Bending seat posts

I’ve bent two aluminum seat posts racing cyclocross. The first was a Zipp Service Course aluminum post and the second was some no name post I had laying around. I’m running posts that are around 330mm in length and have at least 100mm of post inside the bike. Which generally equates to the minimum insertion point being 50mm or 2" below the seat post collar. So plenty of post in bike.

I’m looking at the Thomson Elite 27.2mm posts as those seem to be highly regarded and I’m running one of their X4 stems.

Any other contenders I should look at? Perhaps carbon would offer more rebound should I accidentally remount too hard.

I’m not a light rider and I’ve been very happy with my Whiskey carbon post on my gravel bike. It’s got some give to it, perhaps more than AL bit can’t say for sure.

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I don’t want to be “that guy” but have you considered some coaching to try and minimise impact to the saddle?

(Speaking as a qualified CX Coach you should be aiming to slide onto the saddle rather than land on it!)

Fair question, by I don’t think so as it happens like once or twice a season and at 185lb it’s enough. I also haven’t raced in over a year so…

Thanks for the feedback—I run Whisky bars on my cross bike. I would think that carbon would have more flexion but also rebound to its natural shaped whereas with aluminum I wouldn’t expect it to return as much before failure. But I could be wrong.

Could it be related to where you mount your saddle too? i.e. forward or back and whether you actually land directly on the saddle or slightly front or back?

I think it could, I run 20mm of setback and just looking at where the saddle is in relation to BB it sits quite far back. For that reason I’ve been looking into zero setback posts in order to center the saddle over the BB more.

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Maybe it’s a good thing it bends assuming a carbon frame? Would rather bend a saddle rail or seatpost than transfer that to the frame in a crash.

For what it is worth, I bent two Thompson 27.2posts when I was racing mountain bikes back in the hard tail days. 6’3” and 175lb so I had a good amount of post sticking out of the frame. Pretty sure most of the damage happened on “g-outs”. The posts are really good build quality but might not be ideal for larger riders

Offset post? How far back is saddle? Is your frame cracked? Where is the post bending? Pics…
The Zipp should be in the middle of the flexibility range.

If the frame isn’t cracked, you don’t have your seat all the way back, then you are doing one of the following:

  1. you’re not pre-denting the post. This would be at the clamp. You’re over tightening, using a repair stand wrong, wrong size post, or your frame is cracked there.
  2. you’re twisting the post. Maybe you’ve got a leg length issue or something
  3. You’re post is too low and you’re pushing the post backward as you grind.
    If you’re pushing too hard down, the post will just slide.
  4. you’re tire pressure is too low (hitting rim) or too high, and your ass is too hard.

If none of these are true, a bendy post would likely help as it is strong enough to bend. I’d look at the specialized Z shaped post, as it’ll feel the stiffest. I think the X2 post will be “pre-failed” for you.

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Bike is an aluminum Trek Crockett, no cracks or anything frame related.

Really? Not doubting you, but Thomson goes into great detail online about fatigue resistance and strength of their posts…. “ The Elite is over 40% stronger on “ultimate strength tests” than the strongest production seat posts on the market.

The Elite has double the fatigue life over any other production seat post models we tested.”

See pics of current set up with a replacement generic setback post. 300mm length. The Zipp was the cheap $50 Service Course model that had 20-25mm setback. I don’t put my bikes in a repair stand so that is not the issue.

You can see that the post bent at the point where the clamp was. I always torque to 7nm which is what the Bontrager seat clamp says.

As you can see this frame has a good amount of ST angle, which when used with a setback post really moves the middle of the saddle back. So I can see how a perfectly vertical force could bend a mediocre post.

I recall the remount that I think did it, I jumped too high and remember slamming down on the saddle. It didn’t hurt, but I would guess I was 4-5” above the saddle. Oops. At 185 lbs I guess this is was enough force.



Yup, I don’t doubt their claims, you just need to know how to interpret them.

