It wasn't long ago that we were all riding fully rigid posts and had to stop at the top of each descent to lower our saddles out of the way. The introduction of dropper posts has been, in my opinion, one of the best mountain bike inventions to date. A dropper post will transform your ride and allow you to gain more confidence on the descent, no matter what type of bike you're on. A dropper post is something I simply can't go without!
You may have noticed all the different sizes and versions available. So, how do you know you have one that's compatible with your frame? We'll help you identify compatible posts by giving you tips on what to look for.
Things you need to determine:
- Seatpost diameter.
- Internal or external routing.
- The amount of drop.
Common sizes are 27.2, 30.9 and 31.6. The easiest way to determine what size your frame needs is to look at the bottom of your existing post. The specs are usually printed at the end, if not, your owners manual is a good place to check. Also, you may hear of people using shims to make smaller posts fit frames. Please avoid this, it's not worth the hassle and potential issues that could/will arise.
Example of details printed on seat post. If this came out of your frame, you'll need a 30.9 dropper.
Internal vs. external routing.
New frames are increasingly coming with internal routing for a clean look. This means the cable will actually run from the bottom of the post through the seat tube and exit the front triangle (usually near the head tube). Common terms referencing internal routing are Stealth, Covert, and Integra. If the cable attaches to the post anywhere other than the bottom of the post, then you have an externally routed post. The following photos show the differences.
Externally routed post shown. Note the exposed cable
Internally routed post. Cable attached at the bottom.
It's a little more difficult to identify internally vs. externally routed frames. If the rest of your cables are externally routed on the front triangle, then you can almost certainly assume your frame will not accept an internally routed post. If you have an external frame, unfortunately, internal routeing will not work.
The amount of drop
The amount of drop is extremely important if you are running a smaller sized frame or have a shorter seat post height. The commonly available drops are 100, 125, 150, and 170mm. There are some brands that have sizes in between, but generally, you're going to have something between 100 and 170mm. At correct seat height on smaller frames, you'll most likely be unable to fit 150 and 170mm length droppers. If you can, it may be indicative that your frame might be too small. If you're concerned about fit, an easy way you can determine is to measure your exposed seat height from the top of the seat tube to the seat rails. Remember that you still need to add a few centimetres to accommodate the seat post collar. This is a quick and easy way to get a rough idea as to whether a post will fit.
Somewhat related to the amount of drop, you also need to look at the overall length of the post, more specifically, the amount that needs to be inserted into the frame. Mainly a concern on smaller sizes or riders that run short seat heights. You may measure your exposed seat height and determine there's enough room for 'x' amount of drop. However, you could find that you can't insert the post far enough in the frame. If this is a concern, we'll be happy to help you out.
We hope this guide will help steer you in the right direction for choosing a new dropper post. As always, we are here to answer any further questions and will do our best to find the perfect product for you.
Until next time,
Andrew @ MTB Direct