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One of my bike is this little Dohiker 14" electric bike, but I actually pedal a lot when riding it, and the seatpost is a bit too short. It is about 50cm long, and I would need a few more centimeters. I'm already exceeding a bit the minimum insertion length, but it's not sufficient. Also, such long seatposts apply a lot of force on the frame because of the leverage, so I really have to insert a safe length of seatpost in the frame.

Problem: the diameter is a quite uncommon 30.4mm, and such long seatpost is also uncommon. I searched everywhere for a 30.4mm × 550~600mm seatpost, without success...

So far, after already a lot of research, my best (and only) solution would be to use these two parts:

Do you think this solution is acceptable? Do you have better proposals?


(and extra kudos if you find a similar seatpost (27.2mm × 600mm, color black), but without any branding on it :)

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I have a 20" folder, long legs, and exactly the same problem.

I strongly advise you to put the seatpost to the minimum insertion mark at least, if not further. I've personally cracked and broken my frame at the seat clamp, and fortunately it was weldable being steel.

As for your parts - maybe. The combination of dimensions looks good, but the risk is that a thinner seat post, will be weaker for the same thickness of wall. If its a thicker wall then you're probably good.

Also, the thinner seatpost will have a leverage focus/maximum right at the point it enters your adapter, so if its going to bend it will bend there.

Another solution is to take your old seat post to a machine shop and ask for a price for "one of these, but 700mm long, please"

Mine's got an old-fashioned 2 piece saddle clamp so all I need is a piece of steel tube, necked down at one end to the right size. Your sample has an integrated saddle clamp, which would make it more complex. If you do go this route, buy a cheap seatpost with the right clamp and tube size, and ask your machinist to cut off the clamp and fit it to a longer tube. If you're brave you could offer up your existing seatpost, but that leaves you no bike for the duration.

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    Indeed, my main concern is that the new seatpost, quite long and only 27.2mm large, could bend where you described. After all, it's a BMX seat, designed for short rest, not for sustained pedaling. Thankfully, I weight only 65kg. Also, I'd prefer damaging a seatpost than the frame! About a custom built seatpost, the current seatpost is actually a simple tube as you described, a model like this one, and I don't mind about the type of clamp. But this solution sounds a bit complicated to me (and possibly costly), although it's an interesting one.
    – Gras Double
    Commented May 2, 2020 at 1:09
  • @GrasDouble I reinforced my 25.4,mm MTB seatpost by inserting a solid steel bar up the inside. That worked well and it hasn't bent again, but the seatpost weighs five-times as much now.
    – Criggie ♦
    Commented May 2, 2020 at 2:16

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