MTB seat clamps

How to choose your MTB seat clamp

It is a tiny part, but two mistakes can make it unusable: choosing the wrong diameter or the wrong version when the frame routes a cable through that area.

Diameter: measure the frame

• It is the outer diameter of the seat tube, not the seatpost diameter. Confusing the two is the main reason this part gets returned.
• Common sizes are 31.8 / 34.9 / 36.4 / 38.6 mm.
• A useful guide: a frame designed for a 30.9 mm seatpost often measures 34.9 mm on the outside; with a 31.6 mm seatpost, the outer diameter may be 34.9 or 36.4 mm, depending on wall thickness.
• If in doubt, measure with callipers. An oversized clamp will never grip tightly enough; an undersized one will not close.

Quick-release or bolt-on

• Quick-release : lets you adjust saddle height without tools. Useful on a shared bike, or on a bike without a dropper post when you lower the saddle before descents.
• Bolt-on : lighter, less conspicuous, and nothing to move or go missing when the bike is parked. It is the logical choice if you have a dropper post, since you no longer adjust the saddle height at the clamp.

When the cable is routed internally

On many frames with internal routing, the dropper post cable housing enters the tube right at the seat clamp. You then need a clamp with a hole or cut-out in exactly the right place. A standard clamp would crush the cable housing — and crushed housing is the leading cause of a dropper post that does not return properly.

The fastening

• Follow the specified tightening torque, generally 4 to 6 Nm. Tightening beyond that can deform the seat tube, and on a carbon frame the damage is irreversible.
• The seatpost must never go below its minimum insertion mark: that is exactly where seat tubes split.
• A little carbon assembly paste on a carbon seatpost prevents slipping without having to tighten the clamp further.

To find out more: How to choose your MTB seat clamp

See also: MTB seatposts · Dropper seatposts · MTB saddles