Manic Mechanic – Crank compatibility and reverse threads

Sticker Shock

I have acquired a used Shimano XT770 crankset. The big ring is trashed and I am going into sticker shock at Shimano’s replacement prices. Anyone have luck with other brands i.e., Blackspire, FSA, Race Face for this particular model? I have a hard time throwing down $60-$100 for a chainring.

I sympathize with your plight. Shimano does seem to subscribe to the same OE replacement parts pricing as car dealerships and national defense contractors. The big ring on an XT770 series crankset actually has a 104mm 4-bolt pattern, the same pattern used by most other brands, so other rings will match up, but be careful. Much in the same way you can’t imagine being married to a unicorn or living in a world where dogs can drive, Shimano can’t conceive of a world in which people buy FSA or Blackspire rings and bolt them to their XT cranksets.

If you know the history of Shimano chainrings over the past few years, you’ll appreciate what a miracle it is that even the bolt pattern happens to match now. So the good news is that nearly everyone makes an aftermarket 104mm BCD, 4-bolt ring that will match up to the bolt hole pattern on your XT770 crankset. The bad news is that it won’t work as well as the Shimano. This is partly because Shimano just makes excellent cranksets and chainrings, but mostly because all Shimano parts are designed specifically to work only with other Shimano parts. They don’t intentionally prevent other rings from bolting on, but they don’t test for them either. This means that, even if something else can technically bolt up, it probably won’t mesh all pro-like to the crankset’s spider—it will most likely be slightly too wide or too narrow, and some rings may even need sections filed down if they’re making contact with the Shimano spider or arm itself.

Generally, a Shimano crankset with another company’s ring on it also won’t shift as well, because chains, chainrings and cranksets are one of the things Shimano still does extremely well. So it comes down to you: if you’re the ultra anal-retentive type who demands perfection, you should shell out for the Shimano ring. If, on the other hand, you’re more the frugal rebel type who can smile while pressing a shifter paddle a bit harder to convince the chain to shift into a less expensive big ring, look for the most basic-looking 4-bolt, 104mm BCD ring you can find and get your rogue on.

Reverse Threaded?

I was trying to install a new pair of clipless pedals on my wife’s bike and after 15 minutes of cussing I finally realized one side was reverse threaded. Why is this?

You have encountered the left-handed threading of non-drive side pedals. All proper bicycle pedals and crank arms are designed this way. Theories for this phenomenon abound. A compelling argument exists for the theory that this design keeps the combined weight of the rider and pedaling motion from being able to loosen the pedal and allow it to damage the threads of the crank arm, or even unscrew completely off the arm. The term for this particular stress applied to rotating objects is “precession,” and I’ll leave the definition of that word to much smarter men like Jobst Brandt and our much missed Sheldon Brown.

But even if it looks odd, there’s an actual engineering requirement behind the design, just like the Large Hadron Collider or Donald Trump’s “hair.” I’ve given you the logical answer, but I know it fails to address the frustration you felt in discovering this, so here’s a bonus irrational explanation that, while completely inaccurate, should make you feel better: the Knights Templar did it. Some secret French guy designed bicycles this way just to frustrate Americans. There is, in fact, an underground faux-quasi-neo-Illuminati organization at work within the mechanical designs of things all around us. They’re not only responsible for left-handed threading on nondrive bicycle crank arms, but far more insidious things, too, like cars with the steering wheel on the wrong side, or the metric system.

Every year, hundreds of red-blooded American men like you and me do our part to shake off the shackles of this secretive organization by destroying non-drive crank arm threads or rounding off the head of a metric bolt with an English wrench. Your wife might not understand your struggle, but we do. Looking like an idiot for 15 minutes (we both know it was probably closer to an hour) is a small price to pay for battling evil itself. Respect.

About the mechanic

Chris Currie is our manic mechanic. Have questions about life, love and mountain bikes…or maybe just bikes? Send your letters to [email protected]. You can also read his tech blog at CanooterValve.com.

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