Go Back  Bike Forums > Bike Forums > Fitting Your Bike
Reload this Page >

Seatpost length means what, exactly?

Notices
Fitting Your Bike Are you confused about how you should fit a bike to your particular body dimensions? Have you been reading, found the terms Merxx or French Fit, and don’t know what you need? Every style of riding is different- in how you fit the bike to you, and the sizing of the bike itself. It’s more than just measuring your height, reach and inseam. With the help of Bike Fitting, you’ll be able to find the right fit for your frame size, style of riding, and your particular dimensions. Here ya’ go…..the location for everything fit related.

Seatpost length means what, exactly?

Old 06-22-23, 05:28 PM
  #1  
Newbie
Thread Starter
 
Join Date: Feb 2016
Posts: 2
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Question Seatpost length means what, exactly?

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.
byron.hawkins is offline  
Old 06-22-23, 11:18 PM
  #2  
Senior Member
 
oldbobcat's Avatar
 
Join Date: Jul 2005
Location: Boulder County, CO
Posts: 4,328

Bikes: '80 Masi Gran Criterium, '12 Trek Madone, early '60s Frejus track

Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 488 Post(s)
Liked 423 Times in 321 Posts
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.
oldbobcat is offline  
Old 06-23-23, 08:50 AM
  #3  
I'm good to go!
 
Iride01's Avatar
 
Join Date: Jul 2017
Location: Mississippi
Posts: 14,544

Bikes: Tarmac Disc Comp Di2 - 2020

Mentioned: 50 Post(s)
Tagged: 0 Thread(s)
Quoted: 6001 Post(s)
Liked 4,628 Times in 3,193 Posts
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
Iride01 is offline  
Old 06-23-23, 10:46 PM
  #4  
Senior Member
 
50PlusCycling's Avatar
 
Join Date: Mar 2018
Posts: 1,038
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 517 Post(s)
Liked 686 Times in 361 Posts
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.
50PlusCycling is offline  
Old 06-25-23, 11:46 AM
  #5  
Senior Member
 
Road Fan's Avatar
 
Join Date: Apr 2005
Location: Ann Arbor, MI
Posts: 16,847

Bikes: 1980 Masi, 1984 Mondonico, 1984 Trek 610, 1980 Woodrup Giro, 2005 Mondonico Futura Leggera ELOS, 1967 PX10E, 1971 Peugeot UO-8

Mentioned: 49 Post(s)
Tagged: 0 Thread(s)
Quoted: 1851 Post(s)
Liked 653 Times in 497 Posts
Originally Posted by 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.
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.
Road Fan is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.