A Tale of Two Pulleys

October 20, 2014

Naturally, before each trip it's a good idea to check over the bikes and replace, adjust, or re-lube anything that seems to need it. This time we noticed a strange one. The lower pulley on Dodie's SRAM X5 derailleur seemed to have its teeth honed to knife edges. So it would seem natural to replace it. Our local bike shops are pretty much useless for this kind of thing, so was Amazon.ca (Canadian) and Amazon.com (American), and so were two local bike shops near Laurie's place in Seattle. Amazon, however did suggest various pulleys with prices up to $192. ( I think at that price you get "ceramic" bearings. Still, no sale, unless you can also drink your tea from it!).

Severely worn SRAM X5 lower pulley is seen in the lower right of the photo. A new X5, with its pulley, is in the upper left.

We took the derailleur into REI in Seattle, and were sold a SRAM pulley set for $15. However, when we got home and went to install the wheels, we found that the upper one was not compatible - mounted by two offset holes, not one center one.

So we decided the only way to ensure compatibility was to buy a whole derailleur. Again, forget the local shops, both in Victoria and Seattle. Amazon.ca could ship one, but for $85 plus shipping! Amazon.com could ship one for $56 and free shipping, but would it be in time to coincide with with when we would be in Seattle? We managed to arrange with a local Seattle shop, Gregg's at Green Lake to bring one in from a distributor, quite fast, and $50. Today I went and picked it up.

The puzzling thing is that the lower pulley on the new derailleur does have rather knife edged teeth, much slimmer than the teeth on the upper pulley. No doubt SRAM engineers puzzled over this one and did lots of scientific studies. I have not seen it on other derailleurs, though there are other quirks to observe on various derailleurs - like very big wheels, or wheels of unequal size. Clearly this is a little known but highly developed area of bike design. So maybe Dodie's knife, or needle, like teeth are even better than when they were new?

After a bit of thought and observation, here is my summary of the whole thing:

1. Dodie's pulleys, both lower and upper really were worn out. They were not just slender, like on the new X%, they were almost gone. The fact that the upper pulley was also severely worn says that this was not a case of some sort of chain misalignment. It must just be that she had over 20,000 km on the derailleur!

2. The slender appearance of the pulleys on the X5 must come from the part being mainly advertised as being for 9 speed chain (though the fine print says 7 and 8 speed too). The X4 is advertised as being for 8 speed, and it has slightly wider pulley teeth.

3. The two hole mount for the upper pulley is a feature of the X4, so REI sold us a pulley set for the wrong derailleur. I know about the X4 because as another part of getting ready for the next trip, I did a cleanup in the bicycle section of our workshop. Up popped a new SRAM X4 derailleur that I forgot I had! It would have worked fine for Dodie. Grrr.

Pulley set from a SRAM X4. The upper pulley (right) mounts by two off centre holes, and could never fit Dodie's X5.

Observant readers of this note will realize that on this little topic of a pulleys we consulted six or more shops in two countries, and finally found what would have been a solution in a drawer at home. This assumes there was even a problem in the first place. Clearly we have too much time between trips and better get out of here before we dream up any more goose chases!

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