Seatpost

A bicycle seatpost or seatpin is a tube that extends upwards from the bicycle frame to the saddle. The amount that it extends out of the frame can usually be adjusted, and there is usually a mark that indicates the minimum insertion (or maximum extension). Seatposts can be made of steel, aluminum, titanium, or carbon fiber.

Seatpost generally clamp onto saddle rails.

izes

The size of the seatpost is dependent upon the internal dimensions of the seat tube of the bicycle frame. They come in various diameters, lengths and offsets. Offset is the distance between the centerline of the seatpost tube, and the centerline of the clamp area. Shims are often available to adapt a too-small seatpost to a too-large seat tube.

Diameters

Diameters range from 22 mm to 32 mm. 27.2 is a common size for road bicycles. 25.4 mm (1 in) is a common size for BMX bikes. Documented sizes are almost always a multiple of 0.2 mm and include: 22.0, 22.2, 23.4, 23.8, 24.0, 25.0, 25.4, 25.8, 26.0, 26.2, 26.4, 26.6, 26.8, 27.0, 27.2, 27.4, 27.8, 28.0, 28.6, 29.4, 29.6, 29.8, 30.0, 30.4, 30.8, 30.9, 31.4, 31.6, 31.8, 32.

Length

Lengths range from 75 mm to 400 mm. Mountain bike seatposts tend to be longer than road bicycle seatposts.

Offset

Offset can range from 0 mm to 45 mm.

Types

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

This is the most common type found on bicycles today. 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 modern road and track racing bicycle frames are made from one piece of molded carbon fiber with an integrated seatpost that is cut to length depending upon the rider. 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. [cite web
url = http://www.slowtwitch.com/mainheadings/techctr/compact.html
title = Compact frame geometry and aero seat posts
accessdate = 2008-08-21] In the case of aero seat tubes, there are a variety of clamping mechanisms for such seatposts that include pinch bolts and wedges.

uspension

Suspension seatposts allow the seat 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 seat. The two semicircles are held together with a bolt to attach the seat to the seatpost.

Maintenance

Seatposts should be periodically removed from the frame, cleaned, greased and refitted to prevent seizure of the seatpost in the frame. This is particurly important with bikes which do not have mudguards (fenders) that are regularly ridden in wet conditions.

There is some controversy about whether to grease carbon seatposts or not. There does not yet appear to be a consensus. [ cite web
url = http://www.velonews.com/tech/report/articles/9023.0.html
title = Technical Q&A with Lennard Zinn - More greased carbon
accessdate = 2007-01-18 ] [ cite web
url = http://www.profile-design.com/nice_assets/0000/0495/Carbon_post_012703A.pdf
title = Installation Instructions Profile Design Razor & O3 Carbon Seat Posts
accessdate = 2008-01-21 ] There are now specialty products, referred to as "carbon prep" or "carbon paste", specifically for the interface between carbon and most other materials. [cite web
url = http://www.velonews.com/article/11851
title = Technical Q&A with Lennard Zinn - ... carbon questions ...
accessdate = 2008-08-21]

References

External links

* [http://www.sheldonbrown.com/seatpost_sizes.html seatpost sizing database] by Sheldon Brown