Higher Education: How to Upgrade to a Longer Dropper Post

Simple in concept, complex in design; the dropper post is possibly the most influential invention of the modern mountain bike era. It revolutionized the trail bike and brought a flow to the ride we’d never experienced before. The early years might have been a bit squishy … err … rocky, but in the year 2021 dropper posts are, by and large, solid and reliable. 

However, there still remain issues when it comes to which droppers today’s bike brands brands choose to spec out of the box. Chances are that the dropper that came on your new bike is shorter than it could (and should) be. And that is understandable. Brands tend to play it safe because speccing too long of a dropper will make a bike unrideable, while speccing too short of a dropper is simply annoying. Not all, but most riders can upgrade right away, so how much more travel can you actually fit in there?  

This edition of Higher Education will give you the know-how to answer that question. From insertion depth to cable routing to even your footwear choice, we’re going to be covering everything you need to know to make the most informed decision when it comes time to upgrade your dropper post. 

Relevant terms:

• Seat Tube: The part of the frame that the seat post is inserted in.
• Seat post clamp: Just what it sounds like. The collar that tightens your seat post at a given height.
• Saddle clamp: The part of the seatpost that clamps the saddle rails.
• Seal head: The part of the dropper that houses the dust seals. Part of what is used to calculate the minimum exposed height of a dropper.
• Slam: When a dropper post is able to be inserted deep enough that the seal head rests on the seat post clamp.
• Drop: The amount of vertical travel of a given dropper post.
• Exposed length: In this post, we’ll define exposed length as the distance from the top of your seat post clamp to the saddle rails. Includes the seal head height, drop, and saddle clamp height.
• Insertion depth: The maximum depth that a seat post can be inserted into the seat tube. This is often the determining factor when upgrading to longer droppers.

First things first, when upgrading droppers you’ll need to know your seat tube diameter. Look at your current seat post. Somewhere near the bottom, it will be stamped in the aluminum. If you can’t find it, either visit your bike manufacturer’s website or grab a trusty set of calipers to measure the current post. You’ll probably be looking at either 30.9 or 31.6 millimeters, although 34.9 and 27.2mm are possibilities, though they are uncommon. 

There’s also the question of how the cable will connect to your new dropper, which will be internally or externally. Chances are, if you’re currently rocking external routing, or simply aren’t running a dropper yet, you’ll probably have to stick to external routing. However, it’s worth checking with the manufacturer to see if your frame supports internal routing as some frames have ports for internal routing where you may not expect, either up near the front of the frame or at the base of the seat tube. Many new bikes that don’t come with droppers do support internal routing, especially those at entry-level pricepoints, so be sure to check for that before writing it off. It is by far the best way to go, and has the most options.

Now that you’ve figured out the basics of your new dropper, it’s time to talk about size. When it comes to droppers, length matters most—you probably want to get the most travel out of a dropper, but there are limiting factors to consider. Sadly, not everyone can fit a 200mm super dropper. 

There are two determining factors when it comes to fitting dropper lengths:

1. Exposed length
2. Insertion depth

Sound easy, right? Yeah, not so much. 

Assuming that you’ve long known your correct saddle height with your current seat post, let’s start with exposed length. Say you measure and your exposed length is 200mm. Great, you can fit a 200mm dropper! Right? Wrong. Your exposed length is composed of two other components in addition to the amount of travel your dropper has: dropper seal head and saddle clamp. Your 200mm dropper, when slammed into the frame, probably actually measures closer to 250mm when it comes to exposed length. In other words, it’ll be way too long for you and you’ll probably only be able to run a 150mm or 175mm post (depending on the brand).

So, how do you translate exposed length into dropper length? Well, it’s simple—research. You’ll need to look at the specification of whatever dropper post you’re after to find the distance from the bottom of the seal head to the center of the saddle rails. Most dropper post manufacturers publish this data and make it easily accessible for fitment reasons. One example of manufacturer specifications is below.

Something to keep in mind is that different droppers will, well, be different. You might be able to fit one brand of dropper in a specific travel length, but another brand might use a shorter seal head that makes the overall height eke into the useable range. Be sure to check multiple brands for this reason—if you’re mere millimeters from fitting a, say, 175mm travel post, in Brand A, keep looking around because you might find that you’ll be able to fit a 175mm post from Brand C. We’re not here to pick favorites, but OneUp has a reputation for having some of the shortest exposed lengths on the market.

