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I am about to buy a new seatpost and I am wondering what's the ideal length. The one I have now is 33cm long, even though it only sticks out of 10cm from the frame (measured from the seat collar to the saddle rail).

Because of that, I thought a 25cm seatpost would be a better match and I would save in weight. The question is:

Is there any advantage in having a longer seatpost?

Notice I am assuming the size of the frame and height of the rider is fixed here, so I am only talking about the lenght of the seatpost that's hidden inside the frame. The only thing I could think about is that a longer seatpost is subject to less torque.

  • Well, you need to have enough of the seat post to be inserted beyond the minimum insertion line. – Batman May 29 '17 at 23:24
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    Where is usually that line? how far from the bottom of the seatpost? – Alessandro Cosentino May 29 '17 at 23:28
  • Well, every seat post you can buy will be marked. But according to this question, its about 3-4 inches. Not something I thought terribly much about, and I don't have a bicycle handy to actually check. – Batman May 29 '17 at 23:59
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    I wouldn't generally cut one... someone will want the longer length. Mine are always too short – Criggie May 30 '17 at 1:54
  • It is not subject to less torque. A longer tube spreads the force. – paparazzo May 30 '17 at 11:59
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One thing that is a possible disadvantage is that if your seat post is too long it can run into your bottle/ bidon mounting screw, which won't allow you to push it in all the way. This has come up as an issue for me when packing a bike for flying or sticking it in the trunk of a car.

  • Thanks for moving this to an answer. As I said above, I was hoping for exactly this kind of answers, some reason for a short seatpost that I couldn't come up with by myself. Cheers! – Alessandro Cosentino May 31 '17 at 19:05
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Frames have a minimum insert requirement - 100mm-120mm for carbon is not uncommon. Seat posts also have a minimum, which is usually marked on the post - typically 75-100mm. Both these limits should be adhered to. A long seat post may be too long to lower the seat to the correct position. The difference between the minimum insertion and maximum will vary based on the frame, however it can be quite a small range especially on some more exotic shaped frames (e.g. Softtail MTB's)

The only advantage of going to the minimum insertion is weight savings. However going short adds stress to the seat tube.

The only significant disadvantages of a longer post is weight.

What length post to use between these minimum and maximum insertion depths will depend largely on your own preference for weight saving over strength and your weight and riding style. My lightweight XC MTB has 100mm inserted (100mm minimum), but I am 66kg, and don't do jumps. If I was 120kg and loved big air, it would have 200mm or more.

You can cut the end of the current seat post. Find the minimum insert for the frame and post, measure that much below the current posts location in the seat tube and rip into it with a hacksaw. When I have done this in the past, I also sand off the minimum insertion mark put a new one on the post.

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    Leverage and rider weight is a factor too - I've had a frame crack because the long seatpost was up high and at its minimum insertion distance. – Criggie May 30 '17 at 1:53
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    So if I don't care about weight, no point of risking the shorter one, right? – Alessandro Cosentino May 30 '17 at 7:32
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    If you don't care about weight, and the one you have goes low enough for you, best to leave it as is. – mattnz May 30 '17 at 8:21

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