How to choose and use a dropper seatpost for gravel bikes

What is a dropper seatpost?

On a sports bicycle, power efficiency is increased by raising the saddle height as much as possible when riding on level ground or uphill, and pedaling with your body weight on the saddle. On the other hand, when riding downhill on a MTB, the rider has to stand on the saddle to maintain balance, and if the saddle remains high, the saddle may move up and down over bumps and get in the way, or it may hit the rider’s buttocks.

The dropper seat post is a part used to raise or lower the height of the seat post with a lever at hand to prevent the saddle from hitting the buttocks when descending.

Why do gravel bikes need dropper seatposts?

The advantage of gravel bikes over road bikes is that they can be ridden on unpaved roads in road bike style.

However, gravel bikes are increasingly being used to ride on MTB-like trails, and the introduction of dropper seatposts on gravel bikes is a result of the need to be able to lower the saddle at hand, just like MTBs. For example, Shimano’s GRX gravel component, the ST-RX810-LA left STI lever, is compatible with dropper seatposts.

In a sense, the introduction of dropper seatposts on gravel bikes is a symbol that gravel bikes are becoming more like MTBs.

What is the difference between a dropper seat post for gravel bikes and one for MTB?

Each manufacturer has a lineup of products for gravel bikes separately from MTB dropper seatposts.

The difference between a dropper seat post for MTB and a dropper seat post for gravel bikes is the travel, which is the width of the seat post where it drops down. Therefore, products with 50 to 100mm of travel are mainly used for gravel bikes.

Since the amount of travel is directly related to the weight of the dropper seat post, dropper seat posts for gravel bikes are mainly lighter than those for MTB.

Of course, as long as the seat tube diameter and length are compatible, an MTB dropper seat post can be used on a gravel bike, but since MTB dropper seat posts are heavy, it is more advantageous to use a lightweight dropper seat that adds a little weight to a regular aluminum seat post than to use a seat post with a travel that is not used, especially on paved roads.

Types of dropper seatposts for gravel bikes

The types of dropper seatposts for gravel bikes are basically the same as those for MTB, divided into the following three types

1. wire routing (interior/exterior)
2. amount of travel
3. seat post diameter

All of these types are divided in terms of whether they can be installed rather than in terms of "differences in function or performance.

Not just interior/exterior type! Routing" determines whether it can be installed

When choosing a dropper seat post, the first thing to narrow down is whether the wire routing is internal or external.

Current dropper seatposts are mainly internal, with the wires extending down from inside the seat tube and taking a full outer route to the lever on the handlebars. The ability to route this is very important.

Many gravel bikes have interior frames, but if the BB shell does not penetrate the seat tube and down tube, in some cases, routing may not be possible even with an interior frame.

Also, exit from the down tube is not possible unless the grommets have extra holes for dropper seatposts. Many types have either a front shift or dropper wire here, but some bikes may not have a single front exit for shifting in the first place.

If there are no grommets to secure the outer wire, it must be secured to the frame with a cable tie, and in some cases the outer wire may not be secured properly due to the pull of the wire.

Therefore, if you want to use a dropper seat post on a gravel bike, it is best to choose "a frame that provides dropper seat post routing at the time of purchase.

1. is there a wire routing from the seat tube through the BB shell to the down tube? If not, is there another routing possible?
2. is there an exit in the grommet when exiting the down tube (can it be substituted at the expense of another wire?)
3. if internal routing is not possible, is external routing possible?

How to choose seat post diameter and length (minimum insertion length)

Once you know whether an internal or external type seat post can be installed on your frame, the next step is to narrow down the type of seat post based on its diameter and length (minimum insertion length).

Most dropper seatposts come in the following three diameters

1. 27.2mm
2. 30.9mm
3. 31.6mm

If the diameter of your dropper seat post is larger than these diameters, it can be adjusted by adding shims, but if it is smaller, it cannot be installed.

After narrowing down the diameter, measure the length from the head of the seat tube to the dowel hole on your frame. This should be larger than the “minimum insertion length” listed in the dropper seat post specifications. This is because most seat tubes have dowel holes for bottle cages, and the seat post cannot be lower than the dowel holes.

If the length from the head of the seat tube to the dowel hole is smaller than the minimum insertion length, it is still possible to install a bottle cage, but the dropper seat post will be much higher than the mouth of the seat tube, effectively reducing the amount of travel.

This is a question of whether the dropper seat post can be mounted on the seat tube before considering the performance and function of the dropper seat post.

How to select the amount of travel

Once you have checked the routing, bore diameter, and minimum insertion length, the last step is to choose the amount of travel.

Select the amount of travel (length of sinkage) that is “(length from the bottom of the saddle rail to the dowel hole - frame insertion length*) > amount of travel”.

*Not including the control portion.

Be careful not to make the travel too long so that the pedals will not be able to hold the foot when the dropper is extended.

The best choice is to select a seat post with travel equal to the difference between the height of the dropper seat post at the bottom of the saddle when the saddle is raised to the highest position where you can pedal. This will maximize the travel of the dropper seat post.

Gravel bikes have a longer seat tube than MTBs in terms of geometry, so even if you choose a dropper seat post with 200mm of travel, you will not be able to move the bike that much (the saddle will be too high). It is common to choose a 50mm, 75mm, or 100mm product.

Some products have only 50mm (i.e., 5cm) of travel, which can lead to the situation of "5cm of travel is hardly any travel at all! But this is a matter of consultation with the frame.