Monday, September 6, 2010

Safety Moment : Speed Wobble and Jaw Fracture

Will Cheng is an electrical engineering Ph.D. student at Stony Brook University here in NY. In their free time, he and his sister ride their bikes with a group of retiree friends in Long Island.

Early in August, Will met with a nasty bike accident during a pace-lined group ride that left him with a fractured jaw. He was rushed to the ER where doctors had to perform an 8 hour surgery on him to patch up the severe injury. Some who witnessed the accident had advised him to contact an attorney. Meanwhile, the medical and dental costs for the operation had been tallying up and taking uncomfortable proportions.

He contacted an attorney who recommended him to get in touch with a mechanical engineer who could look into the background of his cycling equipment.

At the time of the accident, Will happened to be riding a 2006 Orbea with Mavic Cosmos wheels fitted with Schwalbe Blizzard tires. I was informed by his sister that Mavic no longer makes the wheelset.  Will's own account of the accident later to her was as follows :

"I was on Clay Pitts road [in East Northport] and I moved up to second wheel in the pace line at the light on Elwood road. After pedaling for a while [at 20mph], I noticed that I was a little to the left of the shoulder. I corrected by moving the wheel a hair to the right so I was heading towards the white line slowly. I then turned the wheel back to the left to straighten out.

That's when I felt no resistance or feedback from the wheel and handle bars. I assumed that I went over a dip in the road and recall crying "WHOA" and thinking that the leader should have warned us. This was followed  by a gross turn of the wheel to the left. I panicked, simultaneously turning the wheel straight ahead and clipping out my left foot.

I felt the bike wobble a little, after which it diminished and stopped. At that point I thought I was OK but a split second later I felt something was wrong. Before I could do anything, I was falling. I do remember that when the wobble disappeared, I was staring at my handle bars and saw that it was straight without signs of the wobble. I thought I was safe and I looked back up at the road. I don't really remember much, but I think I still had my hands on the handle bar right when I hit the ground with my chin. I didn't have a death grip on the bars but my grip was firm and my hands were always on the top of the bars."

As far as I have looked into the wheelset through some internet searching, I haven't found any design related issues and its performance limiter really depends upon who built it. In more cases than not, a crash is what causes a wheel failure. At other times, it is fatigue failure or some very high external load not expected in normal usage of a bicycle that causes spokes to pull through. Wheel experts say something in excess of 2000 N of force is required to pull a spoke out of the rim.

From the attached pictures (see below, and more here), it seems that about 5-6 of the straight pull spokes in total had pulled out and that more spokes pulled out on nut side of the front wheel skewer than on the lever side. This corresponds with weakening and rupture of the wheel rim on the left side, when viewed from the front. Also take note that the rupture occurred right underneath a sticker on the rim so its hard to tell whether there was a hidden crack formation well before the accident.

From the description of the actions of the rider before the accident, I don't see anything particularly out of the ordinary. Steering motions such as this is absolutely normal and is to be expected. I perform more wilder maneuvers on my bike path in order to avoid sharp twigs and bumps.

What may be significant though, is the faint evidence of a speed wobble before the crash. Could a rapid left-right steering correction at 20 mph together with a sketchy road surface amplify an unwanted oscillation? Check out the image of the site of the accident.

Moreover, what gave away first - the spokes or the rim? Another bit of interesting testimony is the loss of "feedback" just before the crash, which almost wants me to question whether Will had remembered tightening his skewer that day.

Unfortunately, these questions are really hard to answer through images. I would check the tension on the spokes with a tensionometer, consult with a metallurgist who would be able to analyze the sample of broken aluminum rim (Stony Brook should have a professor who may help) and try as much as possible to take a similar wheel with the same tire, attach it to the same bike and perform some maneuvers at the speed in question.

Here's wishing Will the best of luck in recovery. Meanwhile, if any of you have had similar experiences, do share some of your thoughts.

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