Brompton rear hinge

[drossall]

Good post with all the links. But I think you should point out explicitly that your BrommiePlus link is to a reamer that is (a) available for sale and (b) cheaper than the official Brompton one.

[rogerzilla]

Coincidentally, I ordered one a few days ago.

I used a standard hand reamer, long enough to go through both sides, when I last did a Moulton F-frame.  In theory the bushes on those should be just right when fitted, but they are all made by little cottage industry suppliers now, so mine were a little tight.  It can be tricky to find one where the parallel section of the flutes is long enough.

[Max-Brompton]

Good post with all the links. But I think you should point out explicitly that your BrommiePlus link is to a reamer that is (a) available for sale and (b) cheaper than the official Brompton one.

Ahh, if any rules have been broken I apologise - I'm happy with a basic adjustable reamer bought on eBay in the UK TBH - proceed with caution and it's reasonably easy and precise enough.

I'm still thinking of adding SS washers - getting anal nerdy here but I'm measuring the following on my Brompton:

Front frame width 68.40mm, + 2 x nylon washers (1.55mm thick) so total 71.50mm, pivot pin 71.80mm in length, 'OLD' rear frame 72.40mm so 0.6mm to play with.

So if adding 2 x 1mm SS washers (between pin end and rear frame) you'd only need to mill say 0.55mm off each end of the pin, use 1.0mm nylon washers and adding the 2 x 1.0mm SS washers it would be spot on with exactly the same 0.15mm excess each end of the pin and the fitment down to 0mm gap.

Errrrrrmmm...I think...   

[drossall]

Not at all. I simply meant that you've found a more reasonably-priced, and available, reamer, which various people have been looking for. But that wasn't apparent until I followed your link out of curiosity to see to what you were linking. No broken rules that I know of, and anyway it's not my place to enforce any.

[Max-Brompton]

If anyone is interested in following this I'll pop up some updates.  As I say I might get the spindle and tolerances tidied up a bit but as the paint was a bit uneven anyway I carefully took off the main thickness with a soft aluminium edge and tidied up the traces with a rather Heath Robinson dare I say '400 grit facing tool' but more to give a nice clean surface to measure from.  It's actually pretty blooming flat now with just a couple of thou' across the face.  Just tighten up and turn the squares gently to clean things up.

[rogerzilla]

The nylon washer performs three functions:

1. Allows for unevenness in the faces of the pivot tube and "ears"
2. Provides a thrust bearing
3  Improves sealing against water and dirt ingress

If you compare a Moulton rear end, they use flanged bushes to provide a thrust bearing but the length of the sleeve is ever-so-critical to give free movement without play, and the sealing is sufficiently bad that they provide a grease nipple.  The Brompton design has its merits; it's just a pain to overhaul.

[Max-Brompton]

Good points.  I'm very tempted to not use Loctite at all and have it freely accessible for access to the rear end not just the 'bearing'.  As you might have guessed  I'm an inveterate tinkerer/fiddler/checker/experimenter and pretty much check bolts etc habitually so torqued up right and checked often there is no concern.

[rogerzilla]

Bear in mind that the RH bolt is usually inaccessible if it comes loose during a ride.  You sometimes get lucky with chainring cutouts.

[Max-Brompton]

Bear in mind that the RH bolt is usually inaccessible if it comes loose during a ride.  You sometimes get lucky with chainring cutouts.

Sure, TBH I have never carried a 5/32" Allen key anyway but might start...

[lkingscott]

I just came across this forum and thread as I am repairing a Brompton for a friend.

The main problem is a broken main frame hinge, completely broken across the hinge pin. I'm going to build it back up with silicon bronze or steel, using a TIG, but that's another story for another thread!

The reason for posting here is that I had to dismantle the rear hinge, which was worn anyway and about the only good thing I can say about it, as an engineer, is that it is lightweight!

I want to modify the hinge to make it easier to maintain (and use metric parts which are more commonly available...!!!).

