Why do crank arms just fall off?

There are few things more terrifying to a mechanic than the sight of a returning, scowling customer whose bike was recently serviced in your shop. If they are in because the bike was recently serviced somewhere else, there is a brief moment of relief, followed by the realization of how that injury to your trade’s reputation is also keeping your paycheck down. Bicycles may suffer from any number of maladies that result of normal wear-and-tear, crashes, manufacturer defects, and poor mechanical work. You, as a bicycle mechanic, have little control over those first few problems, but the last is finally in all of our collective hands.

Does your bike shop have a selection of crank arms in various configurations, colors, and lengths to sell to customers at the apparently inevitable moment that the tapered interface inside the arm rounds out? Most shops seem to have this. Why do taper-fit spindles wear out and go wobbly? Most of the time, the responsibility lies with the last person to put a wrench on that bike.

Cranks with any sort of tapered interface at the spindle (I am talking about any square taper, Octalink, Powerspline, Isis, et cetera) require quite a bit of force to press them together enough to keep them in place. Bike gurus disagree about whether to grease or not grease the spindle/arm interface, If you chose to grease, it does not take much. Don’t paste the spindle like your are icing a cake. Install dry or just smear a thin film on there.

Now, torque. As I said the other day, working on bicycles is a technique, not finger-painting. Engineers designed these- people who probably have a much better understanding the physical properties of the materials you are handling know exactly how crank arms and spindles fit together in such a way as to maximize the effectiveness and life of that unit. Better yet, just like the Holy Hand Grenade of Antioch, it’s written down somewhere in precise, painstaking detail! I have found it handy to research and compile my own list of common torque specs for references and you should too. Square interfaces are usually in the neighborhood of 30 foot-pounds and splined ones are little higher than that. Make your own list, tape it to the wall near your work station or keep it in a handy binder.

Using a the right tool for the job, like I said the other day, is what separates bicycle technicians from sorcerers. This is why a good, calibrated torque wrench is an essential with which no compromise can be made. No one can consistently gauge proper torque on any fitting using their wrist alone. You need a scientific tool to verify that you reached that value, not above, and not below. If you think you can hit the right torque consistently with your wrist, you are mistaken and you are gambling with other people’s bike’s and their safety, not to mention the reputation of your shop and your industry and trade in general. You are effectively sabotaging that bike. Your customers will sense this when their bike fails and they remember whose hands and tools were on it last.

Remember that box of crank arms in your shop? Its presence is a sign that someone is not doing their job right. It is possible that replacement crank arms are needed when a bicycle is ridden for thousands of miles and the materials just fatigue, or the crankset was just poorly made, but those are rare occurrences. Let’s address the ones we can affect.

The failure can most likely be traced back to the assembler at the department store who is paid to crank bikes onto the sales floor as fast as possible, or the bike shop mechanic who touched the bike last before selling it to a customer, or the bike shop mechanic who installed a replacement crank arm on it, or the home mechanic who at least has an excuse not to pay attention to torque specs because he/she is not being paid to do so. The component didn't fail, the installer did. Regardless, almost every time I see a loose, damaged crank arm, it is because someone failed to correctly install it in the first place.

Sure, the shop makes a few bucks selling someone a crank arm and installing a new one, assuming you can convince the customer that the damage was not your shop’s fault (which you might believe, although it’s probably not true). But the value of installing a part like a crank arm is multifaceted:

• Riders whose crank arms don’t fall off are probably going to enjoy cycling, ride more, and buy more parts and services from you.

• Riders whose crank arms do fall off have learned not to trust bicycle mechanics, and by extension, you.
• Correct installation of parts protects your shop, to some degree, against liability. If you can guarantee that your torque wrench was recently calibrated and that you actually use it, problems like a crank arm falling off is the fault of manufacturer or the rider.

Most importantly, the long-term value of a customer’s trust is absolutely priceless. You can’t win them all, but the harder we all work to raise the standards for our trade, the more we can effectively serve customers who value and trust our services.

So make that pile of crank arms a thing of the past. You should not need them. With proper tools and techniques and getting your staff on board with using them like proud technicians will ensure that everyone trusts that you did the best job possible to keep everyone spinning confidently.

Enjoy your ride.