Fatigued mountain bike collapses on dismount, catapulting rider

The Big Bang Mountain Bike Race was on! The track included jumps, bumps and other rough terrain. About 36 so-called professionals went first, followed by a similar-size sport group. A gaggle of nearly 300 beginners, including the soon-to-be plaintiff, brought up the rear.

The accident occurred during a jump over an about 1-ft diameter log. Several witnesses saw the rider go over the log with his front wheel in the air. The front fork collapsed on the landing and the rider went headfirst into the ground. He lost most of his teeth and broke his back. Fortunately he was not paralyzed. The injured, or more properly his parents, sued.

The Investigation

The manufacturer of the front fork claimed it to be the “best mountain bike suspension made.” The 16-year-old rider weighed 200 lb, which is on the heavy side, but the fork should have been able to carry such a load over mountain terrain.

The figure above shows the bike crown and steer tube. The two legs of the fork slide into the large holes and are held in place by the crown bolts. Both legs had broken off very near where they enter the crown.

The various parties in the litigation agreed to a testing protocol that included scanning electron microscopy (SEM), optical microscopy, hardness testing and chemical analysis. The study was mostly done in the MIT labs of a colleague of mine. Over a dozen interested parties jammed the labs to witness the testing. The scene was a zoo.

Chemical analysis showed the legs to be of an aluminum-based alloy containing additions of copper, magnesium and zinc. Hardness tests showed this alloy had been heat treated to a high strength. Optical microscopy showed the fork material to be free of cracks, pores or other defects that could have led to premature failure. The legs had a thin anodized coating. In addition to aesthetic value, the coating would protect the legs against corrosive attack which combined with repeated fatigue loading could lead to very premature failure.

Fatigue is the prime suspect in any failure involving dynamic loading. The SEM is the tool to detect the striations on the fracture surface that prove fatigue failure. See “The Case of the Jelly Roll Blues.”

The Smoking Gun

Sure enough, both fracture surfaces showed pronounced fatigue striations. The failure mode became clear. Fatigue cracks started at the outer surfaces of the legs and proceeded across the fracture surface between a thousandth and millionth of an inch on each stress cycle. Finally, the legs were so weakened by the advancing cracks that they failed on the jump landing.

There was just one problem with the scenario. We expected the fatigue cracks to form as the front wheel hit bumps and proceed from the front of the bicycle to the rear. Instead the cracks proceeded inward from the sides of the bicycle, at a right angle to our expectations.

The mystery was solved by a senior colleague whose teenaged son raced mountain bikes. My friend laughed at my puzzlement and explained how the mountain bikers routinely turn the front wheel sideways to give better braking. Then, of course the fatigue striations would proceed in from the side, as we observed.

The bike in this case was clearly designed for mountain use. Engineering mechanics 2.01 (or common sense 1.001) tells us that the sideways braking is a whole lot more stressful than the straight-ahead kind. But, did the manufacturer design the bike for the sidewise braking? The sidewise braking clearly came under the doctrine of foreseeable misuse, even if the instructions recommend against it.

I know neither the size of the eventual settlement nor how the award was split between the several defendants.

Author Information:
Ken Russell ([email protected]) is professor emeritus of Metallurgy and Nuclear Engineering at MIT. He specializes in physical metallurgy, forensic metallurgy and failure analysis. Cases presented here are drawn from his actual forensic files.