How to Measure Seatpost Diameter

The seat post which is also known as a “verb” is the tube that connects the saddle and frame of the bicycle. In the instructions for Soviet-era bicycles, this part was called a “saddle holder. This term is still used from time to time. It is a vital bicycle part that should not be skimped on. Therefore, before determining the size of the seat post, you need to recall these parts:

  • Brakes
  • The seatpost
  • Handlebar
  • Handlebar stem
  • Locks, clamps

Their breakage or malfunction can lead to very serious injuries on the road, up to and including tragedy. Pin length has a big impact on the cyclist’s riding position, along with handlebar extension. There is a lock at the top of the pin to which the saddle is attached, and the lower part is inserted into the seat tube of the frame. The seat post is clamped in the tube with a special bolted or eccentric seat post clamp.

Clamps with an eccentric are much more convenient, you can with one movement unfasten the pin and set the desired height. For clamps with bolts, you need either Allen keys or a screwdriver with wrenches. By the way, it is very convenient when all the necessary cycling tools: hex keys, screwdrivers, and others are collected in one compact multi-tool, which the cyclist can put either in the saddlebag or in his pocket.

Types of Seatposts

  • Hard Seat Posts
  • Rigid Seat Posts.

This is simply a tube with a saddle lock on one end. The most common type today. They are fairly easy to use, require no maintenance, are strong, lightweight, reliable, durable. Their only drawback is limited comfort, which, however, is enough for 90% of cyclists. After all, if you put correctly selected and adjusted saddle, correctly pump the wheels, the ride will be quite comfortable.

Rigid seat posts are made of:

Steel – the cheapest, quite common, and reliable option.

Aluminum alloys – are also quite common. Such prongs are lighter than steel but more expensive. Naturally, all aluminum alloy products are much less susceptible to corrosion.

Carbon – Lightweight and durable. On average twice as light as aluminum, but more expensive. Although recently we can see a downward trend in the price of carbon products, so assess your finances and choose.

Titanium and scandium – They are very light and durable but also very expensive. Used only by athletes or rich connoisseurs of “art”. Sometimes to reduce weight butted tubing is used. These pipes have different wall thicknesses in different places.

The normal practice is to use thicker walls for the seat posts at the point where they enter the seat tube and thinner walls closer to the top. Batted seat posts are more expensive than their standard counterparts, but lighter, so if weight is critical to you, look for these models.

Seat Post Strength and Reliability

Your health depends on its strength and reliability. Imagine how you will feel if it breaks right during a ride? Imagine. Agree, not very well, if not to say more.

The strongest, lightest, and most reliable are titanium ones – they are unlikely to break at all, but for an ordinary cyclist, they are too expensive.

Then go steel – these are also very rare to break. Also, unlike aluminum, they do not break immediately. Aluminum tends to “accumulate fatigue”. It breaks at once, and steel will first give a crack, and then, in this place, it will break. It is just this property of steel as a metal that allows you to notice the crack in advance and replace the pin. Of course, if the bike is not inspected from time to time, the steel pin will also break on the next bump.

In addition, our team personally doesn’t find the argument that the aluminum rod is lighter than the steel one very weighty. An extra hundred to two hundred grams will not save you.

Carbon poles stand out. Carbon itself is a very strong and durable material. The peculiarity of these pins is to buy real carbon pins, not a fake from a Chinese garage for a “promo price”.

Carbon is also quite a fragile material, so when you clamp the stem in the seat tube, don’t overdo the “Bogatyr’s strength”. Otherwise, it will crack at the clamping point and then break at the most inopportune moment.

We would advise buying a carbon pin only for bikes with carbon frames. And most likely someone who has enough money for a real carbon frame will also have enough money for a quality carbon post.

For everyone else who likes to ride, We would advise getting a regular steel frame as the best value for money. It will serve you well for decades, and you’re unlikely to feel it with your heel.

Suspension Seat Posts

Differs from the rigid by a special mechanism that softens jolts and bumps occurring during the ride.

Cushioning systems are very diverse and some examples are shown in the picture below. We will not describe them all. Some models compensate for shocks by moving only up and down. Others, receiving the shock from below, swing the saddle down and back, which provides more effective cushioning. On the other hand, this increases the distance to the handlebars, which is not very comfortable for some riders.

When choosing them, consider that they are quite expensive, weigh more than rigid and some models require regular maintenance.

As a disadvantage of shock-absorbing pins can be noted the following:

  • When adjusting the height of the saddle, you need to consider a slight sagging of the pin when the cyclist sits on it. And over time it may increase. But bikers get used to this quickly.
  • Cushioning most often works in the direction of the pin axis. Note that it is installed in the bike with a slope to the back, and the direction of shock when riding is mostly vertical and to the left, to the right of the handlebars. In such cases, cushioning helps poorly. I.e. the opinion that the shock absorber pin protects the fifth point from all shocks is not correct.
  • If the shock absorber itself is very stiff, it will weakly dampen small shocks.

You can find more information about them on the internet if you need to.

Height Adjustable Seat Posts

They are peculiar in that the sitting height can be altered during the ride. This is useful when, for example, going uphill, you want to sit higher and when riding flat or downhill, you can lower the seat post to reduce resistance to airflow.