Does anyone have the outer dimension of the bushes or ID of the tube they are inserted into?

The reamer dimensions are 9.5 to 9.55mm and the worn bushes on the bike I am fixing are close to 10mm inside diameter.

My idea, to make it easier to maintain, is to use stainless M6 countersunk cylinder nuts, which have an 8mm outer diameter and I'd put an M6 threaded rod between. I would make or buy close and machine bushes with 8mm inside diameter. I would get decent cylinder nuts, but I expect I'd need to polish them, to minimise bush wear.

I would clean up the tube ends as in one of the previous posts and use 8mm ID washers for endfloat. My gut feeling would be to use brass or oil filled bronze washers. The countersunk nuts would be locktited to the threaded rod (and maybe csk) and tightened to the point where the end float is taken out, but rotation is still good. One of the nuts could be permanently welded or brazed to the threaded rod, etc.

Top hat bushes could also be used, but I suspect the end wear would be way more than the bush wear, so it would be nice to make the washers separately replaceable.

This would then allow the rear hinge to be easily disassembled by removing one nut for regular greasing.

I've priced it up and parts cost is less than a Brompton hinge kit with bushes!!!

[LittleWheelsandBig]

If you add a grease nipple into the frame, bush wear stops.

[rogerzilla]

I don't like the need for a special reamer (which Brompton will no longer sell you, bur Brommieplus will) but it is surprisingly long-lived compared to the similar Oilite-bushed arrangement on a Moulton TSR.  The glacier bushes have an incredibly high load capacity, it rarely corrodes because the nylon washers make a good dirt seal, and it retains grease well.  It also benefits from the frequent folding action, which redistributes grease around the whole bearing.  These pivots barely move when riding.

Overall it is better-engineered than Moulton's rear pivot (little changed since 1962), it's just not very home mechanic-friendly.  Best left as designed.  The bushes, bolts and washers are standard engineering parts and the sleeve/journal barely wears so can be re-used.  The long piloted 3/8" reamer is the challenge.

[lkingscott]

If you add a grease nipple into the frame, bush wear stops.

It's still a PIA to dismantle and replacements are still expensive from a small number of outlets. A grease nipple could be used on my proposed arrangement.

[lkingscott]

I don't like the need for a special reamer (which Brompton will no longer sell you, bur Brommieplus will) but it is surprisingly long-lived compared to the similar Oilite-bushed arrangement on a Moulton TSR.  The glacier bushes have an incredibly high load capacity, it rarely corrodes because the nylon washers make a good dirt seal, and it retains grease well.  It also benefits from the frequent folding action, which redistributes grease around the whole bearing.  These pivots barely move when riding.

Overall it is better-engineered than Moulton's rear pivot (little changed since 1962), it's just not very home mechanic-friendly.  Best left as designed.  The bushes, bolts and washers are standard engineering parts and the sleeve/journal barely wears so can be re-used.  The long piloted 3/8" reamer is the challenge.

There are countless bikes with hinges for rear suspension and I have never come across one which was such a PIA to dismantle and would wear so much yet be so difficult to renovate. Generally wearable parts should be easy to replace. Many also use bushes, but they also have proper seals, so wear even less.

Ideally the bushes should be recessed so that seals could be fitted and then the endfloat washers, which will wear as they are soft, would only have to be endfloat washers.

I have owned many 1960s and 70s British cars and this has so many of their PIA to work on, or weird parts characteristics. They even had the mix of metric and imperial! I know many standard bike parts are still imperial, for historical interchangeability reasons, but why on earth do Brompton use imperial parts in some of the custom parts and not others?

Ease of dismantling for maintenance is my primary motivation for changing it. Secondarily the ease of obtaining parts.

Using metric csk cylinder nuts will make removal a 2 minute job.

