tarckee's mast-style Brompton seatpost thread

Tarckives Bikes
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I figured I’d start a separate thread on this.

The only thing I have been un-stoked about with my Brompton is the seatpost. I have the extended version and even with the pentaclip at the highest position and clamped at the highest safe spot on the column it is slightly too low and setback is not quite enough. Also, while the pentaclip is about as good a design as that kind of thing can be it creaks and has slipped a few times.

The Brompton post diameter is 31.8 (1.25"), which is a commonly available size for titanium tubing, so what I’m considering is rigging up something involving a ti tube and a topper for a mast-style seapost. Maybe this KCNC which has mondo setback:

…or start cheap with a Ritchey which are fairly plentiful on ebay.

34.9 seems to be the most common size.

USE and a few others make a 31.6 to 34.9 seatpost shim. They are plastic in the larger sizes, but with the slight extra give that would afford I wonder if it would be close enough for rock n’ roll (or, assuming it will fit over the 31.8 tube be sandable to take down whatever slight larger outer diameter resulted).

The idea behind this is that the bolt would not really bear any significant load for the height adjustment since the tubing would be bottomed out on the topper. The topper bolt would just keep the saddle from twisting side to side.

One feature of the stock Brompton post I’d like to replicate is the slight flare at the bottom. This prevents you from exceeding the minimum insertion, and also trips like locking for the rear triangle during the folding process.

The way I have the current setup my saddle height is at the max possible extension, which is actually great because I can recall my saddle height instantly. The idea with the mast style post is I would start a little long, experiment with saddle height, find one I was happy with, then cut off the difference between that and max extension.

Assuming you guys don’t think this is too crazy an idea, what should I be looking at tubing-wise in terms of alloy and wall thickness? Is it going to be cheaper to just start with something like this and replace the hardware?

What about the flaring aspect? Is that going to be something a typical fabrication place or framebuilder that works in ti is going to be able to do? Fred also mentioned a plug that could be bonded in? Maybe I could hold it in with a star nut? Any lathebros want to help me with that?

I suppose I could start with 4130 or something alloy too just to test before I spend a bunch on materials.

What does tarck think?

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I would start with a hunk of relatively thick-walled 1.25" OD x .125" wall x 1" ID aluminum tube from Online Metals (which is by the south end of the ballard bridge and so you can order online but pick up at will-call).

http://www.onlinemetals.com/merchant.cfm?id=71&step=2&top_cat=60

Then , either A) add a mast-topper or B) insert a stubby 25.4" seat-post and use a 31.8 clamp (or bettery yet epoxy) to keep it in place.

Test away until you find what you like, then you can explore more spendy options.

If you are lucky you can find an old SR MTE-100 in 25.4, which will allow you lots of setback for test purposes.

Do you need that perfect flare at the bottom all the way around? Or could you drill/tap and use a small set screw or two as a stop? Otherwise I like the idea of a star nut+top cap.

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[quote=jimmythefly]I would start with a hunk of relatively thick-walled 1.25" OD x .125" wall x 1" ID aluminum tube from Online Metals (which is by the south end of the ballard bridge and so you can order online but pick up at will-call).

http://www.onlinemetals.com/merchant.cfm?id=71&step=2&top_cat=60

Then , either A) add a mast-topper or B) insert a stubby 25.4" seat-post and use a 31.8 clamp (or bettery yet epoxy) to keep it in place.

Test away until you find what you like, then you can explore more spendy options.

If you are lucky you can find an old SR MTE-100 in 25.4, which will allow you lots of setback for test purposes.

Do you need that perfect flare at the bottom all the way around? Or could you drill/tap and use a small set screw or two as a stop? Otherwise I like the idea of a star nut+top cap.[/quote]

I like the alu prototype idea. Probably a smart way to start. It would be fantastic if a standard top cap was the the correct inner and outer diameter. I’m not holding my breath, but it would be worth making some measurements.

Re: the set screw idea I think it would serve the minimum insertion function. Not sure about flicking the latch.

