Author Topic: Brompton seatpost shim  (Read 3317 times)

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton seatpost shim
« Reply #25 on: September 05, 2020, 08:21:11 pm »
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.
Wheel meet again, don't know where, don't know when...

Re: Brompton seatpost shim
« Reply #26 on: September 09, 2020, 04:02:44 pm »
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...
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton seatpost shim
« Reply #27 on: September 09, 2020, 05:03:59 pm »
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.
Wheel meet again, don't know where, don't know when...

Re: Brompton seatpost shim
« Reply #28 on: September 10, 2020, 06:44:47 am »
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.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton seatpost shim
« Reply #29 on: September 10, 2020, 08:35:26 am »
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.
Wheel meet again, don't know where, don't know when...

Re: Brompton seatpost shim
« Reply #30 on: September 10, 2020, 11:11:53 am »
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.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.