Lowering and raising your saddle used to be less convenient than it is today. Everyone used to stop at the top of every extended downhill section and lower their seat a few inches, only to stop again at the bottom of that downhill section and raise their seats back up to the proper height for pedaling. These days, dropper seatposts allow us to do the same, only now we can do it on the fly at the push of a button without stopping, providing us the opportunity to always have our seat at the proper height for the changing terrain in front of us.
Many people consider dropper seatposts to be one of the most game changing innovations in the mountain bike industry, literally changing the way we ride. We certainly won't argue with that. We feel that a dropper post can dramatically improve the way you ride while also making it easier and more fun than it already is. There are a number of things to consider when shopping for a dropper post and we hope to make sense of some of that below.
Types of Dropper Posts
Infinitely adjustable dropper posts allow riders to adjust the height of the post anywhere within its travel range. All of the dropper posts in our most recent dropper post test feature this type of adjustability. For example, the Editor's Choice Award winning 9point8 Fall Line will lock into place all the way up, down, or anywhere in between based on how far you compress it with your weight or allow it to extend when pressing the remote lever.
Multi-Position Dropper Posts
Thes types of dropper seat posts are becoming less common, but there are still several multi-positional posts available on the market. These posts typically have three positions, fully extended, fully compressed, and one in between. While they may not offer the range of adjustment that an infinitely adjustable post can the positions are typically adequate to improve your mountain bike experience nonetheless.
Height: Travel, Ride, and Stack
When sizing a dropper post there are several measurements that are helpful to know to determine the size you can fit on your bike. The length of your inseam, the length of your seat tube, and the length of the seatpost at full extension are all factors that need to be measured and considered along with the measurements listed below.
First the length of travel is important. Dropper posts come in various lengths, typically, 100mm, 125mm, 150mm, and now up to 175mm and even a whopping 200mm. This length measures the amount of travel between the bottom of the saddle clamp and the top of the seatpost collar. We generally want to the most drop we can get, usually 150mm, although riders with shorter legs may be limited to shorter lengths of travel.
The ride height is the measured length of the seatpost at full extension between where the seat rail is connected to the saddle camp and the bottom of the post collar. The measurement represents the minimum length that will be outside of your bike frame above the seatpost clamp. Stack height is the measured length of the post at full compression between where the seat rail is connected at the saddle clamp and the bottom of the seatpost collar. This number can also be figured out by subtracting the travel length from the ride height. The Crank Brothers Highline had the lowest stack height of all posts tested at 47mm.
It is also important to measure the usable length of a bike's seat tube. Modern suspension designs often interrupt the seat tube and may limit the length of post that can be inserted into it. The overall length of a seatpost at full extension may dictate the length of travel that you may be able to fit onto your frame. Dropper posts with shorter ride heights/stack heights may help to squeeze a little extra travel onto yours.
Seat Tube Diameter
Dropper seatposts are typically offered in the most common seat tube diameters of 30.9mm and 31.6mm. A limited few are also now being offered in 34.9mm diameter. It is of the utmost importance to purchase a model in the diameter that matches your frame's seat tube. This is easily measured, or can usually be found by searching your bike's model and specs online.
Hydraulic vs. Cable
The RockShox Reverb Stealth is the only model on the market that actuated hydraulically, with hydraulic fluid that runs from the remote to the bottom of the seatpost. All of the other contenders we tested are cable actuated. Cable actuated models use a standard derailleur cable that runs through standard derailleur housing from the remote to the bottom of the post. Cable actuated posts vary in difficulty to install, but most are easier to deal with than the hydraulically activated system. Standard derailleur cable and housing is also inexpensive and easy to replace, and a damaged cable can often be repaired in the field in a pinch.
Dropper seatposts are available in two different routing options, internal or external. All of the contenders we tested are internally routed, which is becoming the norm for dropper posts on modern frames which feature port holes for running the cable and housing internally to an attachment at the bottom of the post. Internal routing limits the number of cables attached to outside of the frame for a cleaner appearance. Running housing through the frame is often the biggest challenge associated with installing internally routed dropper seatposts. Externally routed dropper posts are available for older frames or those without port holes for internal routing. The cable and housing of externally routed dropper posts runs on the outside of the bike frame and attaches to the post somewhere above the post clamp.
We hope the information above is helpful when it comes to deciding what size dropper you need for your bike. For a detailed analysis of the dropper posts we tested, read our full Dropper Post review here.