The first thing you need to know when looking for a new seat post, fixed or a dropper, is to know the diameter of the frame’s seat tube. Often times the diameter is printed on the seat post (this is only accurate if the frame doesn’t have a shim), alternatively you can find this information on the brand’s website, or you can reach out to your local dealer. Another option, if you have a caliper tool or a ruler and math skills, is to measure the seat tube at home.
[Example of a digital caliper]
Now that you know the width you’re looking for, it’s time to determine the length you will need.
Fixed posts are a little easier here, as long as the seat post you have now is a good length, then you can remove it from the bike and measure it. However, if you get one too long, the bottom can be cut (just be mindful of the minimum insertion length), get one too short though and you’ll need to replace it.
For dropper posts, assuming your current seat post is already at your desired height, the best place to start is to measure from the center of the saddle rails to the seat collar. This will help you determine the full extension, or “drop length”.
[example of full extension]
But before you start your search, there are a couple more measurements to find; the full insertion length and maximum overall length.
The full insertion length is the distance a seat post can be inserted into the seat tube before it hits an obstruction. To find this, measure from the seat tube collar to a point where something would keep a seat post from being able to go lower; i.e. a bend designed in the frame.
[example image of insertion measurement]
The maximum overall length is the longest dropper post you will be able to put on your bike before it is too long for you. To get this number, add the full extension and full insertion lengths together. Note: unlike a fixed seat post, the size of a dropper post can not be altered.
[full extension + full insertion = maximum overall]
How to Find Your Saddle Height
If you are unsure of how to find the most efficient and effective saddle height, it’s ideal to follow this general guide; your heel should maintain contact with your bottom pedal while your leg is fully extended when relaxed. This way, when you’re pedaling on the ball of your foot you know there will always be at least a slight bend in your knee. Having a proper saddle height is important because it allows the rider to get the most out of each pedal rotation while preventing joint and muscle pain.