Now that you’ve determined the longest travel dropper that your exposed length allows, let’s move on to crushing your hopes and dreams with insertion depths.

Most frames limit the depth a seat post can be inserted into the seat tube by way of pivots, bends or water bottle bosses. Your exposed length might allow you to run a full 200mm of drop, but if your seat tube only allows 150mm of post to be inserted, you’ll be SOL. 

There are three ways to determine insertion depth, all of them have flaws.

1. Manufacturer specification
2. Measuring tape and flashlight
3. Actual seat post insertion
4. Let a bike shop do it

Many bike manufacturers publish the maximum insertion depth in their geometry or specs chart. Generally, these are pretty accurate, but they’ve been known to be wrong on occasion and when 5mm can mean returning a $400 dropper post, it sucks to be wrong. How do you avoid this? Double check the published number with your own measurements.

This is where a measuring tape and flashlight can be your best friend. Actually look down into the seat tube and see what might obstruct a seat post. Is it the insert for a water bottle bolt? A frame pivot? Great, you can stick a tape measure on those and get a cold, hard number. Is it a bend in the frame? Well, a tape measure can get you close, but depending on the sharpness of the bend it might not be a trustworthy measurement. 

That brings us to the most reliable method to measure insertion depth: Actually get a seat post and put it in the tube. Before you scoff and click away, hear us out. Insert a long, rigid seat post of the correct diameter until it bottoms out. Make a mark or wrap some tape on the post to measure insertion depth. This method will take into account bolts, pivots and bends, no matter how slight. 

Here’s another tidbit to consider—all internally routed droppers have a device sticking out of the bottom that attaches to the cable and, of course, a cable below that. That device is usually factored into the published insertion depth by the dropper post manufacturer, but the cable itself is not. So, let’s say you have a pivot in your frame that limits depth to 250mm and the insertion depth of your dropper is 245mm. Great! Well, maybe great. Is that pivot cut across the middle of the seat tube? Can the cable bend enough to get out of the way of it? Will the bend cause so much friction that the system doesn’t function? What to do?

The easy answer is to err on the side of caution. In the worst case scenario, cable housing will need to jump from one side of the seat tube to the other to make it from the bottom of the post to wherever the frame manufacturer intended it to pass the obstruction. A piece of normal housing should require no more than 40mm to smoothly make that transition. If you can see a significant obstruction across the center of the seat tube, adding 40mm to your chosen seat post’s claimed insertion depth will give you a safe measurement your frame will need to pass.

If you are closer than that, this is where a bike shop can save you a lot of headache. They can run the trial and error for you. But please, if you are going to have a bike shop install your new post, buy it from that bike shop.

There are a few things to consider, however. 

First, some droppers on the market right now allow for travel adjustment. So, you can adjust a 200mm travel post down to 190mm. Okay…why is this useful? Well, if your frame’s insertion depth allows you to run that 200mm post, but your exposed length calls for 190mm of drop (for that brand dropper). Great, you can buy that 200mm dropper and just run it at 190mm! 

Alternatively, your exposed length calls for you to run a 200mm post, but your insertion depth is 10mm short of being able to fit that post. Instead of bumping down to a 175mm post (based on this hypothetical brand’s offerings), buy the 200mm post and run it shorter. You get the same exposed length with a little bit of seat post extended out of the frame, but with more drop than the 175mm post.

On a final note, most of the time these measurements won’t come down to just a few millimeters. More than likely, it’ll be clear which posts will work and which won’t. But if you do find yourself down to the millimeter, keep in mind that you may run into issues in the future if you change your saddle, your pedals, or even your shoes. Each of these can affect your optimal saddle height, and if you choose a dropper that is at the absolute max, it may keep you from getting those more supportive Five Tens, or maybe even that fancy Switchgrade gizmo. 

And on that note, we’ll leave things here, because internal cable routing is a whole other can of words we’ll open at a different date. For now, good luck, make sure to read the right side of the tape measure and remember, measure twice, buy once!

Phtoos: Ryan Palmer