Seatposts are made in lengths from 75 to 430 millimeters. A seatpost can relatively easily be cut shorter, but a too short seatpost can’t be lengthened, of course.
Still, when setting up the riding position, attention must be payed to not move the seatpost too much out for it’s length. That is: to keep a minimal length of the seatpost inserted into the seat tube. How long is that? That is explained in the next chapter.
First safe bet is inserting seatpost at least 150 mm inside the seat tube. This will securely hold on practically any bicycle. It is shown in picture 2.
Is it safe to go shorter than that? There is one exception, that will now be explained. For “men” frames, with (more, or less) horizontal top tube, minimal insertion depth can be shorter than 150 mm, and is defined by the following two conditions:
a) Seatpost should end at least 25 millimeters below the lower part of the horizontal tube weld to the seat tube.
b) Total inserted length should be at least 90 mm from the top end of the seat tube.
This is shown in the picture 3.
Both of these conditions must be fulfilled. If, for example, lowest part of the top tube weld ends 50 mm below the seat tube top, inserting seatpost 25 mm below it won’t fulfill the requirement b) – of minimum 90 mm total insertion depth (seat post would be inserted 50 + 25 mm – so only 75 mm).
Same goes if the lowest part of the weld is at 80 mm for example. Inserting seatpost 90 mm deep would not fulfill the a) requirement of seatpost ending at least 25 mm below the weld. Therefore, seatpost would have to be inserted for total of 105 mm in this case (80 + 25 mm).
Shortening seatposts is very popular with road bikes, for weight reduction. The minimum safe margin was explained in this chapter. What happens if it is exceeded is explained in the next chapter.
Seatpost not inserted deep enough into the seat tube (as explained in chapter 2.) can cause the following problems:
- It will not stay firmly in place – dropping deeper, or twisting (this is the least of the problems and it happens most rarely, the other problems are more serious).
- Seatpost could bend, or break.
- The top part of the seat tube (the frame) could break.
Most of these problems can be hazardous if they happen when riding at high speeds and/or on bumpy terrain.
When inserting a seatpost, always grease it with proper paste. For metal seatposts in a metal frame, use an anti seize mounting paste (“copper grease” or similar). It will prevent seatpost from welding to the frame over time and getting stuck.
For carbon seatposts and frames, a special carbon mounting paste should be used.