Tuesday, August 10, 2010

Crank Brothers Candy C Pedal Failure : 2

Those Chinese made Crank Brothers Candy C pedals, whose flaws we discussed in an earlier post, has obtained another victim. The pedal failed in the same area again. A reader, who describes himself as a relatively non-litigious person, emailed me his story with pics of his injuries along with the broken pedal spindle. He describes this as a serious design flaw and wrote that had he crashed while going downhill at speed instead of the top of the hill he was riding on, he may not have lived to tell the tale.

Some good people had noticed his crash and rushed to get him to a hospital. There, he would find out that he had broken his right clavicle in two places, not to mention extensive bruises and abrasions. "It hurts like hell, inspite of Vicadin", he wrote. The healing process takes 8 weeks, or 2 months. How would you feel if your summer was ruined like this? We do hope him a quick recovery.

Fatigue is the biggest limiter of products whose design goal is to take cyclic loads and is often the prime reason for failure. Overlooking the design or manufacture of such items to combat accumulated fatigue leads to reduction of its life prematurely, that may result in serious injury or death. A rough back of the envelope calculation of mean and alternating stresses in a pedal spindle was done by me in the previous post for perspective.

Simply relying on safety factors alone does not take care of this important issue. The synergistic effects of product design geometry, heat treatment, loads imposed, environment, residual stress and time can all reduce the lifetime of a product. It is common practice to see companies show off their computer skills with exquisitely colorful FEA. This is not the end to the fatigue design process either. Verification has to be absolutely thorough. If you don't test the item, which is for public use, by simulating real world conditions, you can be held guilty of negligence.

This is the flowchart that the SAE likes to follow for designing against fatigue. Observe that the word "modify" shows up in three different spots.

How safe do you want to be? I hear that something like one major air accident everyday is considered a 99% safety rate as far as air travel goes. Anyway, the company Crank Brothers has babysat the individual in question with a new pair of Candy C's. Everything is safe. Don't you worry, here's a band-aid. This was just another outlier.

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