Wednesday, March 25, 2009

10 Design Case Study : Thomson Elite Seatpost

Readers will remember that a couple of days back, I ran a controversial story of a Thomson Elite seatpost that broke off at one of the bolting ears due to a "fall". I placed known marketing information about the product alongside the given failure situation. Why some readers were irked is hardly surprising, since the Elite is widely considered as one of the best designs in the market. For many, it has provided years of faithful service.

Did you know that the Elite seatpost has an elliptical bore?


So what has made this seat post so popular? It must be in the design, right? Please see the latest update to my previous post. I go through most of the design features of the seat post and comment on its attributes with the help of high resolution photos. I also run through the product manual and high light some of the factual installation warnings that Thomson has made clear pretty upfront. A seat post failure while riding can be absolutely dangerous, no question about that. Hence, installation directions and warnings cannot be taken for granted and ignored however mundane reading them maybe.

See you there.


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10 comments:

  1. Anonymous4:15 AM

    Great writeup. What are those marks on the tube?

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  2. Given the sudden failure, and the fact that the seatpost is 4 years old I think it's a case of fatigue. Aluminum does not have a fatigue limit, so no matter how strong they make the post to test loads fatigue can still be an issue.

    Close ups of the crack surfaces would help verify or disqualify this.

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  3. Hocam, you're right. Aluminum will succumb to fatigue cracking and subsequent failure if you stress it enough times in cyclic loading.

    Here's an animation of the stress-strain curve as a 7000 series specimen in loaded in tension.

    So perhaps depending on how much you ride, the average lifespan of a Thomson seatpost is about 4-5 years.

    However, these limits can be calculated upfront in the design stage. What I feel is often overlooked is how the manufacturing processes affect the fatigue life of the aluminum part. By processes I mean machining, heat treating, strain hardening etc. Much of the knowledge of what works and what doesn't comes out of experience down on the shop floor. Any comments on that?

    Thomson designers in this case are aware of this particular limitation of aluminum and added in an extra factor of safety (or overbuilt it) to cushion against catastrophic failure. They use a special term for this called "bending fuse". In my opinion, I see this bending fuse idea as nothing more than that extra safety factor, which is nothing new and has been known in engineering since the Byzantine empire.

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  4. Anon at 4:15 : I'm assuming those marks came out of Quality Control at Thomson. They could be indentation marks to test a mechanical property, like say hardness.

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  5. Liz, UK5:28 PM

    I just visited your blog for the first time this afternoon. Pretty impressed with the sheer volume of information presented here. I think your choice of the cup of coffee on the top is pretty apt! I think I need a lot.

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  6. Ron,

    I have two Thomson offset seatposts. I use them because I'm a fully shaped guy and have to get back without going to a larger frame size because of my inseam sizing for my bikes.

    I'm a big guy. 190. I've been using these posts for years and no problem.

    There are two things that come to mind for me when it comes to pipe and metals getting brittle. Heat and or chemicals.

    Was the post exposed to either of these over the four years of life?

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  7. That should read... Funny shaped guy... but also fully at 190 I guess.

    -B

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  8. I am sorry but I don't buy the story about how the seat post was damaged. I think some one omitted some details.

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  9. I'm with the cowboy on that one Ron. I've been riding bikes for a lot of years and have put a lot of km on the gear and have never had a failure.

    Cranked too much on one side instead of loosening the other when trying to level the seat maybe?

    Something is not right here. Your talking about a machine shop that invented a post and stem system. not someone who had an idea and hired a factory to build it the cheapest way they could.

    It's not like exploding spokes. How many of these have you ever heard of?

    -B

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  10. Anonymous4:39 AM

    If it came back to the company I am working in, the first thing I would check would be the materials in the post with a keen interest in the grain structure.

    I've now been reading this for some time and I am increasingly getting ever disappointed that there has been no request, mention, or inspection of the material used to make this post.

    As if we should assume that just because this was made in the US the materials are perfect.

    - Ryan

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Thank you. I read every single comment.