Custom machined seatpost?

Hey,

Im after a bit of advice on how viable it would be to have an extra long seatpost machined by someone with a lathe?

I've got a folder and can't buy a seatpost long enough for me in 34.9. (I almost got this one but it went out of stock, and i dont think its coming back) It's gunna need to be somewhere around 650/70omm in length. (Huge i know but its a folder and im 6ft 6)

An option is to get a brompton telescopic and shim it up to 34.9, but that will cost the best part of £100 and was thinking it might not be a huge job for someone with a lathe.

Just wondering If there might be any reasons that it's not viable like not having tooling long enough for a post that size etc.

Many thanks

Unless you have a tame lathe operator in your immediate friend group I'd say the £100 Brommy conversion is going to be the cheapest option, by quite a margin.

The length is definitely an issue, it probably puts it outwith the scope of a lot of hobbyists so you're looking at a proper machine shop and they aren't going to do fuck all for £100.

That said, 34.9mm is 1 3/8" so you can buy material in the correct od, albeit might have some surface imperfections. With a quick google search I can only find 1 3/8" tubing 10 or 16 swg though. 16swg is 1.6mm which probably isn't heavy enough for such a long tube in ali? 10swg is 3.2mm which doesn't come out to a good id to do a diy telescopic.

The tubing I was finding was 6000 series and it looks like that's what a lot of seatposts are made from so that's positive.

Could you do this kind of clamp mechanism,

You'd only have to drill a hole, file the shape into the top of the tube and could rob the other bits from a donor post.

Is corrosion going to be a problem? I imagine ali seatposts are generally anodised to 'seal' them from the elements.

You could get that size in stainless, it wouldn’t be all that heavy and should be strong enough.

The telescopic brompton posts are steel and seem to do the job.

Then it would be a matter of making something to fit the saddle, I would look at a clamp like that slots over the tube or pressing/bolting an existing seatpost inside somehow.

Alternatively you could get a 31.6mm seatpost to slot in the larger and longer tube and use a 35mm clamp to hold it together. That should work assuming wall thicknesses of around 1.5mm.

It will all be a bit of a compromise but ought to be strong and relatively low cost, avoiding any major machining etc

Thanks for your responses. Yeah I've been thinking about a diy telescopic, shouldn't be too hard to knock one up, cut a slit in one and run a QR clamp..

Thanks for the info regarding being able to buy this spec off the shelf, great news. Gunna have a look for some now

If you go for steel tubing then a 0.058" wall thickness is a good slip fit over the next size down so 1-3/8" x 0.058" would fit over 1-1/4, 1-1/4 x 0.058" fits over 1-1/8" and so on.