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Snapped off the rear triangle hinge bolt (Brompton)

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Snapped off the rear triangle hinge bolt (Brompton)

Old 08-06-19, 11:48 AM
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rickpaulos
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Snapped off the rear triangle hinge bolt (Brompton)

Just bought a used Brompton that had some abuse.

One countersunk bolt head snapped off after 15 miles. I tried to unscrew the other side bolt but the internally threaded shaft inside the frame just kept turning. There is nothing to grab to hold it. The other side screw needs to be removed to remove the rear triangle to get the internally threaded shaft out of the frame. Guess I'll try to drill out the broken bolt in place.

Anyone have a rear triangle in decent shape? I see you can buy a titanium rear triangle (or nearly any other parts) for the Brompton but I'm not wanting to pay $750 for that. The rear triangle was damaged up top where the little wheels go and one wheel was torn off.



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Old 08-06-19, 02:50 PM
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You can buy a replacement (steel) rear triangle from Brompton, it is an ordinary spare part and way way cheaper than 750$. Did you ask your Brompton dealer about getting a spare rear frame or just look on ebay? Depending from the country you are in frame parts may not be sold separately to end customers but only for intallation at a Brompton dealership. With the rear triangle this is a good idea anyway as you have to exchange the bushings and this is a bit of a delicate, but not very expensive task. If you have not done it before I'd highly recommend delegating this operation to a workshop anyway.
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Old 08-07-19, 06:46 AM
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I was just wondering about having the triangle spread out and a Nexus/Alfine gear hub installed.

How much does a triangle cost when a retailer orders one from Brompton?
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Old 08-07-19, 07:02 AM
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One option, instead of drilling the bolt out, is to saw through the plastic washer and straight through the spindle. I did this by hand and despite being very careful couldn't avoid scratching the inside powder coat of the rear triangle, though that cosmetic blemish gets hidden when resembled anyhow.
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Old 08-07-19, 11:17 AM
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I tried drilling last night. I used a left handed 1/8" bit. Drilled straight through the entire bolt. Sometimes bolts will come out when using a left handed bit. Not this time. Tried my easyouts. No luck. I suspect someone used locktite on the bolts. The shaft is not hollow clear though. 66 mm overall length (about 2 5/8"). I'd say it's drilled and tapped in about 5/8" depth. That leaves 2 more inches of drilling from the broken bolt side to get the other bolt rendered to fillings. I can keep the shaft from turning with a hex wrench in the good bolt. Or drill the good bolt out. Maybe an easy out would hold the shaft to keep it from spinning while drilling but that grips in the wrong direction.

There doesn't appear to be room for a saw blade between the frame and the triangle. The paint/plastic coat is already in fair condition to rubbed off completely in spots. IMO drilling would mean less to no additional paint or frame damage.

This Brompton has a Sachs 3 speed igh with the 2 cogs. It shifts well. My only note is it is geared a bit high. I may install a smaller chain ring later on. I'll need to calculate the ratios ride it more to decide. This is not a racing bike by any means. Standing to pedal isn't good for stability or for the hinge(s) stresses. I have re-spaced a number of steel frames both wider and narrower. This one has such short stays, I guess it wouldn't tolerate that. The 6 gears are okay for this kind of bike. My Giatex folder has a 6 speed der. The Fuji/marlboro mtb folder is now a single speed. If I need speed, I can get the Ritchey Breakaway out. I'll measure the O.L.D. on the Bromton next chance I get.
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Old 08-07-19, 01:00 PM
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There is enough room: you break off the plastic washer first.

