A bicycle seatpost, seatpin, saddlepole, saddle pillar, or saddle pin is a tube that extends upwards from the bicycle frame

Bicycle frame

A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...

 to the saddle

Bicycle saddle

A bicycle saddle, often called a seat, is one of three contact points on an upright bicycle, the others being the pedals and the handlebars.The bicycle saddle has been known as such since the bicycle evolved from the draisine, a forerunner of the bicycle...

. 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

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

, aluminum, titanium

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

, carbon fiber, or aluminum wrapped in carbon fiber.

Seatposts generally clamp onto saddle

Bicycle saddle

A bicycle saddle, often called a seat, is one of three contact points on an upright bicycle, the others being the pedals and the handlebars.The bicycle saddle has been known as such since the bicycle evolved from the draisine, a forerunner of the bicycle...

 rails, while old or inexpensive seatposts slide into a separate clamp that then clamps the saddle rails.

Sizes

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 or mountain bicycles. 25.4 mm (1 in) is a common size for BMX

BMX

Bicycle motocross or BMX refers to the sport in which the main goal is extreme racing on bicycles in motocross style on tracks with inline start and expressive obstacles, and it is also the term that refers to the bicycle itself that is designed for dirt and motocross cycling.- History :BMX started...

 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(most common), 27.4, 27.8, 28.0, 28.6, 29.2, 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 430 mm. Mountain bike

Mountain bike

A mountain bike or mountain bicycle is a bicycle created for off-road cycling. This activity includes traversing of rocks and washouts, and steep declines,...

 seatposts tend to be longer than road bicycle seatposts.

Offset/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 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

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 high end road

Racing bicycle

A racing bicycle, also known as a road bike, is a bicycle designed for competitive road cycling, a sport governed by according to the rules of the Union Cycliste Internationale...

 and track bicycle

Track bicycle

A track bicycle or track bike is a bicycle optimized for racing at a velodrome or outdoor track. Unlike road bicycles, the track bike is a fixed-gear bicycle and so has a single gear and neither freewheel nor brakes. Tires are narrow and inflated to high pressure to reduce rolling resistance...

  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. In the case of aero seat tubes, there are a variety of clamping mechanisms for such seatposts that include pinch bolts and wedges.

Suspension

Suspension seatposts allow the saddle to move up and down with either a telescoping or parallelogram mechanism and incorporate a spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

, an elastomer

Elastomer

An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...

, or compressed air and possibly a damper

Damping

In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...

 to insulate against bumps. The preload of the spring may be adjustable. These seatposts are most common on hybrid

Hybrid bicycle

A hybrid bicycle is a bicycle designed for general-purpose utility and commuting on a wide variety of surfaces, including paved and unpaved roads, paths and trails...

 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

BMX

Bicycle motocross or BMX refers to the sport in which the main goal is extreme racing on bicycles in motocross style on tracks with inline start and expressive obstacles, and it is also the term that refers to the bicycle itself that is designed for dirt and motocross cycling.- History :BMX started...

 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.

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.

Maintenance

Seatposts should be periodically removed from the frame, cleaned, greased and refitted to prevent the seatpost seizing in the frame. This is particularly important with bikes which do not have mudguards (fenders) that are regularly ridden in wet conditions. Care should be taken not to overtighten the bolt or quick-release lever which clamps the post in the frame, especially where this acts on two brazed lugs rather than a separate clamp-on collar. Overtightening can bend or break the frame lugs or strip the threads in a separate collar. Metal seatposts should be very well greased, with the slot in the bicycle's seat tube also filled with a smear of grease. This helps to prevent water from running down the seat tube.

There is some controversy about whether to grease carbon seatposts or not. There does not yet appear to be a consensus. There are now specialty products, referred to as "carbon prep" or "carbon paste", specifically for the interface between carbon and most other materials.

External links

• seatpost sizing database by Sheldon Brown
  Sheldon Brown (bicycle mechanic)
  Sheldon Brown was an American bicycle mechanic and technical authority on bicycles. He contributed to numerous print and online sources related to bicycling, bicycle mechanics and maintenance, including his own website — and received numerous awards for his contributions.-Biography:Brown...