Choosing a seat post

Seat posts are a less talked about part of the bike than the saddle, and for this reason they're generally less well understood.

It's an area of the bike we frequently receive questions about, in particular regarding our own bike saddles' compatibility with a particular brand or design. Thankfully, whilst there is a lot of variety in seat posts, almost all share a few common features, including the ability to attach to a dual rail bike saddle.

Below we discuss some of the differences between the various seat posts currently on the market, and the pros and cons of different features and material choices.

 

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Diameter

There are different diameters of seat post out there, but your choice is essentially limited to the diameter of seat post that matches the internal dimension of your bike's seat tube. If you have a round seat post, you can quickly and easily measure the internal diameter of your seat tube using a pair of calipers, ruler or measuring tape. The chances are you'll get a measurement that roughly matches one of the following dimensions: 27.2mm, 30.9mm, 31.6mm, 34.9mm.

Image: Your seat post diameter can be measured using a ruler or set of calipers. Be sure that the dimension you're measuring is the distance between the internal walls of the seat tube.

27.2mm is the most common seat post diameter for road bikes, whilst mountain bikes tend to take a 30.9mm seat post. If your mountain bike came with a "dropper" type seat post (i.e. one with an internal suspension) then it may be 34.9mm.

You can attach a narrower seat post to a wider seat tube using a shim, but it's generally easiest, and most elegant, to simply choose a seat post that matches your bike design.

If you have a carbon road bike, it may be fitted with an "aero" type seat post. In this case, a replacement seat post would need to bought directly from the bicycle manufacturer, as the designs are often proprietary rather than standardised. Whilst aero seat posts might marginally improve a bike's aerodynamics, the trend towards proprietary components and away from standard components is seen by environmentalists as a step backward for the cycling industry. Easily replaceable parts, help to encourage bicycle owners to replace components that become worn or damaged rather than discarding or replacing their entire bicycle. 

 

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Material

Most seat tubes are made from either carbon or aluminium.

Carbon seat tubes are generally slightly lighter and have more flex than their aluminium counterparts, which can help with comfort and shock absorption. Carbon seat tubes are generally more expensive however, and will often have a shorter lifespan than aluminium, which is fully recyclable, and has a lower carbon footprint per metric ton.

It's worth considering which things matter the more to you out of performance, weight, durability, affordability and sustainability when making the decision of which material to choose.

 

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Attachment type

Almost all bicycle saddles make use of a dual 7mm rail design, and so almost all seat posts feature a method of attaching to this. The method itself varies between seat posts, the most common being the "micro adjust" attachment.

Most micro adjust attachments have a single hex bolt at the rear. This sort are extremely quick and easy to adjust, so if you're the sort of cyclist who likes to make regular changes to fore and aft, or to saddle tilt, then micro adjust is likely to be the option that works best.

Illustration: Typical micro-adjust seat post

If, however, you don't mind something slightly a little more fiddly, if it enables a more precise saddle tilt setting, then you could opt for a micro-adjust seat clamp with two hex bolts. This type of seat clamp allows you to fine tune the saddle tilt by tightening and/or loosening the front and back hex bolts.

Illustration: Double hex bolt micro-adjust seat post

One issue you may encounter with micro-adjust seat tubes is the saddle tilt position slipping over time. Some seat tube manufacturers, such as 3T, have sought to address this with their Difflock attachment system. With this type of seat tube it will take longer to change or reposition your saddle, but it's a good choice for riders who know their preferred saddle tilt, and want to lock it in long term.

Illustration: Many alternatives to micro-adjust seat post attachments exist

 

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Set back

You will be able to adjust your saddle set back by sliding the saddle forwards or backwards within the clamp, however, often seat posts include some built-in set back as well.

Illustration: Seat posts typically have set back of circa 25mm. Most bike fitters will account for this when selecting a bike frame.

In general positioning the saddle further forward suits an aggressive riding style, whilst positioning the saddle further back suits a more leisurely style. A well fitted bike with the correct seat post should enable you to achieve either of these riding positions by sliding the saddle forward or backward.

In competitive racing there are regulations, stating that the nose of the bike saddle must be at least 5mm behind the bottom bracket, which is something to consider if you're intending to compete.

 

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Length

Most seat posts are between 300mm and 400mm in length. A longer seat post will enable you to position your saddle higher and can be a useful option for taller riders, who don't have access to a well fitted frame.

If you're using your seat post at a low setting, then using a long seat post will simply mean you're carry extra weight however. For this reason it's a good idea to work out roughly what height you're going to be using the seat post at prior to ordering.

Most seat posts will have a minimum insertion limit line. It's worth looking out for this, if you set your seat post above this line, then there may not be enough friction to hold it in place.

 

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How to remove your existing seat post

There will either be a bolt or a quick release latch near the top of your seat tube. Loosening this should reduce the friction on the seat post and enable its removal. If the seat post is still stiff, try first rotating it from side to side - a well attached saddle can be helpful here, as it will give you something to hold on to as you rotate.

Illustration: Seat tube bolt position

If the post is still not budging, try adding chain oil or WD40 around the rim and let it seep in for five minutes. Eventually you should find it starts to move.

 

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How to install a new seat post

Installing a new seat post is simple. Just make sure the seat clamp bolt is loose enough that the seat post slides in easily, and then tighten once you've found the correct height. You want to find a height where each leg is almost but not quite fully straight when the pedals are at their lowest position. Once you think you've found the correct height, we recommend going for a short bike ride and adjusting again if required.

 

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