur

How to Adjust It
by Geoff Slater

The rear derailleur is one of the most complex parts on a bicycle, but adjustments to it are fairly simple. Just a few informed twists of the limit screws and some cable tension tuning and you're away to the races. Most rear derailleurs on MTBs are either SRAM or Shimano and of the 9 speed variety. Here's how to treat them just right.

What Does It Do?
The rear derailleur moves the chain from one cog to another and takes up the slack in the chain when the front derailleur is shifted, or when the bike bounces.

Where Does It Go?
It bolts to a hanger on the rear dropout. Two pulley wheels keep the chain tight and guide the chain as shifts are made. Inside the derailleur are springs that pull the wheels tightly against the chain, creating tension. This keeps the chain from popping off the front chain rings.

Before You Begin
To work on this fine piece of equipment, first elevate the bike - either in a work stand or from the ceiling. Your working position should not only allow you to spin the drive train freely, but also provide a clear view of the derailleur from all sides.

The first thing that must be done is to make sure the derailleur and the derailleur hanger are straight. If the parts are new, there shouldn't be any problems. If the hanger is bent a little from abuse, removing the derailleur and bending the hanger straight with an adjustable wrench can often straighten it. If it is a replaceable hanger (like on most MTBs) replace it instead of trying to bend it.

The Adjustments
 Shift into the middle chain ring up front and the smallest cog in the rear. Adjust the upper travel limit screw on the body of the derailleur to line up the imaginary line formed by the center of the two pulley wheels and the outside (repeat... outside) edge of the smallest cog.

 Turn the barrel adjuster on the derailleur body all the way in so extra cable is available for future adjustments. If the cable is slack, loosen the clamp bolt on the derailleur and snug it up with some pliers. Then, re-tighten the clamp bolt.

 Next, set up the "B Tension"; that is the adjustment that sets the pitch angle of the derailleur body. A screw that is located just below the joint between the derailleur hanger and derailleur body adjusts it.

 Shift to the smallest chain ring and the largest cog. Turn the "B Screw" to get the upper pulley wheel as close to the cog as you can without touching it. Shift to the smallest cog and make sure the upper pulley wheel clears all of the cogs. If it does, move on. If not, back off the "B Screw" a little bit until everything clears.

 Shift to the middle chain ring and the smallest cog. Make sure there is no cable slack. Shift from the smallest to the second smallest cog. If the shift doesn't go, turn the barrel adjuster out until the shift is complete. If the chain moves beyond the second cog, turn the adjuster in.

 Now, turning the crank with one hand, turn the barrel adjuster out until the chain just starts to rub on the third cog. Then, back it off a bit. Shifting should now be crisp.

 Finally, shift up to the largest cog, making sure the derailleur hits its travel stop when the pulley wheel center plane is lined up with the inside edge of the largest cog. Make sure the derailleur doesn't hit any spokes in this gear.

 Take a deep breath and test to see that you have good shifting in all gear combinations by taking the bike through its range of gears. Don't worry about extreme gears - you shouldn't be riding in them anyway.

Remember
Expect some cable stretch. After a few rides, back the barrel adjuster out a turn or two if shifting slackens a bit.

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