Their website says :
"The Thomson seatpost design incorporates a bending fuse to prevent catastrophic failure. All brand-x seatposts we tested - every one of them - failed in catastrophic failure with the seat and clamp components - and sometimes pieces of the tube and head - flying off in all directions. This type of failure would dump the rider."And something about impact absorbing clamps :
"Impact absorbing clamps - clamps, head, and assembly will spread and flex on impact to protect seat, rails, seatpost and rider. Easy on seats - allows seat to survive heavy impact loads without bending rails."
The owner of the seatpost, however, had a contrasting experience. He wrote on his blog after the incident :
"Forget the hype! Despite the claim that their seatposts are over "40% stronger" on ultimate strength test than the strongest production seatposts on the market, the Thomson Elite seatpost is not tough enough to withstand even a simple fall. And here's proof of that! This four-year-old post on my GT broke in the upper tube area, which is apparently 'strong enough to withstand 350 foot-lbs of torque', when the bike took a tumble sideways while it was stationary. It didn't "bend slightly" on impact like what the Thomson folks had you believe would happen but just crumbled like a cookie at the top. Actually, the OEM seatpost that I was using before I got the Thomson in a moment of weight-weenie lunacy seemed to take much harder knocks! And it looks like it could take plenty more! So, if you've got an Elite seatpost on your bike, watch out mate! It could be a disaster waiting to happen. "
I'm not sure what went wrong at Thomson's end to cause the seatpost to be brittle, but I'm just going to have to drop this one in the "Marketing Mishaps" section for now. This doesn't mean your Elite seatpost has the same problem. But it won't hurt to be informed. Out of 1000 apples, its likely that 2 end up being bad and that's the nature of manufacturing. When those bad apples that dont meet the standards turn into the customer's hands, that's where the issue reveals its uglier side. If Thomson stands by their word, they should delve into this incident and redress the issue (i.e If they are responsible. Corrosion, or user over tightening cannot be ruled out at this stage).
Take note that the lighter weight Masterpiece seatpost is essentially a machined out Elite. Also note that I suspect some of the brittleness MAY be caused due to anodizing, since the anodized Aluminum Oxide on the surface of the substrate is pretty brittle, even though hard. When the part elastically deforms, the oxide layers starts cracking and it propagates down into the substrate metal. This will only reduce the part's fatigue strength. [Source : Aluminum And Aluminum Alloys by J.R Davis & Associates (ASM)]
TO THE BLOGGER OF KHABAR BIKE (and user of the seat post) :
Due to the number of requests I'm getting to dig further into this issue, I have to put you in the spot light by flexing my Bee Muscles (crunnch). Please tell us the following if you don't mind :
1) Did you over tighten the seat clamp outside limiting values specified in the product sheet?
Khabar Bike : Not as far as I'm aware of. And it's been untouched since I last changed out the saddle about a year ago.
2) Did you wash the seat post a number of times without letting drying it out? Or have you used the bike a good number of times in the rain?
Khabar Bike : Definitely not. And I hate riding in the rain.
3) Is this the first time that the seatpost fell, taking a hit on the same area on the post in question?
Khabar Bike : Yes, as far as I can remember. And this is the first time I've ever had a broken seapost in the 12 years I've been a serious bike buff, with seven bikes including a lugged steel Pinarello, an old Trek bonded/carbon road bike and a 12-year-old Gary Fisher hardtail. They have a Syncros, ITM or OEM seatpost.
4) Did you buy the post from a recognized Thomson dealer? Brick and mortar shop? Online? Ebay? The possibility of a knock-off cannot be ruled out.
Khabar Bike : Yes. My LBS is one of the most reputable in the area. And the seatpost (paid about $70 for it) came with Thomson's unique but pretty useless bag and folded manual. I also have another Thomson seatpost (with layback), on my hardtail, which has been holding up fine for more than three years.
5) Pretty Useless Bag... the manual was in this useless bag. Did you read it?
Khabar Bike : Yes, I did and didn't find it useless at all.
6) Do you recall how the bike fell (orientation)?
Khabar Bike : It fell sideways. I picked it up by handlebar and the saddle and the back part broke off in my left hand.
7) Were you riding it when this happened (added weight) or did it free fall as it tipped over (just the bike weight)?