The bolts are secured with high strength loctite and apparently if you can't manually break the loctite bond you have to use a heat gun. This is Brompton's way of making absolutely sure that the bolts don't loosen when riding, but since I keep an eager eye on my bike I've used low strength loctite for the last couple of years and not had any problems.
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Old 08-07-19, 01:21 PM
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Originally Posted by chagzuki View Post
There is enough room: you break off the plastic washer first.
So, the shaft is longer than the frame? I couldn't tell with the washers in place.
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Old 08-07-19, 02:37 PM
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Yes, it’s frame plus washers in length.
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Old 08-08-19, 12:22 AM
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Originally Posted by rickpaulos View Post
I have re-spaced a number of steel frames both wider and narrower. This one has such short stays, I guess it wouldn't tolerate that.
It does if you know what you are doing. Juliane Neuss does commercially offer to retrofit an Nexus 8 into a Brompton for almost 20 years now and has done hundreds of those conversions. The hub gets downspacered to a bit smaller OLD than factory and the rearframe get's spread. She and others have also sucessfully manged to bring an Alfine 11 (which cannot be spacered as small as the Nexus) into the factory rear frame.

Last edited by berlinonaut; 08-10-19 at 03:55 AM.
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Old 08-08-19, 01:17 PM
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Just a bit of weirdness. I park 5 miles from work and ride the rest of the way using different routes to break up the tedium. I rode the bike to work on Monday. I took a photo of the bike Monday morning just as I was headed out for it's maiden ride and the bolt is intact. As I returned to my van Monday night, the bike got quite wobbly as I dismounted. I figured the hinge bolt just broke at that moment. Before I loaded the bike in the van, I saw the one hinge bolt was snapped off. I walked back about 30 feet looking for the bolt head but didn't see it. Who knows where it broke or landed. Well, today, 3 days later, I'm unloading another bike to ride to work and I found the bolt head sitting on the rear bumper of the van.

After more thought, I'm of the opinion this live hinge pin and 2 bolts are not a great design.All the vertical weight is supported by the two 5 (or 6) mm bolts in a sheering and twisting action. If the shaft extended through the 2 arms on the rear triangle, the larger shaft would support the weight instead of just the smallish
bolts. Or a larger single bolt (same diameter as the hinge shaft) through the rear triangle arms and through the frame would be far stronger and wouldn't weigh much more and would be much easier to service. The counter sunk bolts do look nice but a protruding bolt head and nylock nut would not interfere with other components. (well, maybe if you use two chain rings, not likely) I read where someone as asking about using a Brompton for loaded touring, I can see a major issue if you broke one of the stock bolts on the road somewhere.

I have no idea of how often riders break the hinge bolts. My experience has been 1 15 mile ride and it broke. The bike is quite used. I don't know it's history.

Alternative hinge bolt? "Long Shoulder bolt". with a hex socket end or a normal hex end.
The challenge would be finding one with suitable dimensions.


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Old 08-08-19, 07:48 PM
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Originally Posted by rickpaulos View Post
Just a bit of weirdness. I park 5 miles from work and ride the rest of the way using different routes to break up the tedium. I rode the bike to work on Monday. I took a photo of the bike Monday morning just as I was headed out for it's maiden ride and the bolt is intact. As I returned to my van Monday night, the bike got quite wobbly as I dismounted. I figured the hinge bolt just broke at that moment. Before I loaded the bike in the van, I saw the one hinge bolt was snapped off. I walked back about 30 feet looking for the bolt head but didn't see it. Who knows where it broke or landed. Well, today, 3 days later, I'm unloading another bike to ride to work and I found the bolt head sitting on the rear bumper of the van.

After more thought, I'm of the opinion this live hinge pin and 2 bolts are not a great design.All the vertical weight is supported by the two 5 (or 6) mm bolts in a sheering and twisting action. If the shaft extended through the 2 arms on the rear triangle, the larger shaft would support the weight instead of just the smallish
bolts. Or a larger single bolt (same diameter as the hinge shaft) through the rear triangle arms and through the frame would be far stronger and wouldn't weigh much more and would be much easier to service. The counter sunk bolts do look nice but a protruding bolt head and nylock nut would not interfere with other components. (well, maybe if you use two chain rings, not likely) I read where someone as asking about using a Brompton for loaded touring, I can see a major issue if you broke one of the stock bolts on the road somewhere.

