Saturday, December 25, 2010

Fixing a Sticky SRAM X7 Rear Derailer

I have a SRAM X7 rear derailer on my Big Dummy, and over the past few months it has stopped dropping into the highest gear. I dumped it in the parts washer a few times and that would clear up the problem for a short while, but sooner or later it would start hesitating again. John had a similar problem, and SRAM replaced his derailer, but I wanted to fix mine. Once you start drilling parts off your derailer, it’s not likely that SRAM will take it back, so don’t give them a hard time if you mess it up.

I replaced the steel pins in the derailer with brass tubes. This reduced friction on the parallelogram enough that even off the bike the derailer’s motion was noticeably smoother. On the bike, the derailer feels lighter and drops faster than before, and is perhaps only a little less crisp. The tubing does introduce a bit more play into the derailer, but this is only noticeable on the bench, and certainly isn’t large enough to impact shifting accuracy.

Step One: Break It More.


The derailer is held together by four semi-tubular rivets, which also function as the pivots for the parallelogram. The shop heads are easy to drill out with a 3/16” bit, and the ridges on the shaft behind the factory head will prevent the rivet from spinning if you use a light touch. Remove the bottom pin in the lower knuckle by drilling out the shop end and tapping it out with an awl and hammer. This pin also holds the spring and once removed unloads the derailer, making the rest of the operation much easier.

With the first pin removed, test the rotation of the other three pivots independently. When I tested mine the top plate was binding at both pins, but the bottom plate rotated freely. I decided to leave the bottom plate rear pin in place, and replace the other three pins. Drill out the pins that you need to replace, and set them aside. At this point the spring should be free, and the top plate of the parallelogram can be removed. This is as far as the derailer has to be disassembled.

Step Two: Make New Pins.

The pins are 3.969mm (5/32”) in diameter, and their length varies by their location in the derailleur. Rather than measuring the length of the removed pins, which may have been too short in the first place, and now had one end drilled down, measure the outside dimension of the plates at the pivot. The top plate is not square to the axis of rotation, so measure at several points for each pivot and use the highest value. Using the high value ensures that the new pin will clear the ends of the plate.

My local hardware store, and most hobby shops, carries round stock and tubing in small sizes. To replace the pins I chose 5/32” brass tube, and to secure it M3 30mm bolts with a pair of washers and a hex nut. If you chose to replace the bottom plate rear pin, note that it is considerably longer than the rest of the pins, and a bolt longer than 30mm would be required. Brass has the advantages of being cheap, available, and very easy to cut and work. The downside of the tubing is that it adds the risk of the tube crimping under load, but with the close tolerances on the plates and the knuckles should support the tube and prevent deformation. An option if you are able to make accurate cuts in round stock would be to use a circlip on either end, which would be simpler and allow the use of steel pins.

Cut the tubing to the width of the pivot plus a millimeter or so. So long as the length of the tube is greater than the width of the pivot the length does not have to be particularly precise. Since the pins length is specific to the pivot, mark the pins after cutting them. For 5/32” brass tube fine tooth hobby saws cut it well. Remove burrs and square the end of the tube with a bit of 200 grit sand paper, and remove any interior burrs with a #2 X-Acto knife.

Cut and deburr a short (15mm to 20mm) piece of brass tube to hold the spring in the lower knuckle during reassembly.

Step 3: Reassembly.

Reassembly has to occur under tension for the last two pins, as it is very difficult to install the spring after the pins. Attach both plates and the spring to the upper knuckle, and spread the plates apart. The exposed hook on the spring will loop around the lower knuckle bottom pin, slide it into the notch in the lower knuckle and slide the short piece of tube you cut earlier into the lower pivot hole until it is securing the spring and centered in the knuckle.


Pull the knuckles apart until you can align the top plate pivot holes and the lower knuckle. Slide the new pin for the pivot in. This will constrain the movement of the lower knuckle and make it easier to slide the lower pin in. Slide the lower pin in, which will push the brass tube holding the spring in place out, and transfer the spring to the new pin.

Once all the pins are in place, center them up on the knuckles and then install the washers and bolts. Apply some blue Loctite to the bolts, then thread on the nuts and tighten them down.


Test the movement of the derailer, the spring should drive it to its closed position from any point without hesitation or binding. There should be only a small amount of sideways play in the parallelogram. If everything checks out, install the derailer on the bike and adjust the shifting.

Aftermath

With the weather I haven’t gotten to out on any long rides, but so far the derailer is performing much better than before. We will see how well it holds up, but for now it was a fun project and improved the shifting for a very low cost. There are a few more photos of various parts on my Flickr.

Update 9/4/11: Still going strong, haven't had to do anything to the shifting since installing the derailer.

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