Seatpost robbing me of power and causing back pain?
I was listening to a Mountain Bike Radio podcast earlier this week. A rep from Thomson was on and discussing some of the Thomson products. The question of carbon seatposts came up. The Thomson rep said that he did not think that carbon makes a good seatpost. He claimed that the flexy nature of carbon posts took away precious watts and, in some cases, causes lower back pain. A coach/bike fitter called in to confirm this.
I have a 27.2 carbon post on my bike and, yes, it does flex. I thought it would be good for comfort. I am building a new bike that uses a 30.9 post. I was just going to shim my carbon post, but now I am rethinking that.
Also, I have been experiencing some back pain this year, which is not typical.
Does anyone have anything to add to this conversation? Any truth to this?
I wouldn't shim a carbon post in the first place. To much stress located on one point.
Nobody has mentioned seatpost extension, which will really affect flex.
The 30.9 post, if the same length beyond the seat post collar and similar wall thickness/material , should deflect less than the 27.2 post due to higher Area Moment of Inertia (from the larger diameter).
Last year I rode with a SAVE seatpost (known to have designed flex) and had less back pain than any post ever. And this was on a hardtail. I have more back pain now on my fully, which BTW has a Thompson elite.
I posted a thread last year asking about this and many people concluded that hardtails leads a person to "protect" the back more (by standing and floating off the seat and such) thus leading to less back pain.
I also know 2 other people who went to Cannondale hardtails with SAVE posts and have experienced less back pain than any other setup.
Last edited by Poncharelli; 06-12-2013 at 01:51 PM.
My guess is that this idea is BS.
If you looked at all the energy from your muscle contraction that gets lost and is not transferred to forward motion, I would guess that the contribution from minute seatpost flex would be less than .1% of that total loss. It just can't be significant.
What about the padding of your seat? Seat rail and seat body flex? the padding in your shoes? the deflection of your tires? Bottom bracket flex, etc.
Even with all the losses, a bike is pretty efficient, so overall the losses are small, and the seatpost loss is infinitesimal IMO.
I would also add that there is no reason that carbon has to have any more flex than aluminum.
Agreed about all that's stated above. The Thomson rep was selling his products but not giving the truth to you.
Carbon seatposts on a hardtail come in a variety of flex qualities but the best, like the Cannondale SAVE, have some give to them. I personally love my Enve seatpost and handlebars. Both have a little vibration damping in them. I also have a Truvative Noir WC on a lower-end hardtail that's light but stiffer carbon.
It's tragicomic to see guys buy a light, expensive carbon frame only to load it down with a boat-anchor Thomson seatpost.
Originally Posted by chomxxo
My Thomson seatpost is lighter than my FSA SL-k Carbon post.
I call BS on that one.
If anything a flexy seat post that helps smooth out the ride is going to allow you maintain a high power output.
"The best pace is suicide pace, and today is a good day to die." Steve Prefontaine
A good article on different seat posts flex and dampening:
From the pages of Velo: Getting the most from your post
A table from the above article
That might be a little small to read so go to the article, but the bottom line is that a carbon post could have better, or worse, damping than an aluminum post.
Actually it says that a carbon post is usually better, and setback posts are usually better at vibration damping.
Seatpost robbing me of power and causing back pain?
There are a few test links and articles in this thread about carbon seat posts, flex and comfort:
Carbon 'bars and seat post for dampening.
Those claims (that carbon seatpost flex costs you power and that a carbon fibre seatpost flex will cause backpain) aren't that convincing. Backpain is more likely to be due to your riding position I'd have thought.
I've had a Thomson Elite layback seatpost on my bike since 2009. I've stuck with it because it has been reliable without breaking or allowing the saddle angle to slip. As it's aluminium I'm also less worried about cranking down the seatpost clamp tightly so that there's no danger of the seatpost slipping down whilst riding also. That reliability and peace of mind is worth the weight penalty for me.
I'm trying to remedy back pain by adding back fitness and flexibility through a few extra exercises after rides, I think it's working.
My guess is that unless the post is exceptionally flexy or amazingly stiff, it would be really hard for anybody to tell the difference. My cx bike has an sworks carbon post with the zertz insert, it works fine and stays up now that I have carbon assembly paste in there.
Somebody should tell the Thompson rep than the pedal stroke force is in the opposite direction from the "force" (your bodyweight) that flexes the post.
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