[rogerzilla]

The stainless tube is 3/8" (exactly).  The bushes are, from memory, 06DX06 which are 17/32" OD and nominally 3/8" ID, but will generally need reaming as they shrink when pressed in.  The bolts are high tensile steel and 1/4" UNF 28tpi.  There's nothing metric in the rear hinge.

Dismantling is a lot easier if you use the correct hex key bit - 5/32", NOT 4mm.  It's also easier when doing it for a second time, as I suspect the factory uses a far stronger grade of threadlock than necessary.  A 7/16" tap will drift out bushes easily, unless they have become corroded to the frame tube.  I've only had to cut and collapse one set.

I don't think the countersinking is an anti-precession measure - it's just a way to fit the pivot arrangement into a very tight space at the back of the bike.

[lkingscott]

The stainless tube is 3/8" (exactly).  The bushes are, from memory, 06DX06 which are 17/32" OD and nominally 3/8" ID, but will generally need reaming as they shrink when pressed in.  The bolts are high tensile steel and 1/4" UNF 28tpi.  There's nothing metric in the rear hinge.

Dismantling is a lot easier if you use the correct hex key bit - 5/32", NOT 4mm.  It's also easier when doing it for a second time, as I suspect the factory uses a far stronger grade of threadlock than necessary.  A 7/16" tap will drift out bushes easily, unless they have become corroded to the frame tube.  I've only had to cut and collapse one set.

I don't think the countersinking is an anti-precession measure - it's just a way to fit the pivot arrangement into a very tight space at the back of the bike.

Many thanks, those dimensions seem right, but the stainless part is not a tube, it's drilled and tapped either ends on the bike I have here.

I had no problems fitting bits into the hex heads using an Irish screwdriver (no malice towards the Irish intended). I could put way more torque than needed, but once one side was loose, unless you have an imperial bolt and nut to put into the loose side (all metric in Germany), it was almost impossible to lock the stainless rod sufficiently to put enough torque on the other bolt. A threaded hole in the centre of the frame tube would allow a bolt to be inserted to lock it, particularly if there were a flat on it in the middle. The same threaded hole could be used for a grease nipple.

I suspect most Brompton dealers use the same locking gunge. Note than many Loctite thread locking products are made to soften at specific temperatures. The red type is around 350C from memory, so a bit high. I have no idea which one Brompton use.

I agree that with the current design, the countersinking is needed to minimise protrusions. However it could have been better designed. I have an ultralight recumbent with 20" wheels made from some grade of high tensile aluminium (much thinner tubes than most aluminium bikes), all well under 10kg. The rear hinge for the suspension uses a much larger outer frame tube, about 25mm or so, with larger holes in the suspension arms and aluminium cups. A relatively standard bolt and nylock nut holds it together with the nut, bolt and bushes completely recessed and sealed. The larger diameter means much larger bearing surface to take loads. It seems like a much better arrangement and dead simple to take apart.

Off topic, but I wonder why there's never been an aluminium Brompton??? Titanium is so expensive and very specialised to weld but needed less redesign. I know aluminium would need a complete redesign and unless a high tensile grade was used, weight is not necessarily saved. However my lightweight recumbent is almost all aluminium and much larger than the Brompton, yet lower weight. OK, no hinges etc. to add weight. Due to back issues, I now prefer recumbents or more precisely semi recumbents and a 'bent which folded to the size of a Brompton would be nice.

I have an aluminium Dahon, which is much heavier than a Brompton, but it feels so much sturdier to ride. I have an ancient Bickerton, which feels like riding something made of chocolate (not ridden for decades), which is why I went for the sturdier but heavier Dahon.

The only problem with my proposed change I can foresee is the possibility of the csk cylinder nuts rotating in the rear frame. The standard design should clamp the stainless rod against one side of the rear frame or the other. Some Locktite should easily prevent that. There are csk screws with grooves to lock them, but That could cause wear, which would not be good.

