Seatpost

Seatposts generally clamp directly onto saddle rails with which they must be compatible, while old or inexpensive seatposts slide into a separate clamp that then clamps the saddle rails. (also known as a "binder bolt") may be built into the frame for this purpose, or a "seatpost clamp" may be purchased separately (but must be sized to closely fit the diameter of the seat tube). Whether integrated or separate, the seatpost bolt can have a simple nut, can be an Allen bolt, or can include a quick-release mechanism, with a handle that releases the clamp without tools. A quick-release allows easy height adjustment of the seat, though increases the risk of seat theft unless it is also used to detach the seat when parking. ==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 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 ( in) at the top. Folding bicycles often have the seat post diameter of 33.9 mm. Sheldon Brown collected and published a list of seatpost diameters on his website, evidence of the common problem of finding compatible replacements. 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. ==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 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. File:Transition VANQUISH 29 - 49025341916.jpg|Extended dropper post. File:Transition VANQUISH 29 - 49025549167.jpg|Lowered dropper post. File:MTB Handlebar Left.jpg|Control lever for a dropper post mounted on handlebars. ==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.{{cite web ==References==