Seatpost Design Discussion for Engineers

[andrewc]

For the sake of reference I attach the photo of the damaged post:

 

 

 

Something has been bugging me and after pulling my post out I now know what it is. The top face of the ear is actually level with the top of the seatpost body. How on earth did the ear shear off here? Impossible. The top face would be black anodised. Something is amiss here.

 

 

 

 

Refer to this image:

 

 

 

 

 

 

I think the other shots are taken of the bottom of the ear. The black anodised portion is visible in the photo I referred to above.

 

 

 

Cheers,

 

 

 

Andrew

 

 

 

I R Engineerandrewc2009-04-17 09:11:28

[Dirtbreath]

I might be sticking my neck out here but this post looks quite different from mine.

 

 

The neck here is much flatter. Also looking at the wall at the back this picture is definitely much thinner than my post.

 

 

[Johan Bornman]

Also' date=' just to note that Thomson do claim they performed fatigue tests on their seat posts:

Over double the fatigue life of any other production models we tested.

- from their site

 

That's good news. Exactly what I would expect from them.

 

 

[Minion]

 

What is that thing? Looks electronic.

 

Any way' date=' back on topic.

 

First off, some background:

Thomson Patent No.1 - 5,649,738

Thomson Patent No.2 - 5,664,829

 

Apart from some slight differences in wording, the first contains all of the claims and figures of the second. It also contains 46 additional claims, including the seatpost 'fuse'. I'm not entirely sure why they felt the need to file the second patent (on the same day as the first, nogal) - it's possible they thought some of the parts of the first might be rejected.

 

Of interest in the first patent is a description of the bending fuse feature. It as not a part, but rather the design of the relative strengths of the post, flange and clamp so that the post fails in a predictable way. When subjected to a heavy load, the post is supposed to bend before the flange or clamp fail. I have included some relevant sections from the background and description below. The full functional description can be found in the two paragraphs starting at line 15, column 8 of the first patent.

 

A conventional seat post and clamp may experience a catastrophic failure during such high loads. Catastrophic failure means that the clamp or end of the seat tube fails, thus allowing the seat to fall away from the bicycle. In other words, since the seat tube may typically be stronger than the clamp, the clamp, the fasteners, or the head of the tube may break allowing the seat to separate and exposing an upper end of the seat tube, for example.

...

another aspect of the invention is that the tube, the flange and the clamp may have relative strengths so that the tube of the seat post bends, responsive to a relatively large force offset from an axis of the tube, and prior to failure of one of the flange and the clamp. Stated in somewhat different terms, the seat post and clamp apparatus in accordance with one aspect of the invention may be considered as having a bending fuse, wherein unusually large forces deform the tube rather than fracture the flange or clamp

 

More to follow...

 

 

[Minion]

 

 

 

Damn, it's a mission trying to take pics that show surface detail on a glossy black tube.

 

Anyway, after numerous attempts I now have some reasonable pics and it's time for a comparison of the Elite, Masterpiece and a mystery guest.

 

I'll call this: How do they get it so light?

 

First comparison:

You can see that the Masterpiece has had extra material machined off from the arrow down.

 

Comparing from the bottom:

Elite - The elliptical inner cross-section provides greater bend strength front and back than side to side. Weight is saved by removing material where it's less needed.

 

Masterpiece - Note how thin the walls are. You can't really see it with the naked eye, but it does still have an elliptical inner section - the front-back walls are about 0.25mm thicker.

 

The mystery guest - 2mm tubing helps push the scales to 350g

 

The mystery guest is revealed. If you compare the clamp assemblies, you can see that the ITM is beefier (that ain't plastic - it's a solid chunk of metal that extends 4cm into the tube).

 

Both seatposts have a similar setback amount. By positioning the setback right at the saddle, ITM increases the bending moment in the saddle clamp, just above the tube-clamp joint. The clamp and joint need to be stronger to accommodate this.

The Thomson saddle clamp transfers the load directly into the tube, reducing the amount of bending load it has to support.

The one-piece design of the Thomson post also removes the need for the 4cm saddle clamp boss on the ITM.

Note that in both cases, the peak bending load should occur where the seatpost enters the frame.

 

I've run out of time now, but if I get a moment later I'll post my thoughts on the two seatpost failures.

Edman2009-04-17 13:02:28

[Climate]

Sorry I didn't see this thread earlier ... could have saved some keystrokes by the conspiracy theorists. I've attached a picture of the same seatpost (except that it was my wife's - she weighs 55kg and did NOT have an accident to cause the failure). This post was about a year old, when it failed. The bike was hanging in the garage when the failure must have happened, because when she took the bike down for the next ride, the saddle was dangling!!! I suspected the bolt must have been over-tightened, but without proof, simply replaced the seatpost (with the same) and it's been good ever since. I also use a Thomson Elite (4 years of regular use and 72kg load) ... no problems.

[GoLefty!!]

These failures appear to be caused by embrittlement of the aluminium.

 

How?

 

Well the post is forged and then machined.

I suspect that the embrittlement is due to either over heating due to too fast machining or taking too much material out to shorten the production time (costsaving).

 

Over tightening of the bolt is a contributing factor but the material has been compromised during production. If it was ductile enough the ear would have bent without ripping a huge chunk of material out of the tube.

 

Perhaps something Thomson should take a look at.

[vanniri]

A general remark on cockpit items - when I started Mtb it was common knowledge that the handlebar should be replaced every two years (normal riding - more if aggresive or frequent). The seatpost was said to be within 6 months from the handlebar.

 

Apparently the above is no longer promoted - maybe since most people ride their bikes only 50km before selling them (look at the for sales section - only "hardly being used" bikes for sale).