First, The “ultimate strength tests” may be relevant, or might not. Your post did not exhibit ultimate failure, rather it “yielded”. We define yield as the point where plastic (non-recoverable) deformation occurs. Essentially you can load a material/structure below its yield point, and it will return to the original shape. Once you exceed the yield point, the material continues to harden, requiring more load to impart further plastic deformation until the ultimate strength is reached and the material fails catastrophically. So while having high ultimate strength is a good thing (jagged aluminum and soft bits don’t mix well), it doesn’t necessarily say anything about the yield strength of the material.

Second, the fatigue resistance/life is not applicable here. Fatigue failure manifests as cracking due to many (say millions) of cycles where the loading is less than the yield strength. In your case, you have simply overloaded the system in one cycle.

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Maybe Thompson will warranty it? I was looking on their web site and saw an interesting blurb about their “bending fuse”. it sounds like you’ve hit the limit.

The Thomson seat post design incorporates a bending fuse to prevent catastrophic failure. All competitive seat posts we tested failed catastrophically with the seat and clamp components, and sometimes pieces of the tube and head flew off in all directions!

The Thomson seat post 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. Under severe impact the Thomson seat post will bend slightly and allow the rider to come to a safe stop or finish the ride.

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Ok, makes sense. I just bought a 27.2x410mm Thomson Elite post. At $65 lightly used the price was/is right and I’m going to wager my money on their quality (I own an X4 stem and it’s nice) and see if it can hold up to the abuse. Have to try something.

FWIW, the posts I’ve bent have not been Thomson. The first post was an aluminum Zipp Service Course (the $57 model) and Zipp said they would warranty it. The second post was some off brand that I had from an older bike.

So that wasn’t done from pounding on it vertically. I’d say either you’re pushing your saddle backward as you’re pedaling, you’re bouncing in the saddle as you’re pedaling, you’re bending the post in the repair stand, or the frame is damaged.

You’ll note in your pic that the bend is below the clamp mark. That means it is pivoting there. Maybe check how the post clamp is sitting in on the seat tube. Check the post diameter, check the tube diameter. I had an issue with my 2013(?) Crockett where the frame failed at the seat post binder. https://i.imgur.com/dbF42SL.jpg I was using the Specialized Cobl post. The clamping area is probably just too flexible

It could have been from a bad remount, but generally speaking I don’t think remounting really adds a lot of stress. Granted this is CX, there is no doubt a LOT of pedaling on rough terrain with a lot of jarring, and jostling on the saddle. I’m not a light rider by any means and run a lot of post so it makes sense that on a cheaply made post this type of repeated riding could bend the post. Still somewhat surprising.

I moved my saddle forward 20mm today to simulate the saddle position I’ll have with the 0mm offset Thomson I just bought. I did a training session and it felt really good in a more forward position. I actually liked being more “on top” of the bike and I think moving forward a bit puts more weight on the bars and increases front wheel traction. All good.

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I’ve had Thomson’s for years across many bikes. They’re great. I would try one for sure. The only bikes I have that don’t have Thomson’s, are where I’ve got a carbon post for the flex.

In any case, …

I’d get a carbon post rather than AL here as the AL one is most likely to fail. The carbon one can take this beating much better. I’d also get a longer one. This might be the cheapest Contact Composite Seatpost | Giant Bicycles United States
I’d also shim the bottom of the post a bit. This should work (.1mm thick metal tape - apply some grease around it.) https://www.amazon.com/3361-Silver-High-Temperature-Stainless-Steel/dp/B00L48YL20/ref=sr_1_3?dchild=1&keywords=stainless+metal+tape&qid=1634238750&sr=8-3

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Thanks for the advice. I have a carbon post on my road bike that I can switch.

I have nothing experience-wise to add, but looking at the pic that setback would create a lot of leverage on the seatpost at the point right above the clamp wouldn’t it? Is it common to use such a setback in CX?

I suspect the zero offset post will solve your issues. It’ll be interesting to see.

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