Wednesday, November 17, 2010

A Broken Rim Mystery

It was a dark and stormy night...

Really, it was. It was a Monday evening in November. The wind was whipping the orange and yellow leaves off the trees and the wind was howling up the valley. The lights in the shop had flickered ominously several times, but it so far the power was still holding and the phones were still working. It was the kind of night where I'm glad that my current commute is only four blocks long.

One service we don't offer at our bike shop is on the road pick up, but when the call came in the fellow on the other end of the line sounded so soaked, stranded and stuck I told him I'd see what we could do. As luck would have it Chris, one of our off-duty mechanics, lives not far from where this guy was broken down and Chris happened to be home, happened to answer the phone when I called him and happened to have a truck big enough to bring man and bike back into Issaquah on this particular dark and stormy night.

What stranded man and bike wasn't a routine puncture, it was a rim blowout. I've written previously about rim blowouts, but they are usually caused by the rim being thinned by braking. In this case, the rim failed at the rim wear indicator but this bike uses disk rather than rim brakes, so the rim should not be subject to any wear at the rim. The bike's owner tells me that this is the second rim failure he's had on this bike in the year or so that he's owned it. Rather than having his recumbent shop replace the rim again with the same thing, he's opting to have us rebuild the wheel with what we all hope will be a stronger rim. The new rim has no wear indicator, and stronger construction. Since the new rim has a different E.R.D., we're replacing all the spokes as well.

The mystery of why this bike is blowing out rims is still a mystery. I have theory, but I'm posting this story and a couple of photos here in the hopes that folks might say "oh, I've seen this before" or "I bet this is what's going on."

The bike is a Rans Cruz semi-recumbent and it has a Bionix electric rear wheel. The design of the bike puts a lot of weight on the back of the bike. The wheel weighs about 16 lbs and the battery (which the customer took with him and I didn't get a chance to weigh) rides on a rack right above the rear wheel. Most of the rider's weight is carried by the rear wheel and the electric motor puts out a lot of torque.

My best working theory (which is really just a wild-ass guess) is that the combined weight and torque is placing a greater stresses on the rim than what an unassisted rider would generate on a conventional bike. Unsprung weight over the rear wheel means that every bump transfers more pressure via the tire bead to the rim. The wear indicator on the existing rim not only serves no positive function on a bike with disk brakes, the groove of the wear indicator actually creates a stress riser in the rim. Over time, the combined stresses lead to a blowout.

So like a Stephen King mystery that I bothered me a lot when I read it but I still sticks in my head, I have a problem and a theory but no certain solution. We've got a rim and spokes on order and we'll lace 'em up, advise running a bit lower pressure in the tire and hope for the best. I guess that's how life goes. We never get all the answers but we get interesting questions.

Keep 'em rolling,

Kent "Mountain Turtle" Peterson
Issaquah WA USA
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