If you want to put a dropper post on your bike, we don’t blame you. The seamless transitions between the ups and downs completely change the experience of mountain biking. Dropper posts mean never having to drop your seat post before a big downhill again. As it goes with pretty much anything bike-related, finding a dropper post that will work is sort of complicated. From the diameter of the seat post to insertion length, amount of drop, and the type of routing, there is plenty to consider when purchasing a dropper post. Let’s walk through all the steps to selecting the right dropper post that will work for your bike.
Internal or External Dropper
It’s becoming common practice for mountain bikes to have internal routing. This means the cables go inside the frame of the bike and lead to the derailer, brake caliper, and seat post. If your bike has internal routing and then go with an internally routed dropper post. This is the most common post out there. An internally routed dropper post will keep things looking clean. If your bike doesn’t have internal routing then you’ll need an externally routed dropper post. This will likely be the case if you have an older bike.
Dropper Length
Figuring out how much drop you need/want is the next step. Ideally, you want to be able to drop the seat all the way down to the seat post collar. You can figure out this length by measuring the distance from the seat post collar to the saddle rails when your seat is set to the correct height. Most dropper posts will come in lengths of 125, 150, 175, and even 210 mm. The number you measured should be somewhere in that range. PNW and other dropper post brands make it easy to find the perfect length with adjustable shims of 5 to 10 mm increments.
Insert Length
In some cases, you might not be able to stick the dropper post all the way into the seat post. If the seat tube bends or curves, it limits how far the post can be inserted. Figure out the maximum insert length on your bike by seeing how far you can push your current seat post in or using a measuring tape. If the seat tube is 450mm long, and the dropper post insertion length is 500mm long, the post will stick out 50mm. If your ideal seat post height is 150mm from the collar, you’ll need a 100mm dropper since the post is sticking out by 50mm. Most manufacturers will list what the minimum insert length is. OneUp has one of the shortest insertion lengths, making it compatible with a lot of bikes. Posts like the RockShox Reverb run longer making it more difficult for a perfect fit.
Seat Post Diameter
The next piece of the puzzle is finding a post with the right diameter. The three most common sizes are 30.9, 31.6, and 34.9. You can put a smaller post in a wider diameter seat tube by using a shim that fills the empty space. You can’t put a bigger post in a smaller tube.
Final check list
Do you need an internal or external post?
Does the dropper post length match your current setup?
Can the dropper be inserted all the way into the seat post?
Is the seat post diameter the right size?
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