byron.hawkins

Seatpost lengths are usually like 300mm or 400mm, but I can't find a clear explanation what exactly that length includes. Is it just the length of the cylinder, or does that measure include the mount bracket as well? I realize it's only a few mm difference, but I'm trying to fit a longer stem on a bike with no seat tube (folding bike, split suspension), so I need to be careful about clearance. Unfortunately the shop doesn't have any long seatposts in stock, so I have to order and pay before I can actually see one and measure it myself.

oldbobcat

Generally, the distance from the bottom of the post to the slot that holds the saddle rail. When you're talking about something that's 30-40 cm long (that's 9-10 inches), it's point less to quibble about a couple millimeters. Shorter posts are generally for bikes with horizontal top tubes. People who ride bikes with slanted top tubes (or who ride bikes that are too small for them) choose longer seat posts. When measuring for your fit, be sure to use the minimum insertion point, etched a few centimeters up from the bottome.

Iride01 

I don't know that there is any standard for measuring. You just have to figure out how each manufacturer measures theirs. But as mentioned, it's probably only quibbling about a few millimeters with any.

Back long ago when I was looking for seat posts, I was able to find diagrams and drawings that showed dimensions on the manufacturer/brand website of them with a little bit of effort for some.

Welcome to BF

50PlusCycling

The length will depend on the design of the frame. The seat tube on a Brompton or Birdy is around 530mm, it’s better to error in getting a longer post than a shorter one, as you can cut it down if its too long.

Road Fan

If you need to nail down the exact length of the seat post you have to buy, while for the obsessive it is possible, but not even I do that. The following is the farthest I would go, as far as obsessive processes, and I'm an engineer:

1. Figure out how far above BB axis the top of your saddle needs to be, based on measuring along the seat tube. Probably the best way to do this is to measure that distance on a bike you have which is already comfortable.

2. measure the length of your seat tube from the BB axis to the top of the seat tube.

3. Subtract the smaller number from the larger number. This is the exposed length of seat tube plus saddle.

4. Round that number up to the nearest inch (i.e. consider a 12.5" length to "really" be 13".

5. Add 3 or 4 inches to that to make sure you will have adequate (actually, extra) seat post length inside the seat tube. This is critical to your safety, to keep from breaking the frame, and to make sure there is no damage to the seat tube as you bounce the bike roughly over bumps at speed.

6. Whatever length that seat post is supposed to be with that length, try to buy one. If you can't find the correct length, buy the next longest one you can find.

7. If you cannot get one which is long enough, do not use a post which only has less than 2.5" of seat post down in the seat tube.

Keep in mind you also need to find a seatpost which will allow you to set the saddle back the right amount (not "slammed," rather it may need to be adjusted as time goes on), as well it has to be a fit for the inside of your seat tube.

After all this, just use those specs at Tree Fort Bike (no affiliation) or other well-stocked parts seller.