Khabar Bike : It was leaning against the wall when it tipped over.
8) Did the seatpost also have the saddle attached during that time?
Khabar Bike : Yes, the clamp was secure and undamaged, and still attached to the saddle.
Finally, I'd just like to say that I'm not as upset about the whole thing as I would've been a few years back when I was really into lightweight products. These days, anything that doesn't cost and arm and a leg and lasts is just fine with me.
DESIGN FEATURES OF THOMSON ELITE SEATPOST.
THIS IS A WELL REGARDED SEAT POST AND MANY CONSIDER IT TO BE AMONG THE BEST DESIGNS.
KNOWN FACTS ARE GIVEN IN BLUE FONT. ANY EXTERNAL REFERENCES TO LINKS ARE IN RED.
One of the apparent design goals was to make this seat post able to withstand 350 in. lbs of torque, with bending taking place at 250 in. lbs of torque. There appears to be shoulder fillet at the tube-head interface (machined). The bigger the fillet radius, the more the stress concentration factor is reduced. Thomson designers should have selected the fillet radius for a desired stress concentration factor for this shaft for the particular loading case (torsion, tension, bending etc). The critical numbers here are the D/d ratio and the r/d ratio which can be referenced to known and published charts to find the stress concentration factor (you can work backwards in design as well). See Peterson's Stress Concentration Factors by Walter Pilkey for more information.
Moment of Inertia (or Second Moment of Area) of a hollow elliptical section beam about a horizontal axis passing through the center of gravity. Practically, it is the measure of the capacity of a cross-section to resist bending. See The Design Of Structures By Samuel Anglin, C.E
The same for a circular hollow cross section
The product sheet says : "All Thomson seatposts are designed with positive metal to metal stops. These positive stops and bolt lengths are designed to work together to allow a maximum 5 upto 29 degree down tilt. This means the tilt adjustment of the clamps will be stopped by the positive stop before the back bolt runs out of the nut. This prevents damage to bolts, nuts, ears, and clamps that may otherwise occur with over adjustment."
A warning in the product sheet cautioning that if the clamp lip touches the tube, the seatpost could dimple during clamp tightening.
Finally, the highlight of the Thomson marketing is this statement :
"The Thomson seatpost has a clamp, head, and upper tube strong enough to withstand 350 foot-lbs of torque. The tube will start to yield and bend at the seat tube clamp at about 250 foot-lbs of torque. Remember all brand-x posts we tested flew apart at less than 150 foot-lbs of torque. Under severe impact the Thomson seatpost would bend slightly and allows the rider to come to a safe stop or finish the ride. The ride could continue."My thoughts on this statement are that putting this alongside with what was actually observed by the user who broke his seat post (see top picture of failure), the tube did not withstand the force and there was no observation of bending. It appears that the impact forces approached ultimate strength for the broken area in question and broke catastrophically. It looks like the rider was not injured in any serious fashion.
What remains to be researched is how the bike with the seat post and saddle fell, validate the claims of the user (see above) when he replied that he did not overtighten any of the two bolts, validate the claims of the presence of a bending fuse and the actual strength of the product from the manufacturer, do a small paper and pencil analysis of the forces and stresses encountered and compare this with known the known design limits of the seat post.
The owner has made it known to us that for about a year, he did not have to change the saddle on the seat post. It is likely that during this time, he may have lost notice of the critical areas of the seat post. A stress riser may have led to the formation and propogation of a tiny crack, which, upon the post striking the ground in this fall, led to the severing of one of the bolting spots from the tube due to the torsional shear stresses encountered.
What was nice from writing this post so far has been the fact that this issue has been exposed to the world. Users must remain careful of how they install and use cycling products. At the same time, competition in the cycling industry is cutthroat. Over the years, we've seen an increasing number of bad designs with bold marketing claims and promises. Few have stood the test of time.
A reader notified me of another Thomson seatpost failure incident, only this time it was a setback post and the failure occurred on the post body itself, very close the bolting ears. These pictures were shown on Mtbr.com Forums, by user Apacherider. Incidents like these are not safe and opens up a range of possibilities for injury. Since the above incident and this new one is not very far apart in occurence date, Thomson must really dig into the backgrounds of the causes. It seems to me as if these posts are in need of more reinforcing in the areas around the bolting ears, including the ears themselves.