Having a dropper post is a huge luxury for every mountain biker. Since dropper posts come in different travel options, different lengths, and different seat tube diameters, this is a tell-tale sign that not all dropper posts are the same and you need to find your fit before purchasing. To make sure you pull the trigger on the proper size dropper seatpost for your bike, you will need to know:
Dropper posts come in diameters of 27.2, 30.9, 31.6, and 34.9mm. The first step to choosing your correct post would be to match the seatpost diameter to the frame’s seat tube diameter. You can do this by pulling your current post out and looking for the printed/engraved numbers on the post, looking online at your bikes specs, or if those two routes fail, you can take a caliper and measure the diameter that way.
Dropper posts come with two types of cable routing, a cable that connects to the top of the post near the clamp or at the side of the post at the bottom of its stroke. Both of those are usually called "Non-stealth" since the cable is exposed and you have to route it outside the frame. Most bikes made before 2012 do not have "stealth" routing and in this case you need this kind of cable routing on the post. Most new'ish bikes have stealth routing for dropper seatposts and this is where the cable connects directly to the bottom of the post and is internally routed in your frame. This is becoming so common among new bikes, companies who just released a dropper post like RaceFace and Easton do not even offer non-stealth versions. Your frame basically has cable routing for a stealth dropper or not, you need to know this in order to get the correct post for your bike. The RockShox Reverb was one of the first dropper posts and they came out with the term "Stealth" and it has become the commonly used term even though no other brands call it that. KS calls their "Stealth" routed posts the "Integra" versions. Other brands just call it "Internal routing" or some variation of that.
You will also need to know the length of the post. The overall length of most posts is measured from the bottom edge of the post to the top clamp of the cradle (where the seat rail would be). For dropper posts, this is always measured when the post is at full extension. Knowing this length of your current post will help you know if the dropper post you are considering will fit. Depending on your stack height (mentioned later) and the insertion length, this will help you determine what the max travel of a dropper post you can run. Travel on these posts ranges from 65mm - 170mm. (125mm is the most common and the length about 85% of riders go with.)
The length of the post that comes out of the frame is known as the stack height. This is the distance from the top of the seatpost collar to where the seat rails would be in the saddle clamp if the post was all the way inserted in the frame. You need to know this to determine what travel length you are able to run. To choose the correct drop, measure from the center of saddle rail down to the top of the seat collar on the frame with the post height at the normal climbing height (This is where you are set up for full pedaling mode where your knee has a slight bend when the crank is at the bottom of the stoke. Just like you'd set up a road bike seatpost height, the height is where it is for maximum pedal efficiency and nothing else), then subtract 50mm. For example, if the stack height measurement on your bike was 180mm, you subtract 50mm to get 130mm. This means you can run a post with 130mm travel or less, but not more. The reason you don't want more is because if you tried, you'd have the post as far as it goes in the frame and it would be too high for you at full extension and you would not be able to reach the pedals unless you put the post down some. This is simply not ideal and not the way to do it - you want a post that has the right amount of drop so that you have a bit of exposed post and can extend it to full extension and once at full extension, it will be at your perfect climbing height.
When talking about the insertion length, this is the portion of the dropper post that is inside the frame: it is the distance from the top of the seatpost collar to the bottom of the post. Since some internally routed posts have hardware that protrudes out of the bottom of the post (i.e. Rockshox Reverb stealth and KS Integra versions) you will have to figure that into the measurement as well. This is more of a concern when using a long travel post on a small frame. To figure out your max insertion length, measure from the frame bend, routing port, pivot bolt that stops the post from going down etc. Basically the spot where the post bottoms out in your frame, measure from there to the top of your seat tube. This tells you the maximum insertion you can fit in your frame, most dropper posts will tell you the minimum insertion it has to go into the frame and that of course needs to be smaller than your max insertion. Seeing a dropper post with a min. insertion that is greater than your max insertion is rarely a problem, but some frame designs can be problematic or short seat-tubes on size small or extra small frames can also come into play here.
The last step is to know/measure your current seat height so that way you are able to make sure that your new dropper post will allow you to match your current set-up. Make sure to keep in mind all the measurements from above. The proper way to measure your seat height is to go from the center of the crank spindle/bb to the top of the saddle.
Some helpful tips when it comes to picking out the right dropper post is to not try to “cut it close” or have the post slammed on the frame as this is risky. Also, anybody wanting to match a smaller frame with a longer post, keep in mind the minimum insertion line and make sure to never install/ride a post past that line.
When all's said and done and you switch over from a rigid post to a dropper post, you will be extremely satisfied that you made the upgrade. Every rider we know that made the switch is happy they did and we highly recommend this upgrade to anyone in the market for a new seatpost. Personally I would rather not ride my mountain bike if I didn't have a dropper on it. It is considered as necessary as having air in the tires to most riders :)
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