Diameters

Seatpost diameters generally range from 22 mm to 35 mm in 0.2 mm increments. The most common size is 27.2 mm (1.07 in) for most bikes, especially for the higher-quality models. BMX bikes commonly use 25.4 mm seatposts. In some modern bikes with thicker alloy or carbon tubing, larger diameters such as 30.9 mm are used. Tapering seatposts often have a diameter of 22.2 mm (7⁄8 in) at the top.

Sheldon Brown collected and published a list of seat post diameters on his website, evidence of the common problem of finding compatible replacements.^[10]

Length

Lengths range from 75 mm to 430 mm. Mountain bike seatposts tend to be longer than road bicycle seatposts.

Offset or layback

Offset or "layback" can range from 0 mm to 45 mm. A seatpost with offset is necessary when the seat tube angle of the frame is too steep to give the desired saddle setback (the horizontal distance between a plumb line hung from the nose of the saddle and the bottom bracket spindle). Conversely, an "in line" post may be required if the seat tube angle is too slack. Some saddles, notably Brooks leather saddles, have relatively short rails, allowing less adjustment of setback, and changing the seatpost or inserting a saddle adjuster may be the only way to achieve the correct position.

Plain

This type, usually found on older bikes, less expensive bikes, or kids bikes, consists of a tube which may decrease in diameter for the last inch or so (2.54 cm) and a separate clamping mechanism at the top. One bolt tightens the clamp to the rest of the seatpost and to the saddle rails at the same time.

Micro-adjustable

They can be divided into two types; ones which can adjust the saddle angle continuously, and ones in which the saddle angle can only be adjusted to a certain number of positions.

Integrated

Some high end road and track bicycle frames are made from one piece of molded carbon fiber with an integrated seatpost that is cut to length depending upon the rider, also known as a seat mast. The advantage is that it is lighter, can be molded into an aerodynamic shape, and removes the need to clamp an irregular tube shape. The disadvantage of this setup is that the seatpost height is not as adjustable. There is usually 2-3 centimeters of adjustment with the clamping device.

Aero

As alternatives to the integrated seatpost mentioned above, some seatposts merely have an aerodynamic shape that either matches the shape of the aero seat tube or is only not round above a certain point.^[11] In the case of aero seat tubes, there are a variety of clamping mechanisms for such seatposts that include pinch bolts and wedges. Aero seatposts are typically proprietary designs for a specific frame model and thus cannot be mounted on bikes of other manufacturers.^[12]

Suspension

Suspension seatposts allow the saddle to move up and down with either a telescoping or parallelogram mechanism and incorporate a spring, an elastomer, or compressed air and possibly a damper to insulate against bumps. The preload of the spring may be adjustable. These seatposts are most common on hybrid and mountain bikes. Suspension seatposts usually come in fewer diameters, and shims are more likely to be necessary.

Pivotal

Pivotal seatposts are common on BMX bikes. They have a concave semicircle of ridges at their top that matches the convex semicircle of ridges on the bottom of a pivotal saddle. The two semicircles are held together with a bolt to attach the saddle to the seatpost. Pivotal seatposts are currently^[when?] expanding rapidly in popularity with mountain bikes.

Seatmast and cap

Some bikes, such as Trek Madones, provide saddle height adjustment with a seatmast and cap arrangement. The seatmast is extension of the seat tube above the top tube, and the cap slides into it, clamps in place, and attaches to the saddle.^[13][14]

Dropper

Dropper seatposts (also known as dropper posts or droppers^[12]) on mountain and gravel bikes can be remotely adjusted while riding using a lever or switch on the handlebar. This can be used to quickly lower the saddle and position it out of the way to allow for better body positioning and maneuverability on technical sections. The same lever or switch can then be used to return the saddle to its normal position for better pedaling efficiency. Most dropper seatposts use a hydraulically damped air or coil spring with cable, hydraulic, or electronic actuation.^[15]

Matej Mohorič used a dropper seatpost on his road bike to help him win the 2022 Milan–San Remo one-day race.^[16]

• Extended dropper post.
• Lowered dropper post.
• Control lever for a dropper post mounted on handlebars.