Introduction

Height-adjustable seatposts add the cool feature of being able to quickly get your saddle down and out of the way for descents on your mountain bike. However, choosing the wrong one can put your saddle in a position that is too high or too low for normal riding, and few manufacturers provide the necessary measurements to avoid this mistake. This guide should help you in picking the right post the first time.

The factors at play

There are several distances to take into consideration here. Not all of them will be applicable to every bike, but for the sake of being comprehensive I've included them all:

What is your current comfortable saddle height?
When fully inserted, what is the saddle height of this dropper?
How far can you insert a post into your frame?
How far can this dropper be inserted into a frame?
How far must you insert a post into your frame?
How far must this dropper be inserted into a frame?

Length a is the most important. When we're finished, you'll be able to take this number and verify that it is within the saddle height range that we've calculated for each dropper post. To measure this, use a metric tape measure to find the distance in millimeters between the middle of the saddle rails down to the to top of the seat clamp where your post enters the frame. If a < b, you can quickly rule out that dropper since it will not give you a low enough saddle position even when fully inserted.

Length b is the next most important measurement. It's the distance from the middle of the saddle rail clamp on the dropper post (at full extension) down to whatever protrusion on the post that prevents it from being inserted any further. It's easy enough to measure, but if manufacturers would only publish this "Minimum exposed post" number then it'd be easier to confidently purchase an expensive dropper sight unseen.

Length c only needs to be measured if you have obstructions in your frame's seat tube sufficiently high enough to prevent the full insertion of a seatpost. These obstructions could include a bottle cage rivnut, suspension pivot, or other stop. Loosen your seatpost binder and push your saddle down into your frame until it stops. If it stops at the saddle clamp, you're probably fine. If it stops before then, mark your post with some tape right above the seatpost clamp, pull the post out and measure from the tape to the bottom. This is your maximum frame insertion.¹

Length d is the other one of those measurements I wish manufacturers would publish. It represents the distance between the bottom of the dropper and the protusion right below where the upper telescoping tube enters the lower stationary tube. They could call it something like "Maximum post insertion." Combine it with length b, and you have the full length of the dropper post.

Lengths e and f are both minimum insertion, but there's a subtle difference. The first is the minimum safe insertion length required by your frame, while the second is the one required by your post. Always insert a post to whichever number is greater. The post will be marked, but the frame probably won't be, although it's unlikely that length e will be greater than f unless you have an unusual seat tube construction, such as external seat post clamps à la Cannondale Raven.

Crunching the numbers

Here's where we get down to it. You have the measurements in hand, now we're going to do the math and figure out what the saddle height range is for a given dropper post in your frame. Good thing it's just addition and subtraction.

If c > d, then Minimum saddle height = b
If c < d, then Minimum saddle height = b + (d - c)
If e > f, then Maximum saddle height = b + (d - e)
If e < f, then Maximum saddle height = b + (d - f)
If Minimum saddle height ≤ a ≤ Maximum saddle height, then the dropper has a useful saddle height range for your frame!

Measurements for Some Dropper Posts
Model	Minimum exposed post (b)	Maximum post insertion (d)	Minimum post insertion (f)
FOX D.O.S.S. 4"	165mm	167mm	99mm
FOX D.O.S.S. 5"	190mm	193mm	99mm
GravityDropper 2" 275mm 300mm 325mm 350mm 375mm	133mm	142mm 167mm 192mm 217mm 242mm	76mm
GravityDropper 3" 275mm 300mm 325mm 350mm 375mm 400mm	159mm	116mm 141mm 166mm 191mm 216mm 241mm	76mm
GravityDropper 4" 350mm 375mm 400mm 425mm	184mm	166mm 191mm 216mm 241mm	76mm
GravityDropper 5" 400mm 425mm	210mm	190mm 215mm	76mm
Rock Shox Reverb 100mm/355mm	165mm	185mm	80mm
Rock Shox Reverb 125mm/380mm	190mm	185mm	80mm
Specialized Command Post 125	190mm	230mm	100mm
Thomson Elite Dropper	188mm	212mm	100mm

^{1. A slightly more complicated frame situation in which you'd have to measure
maximum frame insertion is when you have a full-suspension bike with a truncated
seat tube which opens shortly above an upward-moving rear shock or swingarm.
The most common examples of this would be the Specialized FSR and Trek Y-bikes.
For this style of bike equipped with an air shock, it's as simple as depressurizing the
shock and fully compressing the suspension, then reinserting the post until it
touches the first thing in its way, be it shock, swingarm, v-brake cable or tire.
For a bike with a coil shock where the coil itself is obviously the interference,
you may as well just make sure the post doesn't extend out of the seat tube,
since removing the coil to compress the suspension eliminates the point of
reference, and there's no way you could compress it far enough by hand or by
sitting on it to simulate its full range of motion.↩}