FYI I have made my own bikes and trikes. My daily ride is the opposite end of the spectrum to the Brompton, a >50kg non electric, self built semi recumbent trike. The seat is a converted garden sun lounger and it can carry 2 crates of beer in the boot! It has over 20,000km on it mostly around town, although I've done some day rides over 60km. I weld and machine everything myself, so I know what I'm talking about, although it's not my day job. My current project is a carbon leaning trike for day touring.

[rogerzilla]

The hinges, which are normally very effective, have to be very hard, and tube diameters are limited by the need to fold, which is why they can't be aluminium.  I suspect the titanium ones will also suffer recalls while they sort out teething troubles.  The basic product hasn't changed much in 30 years, and Brompton have a bit of history in failed updates.

[lkingscott]

The hinges, which are normally very effective, have to be very hard, and tube diameters are limited by the need to fold, which is why they can't be aluminium.  I suspect the titanium ones will also suffer recalls while they sort out teething troubles.  The basic product hasn't changed much in 30 years, and Brompton have a bit of history in failed updates.

The tubes can be made oval, which also has other advantages. There is a carbon fibre Brompton copy which uses oval tubes.

The frame hinge on my aluminium Dahon is a much nicer mechanism, using an adjustable over centre lock, similar to some old suitcase latches. No winding of nuts or bolts needed to lock in place and no hard metal mating surfaces needed.

[lkingscott]

The hinges, which are normally very effective, have to be very hard, and tube diameters are limited by the need to fold, which is why they can't be aluminium.  I suspect the titanium ones will also suffer recalls while they sort out teething troubles.  The basic product hasn't changed much in 30 years, and Brompton have a bit of history in failed updates.

The T-Line has been thought about, but owner self maintenance appears never to be a Brompton consideration.

The main hinges on the T-Line use bushes (presumably because titanium would wear quicker than cast or forged steel) and the pin is a bolt with a nut on the end. Much nicer than the main hinges on the steel frames as it is easy to maintain and replace the bushes with no custom tools.

The rear hinge on the T Line seems to be identical to the steel frames, the parts are even interchangeable.

[rogerzilla]

My main concern would be cracking at the welds.  Titanium is treacherous stuff unless you have the very best (or automated) TIG welding, with no overheating and absolutely no oxygen contamination.  Brazing hi-tensile steel is child's play by comparison.

[lkingscott]

My main concern would be cracking at the welds.  Titanium is treacherous stuff unless you have the very best (or automated) TIG welding, with no overheating and absolutely no oxygen contamination.  Brazing hi-tensile steel is child's play by comparison.

Yes, apart from the cost, that's why it's rarely used, except single piece cnc'd parts.

However the Brompton web site shows pictures of the titanium frames being welded and you can see the fittings to fill the tubes with argon while welding, etc.

The titanium is thicker than the steel frames (it only needs to be about 10% thicker, but it looks more). It's particularly noticeable on the main hinges, which look chunkier and because they use bushes, it looks like 8mm OD bushes, they need more metal around them.

The Ti frames do look much more professional, as they look more like moulded parts. The steel frames still look like pre 80s non waterformed / non welded (brazed with brackets) bikes.

Typical old British manufacturer under investing.

However I applaud them for not taking the easy route and move manufacturing to China.

[Asterix, the former Gaul.]

There must be a Chinese copy available somewhere.

[rogerzilla]

There are, in varying degrees of crapness and unsupportability.  Some are better than others.  The aluminium Merc was supposedly quite good, if you were lucky enough to get one where the seatpost didn't slip.  The 3Sixty is the current clone of choice.

[lkingscott]

There must be a Chinese copy available somewhere.

This one's not Chinese, Korean carbon fibre. Actually looks pretty good. Not a huge weight saving though.

http://chedech.com/?ckattempt=1

[lkingscott]

I'm posting my hinge modification on another thread, because it's associated with the main hinge repair I've been doing.

https://yacf.co.uk/forum/index.php?topic=124758.25