It's just a tube right? Well, kind of, but it's an important tube and materials, diameters and bolting systems vary greatly. Here's what you need to know...
The main factors
Firstly, make sure the seatpost is the correct size (diameter) for your frame's seat tube - not all posts come in all sizes. Before you ask, mere fractions of a millimetre really do matter when it comes to seatpost diameter because you risk causing serious frame damage if you try to fit a post that's too large or small.
It's also important to choose a seatpost that has a secure saddle clamp, is easy to adjust, and is strong enough for the type of riding you do. Having a post collapse, splinter or just let go of the saddle can have serious consequences for future generations of your family. Always err on the strong side of caution rather than risking injury. Even if the post doesn't break, knowing that it might slip at any moment, spill its guts all over the trail/kitchen if you try to adjust it, or that you won't be able to get the exact saddle angle you desire is not good. We're big fans of the simple, fine adjustment and security of twin-bolt fore and aft set-ups but not all are created equal so check out our reviews.
A seatpost doesn't have to be heavy to be strong, and some heavy posts can be quite weak. As long as the post is strong enough, excess weight will always be a downer. With many DH racers and freeriders running lightweight and minimal but strong posts from companies like Thomson, a lot of 'burly' posts are left seeming excessively lardy.
You also want a seatpost that lets you create your desired saddle position. Most bikes have lay-back posts as standard so you'll need plenty of saddle rail to play with or you'll have to replace like with like. On the other hand, if you want a more forward weight position for aggressive cornering traction, try an inline-clamped post.
Make sure the clamps will fit the rails of your saddle. Heavy-duty seats often use 8mm rails which benefit from matching 8mm cradles, while lightweight hollow rails need as much support as possible.
Finally, there are a few seatposts out there for special requirements, such as restricted raising/lowering room on frames or riders with a perpetual need to fettle seat height.
The post's bolts control saddle clamping and tilt angle. We're fans of twinbolt front and rear set-ups. They can be difficult to reach without a ball-ended allen key but are secure. Single-bolt set-ups tend to be poor in comparison.
The business end of the post that holds the saddle. One-piece shaft and head constructions are generally a lot stronger than plugged and glued creations.
The pieces that support and clamp the saddle rails. The lower cradle is rounded so that it can roll for saddle tilt. Smooth-radius cradles allow finer adjustment than toothed ones.
The long post section - ranges from 25 to 34.9mm in diameter to fit different frames. Diameters vary in increments of 0.2mm so it's important to fit the correct size. The materials, manufacturing techniques and thicknesses also vary widely to provide different strength/weight yields at different prices.
ANODISING Chemical surface treatment that's tougher than paint.
BUTTING Changes in the wall thickness of a tube to alter its strength and weight at different points.
COLD FORGED Cold metal crushed into shape by huge pressure, which also realigns the metal grain to maximize strength.
HEIGHT MARKS Useful numbers on the shaft to show what height the post is sitting at for later reference or re-adjustment.
INLINE The saddle clamp is directly in line with the shaft of the post rather than offset - or laid-back - to the rear
LAY-BACK The position of the centre of the saddle cradles in relation to the shaft. Most 'original equipment' posts supplied with bikes have close to an inch of rearwards offset but lots of the posts here are inline designs with no setback at all. You'll be surprised how different more/less layback feels too, so don't think it doesn't matter.
LENGTH Some posts come in different lengths and most can be chopped shorter too. Always make sure you have at least 4in/10cm of post inside the frame or you could crack the seat tube.
MACHINED Shaved and shaped by a milling machine to reduce weight.
SHOT PEENED Bead blasting the post to equalize and reduce any residual stresses in the surface of the post.
T-BAR Horizontal bar that sits across the top of the saddle rails with the bolt threading into the centre; can be fiddly to set up but it's usually light.
UP-AND-UNDER An older seatpost design using two T-bars either side of a narrow lower cradle. Light, but awkward to use and puts high stresses on the saddle rails.