How to choose the right seat post and seat clamp

There are various different types of seat post to choose from, some that can significantly alter your ride position, some that are designed to cushion the ride, and others can be lowered and raised with the push of a button.

If you're replacing the seat clamp, there are different styles to choose from and you need to ensure you get the right size.

Here's how to make the right choices.

Seat post diameter

The majority of seat posts have no moving parts to wear out as you ride and will last the lifetime of your bike, but if the post gets knocked or the saddle clamp at the top gets damaged, replacements are generally inexpensive. 

While most bike frames take a seat post with a round profile you also need to remember that seat posts come in different diameters – 27.2mm being the most common – so make sure you get the right size for your bike. B'Twin offers lots of options.

The diameter you need will usually be printed towards the bottom of your old seat post, but it's easy enough to measure if this isn't visible. If you’re in any doubt, bring your old seat post into the Decathlon store with you to check you get the right replacement. 

Some frames don't take round seat posts, though, and require one that's specially designed to fit. You can get a carbon post that's made to match the unusual shape of a Van Rysel/B'Twin Ultra seat tube, for instance.

Seat post length

Seat posts come in different lengths too. As well as the seat post that you can see outside of your frame, you need a certain amount extending down inside the seat tube to avoid damage (the exact amount varying between bikes). 

If in doubt, your best bet is to replace your seat post with another of the same length or slightly longer – you can always cut down a seat post that's too long.

Most seat posts come with handy markings at the back to help you get the right height. These also allow you to check that your post isn’t slipping.

Folding bikes usually have a compact frame with a very long seat post to provide the saddle height you need. The replacement seat post for the B'Twin Tilt 500 and 900 folding bikes is 570mm long, for instance, which is much longer than you'd ever need on a standard bike.


This is the distance that the saddle clamp (at the top of the seatpost) sits from the centre of the seat post. Changing your existing seatpost for one with a different setback is one way of altering your position on the bike. If the saddle clamp sits directly over the centre of the seat post, it is said to have zero setback. This is sometimes called an inline seat post. Most seat posts come with some setback, 25mm being common.

The B’Twin 29.8mm is an inline seat post, for example, while the Rockrider 29.8mm seat post has setback.

Swapping from a seat post with setback to an inline seat post is one way of shortening your riding position if you feel uncomfortably stretched out on your bike. Another option would be to swap to a shorter stem.

Adjusting your saddle position

Most of our seat posts use a single bolt clamp so it’s very easy to adjust your saddle position. 

Once you’ve loosened the bolt, you can move the saddle forwards and backwards on its rails, or alter its tilt before retightening.

Seat post care

Seat posts can sometimes seize inside a frame and become almost impossible to move. You can avoid this by lightly greasing the section of an aluminium seat post that’s going to be inside the frame, or by using an assembly compound for carbon frames/components.

Keep the saddle clamp bolt(s) greased to avoid seizing too.

Dropper seat posts

A dropper seat post lets you alter your saddle height at the push of a button/lever while mountain biking.

When approaching a technical descent you can lower your saddle out of the way, allowing you to shift your weight around more easily for better control and manoeuvrability. 

The Rockrider Telescopic seat post replaces a standard 31.6mm diameter post and allows you to drop your saddle height by 100mm on challenging terrain.

Suspension seat posts

Often found on hybrids and hardtail mountain bikes rather than road bikes, suspension seat posts allow the saddle to move up and down to help absorb bumps and add comfort. 

The B'Twin Seat Post with Suspension offers 40mm of travel. It comes in a 27.2mm diameter with adapters for use with bikes designed to take larger seat posts.

Seat clamps

The seat clamp (or seat post clamp) is the collar that holds your seat post at the right height. It is tightened by either a hex bolt or a quick-release mechanism. 

A quick-release design allows you to alter your saddle height in seconds without the use of tools, although it does mean your seat post and saddle are more vulnerable to theft if you lock your bike in a public area.

Seat clamps come in different diameters to match different frames, so make sure you get the right size for your bike. Beware, though, that the size of the seat clamp isn't the same as the size of your seat post, it's slightly bigger. For example, a B'Twin 28.6mm seat clamp is suitable for a 27.2mm seatpost.