Flexy seat posts

04/04/24 00:36 Filed in: Gear

My first experience with a seat post specifically designed to make the ride more comfortable was the Specialized Roubaix carbon post back circa 2004. That one made a remarkable transformation of my oversized aluminium Cannondale CX frame from its original aluminium post. Even before that I had used posts like the Syncros one, which was primarily aimed at being light in weight, but as a consequence of being made so thin-walled it also flexed quite a bit. Unlike modern flexy posts, it flexed evenly in all directions.
Fast forward to my first Trek Checkpoint. Like most Trek road bikes it used a seat mast rather than a seat post. These very large diameter units slide over the frame and must be close to inflexible for it. But the Checkpoint had Isospeed decoupling where the seat tube is not rigidly joined to the top tube/seat stay junction (instead an axle at that joint permits the seat tube to flex like a leaf spring). I found this bike to be perfectly comfortable. There is a lump in the pavement in my neighbourhood that is a good indicator of how a frame transmits harshness through the seat. On that Cannondale, even with the Specialized post, it was sharp enough that I tried never to hit it. On the Checkpoint, it was a dull noise and not much of a feeling at all.
And now everyone wants a dropper post on their gravel bike for the same reason everyone needs a dropped post on their MTB - it just works better. So my new Checkpoint uses a seat post again - and has routing room for a dropper post actuation cable. The post I put in it is the Bontrager RSL - a very high-end post designed to be flexy fore and aft but rigid side to side. I haven't ridden the Checkpoint without this post, but it does float over my harsh neighbourhood lump approximately as well as the old Checkpoint did.
I've been contemplating this as I ride it for the past week. There is no indication that this post flexes at all. However much it absorbs, it does so in conjunction with the Isospeed and it is not materially different than the older frame.

As a corollary of this, my Madone race bike has its own version of Isospeed - the seat post floats inside the seat tube in a carbon leaf spring mechanism that permits adjustment of the spring rate in that spring. You can have more or less movement for a given force to reflect your body weight or preferences. It came in the middle of the adjustment range and I haven't adjusted it once - it was fine on day one and remains fine. Interestingly to me, it feels a lot like the first Checkpoint (and therefore the second Checkpoint) despite the deep aero sections of the frame. It suggest that Trek know what they are doing with the Isospeed thing. I wonder what the new Checkpoint would be like without the flexy post? I don't have any other 27.2 posts, nor do I want to fuss around with the swapping of posts, so I won't find out any time soon.

The most extreme looking flexy post is the one that Canyon puts in their gravel bikes. It is a two part leaf spring that is held together on one end by being in the frame and at the top by how it bolts to the post head. An idea whose time is here. If you haven't tried a designed-flex carbon frame you should. They're brilliant in their ability to absorb bumps and still feel firm underfoot when you pedal.

Michael Hanslip Coaching

If you want to go faster, you have to pedal harder

Flexy seat posts