I have seen a great seatpost online that i would like to fit to my brompton. The only trouble is that the OD is 31.6mm instead of 31.8mm.

The Brompton seat mechanism uses a plastic shim to help the seatpost rise and lower smoothly.

bromtpon seat shim Anyone know the dimension of the brompton seattube and if it is possible to buy a new shim and fit to brompton frame so that I can fit this seatpost properly?


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2 Answers 2


It might fit perfectly as is. If for some reason it doesn't fit perfectly, you could make a shim out of a soda can (which is about 0.1 mm in thickness). I don't think anyone makes a commercial shim of that size.

  • Thanks Batman but the intention was to replace the original factory fitted Brompton shim with one 0.2mm larger. As the seat is always going up and down the beer can would not work Commented Sep 2, 2016 at 5:55

so i finally worked out how to do this. i bought a stock 34.9 -> 31.6mm aluminium shim. it's much better quality and much longer than the plastic PoC shim that comes with the brompton. The 34.9 shim was slightly too small for the frame but with 2 layers of drinks can it fits perfectly (with a bit of help from rubber mallet). so now i have my aluminium seatpost and railed saddle without that horrible steel post and pentaclip and without the ridiculously priced bromptification Ti post. hope this is useful

  • Glad you found a solution. Just a bit of caution. Aluminum likes to cold weld itself to other metals. You want to apply some grease to prevent this.
    – RoboKaren
    Commented Oct 4, 2016 at 3:19

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