Your seatpost can seem like an incongruous part of your bike – a simple tube that connects frame to saddle – but it can have an important role to play in how comfortable your bike is to ride, and can also be a potential area for a weight-saving upgrade.
Which seatpost is right for you?
While the vast majority of standard seatposts are identical in form and function – being hollow aluminium or carbon tubes upon which the saddle is clamped – there are a few essential differences to know about when choosing or replacing a post.
These include sizing, materials, clamp type and the amount (or not) of layback.
• Sizing: The most important dimension to consider is diameter, as it must correspond to the internal diameter of your seat tube in order to have a snug fit. While there are any number of post diameters out there, most modern road and MTB bike frames accept a seatpost of either 27.2mm in diameter (‘standard’), 30.9 or 31.6mm (‘oversize’). An oversize post is regarded as adding stiffness and strength for optimum power transfer as well as resistance to bending/failure, but a narrower post is accepted as being more comfortable over rough surfaces, as it will tend to have a little more ‘give’. You can use a shim to enable a 27.2mm seatpost fit into a frame taking a larger standard, but not vice-versa for obvious reasons.
Seatpost length is a further consideration. While a longer post is generally regarded as offering more comfort, the amount of post ‘sticking out’ of the frame will largely be dependent on your frame size/geometry and your own dimensions (inside leg length etc). It is essential to have a minimum amount of post remaining inside the frame (or the leverage ratio will be too high and you risk cracking the seat tube) – most posts will have a ‘minimum insertion’ line etched on the shaft. If you wish, you can also cut down your seatpost to reduce excess material, but you may want to consider a carbon-specific hacksaw/saw guide with a carbon fibre post, as any mistake made here could be expensive.
• Materials: The most common seatpost materials are aluminium and carbon fibre, with the former being less expensive and more commonly found on low- to mid-range posts and bikes, and the latter being the option of choice for premium bikes. Carbon fibre is generally regarded as being more comfortable due to its vibration-absorbing characteristics, while top-end carbon posts are also the lightest on the market. However many – especially trail and gravity MTB riders – still favour premium aluminium posts for strength and confidence.
• Clamp type: The majority of posts feature a clamp designed to accommodate twin-rail saddles, with one or more bolts to secure the top part of the clamp to the forged bottom (two is the most common standard). The clamp bolts can be loosened to allow you move the saddle for and aft to your preferred position, while most posts will also also for a degree of tilt adjustment to the front or rear, enabling you to fine-tune your fit. Some heavy-duty saddles may use oversized rails (the standard is 7mm) so you may need to double-check if your clamp can accommodate these. Also, a small number of saddle/seatpost manufacturers use a proprietary clamp design (e.g. a single-rail iteration) which requires the use of a specific saddle. However these haven’t really caught on as most riders prefer to be able to chop and change to the saddle of their choosing.
• Layback: A seatpost shaft can be inline (i.e. straight, with no kinks or bends) or layback (with a shallow-angled bend towards the top of the shaft which puts the saddles slightly farther back relative to an inline post). This is entirely a matter of personal preference – riders who cannot achieve their optimum seating/saddle position with an inline post may find that a layback post puts them where they want to be.
NOTE: Layback is slightly different from setback, which is the degree to which the post clamp sits rearward of a straight line drawn through the shaft. A layback post will have a visible bend in the shaft, while most posts will have some degree of setback – i.e. the clamp sits not directly on top of the post, but a few mm to the rear. However the terms are often confused and practically interchangeable. A ‘zero-setback’ post is sometimes used to refer to a post with no layback in the shaft and the clamp directly in line with the shaft centre. Inline/zero setback posts are more commonly used in MTB, with most road posts having the clamp positioned with some element of setback.
Finally, there are a number of ‘non-standard’ seatpost designs worth mentioning, the primary ones being aero posts (for road and TT racing) and dropper posts (for trail and gravity MTB riding).
• Aero posts: While most posts (and seat tubes) are circular in cross-section, many manufacturers are now making aero posts for road and TT racing, with an elliptical ‘blade’ cross-section which offers improved aerodynamic performance by offering less frontal area to the wind. However in the case of ‘full’ aero seatposts you will require a frame with a matching seat tube, which limits the options when it comes to changing or upgrading. Some manufacturers get over this by offering posts with a shaft featuring a ‘normal’ bottom half, with the part above the seat clamp then shaped to a bladed aero profile.
• Dropper posts: ‘Dropper’ seatposts – telescopic hydraulic posts that are instantly height-adjustable by means of a saddle- or bar-mounted switch – have become hugely popular among trail and Enduro MTB riders.
For steep, technical descents the rider has the option of activating the switch and dropping the saddle out of the way, enabling them to more easily position their body over the back wheel for improved control and weight distribution (previously riders would have had to hop off the bike and manually release the seat clamp and lower the post). Once at the bottom, another flick of the switch and compressed air causes the seat to rise back to its original position for optimum pedaling dynamics.
Suspension seatposts give riders the best of both worlds –a fun and flickable bike on the way down and optimum pedaling position on the way back up.
Once a popular option to add a little more comfort to your bike, suspension seatposts offer some degree of travel via an internal spring or other cushioning system. However they have been in many ways superceded by improvements in fork and rear suspension systems and have largely fallen out of favour.
They may however remain a simple and inexpensive option to add a little more comfort to your hardtail MTB or city bike.
Get spare parts for your ‘standard’ seatpost including replacement rail clamps, bolts and more.
Owners of ‘dropper’ posts will also require specific parts for their care and maintenance including bleed kits to remove and replace the hydraulic fluid inside, replacement switches in the event of crash damage to that part, replacement springs and other parts.
A seatpost shim is a hollow aluminium ring that allows to use a specific-diameter seatpost in a frame that is nominally too big for it.
For example, using the appropriate shim on a 27.2mm diameter seatpost will enable it to be used in a frame with a seat tube diameter designed for 31.6mm posts. This allows you to use a favourite or lightweight seatpost with a new frame of a different tube diameter, or to match a seatpost to an older frame with an ‘odd’ tube diameter.
In order to choose the correct shim you must first know the diameter of your seatpost, and secondly the internal diameter of your seat tube. Of course, it’s only possible to use a shim to increase the effective diameter of your seatpost – if you are trying to fit a larger seatpost into a smaller frame tube, you’re going to need to buy a new post.