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The Case of the Spurious Spring
Replacement kickstand part causes scary spill on modified motorcycle
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, April 13, 2009
A young man took his motorcycle for a spin. He started to navigate a gentle left-hand turn (about a 1,000 ft radius), lost control of the bike and piled up. He was injured badly enough to sue. I was retained by counsel for the bike manufacturer.
The Investigation
The spring on the kickstand was found to be missing after the accident. The operator claimed the absence of the spring caused the stand to drop down and distract him, causing the accident. The configuration of the motorcycle was such that the end of the dangling stand could not touch the surface of the road.
The bike had gone through several owners and was extensively modified prior to the accident. The plaintiff claimed to have replaced the spring a couple of days before the accident.
The figure, above right, shows an exemplar spring obtained from a dealer. The hooks are separate pieces that are held inside the tapered ends of the spring by so-called spades. The three-piece construction is clearly superior to a one-piece spring.
The stress in the spring depends crucially on the number of coils and diameter of the wire. In later versions, the number of coils were increased and a lighter wire used. Both changes would make for a longer spring life. The bike manufacturer claimed to have had no trouble with spring fracture; however, the number of springs being sold and design modifications suggested otherwise.
Structural parts subjected to cyclic stresses must be designed to withstand so-called fatigue loading. Design may be for high-cycle fatigue or low-cycle fatigue. Engine parts such as connecting rods must withstand many millions of cycles, which constitutes high-cycle fatigue. Many other parts are subjected to so-called low-cycle fatigue, which involves only hundreds or thousands of stress cycles. Low-cycle fatigue loading is intermediate in severity between static loading and high-cycle fatigue.
It was fairly easy to calculate the stresses in the spring from the spring geometry and extension during use. Calculations showed the spring to be suitable for low-cycle use, but not high cycle. The kick-stand had only been used a few times between the installation of the new spring and the accident, which is clearly very low-cycle fatigue.
It was entirely possible we were barking up the wrong tree in examining the springs from the manufacturer. There was no record of the plaintiff having bought a spring from the local dealer. The plaintiff's father said his son got the replacement out of a box of springs.
The Smoking Gun
The father's comment rang a bell with me. I kept old cars running for many years and still do much of the maintenance and repairs on my house. I have several boxes labeled "doodads" which contain various bits of hardware that may be of some future use. Faced with a missing spring I would have grabbed some kind of a replacement and tried to make it work. I suspect the plaintiff did something similar.
I reported my findings to my client, but never testified. My client reported he won on a matter of law rather than evidence, but did not give me any details. The plaintiff appealed the verdict and my client expected the appeal to be upheld and for the case to go to trial. This was the last I heard of the case, which I suspect was settled out of court for a small amount.
This kickstand spring, obtained from a motorcycle dealer, shows what the spring on the young man
