Increasing length of seatpost on a foldable bike 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:


This 30.4mm → 27.2mm adapter (I'm considering this model because it's 10cm long, whereas other models are 8cm or 6cm long only…)
This 27.2mm × 600mm seatpost (I think it's initially intended for BMX)


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 :)
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.