The few cases of fatigue cracks that we are aware of to date have started at the small notch in the box frame at the end of the bend under the foot stretcher. This notch was necessary to prevent the metal from tearing when the box frame was being bent into shape. Our pre-production testing of this part showed that we had adequate strength to avoid a fatigue problem. In hindsight this testing was not quite enough since at least in certain loading situations the frame has not been strong enough.
When we first became aware of a failure, we realized we had to devise a better test that could duplicate this fatigue crack we were seeing in a relatively short time. In order to get results quickly we had to both speed up the cycle rate and use a very heavy load. The test consists of a rapidly alternating load of 600 lbs (275kg) on the rail near the foot stretcher. With this test we started seeing a fatigue crack somewhere between one and four hours. Fatigue is a fairly random process so each part will be different.
Once we were able to duplicate a fatigue crack, we then tested possible solutions. Ideally, the solution needed to be applicable to both new production indoor rowers as well as indoor rowers that were already shipped. And for the units already shipped, we needed a reinforcement kit could be easily installed by the customer. After a number of dead ends, we designed a kit that exceeds the criteria. We installed the reinforcement on a box frame that had developed 2cm cracks and had it go 60 hours with the 600lb fatigue test without any further cracking. At 60 hours we had to stop the test because the box frame was breaking up in other places where the design is the same as on the 14 year old Model C that we have never seen fail.
We used this method of reinforcement on new production indoor rowers while working on phasing in a box frame that did not need this reinforcement. This design is also required to outlast the 600lb/60hr fatigue test.
We also conducted max load testing on the frame. This consisted of loading the indoor rower on the rail near the foot stretcher with as much load as we could until something failed. The results might be a surprise to some people longing for the "sturdy old Model C"—the Model C failed at 700lbs. The mode of failure was the monorail hanger bolts plowing their way through the rail. The Model D and E shipped between August 2006 and April 2007 failed at 1000lbs. The mode of failure was buckling along the top edge of the box frame between the foot stretchers. Note that the failure is not at the notch where we had seen the fatigue cracks. The D and E monorail connection was redesigned from the C to get the additional strength. The Model D and E with the reinforcement kit installed did not fail at 1800lbs.
The last part of this process was how to roll it out to customers. To tell you the truth, the cheapest and easiest way would be to quietly eliminate the possibility on new units, do nothing about the shipped units, and just replace the few that fail in the field under warranty. This would be a viable possibility since this type of failure does not involve physical danger. In fact, most of the indoor rowers that might eventually fail would likely fail beyond the 5 year frame warranty. The problem with this solution is that it could damage our reputation and it does not agree with our company mission. The applicable part of the mission reads: "To design, sell, and service unique products of high quality and value for the benefit of the Concept2 community including customers, employees, families, suppliers, and the local community."
So we decided the best thing to do was to be proactive and eliminate the possibility of this problem occurring by offering a free reinforcing kit to all customers with affected rowers. We consulted a PR expert who said we would be lucky to get 5% of customers to install the fix. We are hoping we can get 20 times that rate.
Dick and Peter Dreissigacker, Concept2 founders