Paragon sells 1.25" ti tubing by the inch. No idea if the .035"wall thickness is adequate. Being a full on machine shop they might be equipped to do the flare.

https://www.paragonmachineworks.com/cgi-bin/commerce.cgi?preadd=action&key=TI0120

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Your shim idea should work with some epoxy and maybe a set pin to stop rotation. How long a post do you need. I might know of something.

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How tall is the “seat tube” part of your Brompton frame? I have an idea.

Also, holy hell, trying to google this information, there is a whole world of Brompton stuff out there.

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What about a seat post in a seat post for maximum stowage?

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brompton already makes this, but it’s heavy and expensive

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Made out of titanium?
I think not
1/2 ain’t bad
I mean you’re riding a brompton, ball hard or

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Titanium bottom post, carbon top

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no

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Dropping a standard post into the telescoping model had occurred to me. Would be the simple option, but yeah heavy.

In a perfect world I’d get a 31.8 topper CNCed or 3D printed.

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look at where the upper clamp is — the kcnc really has thomson-level setback, the ritchey would be more than a cm more

this should work awesome

flaring would be obnoxiously difficult to do as a one-off, would need a bunch of scrap tubing to test with

turning an insert to jbweld in would be super easy, but really you have a number of options for found objects — headset parts are a goldmine, someone already mentioned topcaps but the upper ‘cup’ from a 1" headset should be exactly the right size and shape (a traditional headtube is 1.25 x 0.035")

the place a lathebro would come in the most handy would be for parting/facing the tube squarely to your preferred length — using a hacksaw on ti would be pure suffering

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I’d just use a clamp on the bottom to function as a stop.

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I think that might impede the fold slightly because the post wouldn’t have as much room to drop — but it would make the bonded head idea feasible, and make it easy to tweak the saddle height

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BOOSH

http://www.titaniumjoe.com/index.cfm/products/tubing/

half the price of paragon for 1.25 x 0.035"

plus a bunch of other wall-thickness options

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[quote=match avatar]BOOSH

http://www.titaniumjoe.com/index.cfm/products/tubing/

half the price of paragon for 1.25 x 0.035"

plus a bunch of other wall-thickness options[/quote]

Sw8. That was the place I was trying to think of. Stumbled on that a while ago and then couldn’t remember the name.

Internetting/napkin math tells me 24" should do the trick as the extended post is 59cm-ish. I will measure to be sure.

Any idea what I should be shooting for in wall thickness?

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[quote=tarckeemoon]
Any idea what I should be shooting for in wall thickness?[/quote]

Back of envelope:

Current post material, wall thickness, maximum rated rider weight?

Calculate section modulus Z, and moment of inertia I.

Calculate maximum deflection and maximum fibre stress in current post for max rider weight. Compare to proof/yield stress. Calculate safety factor as proof÷fibre stress.

Sub in Young’s modulus of (110GPa) for Ti and vary wall thickness in I and Z until fibre stress / deflection is acceptable with a similar safety factor.

Or, y’know, get someone to measure a ti brompton seat post. (Or back calculate from quoted weights/lengths of ti posts, minus hardware.)

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Lulz.

I’ll see if if i can take a stab a measuring the steel post wall thickness.

The old hellishly expensive Brompton ti posts are notorious for cracking where the pentaclip clamps on.

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The calcs are pretty easy, especially for your load cases. I could build you a google spreadsheet in 10 min with them.

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Blakey what is your consulting fee?

Anyway, I finally took some measurements. A 24" length will be perfect - just slightly on the long side.

I’m thinking of 0.0650" or 0.0700" wall thickness just to be on the safe side. Wall thickness on the steel steerer tube I measured was 1.55 or so mm, so 0.06102362".

Also in my experimentation simply moving the post down and locking the qr seems to lock the fold together. The flare doesn’t seem to offer anything special in this area. Since I would be a good 1" from exceeding the max height at anything approaching my normal saddle height maybe I should just mark my saddle height with a sharpie and call it good?