Guide Before Buying or Replacing a Bicycle Seatpost

Guide Before Buying or Replacing a Bicycle Seatpost

Seatpost is a saddle support tube (bike mount) that is inserted and mounted on the bicycle frame. The diameter and length of the seat post are the main determinants of getting a suitable seat post. But there are offsets (laybacks), features, and types of seat posts that make seat posts have different functions and abilities. Read on below if you want to know how to choose a suitable seat post and what things to consider when buying or replacing a bicycle seat post.

Seatpost Size

The seat post or “stand post” may appear to be just a truss. However, when it comes to replacing or buying a seat post, there are several parameters that we must pay attention to so that it can be installed on the size of our bike. Not only that, there are also seat posts that are specially designed to create different cycling abilities.

As we know from “the influence of bicycle geometry”, the angle, length, and height of the sitting position will affect the feel and ability of the bike. The angle of the seat post will follow the angle provided by the bicycle frame, but some seat posts also have additional angles, which may improve the performance of the bicycle or make the performance of the bicycle less comfortable.

The following are the parameters that need to be considered when determining a suitable seat post.

Seatpost Diameter

Diameter is the main determinant of a seat post can be installed or inserted into the frame. The diameter of the seat post must be the same as the diameter of the bicycle frame. Most bicycle seat posts are round in shape which can be measured or calculated in diameter, except for the aero seat posts which are flat (discussed later).

To find the diameter of the bicycle seat post, measure:

the inside diameter of the bicycle frame, or outer diameter of seat post
For the right measurement, you can use a caliper (caliper), or a seat tube size rod (a special rod for calculating the seat post diameter), or other diameter measuring tools.

A simpler way is to wrap the existing seat post with thread/string or paper, then calculate the length of thread/paper needed to wrap a full circle. Using the circle formula, where the circumference of the circle is 2π x r or x d, then the length of the thread is the circumference of the circle, to get the diameter (d) divide the circumference (thread length) by 3.14 (π).

For modern bicycles, the units of measure are usually multiples of 0.2mm. The most widely sold seat post diameter sizes for mountain bikes and racing bikes, BMX, folding bikes are 25.4mm, 27.2mm, 30.9mm, and 31.8mm. Folding bikes, such as Brompton folding bikes, usually use a large seat post diameter of 31.8mm.

For the seat post diameter which is larger than the bicycle frame, it will not be able to be installed. If forced to install, it will damage the frame or seat post (whichever is weaker).

If the seat post is smaller than the frame, there is still a possibility that it can be installed using a seat post converter/adapter, called a shim.

This seat post shim is like a tube or a thin layer that will cover the seat post so that the total diameter is larger, and can be mounted on bicycle frames with different diameters. Not all size conversion sizes are available, so pay attention to the value of the size converted, for example: 25.4mm seat post adapter to 28.6 frame or 30.4-27.2mm adapter, and so on.

Installing a seat post size that is smaller than the inside diameter of the frame without a shim is not recommended, because there will be excessive pressure at one point on the seat post tube and one point on the bicycle frame, which can damage both.

Seatpost Length

The length of the seat post also needs to be considered, because it will affect the maximum and minimum height for the bicycle seat, as well as its weight. For the same type of material the longer the heavier, the shorter the lighter. But the short seat post has a disadvantage in the flexibility of adjusting the seat height. 

The seat post also has an insertion limit written on the tube. Seat posts must be inserted into the frame beyond that limit, so that the bicycle frame has a foundation / grip that is strong enough to support the load on it. So the seat post can’t be too short, we force it to get the height we want with the insertion limit on the outside of the bicycle frame.

Choose the length that suits your needs, for racing bikes you may need a long seat post so your legs don’t bend, for downhill or extreme offroad you may need a short one so you can easily stand on the pedals, and so on.

Also pay attention to the bicycle frame, some frames have a bent or curved shape, so not all tubes can be screwed in. Additional bolts for bottle cages or drinking containers can also prevent the seat post from getting too deep.

The length of the seat post varies from 75mm to 400mm. Some use letter units (XS, S, M, L, XL), which can be selected according to the size of the bicycle frame, but it doesn’t always fit if you use font size, you should also find out the length.

For folding bikes, the seat post can be very long up to 600mm. Because folding bikes with wheels are small but are used by adults who have a long leg reach. so it requires a sitting position that is much higher than the height of the bicycle. 

Unit labels or markings like the ruler on the seat post can also help make it easier for us to remember or find the ideal seat post height. So we don’t have to measure, guess, or scratch the seat post to adjust the height of the saddle if its position changes.

Read : 7 Things That Are Usually Upgraded On New Gravel Bikes

Offset/Layback/Setback Seatpost

Another feature that may also be found on a bicycle seat post is the setback or layback or offset. Layback on the seat post states that there is a bend or change in the angle at the top of the seat post (where the saddle attaches), expressed in angle units (degrees), generally around 0-35 degrees.

Setback or offset states the position of the saddle clamp whose position is further back from the straightness of the seat post, to get a more backward saddle position, expressed in units of distance (mm), can be up to 45mm.

The terms setback and layback are often used interchangeably, but it is important that we understand how much the saddle position changes due to this angle or offset. For a straight seat post without a setback, it is sometimes called a zero setback.

With this offset and layback, the sitting position will be further back from the straightness of the seat post pole. Depending on your needs and comfort, this can be useful or uncomfortable.

This seat post is also useful for getting a more ideal position, if the size and dimensions of the bicycle are still not optimal, and cannot be changed anymore.

This setback is usually useful for maximizing the quad and hamstring muscles. Or on a bicycle frame where the sitting position is too forward/close, with a setbak the sitting position will be further back and increase the reach of the feet.

