Dulwich Hill Bicycle Club

I'm currently building up my fixie for the up-comming Fixie Century.

I found a cool tool at work which measures seat-post diameter, but mine measured in at 24.5!!!

I've checked on all of our suppliers lists, and the closet I've seen is a 25.4

Any ideas? Hidden stashes of crazy skinny seat-posts?

There is always the option of getting one with a good wall thickness, putting it in a lathe and turning it down to the right diameter. The 25.4 will be more common since that's 1 inch.

Chances are that the seat tube is 25.5 when the clamp is undone and 25.4 once its done up. Its suprising but as little as 0.2 of a mm is the difference between something sliding or being fixed. Id say test a normal 1 inch seat post in it and it should be fine.

Are you sure that cool tool works properly???

Colnago made a bike around 1982-1983 called the Oval CX which used a 20mm seatpost, so you can go thinner. I believe some Peugots, British folding bikes and BMX bikes used the 24.5mm size too though.

The obvious question to ask is what's wrong with just using the seatpost you measured?

Did you use one of these...?

or maybe you used one of these...?

in which case you are actually measuring the frame not the post. It is possible to ream the frame out to say 25.4mm to fit a more conventional post.

I know that Thompson have made posts in 24.5mm in the past, but I think your best bet would be to ask the Guru on such matters (Peter Bundy) and if that fails, ring around a few BMX specialty bike shops. (I think there is one in King Street Newtown).

Oooo, thanks for all your help! I now have lots of ideas.

Toff, the tool I used was the first one. If you see the top half of it, you stick it "inside" the space where the seat-post goes. Sad but true, there is no seat-post currently on the bike, but the bmx idea will be worth looking into.

Cheers!!!

If you used the calipers, then you need to take multiple readings at different angles of rotation, and take an average. It is unlikely to be 24.5mm al the way around...

Really good thinking. Will re-measure again tomorrow.