Replacing your bicycle dropper can be daunting if you don’t know how to determine Seatpost size. A good Seatpost is the key to achieving a comfortable ride. Further, it enhances the aerodynamics and usability of your bike. The Seatpost is one of the most overlooked components of the bike, yet critical for efficiency and performance. Installing the incorrect size not only damages the frame of the bike but can also lead to back pain. How then do you determine the size of your Seatpost to avoid all this? We have prepared a step-by-step guide that will show you how to measure your saddle post so that you can get the full benefit out of your workout.
How to Determine Seat Post Size
Look at the Old Seatpost
It might seem obvious, but it is worth saying, you can determine the seat post size by looking at your existing or old saddle post. This is the simplest way to know the size of your dropper.
If you plan to change your Seatpost and don’t have a clue about the measurement, then you need to pull out your old post and check. Mostly, the sizes are usually stamped, or laser marked near the end of the insertion line.
Measure Your Old Seat Post Diameter
Due to old age and frictions, the marking can be erased. Thus, how do you determine the size of your dropper? Well, this is where we get hands-on and take the measurement. You can easily measure the size of the post using a digital caliper and metric calipers to check the diameter of the existing seat post.
First, if you have an old saddle-post that attaches firmly, then you can measure its outer diameter for an accurate estimate of what you need to buy. However, if the old seat post is worn out and you are unable to make reasonable estimates from it, then you need to follow the next step, where we will show you how to measure the seat tube to determine the correct seat post size.
Find Your Seat Tube Diameter
How do you measure your bicycle seat post size if you don’t have an existing seat post? Well, it is simple, read on.
A seat tube is a frame that holds the Seatpost. Put it another way; it is where you mount the seat post. When measuring the seat tube, you should pay close attention to the internal diameter of where the seat post will be inserted. Ensure you don’t measure the external diameter as it does not help measure Seatpost size.
We recommend using a Vernier caliper to measure the diameter of the seat tube. Vernier caliper is extremely precise when taking either external or internal measurements. You can check for calipers with digital readouts, which you can set in metric or inch at the click of a button.
Do You Want an Internal or External Routing?
Cable routing of the Seatposts can be internal or external. Internal routing (as known as stealth) is the most common type of cabling in mountain bikes. In internal cable routing, the Seatpost is linked to the remote level at the shaft of the post. Contrary, in external cable routing, cables are routed on the outside of the tube and attached to the shaft or the head of the post.
The routing you have will determine the type of post size that you will get for your bike. I suggest you go for the seat post, which has the cable routed to the shaft as opposed to the head of the post. This makes it less prone to damage and looks great aesthetically.
Determine the Travel Needed
The next crucial factor in determining the seat post is the length of the dropper. The length of the dropper is always measured when the post is at full extension.
Also, you need to know how much “drop” you need. Most bikes will have an allowance of 100 to 150 mm. It is advisable to get as much travel as you can get. This means that you have the freedom to move the seat, and you can lower it for more comfort and fun. Ultimately, you want a seat post that will maintain your saddle in a position suitable for pedaling. Avoid going for a seat post that leaves the seat too high even when fully inserted in the frame.
Determine Your Maximum Extension
To measure the size of the seat post correctly, you need to determine the height you want to have when your seat is extended to the maximum. You do this by measuring the seat from the collar to the center of the seat rail. When choosing a post, you don’t want it to be too long since it can affect the efficiency and transfer of power when pedaling.
Determine Your Minimum Insertion
How deep can you insert your Seatpost into the frame? You can determine the size by measuring from the top of the saddle collar down to where you feel there is nothing that can go further. It is essential to know the minimum insertion because it helps you to avoid focusing on extension numbers that can mislead you and make it difficult to adjust your post to lower points.
Determine the Overall Length
The overall length of the dropper is crucial when determining the size. Seatpost vary in length, and if you are looking for one, you should make sure that the length is sufficient for your needs. A short post can be uncomfortable and ergonomically unfit. Most Seatpost lengths range between 75 mm to 400mm.
Short Seatpost tend to be lightweight but less adjustable, while longer Seatpost tend to be more flexible. From observation, most mountain bikes have longer Seatpost, which makes them easy to adjust for uphill and downhill terrains, making them safe and offering you protection against injury.
Use Seat Post Sizing Rod
How else can you measure Seatpost diameter? We have additional ways that you can determine the size of your seat post. One of those includes the use of seat sizing rods. To measure, you place the rod inside the seat tube and match the diameter. The rods are designed to have an increasing diameter from one end to the other. Therefore, the correct reading, which matches, is the one above the end of the seat tube.
Check Manufactures Website
In this age of the internet, you can determine the size of your post by checking the manufactures website or through Google. Most manufacturers will put a catalog of the saddle size for their model, and the seat tubes they can fit. So, if you are planning to replace your post, I suggest you visit the manufacturer’s website for details. You should also read below for more information on post sizes and their fit.
Additional Measuring Hacks
- In case you don’t have a Vernier caliper, you can use an adjustable wrench to measure.
- Measuring is better than guessing. However, if you don’t have the tools to measure, you can use the chart developed by Sheldon Brown (or the chart provided in the FAQs section)to compare and determine the size.
What Size Seatpost Do I Need?
The size of the dropper depends on your needs. Nonetheless, you should be aware that the size of the post can affect the efficiency and the comfort of your bike. For instance, an oversize post makes a bike to be stiff though it enhances power transfer when pedaling. On the other hand, a narrow post is more comfortable on a rough surface.
Lastly, you need a Seatpost whose diameter can fit with the diameter of the seat tube for a perfect snug fit. Going by the industry standards, the most common size that fits most bikes’ frame is 27.2 in diameter (which is standard), and 30.9 or 31.66mm
Will A 31.6 Seat Post Fit A 30.9 Frame?
No. You need a larger frame. Trying to fit a big seat post in a small frame can lead to damage. As a general rule, if the seat post is bigger than the frame’s tube, then it cannot fit. While there are some instances when manufacturers’ sizes differ, it is not advisable to fit a larger post on a smaller frame.
However, on the flip side, a small Seatpost can fit in the big tube, but you must add shims to fit. Therefore, you need to follow carefully the steps we have provided in this article, so you don’t make the mistake of buying a Seatpost that does not fit in your frame.
Standard Bike Seatpost Clamp Size Chart
I recommend you have this chart as your checklist before buying a Seatpost. The first row shows the standard size of the seat tube and the different sizes that can fit. For instance, the 25 in the second column can fit sizes 25.4, 25.5, and 25.8.
Can a Wrong Size Cause the Seatpost to Get Stuck?
Yes. Your Seatpost can get stuck either because of improper installation or the wrong size. Getting the right size eliminates the need to force in the post. An oversize dropper will necessitate you to force it down, thus causing a bulge in the seat tube, while a thin dropper will necessitate you to overtighten its bolts, hence causing a deformation: This is what leads to post getting stuck.
A Seatpost that fits well in the bike is vital for a comfortable ride. With a good post, you can reduce high-frequency road vibration, balance weight, and increase the aerodynamics of the bike.
To choose the right Seatpost, you need to measure the diameter of the frame accurately. Having the correct size will give you the freedom to conquer varying terrain and have fun while at it. On the other side, getting the right measurement will save you the cost you would incur in purchasing a dropper that does not fit or one that damages your frame.
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Last Updated: June 2, 2022