Are in the form of two tubes inserted into each other with a clip – the so-called telescopic pins. They cannot be very handy, because in order to change the saddle height you have to stop, get off the bike and adjust the height.

Usually, they are installed on bikes with a frame that has restrictions on the length of the prong, but when the saddle needs to be raised high. Although there are models where the height adjustment lever is located directly under the saddle, which allows you to change the height on the go, how convenient it is, everyone decides for himself.

This disadvantage is deprived by hydraulic seat posts with the possibility of smooth adjustment directly during the ride with a special lever or a handlebar-mounted hand lever. Then the saddle height changes “with the slightest movement of the finger of the hand”. They are, of course, convenient for mountain biking, but they are very expensive, heavy, and require regular maintenance.

Disadvantages of Height-Adjustable Sea Posts

Just like shock-absorbing prongs, telescopic prongs can sag a bit under the biker during rides. The strength of such prongs is lower than that of rigid prongs. And sometimes, with low-quality models, there is a backlash between the tubes, which leads to rattling while riding.

Height Adjustable Seat Posts with Wireless Control

Things are changing so much that some brands are now putting wireless control seat posts on the market. By pushing a switch on the handlebar, the rider can raise or lower the saddle while riding.

How to Insert the Seat Post into the Seat Tube of the frame?

When adjusting the saddle height it is very important not to insert the seat tube in the frame above the marking on the seat post. It has to stay at least 10-15cm inside the seat tube, otherwise, the seat tube will simply break in time from the constant heavy loads on its top. Depth markings on seat posts

Adapters for Different Seat Post Diameters

Serious manufacturers usually put special depth marks on their products. Second point. If the seat post does not go down and you feel that something is in the way of the tube, do not hammer it in. First of all, you can break the pin as well as the frame.

Secondly, it is quite possible that when the frame was made there was a drop of metal leftover from welding, or the weld was not welded accurately. You will not cut the drip off with these blows, but this place in the frame will become a stress point and that is where the frame can break over time.

There are different diameters of both the pins themselves and the seat tubes on the frame. Of course, before buying a seat post you should check if it corresponds to the diameter of the seat tube on the frame. If it is larger, you will definitely not be able to fit it into the frame. But if the diameter of the seat post is smaller than the tube, you can use special adapters for their reliable fixation to different diameters. There are several diameters of seat posts.

The most common diameters are:

  • 0 mm. – For one-inch (25.4 mm) seat pipe. It is also called the Soviet diameter. It was very often, but not in all models, used in bicycles made in the USSR.
  • 2mm for a 1.1″ tube.
  • 6mm for a 1.25″ tube.

Some manufacturers produce frames with their own seat tube diameters and, accordingly, you should look for the appropriate diameter pins. For example, such diameters as (in mm): 26.0; 26.8; 29.2 (Trek, Stels steel); 29.6; 30.8 (Giant); 30.9 (Specialized); 31.2 (Atom fxc); 31.4; 31.8; 33.9 (Shulz)

In general, almost a complete list of seat post diameters in mm looks like this: 25.0 / 25.4 / 26.0 / 26.4 / 26.6 / 26.8 / 27.0 / 27.2 / 28.6 / 29.0 / 29.2 / 29.4 / 29.6 / 29.8 / 30.0 / 30.2 / 30.4 / 30.8 / 30.9 / 31.2 / 31.4 / 31.6 / 31.8 / 32.4 / 33.9 / 34.9

When selecting the adapter, be careful to ensure that the stem sits firmly in the seat tube of the frame, and does not wobble. Even a small gap of half a millimeter, in which the seat tube swings, will eventually destroy the seat tube. And it is not comfortable to ride on a swinging saddle. If it is difficult to find such an adapter, try simply wrapping the pin with duct tape or using thin metal plates to eliminate the gap between the pin and the tube.

How to Determine the Diameter of the Seat post?

There are several options: If you have an old one, you need to find out its diameter. How?

The most accurate and reliable way is to measure the diameter with a caliper. Measure with a simple ruler – a school or construction ruler. The method is not very accurate, but basic diameters are suitable.

Another method for those who are not looking for easy ways in life or if you do not have a caliper. We take a thin thread (the thinner – the more accurate the result) and wind 10 turns around the old pin. After that, we measure exactly the length of the thread, divide by 10 and by 3.14. Why 10 turns? To more accurately measure tenths of an mm of the diameter of the pin. Why divide by 3.14? It’s a high school geometry course: circumference length = diameter multiplied by the number Pi.

Wrap Up

A couple of other folkways to measure the diameter of the seat post using a construction angle and a school triangle or with an adjustable wrench are also quite popular among the people. But we are not mentioning them here because all the above-mentioned measures will be more than enough for any type of person.

Finally, if you are buying a seat tube for a new frame, you need to measure the inside diameter of the seat tube. The only option here is a caliper or ruler.

Jonathan Tim
Jonathan Tim
A bicycle geek since early childhood spent his twenties as a mechanic in bike shops. His passions include flatland BMX, unicycles, cycle touring, mountain biking, and road riding.