Seattube top design for carbon seatpost safety

[Ben Hudson]

Salonistas,

I had a bit of distressing feedback on a recent build - within a couple of weeks of riding, the rider had a failure on his carbon post:

CDA8E7BC-2375-4A62-8FE3-1F702B466D04.jpeg

The damage is to the rear of the post, where the seattube is split. By the sound of it, the failure was just above the top of the seattube. It was a parts bin seatpost of no especial pedigree and I didn’t put the bike together so I can’t rule out pre-existing conditions (and can’t comment re. torque and assembly paste) but it has worried me.

I’ve not got any good pictures of the seattube top style but you can just make it out here:
4BB53712-4616-41AF-8244-105177BF1FAB.jpg

It’s very similar to this Kirk, but rounded to the front rather than pointed. At the rear there is an curved cut away that leads into the slot.
DSC_1844.jpg

The seattube is a 31.8mm ID externally butted job with an aluminium shim pressed and Araldited into it and reamed to 27.2mm. The slot is probably an inch and a half, and the shim extends another quarter inch below the bottom of the shim. I lightly chamfered the inside top edge and the inside edges of the slot specifically to avoid doing any damage to carbon posts, and used a longer slot than usual in hopes of getting a nice gentle squeeze over a longer distance. The binder bolt is a steel cylindrical one, brass brazed on and with the seatstays shot directly into it, just like the Kirk.

Any advice? Is there anything else I need to do in future?

Ben

[fortyfour]

Welcome and hello Ben.

Personally, I do not like integrated binder bolts for this reason. Every integrated binder I've made and / or owned I've always had to add excessive torque to prevent the post from slipping. That's just been my experience to date however.

Your aluminum insert has a chamfer on the end, but I don't know how much that is either. It should just be a subtle deburring / edge break. Too drastic of a relief and you get a section of post to insert that is unsupported where the post can technically flex below where it's been clamped.

As stated, you do not know about assembly or torque used so we have to trust the customer assembled everything to spec (another reason I really prefer building completes - as the builder, you have control over every aspect of the build and when it leaves the shop, you know every last detail of the bike). But if we take a look specifically at how your cluster is made, there is little to no seat tube exposed to help distribute clamping forces along the length of the post up from where the load on that same post originates at the seat tube/seat stay cluster. The seat post is basically a lever arm, and that cluster is the axis of rotation. It's telling that the post started to fail in the spot it did. So my best advice moving forward when you know that a carbon post is to be spec'd in the build would be to add about 1.00" / 25.4mm of extension above the top of where the seat tube and top tube meet. I've found that makes room for just about every seat post clamp design and allows for even distribution of forces if you are using an integrated binder.

Hope that helps!

Best of luck to you.

[Ben Hudson]

Hey Kris,

Thanks for the advice - I’ll tweak things on future builds.

Cheers,
Ben