[EDITOR'S NOTE: So many of the responses to our tikit stem safety issue surprised us, none more than an email from Richard King, an engineer with expertise in failure analysis and stress analysis. Richard not only offered assistance in reviewing our work, he flew into Eugene at his own expense. Here is his independent review of our efforts.]
My Review of Bike Friday’s Response to tikit Stem Failures
By Richard King, Ph.D, PE
I'm a mechanical engineer with a Ph.D in engineering and 30 years experience. I have specific expertise in failure analysis and stress analysis, including stress analysis by computer with the finite element method. I had always wanted to visit Eugene and see the Bike Friday facility, Hayward Field and other sights. So since I was only a short plane hop away, when I saw the announcement about problems with the tikit on their website, I thought now would be a good time to visit and see if I could help with this problem.
My first impression was that everyone at Bike Friday sincerely wanted to fix this problem right, to “do the right thing,” no cutting corners, solve the problem and get a fix ASAP to the customers.
Bike Friday Line Master Peter Kaspar put together a compact test stand to allow fatigue testing of the stems, and testing of newly designed stems intended to fix the problem. By applying a severe overload in the fatigue tests, it was possible to cause failures in a couple of hours that reproduced how the parts had failed in the field.
ABOVE: Peter Kaspar's testing stand.
Bike Friday Head Engineer Rob English, a brilliant mechanical engineer, was then able to examine these failures and determine what had gone wrong. He then quickly came up with a design that solved the problem. Based on the accelerated testing it appears this part will perform well in the field and will essentially, from a fatigue standpoint, “last forever.”
Above: The new design on the left is one piece, replacing the old two-piece system.
As a side note, when I worked for the Bureau of Standards (now National Institute of Science and Technology), we had versions of Peter’s test stand that cost hundreds of thousands of dollars. That wasn’t a waste of taxpayers money, as these could apply loads of hundreds of tons, with sophisticated control and instrumentation (and were used for important applications like verifying the Alaska pipeline was safe). But it was still incredible to me that Peter could quickly cook up a small scale version of the same type of apparatus in a short time.
Green Gear is a small company with a small engineering staff. The promptness and competence with which they handled this problem is nothing short of amazing to me. From a qualitative standpoint, they did everything right, and I am highly confident that the new design will fix the problem.
The only thing missing is some numerical “icing on the cake.” I intend to do a quick fatigue analysis based on the test results to show that the fatigue life of the new design is more than adequate. I will also do a finite element stress analysis of the part which may give a little more insight into the part.
My overall impression: I cannot imagine how Green Gear could have handled this problem any better. They identified and analyzed the problem, and came up with a good fix, in a remarkably short time.