Although rare, there is also something called set-forward or advancing the saddle position more towards the handlebars. This particular model is usually worn by triathlon or time-trial athletes for maximum strength on flat roads.

Type of Clamp Seatpost

Most seatposts use twin rail clamps to secure the saddle to the seat post, with the bolts turning to lock the block securing the rails to the saddle. This bolt can be loosened to adjust the position of the saddle back and forth on the seat post. Some seat posts also have a feature that can adjust the seat tilt. On some special heavy duty bikes, the saddle clamp can hold rails that are larger than the standard (7mm).

Some seat posts can also have a single rail (single rail) that can only be attached to a special saddle that only has one rail underneath.

Seatpost Thickness

Thickness is sometimes also shown on the seat post specifications. The thicker the seat post, the stronger the seat post, but also the heavier it is, and vice versa. But the strength of the seat post is also determined by what material is used.

Seatpost Weight

The weight of the seat post really depends on the material used and its size, ranging from 100grams to 500 grams.

Bicycle seatposts made of carbon are of course the lightest (about 100 grams), iron that can weigh more than 0.5 kilograms, other materials that are often used are aluminum, alloy or titanium. Each material has its own advantages and disadvantages, the quality of manufacture is more dominant in determining a better seat post.

Some special designs such as seatposts with cavities or holes can also reduce the weight of the seat posts, but of course structurally, their strength is weaker than those without cavities.

Types of seat posts

Not only dimensions and geometry, the types of seat posts that are in circulation have special features that make them unique and have special advantages. Here we look at several types of seatposts based on their special features.

1. Rigid seat post

This is the most standard and most common type of seat post, without any extras, with a rounded stem. Because of its simple and rigid shape, the rigid seatpost has its own advantages, where we can use the seat post as a place for bicycle accessories, such as reflectors and bicycle lights, bag hangers, drink bottle holders, even for cargo (panniers).

2. Telescopic Seatpost

Telescopic seatpost is a seat post that consists of more than one tube that can be arranged into another tube to save space. By pulling out the tube on the inside, it can be extended more than it seems. Telescopic seatposts are usually used on folding bikes, because they have a small frame but are used by adults. The telescopic seatpost will solve the problem of the seat being too short on a folding bike for tall people. Because ordinary seat posts do not necessarily all can be raised to the ideal position for adults.

3. Aero Post

The seatpost is flat, so it can split the wind/air for a more aerodynamic bike. High end racing bicycles, racing, Time Trial usually use aero seatposts to get a higher speed than ordinary seat posts.

Aero seatposts that are flat from top to bottom can only be mounted on compatible frames (the seat post holes on the frame are also flat), which we don’t find on most bicycles. However, there are some products that make seat posts that have a round bottom but a flat top (blade), so that bicycles with a regular frame (round) can use an aero seatpost.

4. Dropper Seatpost

Dropper seat post has a unique function where we can adjust the height of the saddle easily and quickly, it has suspension or telescopin hydraulic in it, which can be controlled with a remote. So we don’t need to get off the bike, open the clamp, attach the clamp to adjust the height of the saddle. The position of the height of the saddle can be done while cycling because the suspension inside provides pressure that pushes the saddle up, the opening and closing of the suspension pressure is controlled by a special additional lever / button (remote). When the dropper lever is pressed, the suspension will move freely upwards, we need to hold with our body weight the position we want, then release the lever to lock the seatpost height position at the position we wanted earlier.

This feature is very useful for cyclists who need to change the height of the sitting position according to the area they are facing. As we know, the height of the seat post will affect the center of gravity (gravity) of the bicycle, and also the position of the foot against the saddle. Most dropper seat posts are used on mountain bikes where the terrain can change, but dropper seatposts for racing bikes are also available. Mostly used on mountain bikes, because they have a very varied terrain, so changing the seat position relative to the terrain at hand can provide more performance.

A low saddle position will create a lower center of gravity of the bike for more stable cornering, while keeping your feet closer to the ground in highly technical areas. The high saddle position will be efficient to maximize leg strength, making it suitable for use on flat or incline roads.

For frames that support internal cabling, the dropper seat post control cable can be inserted into the frame, so it looks neater and cleaner, and looks like an ordinary seat post. With the dropper seatpost that can adjust the height of the saddle easily, we can maximize muscle strength and efficiency throughout the trip without wasting time. Like a fork suspension or rear suspension, dropper seat posts also have travel or how far the suspension can push the seat post.

5. Suspension seatpost

The soft saddle will certainly make for a comfortable cycling experience, especially for long trips. But saddles also have a thickness limit, saddles that are too soft are also weird. This is where the role of the seatpost which has a suspension system, to reduce vibration or pressure on the bicycle seat.

There are many types of technologies and methods for suspension systems in seat posts. Some use an elastomer or some kind of rubber to dampen vibrations. Some wear a kind of spring that will move actively when there is a vibration. Expensive seatpost suspensions already use dampers, which can make the seat position more stable, and only the suspension moves up and down when there is a shock, thus keeping the foot in position relative to the pedal during vibration.

Some also only have a design with cavities to dampen vibrations on a flat road surface (road buzz). Suspension seat posts may be of great use on race bikes, fixie or BMX bikes that do not have a special suspension system and thinner tires. That way it will increase speed comfort when cycling using a bicycle without suspension (rigid).

A seatpost is not just a support pole, or a quick hook for bicycle lights, reflectors or other bicycle accessories. It is not only the position or height of the seat post that must be adjusted, but choosing the type and features of the seatpost will also affect the comfort and ability of cycling.