Brompton seatpost shim

[LittleWheelsandBig]

I have never pulled the seatpost of an Oliver frame (only seen one of them in the flesh). It was a Frezoni frame. It takes minimal heat, even with silver brazing, to not shrink the seat cluster down to 27.2mm. Too low a heat risks not getting full braze penetration. Too much heat means a metal puddle on the floor. Brass has a wide temperature range between too hot and too cold; silver has a much narrower and lower range.

[rogerzilla]

Tony Oliver silver brazed all his frames, and all the 531 frames take a 27.4 pillar (he expounds upon tnis in his book*).  AIUI he jacked it in because the fumes were getting to him.  I don't know how many he built in total but I imagine it was only in the hundreds.

*which contains quite a lot of opinion and some inaccuracies; the description of steering geometry is completely arse-about-face and I wouldn't like to ride his radial drive side, narrow-barrel hubbed, rear wheel!  That trick is ok with a big fat hub designed to cope with the torsion...

[LittleWheelsandBig]

How many 531 frames out there use a 27.4mm seatpost? Vanishingly small numbers in my experience.

Most bicycle frame tubes have a subtle curve and applying heat to the top tube, seat tube and seat stay junction is not usually done in a single heat cycle. More heat cycles and higher temperatures mean more distortion.

There is a reason virtually every brazed or welded frame needs its seat tube reamed, usually with a fixed 27.2mm reamer. You could always ream out to 27.4mm but that means less wall thickness and there isn’t a huge amount of steel to start with. A perfectly reamed seat tube that matches a specific post (there are manufacturing tolerances in seatpost diameter) is lovely though. I worked on a friend’s Team Fat Chance that had a sealed seat tube at the BB. Grease on the seatpost would bubble out because of the trapped air being compressed as you lowered the seat.

Frezoni, like early ICE trikes, likes to use eutectic silver brazing for lugless frames. That takes pretty good torch control to avoid puddles of silver on the floor. The capillary action of nicely matched lugs and tubing makes silver brazing a bit easier.

[rogerzilla]

Oliver didn't ream the seat tube, or so he claimed.

A sealed seat tube suggests Chris Chance was building for the California market (despite being an east coast business), or didn't understand how water usually gets into a frame.  Such a frame runs a big risk of rotting out in a wetter climate.

[LittleWheelsandBig]

Fat City Cycles sealed most of their frame tubes at both ends. Even the bidon bosses were blind fittings. You are right that a sealed seat tube is asking for water to accumulate and rot it out at the BB.

https://raleigh-sb4059.com/2017/02/01/reynolds-tubing-wall-thickness-tube-gauge-and-seat-pin-size/ is an interesting read. I knew there were some dimensional quirks but not quite this many. To avoid reaming the seat tube entirely is tough though requiring minimal reaming is reasonable.

[rogerzilla]

It used to be common practice to wedge a greasy rag down the seat tube to keep any water creeping down the seatpin out of the BB bearings.  Decades later, if the frame is still being ridden, it rots from the inside out where the rag has retained all the moisture.  If water can get in, so can oxygen.

[fiveofthem]

Some more Brompton heavy metal has arrived.



After picking up a couple of unreamed sleeves from Stows Cycles, everything should be sorted soon with a sleeve to spare.

Hi there,
I was told that the original Brompton reaming tool set was only sold to commercial sellers. Where did you get yours?
Regards
Jan

[LittleWheelsandBig]

Your understanding is correct, so that is not a question I can answer.

Try searching for Brompton’s part number for the tool. https://www.vlerickfietsen.be/download-asset/158a1670-1731-11ea-9eb5-005056985c04 might be helpful to you.

[rogerzilla]

I bought an unused kit from a private owner.  The Brompton tools are sold to the public in other countries, normally on the understanding that they won't ship to the UK.  Brompton are not my favourite company.  They also engage in something very close to retail price maintenance, and have done so since the early days.  The replacement value of an M3L with lighting on the hire scheme is only just over half RRP, which makes you think.

[velosam]

I bought an unused kit from a private owner.  The Brompton tools are sold to the public in other countries, normally on the understanding that they won't ship to the UK.  Brompton are not my favourite company.  They also engage in something very close to retail price maintenance, and have done so since the early days.  The replacement value of an M3L with lighting on the hire scheme is only just over half RRP, which makes you think.

Well someone has to pay for BWC! lol

[drossall]

I notice that Cyclus make a 31.8mm reamer. Would .2mm matter? Would that with its holder be usable? I think I've read somewhere about clever tricks to insert something in the bottom of the seat tube to get good alignment for the reamer.

Or adjustable reamers?

[drossall]

Loctite do a 3g bottle of suitable adhesive for £3-4, which should do nicely.

Could I ask which Loctite you used?

[LittleWheelsandBig]

Loctite Super Glue Power Gel, from memory. A rubberised cyanoacrylate.

[drossall]

  That looks good.

[james]

I recently successfully replaced and reamed with a generic 32mm reamer (a big 4MT taper one). I used Loctite Power Flex (I had the 'mini trio gel' set where one tube seemed about the right amount for a shim).

The only problem I had was that it grabs very quickly, whereas I think the Brompton one gives you more time to align everything, so it's worth seeing if you can find something that gives a bit more handling time. I found that I couldn't get the alignment tab to click into place before it was impossible to twist the shim, so ended up knocking it out and using the second one that is supplied in the kit without the tab.

I ended up just using the remains of the previous shim as a guide to keep the reamer straight, and used a simple toolmaker's clamp on the tang of the reamer to turn it. I was a bit worried in advance about the alignment, but it seemed to work well.

If anyone wants to borrow an old 4MT 32mm reamer, and a piece of brass machined to 31mm to clamp onto while the glue sets, let me know!

[drossall]

Well I finished it off last night, and today had the not insignificant pleasure of riding on a seat that stayed where I put it

I used Loctite Super Glue Power Gel as recommended by LWaB. I think I didn't use enough glue first time, and maybe also didn't wait long enough - the reamer pushed the insert out. So second time I used more glue and waited overnight. For good measure, I left the mandrel partly in place, so that the insert couldn't really go anywhere, until the reamer was half way along it.

Of course, no job is ever finished. In the process, I found that my saddle height insert was breaking up and my O ring was failing, so I've been back to Condor's today to get replacements for those.

Thanks to all who have contributed here.