Trying to wrap your head around seatpost clamps? Then you’ve come to the right place. We’re going to go through what these bike components do and the different varieties of clamp. We’ll then look at the different sizes they come in and how to measure your bike so that you choose the right one.

What is a seatpost clamp? What does it do?

Seatpost clamps are the unsung heroes of the cycling world. Under normal circumstances, you’d never even know they were attached to your bike. But, if they fail, or they’re the wrong size, or the wrong type, then it can turn a great day out cycling into the day from hell.

So, let’s start with a quick definition.

What exactly IS a seatpost clamp and what does it do?

The job of the seatpost clamp is to hold the seatpost firmly in the seat tube.

Without it, you’ll find that your saddle will sink lower and possibly spin around as you sit on it and pedal along. Certain types of seatpost clamp also provide an inherent security feature, stopping thieves from stealing your saddle and seatpost.

Unfortunately the name “seatpost clamp” is a bit of a misnomer.

Why? Well, the clamp doesn’t actually clamp onto the seatpost itself. Rather it clamps onto the seat tube where the seatpost slots into it.

When the seatpost clamp is fitted onto the tube and tightened up it fixes the seatpost at that height and orientation.

The two main types of seatpost clamp you need to know

Seatpost clamps come in two flavors: a bolt clamp and a quick-release (QR) clamp. Both varieties have a collar that fits around the seat tube and a method of tightening this up firmly.

The bolt clamp uses an Allen/hex head bolt to close up the clamp. This obviously requires a hex wrench to operate the bolt and therefore adjust the height of your saddle. This makes it a somewhat trickier process to carry out and means that you need an appropriate tool to do the job. It does however have a major advantage in that it stops opportunistic thieves from stealing your saddle and seatpost when your bike is left unattended.

The quick-release seatpost clamp has a lever-and-cam operated mechanism that allows for quick and easy adjustments to be made to your saddle height. It also means that you don’t need to carry tools to do the job. The lever and cam assembly engages the clamp around the seat tube, fixing it firmly in position. Open the lever and the cam releases its hold, allowing adjustments to be made. This allows for on-the-fly variation in your saddle height (particularly useful for mountain bikers) but can be tempting for thieves.

Is there a standard size seat post clamp?

Unfortunately not! Seat post clamps come in a variety of sizes. However there are some that are more commonly used than others.

There are six seatpost clamp sizes available and these are all measured in millimeters:

• 28.6mm
• 29.8mm
• 31.8mm
• 34.9mm
• 36.4mm
• 38.6mm

You’ll see from the list above that there are some very small size differences between some of the clamps. So, how do you know which one you need for your bike?

How do you measure what size seat post clamp you need

There are three ways to determine which size of seatpost clamp you should get:

1. Check the manufacturer’s specification for your bike
2. Check the existing seatpost clamp for a marked size
3. Measure your bike’s seat tube external diameter

Check the manufacturer’s specification for your bike

If you have access to the original specification for your bike, then it might show the size of seatpost clamp that you need. Sadly, most published specs never go down to this level of detail. Any, anyway, if you’re like me you probably threw the paperwork out years ago!

Check the existing seatpost clamp for a marked size

The next step is to check the existing seatpost clamp. Sometimes these have the size printed on the collar. In some cases they’ll also have a useful indicator of the maximum torque you should use when fitting them.

Measure your bike’s seat tube external diameter with digital calipers

The best way to check what size seatpost clamp you need is to measure the seat tube where the clamp will be fitted.

This is easy to do, but requires some accurate measuring with either a ruler or a set of digital calipers.

The measurement that you need is the external diameter of the seat tube.

This is different from the seatpost diameter and also the internal diameter of the seat tube – both of which will be smaller than the diameter that you require.

Seatpost clamp common sizes chart

Now that you have the information and the measurement you need to choose your new seatpost clamp, here’s an easy chart showing where to buy each size. Decide whether you want a bolt-on or quick-release style clamp and click the corresponding button next to the diameter of your bike’s seat tube.