Every bicycle has a seatpost. The seatpost is a tube that connects the frame of the bicycle to the saddle, where the rider sits. Seatposts are useful because they allow for adjustability. In this way, the same frame can accommodate riders of different sizes or can be adjusted for different riding styles or terrain types.
This article will go over basic information about seatposts, such as different types of posts, how to choose the right one for you, common seatpost issues and more.
Types of seatposts
Seatposts vary in type as well as size. Many older bikes, for example, feature what is known as the "plain" seatpost. In this design, the diameter of the seatpost decreases at the top, where a single bolt tightens a clamp that fastens the saddle to the post. This design has largely fallen out of favor.
Here are the more common types of seatposts in use today:
The most common is the rigid seatpost with an integrated micro-adjust clamp at the top of the post. This clamp fastens to the rails of a saddle via one or two hex-bolts, allowing for quick and simple saddle adjustments. The micro-adjustable seatpost design has the advantage of being simple, relatively lightweight, and (as the name implies) adjustable.
Suspension seatposts are somewhat common on hybrid and mountain bikes and have a built-in shock absorber to soften bumpy roads and trails. This style of seatpost tends to be heavier in weight with fewer size options.
Another style of seatpost is the dropper post, or adjustable seatpost -- a style used for mountain biking. Adjustable seatposts use internal cable routing to allow a cyclist to drop their seat when they approach technical terrain without having to get off the bike and operate a quick-release mechanism.
High-end time-trial or triathlon bikes are designed for aerodynamics and low weight. Most of these bikes use an aero seatpost which is shaped to match the specific aerodynamic shape of a specific frame's tubing. While most the other seatposts previously mentioned can be used on a wide variety of bikes, aero seatposts typically can only be used with one matching frame model. Some TT/tri bikes don't even have a separate seatpost. Instead, the seat tube is one section supporting the saddle. This kind of seatpost is called an integrated seatpost.
Important seatpost dimensions
The area where the seatpost mounts in a bike frame is called the seat tube. The dimensions of the seat tube can vary from bicycle to bicycle, so when choosing a seatpost, it's important to get the correct diameter. The most common diameter is probably 27.2mm, but other sizes between 21.15 and 35mm are often found, as well.
Seatposts also vary in length. A shorter seatpost will typically be lighter in weight but less adjustable. Mountain bikes tend to have longer seatposts, to allow for adjustment between higher positions for uphill and cross-country rides, and extremely low positions for downhill sections where the rider will be standing and a low seat means less potential for groin injury.
Seatpost lengths range from 75mm to more than 400mm. If you're looking for a new seatpost, make sure the length is sufficient for your needs. A too-short post can ruin the ergonomics of a bike and make riding very uncomfortable. Make sure not to overextend a short post, either. Just about every seatpost will be marked with a minimum insertion amount; make sure it goes in farther than this.
The final element of sizing a seatpost is setback. This refers to any bend or curvature in the upper section of a seatpost and is meant to offset the effect of a frame with a seat tube which is very vertical. Some riders also prefer a setback because it puts the saddle in a position to help them utilize quad and hamstring muscles more effectively.
Your frame may need a setback post if a straight post would place your seating too far forward. Setback varies from frame to frame and is measured up to around 45mm. Check the specs of your frame or take it to a bike ship to ascertain how much setback is needed for your frame and body.
There are a few seatposts designed for “set-forward,” so the rider is positioned more over the pedals. These specialized models are usually used by triathlon and time-trial cyclists to provide maximum power on flat terrain.
Choosing the right seatpost: cost vs. quality
Like all bike parts, seatposts are available for a wide range of prices. Cheaper seatposts are generally pretty basic in design and material. A decent quality seatpost at this level might cost $30-$50. Higher quality seatposts with basic designs often cost around $100.
More expensive seatposts offer better features, like advanced lightweight materials, dropper features or suspension. If you want the lightest possible weight, you should look at a carbon fiber seatpost. These designs are flashy and fast, and can save quite a bit of weight over an aluminum model.
Suspension, dropper and carbon fiber seatposts can cost $150-$400 or more.
A final note on finding the right height
The most common issue people have with seatposts is simply setting them too low. Pedaling with a low seatpost is very inefficient and doesn't allow you to fully use the strength of your legs. Instead, you should raise your seatpost high enough that when you reach the bottom of the pedal stroke, your leg is fully extended (but not overextended). This is the most efficient position for pedaling.
As mentioned above, if you ride a mountain bike on any sort of trails, you should make sure to lower your seatpost when you approach any technical terrain.