One of the seatpost's jobs is to provide a pillar for the saddle to rest firmly on, the other is less seen, and often un-noticed by casual riders. It is to add an area of structural rigidity to the seat stay / post / clamp / top tube assembly. When you land on a bike there is tremendous pressure placed on this one, joined area. The seat stays try to push into the seat clamp, the top tube similarly pushes backward into the seat tube. The seatpost is the object that makes this hollow seat tube "solid", which is why the diameter of the seatpost is so important it is measured in tenths of a millimeter. If the seat post is under, or over, the correct sized diameter, there is certain to be real fatigue at the seat clamp area. Because the seatpost is so important, in terms of strength in this area, we should tell you how to find out what the present diameter of your post is. The seatpost diameter is almost always stamped into the base of the post usually at the rear, so you will need to remove the one you have and actually read it. While we are on the subject of marks stamped on the post, there is another marking on all seatposts, a line with the words "max height" near it, as kind of a hedge against liability suits. This line marks the point above which it is unsafe to raise your seatpost. It is generally between 65mm and 80mm from the physical bottom of the seatpost. Each post is made of different materials, different aluminum or steel alloys, that have a particular shear strength for that material, in that thickness. This shear strength is taken into account as the "max height" is developed for each seatpost. It is based on the likelihood of failure from a forward or rearward motion with body weight on top. The Maximum Height marking provides for a safe amount of seatpost in the frame, to both support the seat tube stress and to anchor the post against the rider merely shearing off the post, accidentally, in abusive use. There is a current custom for mountain bike riders to buy the longest seatpost possible, the primary reasons for this are actually stated above. The desire is to have a mass of seat post material buried in the frame for solid support of both the rider and to relieve structural stress. There are also many riders that are very bike weight conscious; an acceptable, but overlooked way to reduce weight might be to cut off the surplus seatpost tubing section you may have. Rarely does anyone, in real use find they need all of the 400 plus millimeters, and even structurally it might be difficult to justify. Note what the required Maximum Height is on your post, and make certain that much, plus a little, is there beyond what you need outside the frame for comfort and use (travel height, from moving the post up and down for different terrains to be ridden over). Some may contradict the thinking, and to them the question may be, "what if I merely had a shorter post from the outset?". We have discovered that there are two types of measurements used in measuring a seatpost length, the manufacturers, and reality. Manufacturers will claim a nominal length like, 250mm, 300mm, or 350mm and when you measure the useful area of the post, it's not the same. The useful area of a seatpost, as a practical matter, extends from the center of the saddle rails to the bottom of the seatpost, that still maintains the desired outer diameter, which may not necessarily be the bottom. There are occasions where the very end of the post is a little under the dimension, in our center of rail to bottom of post measurements, this part isn't added in. The measurement is also listed in the seatpost table as "Bike Pro Length", again this is the actual useful length on each seatpost, expressed Many seatposts we discuss here, have a "micro adjusting" head. This is a term for seatposts using a single bolt, usually from the bottom, to hold the saddle in the clamp tightly at any angle adjustment. Lately we've seen another style of post, called a "revolving clamp" style. In this type of post there typically are two half round aluminum clamp pieces that are can revolve freely within a tube welded on top of the seat mast. This "cradle tube" has a stem-like binder fitting welded to it, which cinches the clamp with the saddle rails in place. This "revolving clamp" style permits tiny, infinite adjustments, and allows the maker to achieve some of the lightest seatposts on the market. Many of the clamp pieces are of cast aluminum, which for clamp pieces is an acceptable way to do it. Hand machining is more desirable, and more expensive. Casting may sometimes introduce a weakness that is unseen from the outside. Another way cast is described is "melt forged". This term is used all over this industry to disguise the casting technique. Casting is performed in a few ways, one is to pour molten aluminum into a mold. Another is to squirt molten aluminum into a mold. There is also a technique of pouring aluminum powder, or filings, into a mold and heating it. Each of these styles can create a defective part, and the defect is usually obvious at sight and rejected. Machining represents the height of the workmanship possibilities, and is the more desirable method. The parts of a seatpost, (as we discuss them), are these; Upper Clamp Piece, this holds the saddle rails from the top. The Lower Clamp Piece is the lower cradle piece that supports the bottom of the saddle rails. The Fastening Bolt or bolts hold the seatpost clamp assembly and saddle together on the post, on top of what is called the Head Piece. The Head Piece is the part of the post that joins the clamp assembly to the tubing section. The Tubing Section is what is actually in the seat tube of the bike. These observations were made, in those instances where the description says "from the inside", by using a grain of wheat flashlight right up inside the tubing section . All weights presented were made on a digital scale using a finished production seatpost. They are "off the assembly line", production versions of exactly the same quality the maker sends out every day, not prototypes. The weights are dead accurate. Lastly, some makers have chosen to paint the head piece on their seat post. Nearly every seatpost has knurls, (small ridges), that in many cases seat against knurls on the bottom of the lower clamp piece. While it may look good to paint the top of the Head piece, or the bottom of the lower clamp section, the paint makes a weak surface for the two parts to make tight contact. Removing it should help the post's performance.
