Diameter and length
Diameter and length
Putting aside the weird and wonderful dimensions of aero seatposts (more on them later), the most common seatpost diameter on performance road bikes these days is 27.2mm. Unless you’re also changing your bike, you’ll need to match the seatpost diameter to the clamp on your frame.
While 31.6mm used to be the standard, that’s shifted towards 27.2mm as bike brands look to boost comfort, with the thinking being that a skinnier seatpost is able to offer up a little more flex. Still, whether your bike accepts a 31.6mm or 27.2mm seatpost, the majority of manufacturers will offer posts in both diamaters. Having some kind of a universal standard is only a good thing, as non-standardisation can become a veritable minefield as anyone whose ever tried to replace a bottom bracket will tell you.
Still, some bike brands like to mix is up and Cannondale went with a 25.4mm post on their Synapse endurance bike to add even more comfort at the back. The thinner post works in conjunction with the similarly skinny seat tube and helix-shaped rear stays (in what Cannondale call the SAVE rear triangle) to add more flex to the back of the bike. It makes changing the post a bit of a nightmare, though, with limited 25.4mm aftermarket options.
Length is another thing to think about, especially if you like to ride a smaller frame size. All seatposts have a ‘minimum insert’ – in other words, the minimum amount of the post you need clamped inside the seattube to be safe. Basically, the line is there to make sure that the amount of post inside the frame can handle any load placed upon it during riding. If not, there’s a chance you could damage the post or even snap it in extreme circumstances. Needless to say, that’s definitely not something you want happening.