I have no idea of how often riders break the hinge bolts. My experience has been 1 15 mile ride and it broke. The bike is quite used. I don't know it's history.

Alternative hinge bolt? "Long Shoulder bolt". with a hex socket end or a normal hex end.
The challenge would be finding one with suitable dimensions.


its a great desing, you bike was just severely beatup.
its first time i hear this part was snapped on a brompton
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Old 08-09-19, 11:00 AM
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Originally Posted by rickpaulos View Post
Just bought a used Brompton that had some abuse.
One countersunk bolt head snapped off after 15 miles.
Originally Posted by rickpaulos View Post
After more thought, I'm of the opinion this live hinge pin and 2 bolts are not a great design.
It is not a bad design. More than 1/2 a million Brompons have been built since the late 80ies and the issue that you are suffering from is a very rare one. I know some cases but only a handful. You bought a bike that - judging from the pictures - is at least 12 years old (or even older), has been neglegted, beaten and misused for years and years w/o love or maintenance. Surprise: This has consequences. Judging on the design with literally no knowledge about the construction from a single coincidence on a obviously neglected bike seems a bit misled, to say it politely. The rear frame bearings are prone to wear, depending from the rider and the situation they tend to last 20.000 - 40.000kms, though some people suffer from the need for exchange already after 5.000 - 10.000 km. Exchange is cheap, but not easy and if not executed properly a new bearing won't last long. Thus it is better to leave the exchange to someone experienced.
So my first question is: Did you check the wear on the rear triangle when you bought the bike? Judging from the pictures chances are big that a regular exchange of the bearings has been long overdue but was ignored. Surprise: this has consequences.
Regarding the construction I was asking myself if the wear could be avoided by a different construction but learned from a metalurgist that the construction is solid state of the art and properly made.

Last edited by berlinonaut; 08-10-19 at 03:57 AM.
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Old 08-09-19, 08:36 PM
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Originally Posted by berlinonaut View Post
So my first question is: Did you check the wear on the rear triangle when you bought the bike?
No. It was a purchase on the spur of the moment. Never owned or ridden a Bromtpon before and just quickly looked at the typical items like straight rims and obvious damage. I took it for a short test ride around the block and it rode okay.

Last edited by rickpaulos; 08-09-19 at 08:47 PM.
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Old 08-10-19, 03:25 PM
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I agree it’s a problematic design, though I’ve never heard of anyone snapping a bolt. I wonder how it’s designed on the Dahon Curl. Andrew Ritchie designed the brompton around the limitations of basically putting it together in his shed, and not much has changed. There are a benefits to that style of design though, e.g. the hinges/hinge plates can wear and still be perfectly functional, it’s a robust machine in that way.
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Old 08-11-19, 02:11 AM
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Originally Posted by chagzuki View Post
I agree it’s a problematic design, though I’ve never heard of anyone snapping a bolt. I wonder how it’s designed on the Dahon Curl. Andrew Ritchie designed the brompton around the limitations of basically putting it together in his shed, and not much has changed.
That's one of those common beliefs. In fact, when comparing an early MK2-Brompton from the late 80ies with a current one in detail you'll recognize that in fact there are only a small handful of things hat have NOT changed. The only two that I am aware of are the standard steel seat post - and the rear hinge. Every other part HAS changed, at least once, many more than once. Maybe I overlooked some constants but it can't be many. (Edit: I was informed that the seat post did indeed change over time. The second bit that did not change is the little plastic knob on the stem that keeps the folded stem in place /Edit). Considering that I assume that the design is considered to be not that bad and - as often - many of the people critzicing it do not fully understand how the thing works in terms of loads etc, (no affront intended), they just see a relatively small bolt and judge "bad design". Which is not how engineering works.

