Part of being a sustainable bicycle component is having a long life span, or at least having a long life span worked into the design. Failure analysis is an important aspect of the design process in order to gain long life span. For parts or components that undergo load, a good design process usually involves failure analysis after the testing phase. A lot of this happens in the bicycle industry. Many bicycle and component manufactures have an entire branch of their company that tests their product to a point of failure, learns from that failure, and works that knowledge into the continued design. If the company does not have a testing department, they contract the testing out to a third party such as EFBe. Another important reason that frames and components are tested is to make sure they meet certain safety standards, or performance standards. This kind of testing usually doesn’t lead to failure.
Cycling is a good place to find failures after the product has hit the shelves. The reason for this is that the weight of the final product is an important design parameter. So important that vast amounts of research go into removing mere grams off of components. Jobst Brandt said it well: “Parts which give reliable service are often considered “overbuilt” and are redesigned to save a few grams.” Unfortunately other design parameters are sacrificed to obtain desired weight. These can include ultimate strength, fatigue strength, and lifespan. Visit Jobst’s article Some Bike Failures to see more photos of failures and some more discussion as to the reason why bike parts fail.
Failures are seen on just about every part of a bicycle, but one of the most common failures that I see on a regular basis as a mechanic is pitting of bearing races such as this one:
This pitting can happen when abrasives contaminate the grease surrounding the ball bearing and race. Put simply, a ball bearing is sandwiched in between two smooth surfaces that are parallel to each other. These surfaces are called “races”. Since it is almost impossible to get the two races perfectly parallel, the ball bearing is always a little bit closer to the races at a single point in its rotation around the surfaces. The space between the ball and race is filled with grease, so when this grease is contaminated with abrasives, the abrasives get compressed against the race and the ball at the point in the rotation where the ball is closest to the race. When this squeeze happens under load, with thousands, upon thousands of rotations, it causes the pitting you see in the picture. This is a common failure and is the result of poor maintenance and poor bearing adjustment. With proper maintenance and adjustment, bearing systems of this type will last longer than their modern brethren.
Modern bearing systems are usually sealed. This means that the two races and ball bearings come in a neat adjustment-free package that just needs to be pressed and seated into a recess, like this:
You can see two metal rings. These are the races. The balls are under that blue piece of plastic. When these types of bearings get contaminated and go bad, you remove them, throw them in the recycle bin, and put in new ones. To maximize their life span they must be installed correctly (parallel and seated), and carefully wiped clean every now and then. You want to be careful as to not push contaminants beyond the plastic seal.
Once in awhile I’m blessed with an uncommon failure. Recently I was working on a Masi Gran Criterium that was upgraded in 1985 to full Campagnolo Super Record components. The rear derailer (1984 Super Record) had a small crack in its outer cage:
This is a strange spot for a crack failure since this part of a bike is under minimal load. Since the crack lies at the back of the cage, then the maximum load scenario would be when the chain is in the big-big combo. This would place the tension spring at its maximum potential to compensate for the lack of slack in the chain. In other words, the derailer would look like this:
When the derailer is in this position, the tension spring is counteracting the chain by trying to pull the tension pulley back to its neutral position. This puts tension on the back surface of the pulley cages. If you have trouble visualizing this, imagine the bottom surface of a beam under load. This surface is in tension while the top surface is in compression. In this scenario the back of the cage is in tension and the front of the cage is in compression.
Now, I don’t think that this is the only factor that caused the failure. In the picture of the cracked cage, you see a nut recessed into the cage where the spring cage bolt screws into. When the bolt is tight up againast that nut then there is a portion of the cage that gets compressed. The cage material is a very lightweight aluminum alloy, and when compressed with a recessed nut that has 90 degree angles for edges it will develop microscopic cracks. Points like this are called stress risers. To get a better look, I pulled the cage apart at the crack:
The thin part of metal that is below the nut seat in the above picture is compressed when the derailer is in use. The angle between this section of material and the non-compressed material is around 90 degrees. At the vertex of that angle, the compressed material is being pulled away from the non-compressed material forming a high stress point which, over time, will develop microscopic cracks, especially in lightweight aluminum alloy.
Add one more factor to the mix, and it all becomes clear. Material flaw. There is a good chance that this part may have had a flaw on its back outersurface, such as impurity or void. I didn’t see any evidence of one, but I don’t have a scanning electron microscope at my disposal, so I may never know. It is, however, likely.
Design flaw, material flaw, and one too many times in the big-big combo sent this derailer cage to the graveyard. The problem is I don’t know the history of the bike, the rider, or when the crack developed (my customer just bought the bike), so I don’t know if it had a long life or not. I do know one thing. The bike it was on did not get ridden all that much. This crack may have developed in 1985 for all we know.












