PEMBREE DBN V2 seatpost clamp — sizes 34.9, 38.6 and 39.7 mm

A seatpost clamp is a small part with a big job: it holds your saddle height exactly where you set it, ride after ride. Get the size wrong and it either won't fit or won't hold. This seatpost clamp size guide explains what the size refers to, how to measure your frame and how to choose the right clamp.

What seatpost clamp size actually means

This catches a lot of riders out. Seatpost clamp size is not the same as your seatpost diameter.

  • Seatpost diameter is the outside diameter of the post that slides into your frame, for example 30.9 mm, 31.6 mm or 34.9 mm.
  • Seatpost clamp size is the inside diameter of the clamp, which must match the outside diameter of your frame's seat tube at the top, where the clamp sits.

Because the seat tube wall has thickness, the clamp is always larger than the post. A frame that takes a 31.6 mm post, for example, will need a noticeably larger clamp. Always measure the frame, not the post.

Common MTB seatpost clamp sizes

Mountain bike seat tubes vary by frame design and material. Our DBN V2 seatpost clamp is available in three sizes to cover common MTB frames:

DBN V2 size Fits seat tube outside diameter
34.9 mm 34.9 mm
38.6 mm 38.6 mm
39.7 mm 39.7 mm

It has a deeper clamp and a wider nut than our original DBN for a secure hold, and comes in six colours.

How to measure your frame for a seatpost clamp

Option 1: check the existing clamp

The easiest way is to look at the clamp already on your bike. Many clamps have their size laser-etched or printed on the outside. If you can read it, that's your size.

Option 2: check the frame specification

Your bike manufacturer's website or manual will often list the seat clamp size alongside the seatpost diameter. Make sure you're reading the clamp size, not the post size.

Option 3: measure with callipers

If there are no markings or specs:

  1. Remove the existing clamp. Loosen the bolt and slide the clamp off the top of the seat tube. You have to lower or remove the seatpost.
  2. Measure the outside of the seat tube. Use digital or vernier callipers across the top of the seat tube, just below the slot, where the clamp sits.
  3. Measure in two directions. Take a reading front to back and another side to side. The slot can make the tube slightly oval at the very top, so measure where it's round.
  4. Match to the nearest standard size. Your reading should be very close to a standard size such as 34.9, 38.6 or 39.7 mm.

A tape measure or ruler isn't accurate enough here, as the difference between sizes is only around a millimetre in some cases. Callipers are inexpensive and well worth having in your toolbox.

Fitting your new seatpost clamp

  1. Clean the seat tube and post. Remove grit and old grease. For alloy frames and posts, a light film of grease on the post helps prevent seizing. For carbon frames or posts, use carbon assembly paste instead of grease.
  2. Position the clamp. Slide the clamp on so it sits fully on the seat tube. Line up the clamp slot with the slot in the seat tube, or follow your frame manufacturer's guidance if it specifies otherwise.
  3. Insert the post. Check your seatpost's minimum insertion mark and stay well within it. Also check your frame's minimum insertion depth (from the frame manufacturer's spec) and respect whichever of the two is deeper.
  4. Set your height and tighten. Set saddle height, align the saddle straight, and tighten the bolt with a torque wrench to the torque recommended by the clamp and frame manufacturers. Use the lower of the two.
  5. Check after riding. Make sure the post hasn't slipped and the clamp is still tight after your first ride.

Choosing a seatpost

If you're also replacing your seatpost, measure the inside diameter of your seat tube or check your frame spec for the correct post size. For downhill and dirt jump bikes, our ASP seatpost weighs 290 g and is certified by EFBE for strength and durability.

Frequently asked questions

Is seatpost clamp size the same as seatpost size?

No. The clamp fits around the outside of your frame's seat tube, so it's always larger than the seatpost diameter.

What size seatpost clamp do I need?

Measure the outside diameter of the top of your seat tube with callipers, or read the size on your existing clamp or frame spec. Common MTB sizes include 34.9, 38.6 and 39.7 mm.

Can I use a slightly bigger clamp?

No. A clamp that's too big won't grip evenly, and overtightening it to compensate can damage the frame or clamp. Choose the exact size.

What sizes does the DBN V2 come in?

34.9 mm, 38.6 mm and 39.7 mm, each in six colours.

Why is my seatpost slipping?

Common causes include a clamp that's the wrong size, too little torque, grease on a carbon post, or a worn clamp. Check the size first, then follow the manufacturer's torque and assembly guidance.

See the DBN V2 seatpost clamp in all three sizes.