Bicycle Frame Fatigue Testing

February 7, 2009

in Engineering

We do a lot of testing at Seven Cycles:  fatigue testing, destructive testing, product testing, real world testing, and some test riding once in a while. Even our sponsored riders are expected to perform product testing; it’s a tough life.

One of the more interesting testing tools we employ is our fatigue testing machine—named Tessa.  Not many bike companies have in-house fatigue testing equipment.  In fact, I don’t know of any US companies of similar size to Seven that have invested in designing full fatigue testing equipment that can cover the breath of testing that we do at Seven; although there may a company or two that has.

If you’ve never seen a fatigue testing machine in action—bicycle or otherwise—it’s a pretty cool sight. I have some video of our tester in action; I’ll post it once I figure out how to edit and transfer it…  Most fatigue testing machines are designed to accelerate fatigue failure results. In other words, the equipment usually pushes the stressed part to extremes.  Seeing a bike frame on a test apparatus is impressive;

every cycle of the test looks like the frame should be failing at any moment.  It makes you want to wince.  Eventually you get used to this feeling but it is a testament to the durability of bicycle frame design that a typical frame can handle so much abuse.

Our testing apparatus lives in our machining area so our frame builders get a little tired of the metronome like sound of the pistons, “pfffft, pfffft, pfffft”, again and again for 100,000 cycles at a time.

German Standards

The testing standards for the high-end bicycle industry are developed by the German Institute for Standardization—Deutsches Insitut für Normung—the term “DIN standard” is the more common reference.

As I’ve mentioned in another post, Seven invests significant resources in testing.  Not only was our fatigue tester the single most expensive fixture we’ve designed and fabricated, to date, but we also invest in third party testing, and testing from some of our material supplier-partners.  Testing that includes tensile testing, chemical testing, as well as fatigue testing.

Multipurpose

We designed Tessa from the ground up not only to perform DIN testing to the letter, but also:

  • to perform fatigue testing on raw materials:  running tests on materials from every supplier with which we work—including some that we don’t.  Steel, titanium, carbon, aluminum, and even a few hybrid materials.
  • to fatigue test welding, brazing, and bonding:  raw material strength is useless if the joining methodology is faulty.  Joint geometry, transition, and fusion all play critical roles in joint fatigue endurance.
  • to test material finishes:  paint, anodizing, shot peening, polishing, etc.  It’s amazing the role that surface texture plays in the service life of a part.
  • to fatigue test every possible other product Seven offers:  or ever will offer; forks, seat posts, stems, handlebars, and a few other items I’m not going to mention.

When we set out to design Tessa, we knew it would be a challenge to encompass this diverse list of requirements.  It was worth the effort.  Given the research we did before building the machine, we think the apparatus is unique because of its versatility, within the bike industry.

Part II

We’re now in the midst of the second phase of the Tessa project.

Even the DIN standard, while important for the industry, is not 100% realistic.  Much like finite element analysis, looks can be deceiving and create false positives; test results do not necessarily match real world results.  As with any testing that’s not real world—“real world” basically consisting of multi-year riding over many thousands of miles—the testers have to make a series of tradeoffs.  Finite element analysis is a good example of this limitation, as I’ve discussed before.

So, Seven’s primary ongoing focus with Tessa is to continuously improve its relationship to real world bike riding.  Since we’ve got the DIN test well in hand, and all of our bikes far exceed that standard, we’re working on improving our fatigue methodology to be more and more realistic.   We won’t stop until we see results that consistently, and very accurately, reflect real world performance.  We expect to have our new standards fully implements by midyear.  I’ll post an update once we’ve codified the improved standard.

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