Chainlink Tech Corner: Causes for Bearing Failure

Article and Photos by Scott Wilson

This here is an original Shimano bottom bracket spindle off a 1980's Cannondale:

Or rather, that's the diagram, this is the spindle:

This particular spindle is unique because the bearing track on one side is completely worn out while the other is just fine. What does it mean to be worn out? It means that the bearings have created little divots and stress fractures on the "cone" sometimes referred to as the “tapered race” which is the track the bearings run on. This is caused by friction. Normally the bearings swim in a world of grease, creating a near frictionless state on the highly polished cone surface. But if the grease washes out, the bearings are adjusted too tightly against the taper, or a foreign element like sand gets in the mix, a little wear spot will emerge right at the contact patch. Over time, friction and heat caused by the small imperfection will deform the metal, making the race go from this:

To this:

Those striations at the angle are what I'm talking about. They look small, but any tiny pimple is amplified to a boil when bearings are pressed against it.

Here's a different angle. First the good end:

And then the bad end:

In addition to feeling crunchy when pedaling and creating extra drag, a worn out bearing race will eventually chip and cause the bearings to jam. Even worse, the added friction from all those bumps will cause the ball bearings to deform, and eventually break, causing whatever component it’s a part of to lock up. This can happen to the bottom bracket, the headset, and most frequently, the wheel hubs.

How do you know your races are kaput? You'll be able to feel it in your fingers by spinning the component. In extreme cases you can feel a grinding through the bike itself, as well as a noise kind of like a bunch of very small bowling balls being thrown down an endless alley. Bearings should always be smooth as a dolphin on a water slide and quiet as a box of pillows.

If the bearings are new and feel crunchy they might be adjusted too tightly, or there might be a manufacturer’s defect.

Quality bearing assemblies, like Dura-Ace or Phil Wood, tend to be polished to a high degree and last longer. When your bearings go out (and they always do, everything eventually crumbles to dust) replace them with a high quality brand and have them installed with care and you'll be happier for it.

Scott Wilson is an MFA writing student at Columbia College as well as a seasoned professional bike mechanic. Scott’s “wrenching” experience includes bike shops, racing teams, and professional triathletes across the US. The aim of Scott’s technical articles is to explain in detail how bicycles and their individual components work...and in doing so, help you keep your own bikes running better and lasting longer.

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Comment by Scott Chillson on April 30, 2015 at 3:21pm

I should also mention that bearings raaaaaarely break apart. I've only seen it twice on a bicycle: once was on a Wal-mart bike, the other time was on a bike from the 1950s. Also, poorly made bearing assemblies sometimes come pre-ruined, as it were, because the cone is machined unevenly. They still last a few years, but if I were a bike company I wouldn't stand for it (but I might not get rich like that either).

Comment by Steve Weeks on April 30, 2015 at 11:10am

Bearings can also wear from being too loose; this permits the rotating element to overload a smaller number of balls than would normally contact the race. When the load is supported by fewer than the optimum number of balls, the excessive pressure can cause direct damage to the bearing surfaces even in the presence of lubrication.

Ball bearings (as I understand them) are intended to be run with zero free play and zero "pre-load"... this is a tricky state of affairs to obtain, but it can be done.

If the bearings have the balls contained in "cages" (caged ball bearings), it is generally possible to increase the number by eliminating the cage. I've found for a typical 8-ball cage, it is possible to install 11 uncaged balls, which is about a 38% increase. Assembly relies on the grease to hold the balls in place; this involves more work (though not much), which is why cages are used more in high-volume production bearings. Notice that the image of the Shimano bearings above does not show cages. I have Shimano "Ultegra" bearings on a road bike... no cages, and a full complement of bearings. These bearings show no wear after 7,000 miles (2 services), and as SSSB says, these bearings will probably outlive me.

Steve

Comment by Far'arned Retrogrumpalunkus #63 on April 30, 2015 at 6:42am

Nice write-up.

I would only disagree with the statement that bearings "always do" wear out, on a bicycle that is.  Bicycles see very little stress on their bearings under normal operating conditions.  Because of the extremely harsh conditions some bikes are exposed to, and a lack of basic preventative maintenance done by most bike owners, the sad fact is that the grease that protects bearings often gets contaminated and/or washed out with water and is never cleaned or replenished until it is much too late.  Often the only time they get any attention at all is after they begin to fail.

Bike designers know this and bike bearings are made much stronger (larger) than really would be necessary so that they have a chance of surviving a little bit longer with the nearly inevitable contaminated grease and minimal maintenance.  

But because of this over-engineering a bearing on a bicycle that got regularly serviced, with a cleaning and fresh grease well before any appreciable contamination built up, would be practically immortal.  Even a lower-end bearing that was not as highly polished would last nearly indefinitely.  The number of miles it would take to wear out bearings that only ever saw clean fresh grease would be well in excess of what 99% of riders do in their lifetimes.

Folks that tear apart their bikeikes and clean/regrease the bearings on a schedule that keeps ahead of significant contamination, will probably get near indefinite bearing life on a road bicycle.  Once a year servicing works for me.  The bearing grease is still fairly clean and free of contamination or water ingress.  

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