Last edited by berlinonaut; 08-11-19 at 10:41 AM.
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Old 08-11-19, 07:05 AM
  #16  
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Originally Posted by berlinonaut View Post
That's one of those common beliefs. In fact, when comparing an early MK2-Brompton from the late 80ies with a current one in detail you'll recognize that in fact there are only a small handful of things hat have NOT changed. The only two that I am aware of are the standard steel seat post - and the rear hinge. Every other part HAS changed, at least once, many more than once. Maybe I overlooked some constants but it can't be many. Considering that I assume that the design is considered to be not that bad and - as often - many of the people critzicing it do not fully understand how the thing works in terms of loads etc, (no affront intended), they just see a relatively small bolt and judge "bad design". Which is not how engineering works.
Let’s just say it is a “badly” designed component/design...which Im not. There are always engineering compromises of some sort that have to be made in product design- especially machines. And more so for products which are pardigm changers. Show me a car, motorcycle, bicycle, even cell-phones/electronics which dont have some kind of Achille’s heel design-wise, it does t exist (take the iPhone for instance....non user replacable battery) The unique fold of the Brompton required someery unique solutions, which are well proven to work but cannot be taken for granted or treated as if they weren’t unique in the attention/maintenance they require in order to perform their design function throughout the entire service life of the whole.
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Old 08-11-19, 07:59 AM
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Originally Posted by FolderBeholder View Post
Let’s just say it is a “badly” designed component/design...which Im not.
And you are a brave and wise man by not doing so. The construction is as follows:

- inside the hinge part of the main frame there are bronze bearings.
- through the hinge part goes a steel spindle, that sits on those bearings. The spindle is slightly longer than the hinge part of the main frame
- the rear frame is sitting on this steel spindle, so the load goes onto that spindle, not onto the bolts.
- between the rear frame and the main frame there sit slim nylon shims on each side
- the bolts are just there to keep the rear frame on the spindle, very much like the nuts on a bike wheel's axle. They press from the side and do not get any vertical load. Those bolts are conical and they are glued in.

Over time the bronze bushings wear which leads to side play in the rear frame. In this case the bushings have to be exchanged. This is an ordinary, well described maintenance job, the parts are cheap and so is the work. It is however nothing for the ordinary hobbyist or fainted hearted as the bolts have either to be drilled out from it's heads or cut in the gap between main frame and rear frame as they are glued in. The bronze bushings have to be removed, the whole thing has to be reamed with an exactly fitting reamer, then the new bushings come into place along with a new spindle and the whole thing goes back together. If the job is not done properly the new bushings will wear quickly or will not stick, the play will not be gone, the whole thing will not fold properly or you'll loose the bolts. Thus it is a good idea to give this work to an experienced workshop that knows what he is doing because attention to detail is relevant during this operation. Many Brompton dealers do not know how to do it properly and delegate this work to someone who knows (which is a good idea) while others in the same boat deliver bad work. Ignoring massive play for too long can lead to a state of unrepairability due to overwear and a damaged main frame.

When a bolt sheers this is not only very, very rare but due to the fact that the glue that holds it in place was either not sufficient, had not enough time to harden or was dispersed. The latter happens if the owner of the Brompton has i.e. the idea to lubricate the thing with oil. Longterm it will loose the bolts and there you go. If you leave it alone as you are supposed to nothing will happen. When the bolt comes loose the rear frame can move sideways on the spindle and give load to the bolts that they are not designed and intended to handle and so it sheers. This is however a process and it needs a bit of an ignorant rider not to recognize while riding that something is seriously wrong when a bolt has started loosening. So in most cases nothing sheers, you glue the bolt back in and tighten it, job done. And even that is a more than rare event.

Same goes with the bushings btw. - if you lubricate those (and possibly overlubricate) they will come loose and the trouble starts.

My question was if there was not something longer lasting and more sophisticated than the bronze bushings but I was told by an material expert for those kind of things that this way of handling the thing is still state of the art and perfectly fine for the application.

The people who judge and claim that the whole construction would bad because the bolts are so tiny just missed to understand how the whole construction works. If they had they would know that the bold does not carry transverse load and is perfectly fine for the purpose that it is intended to deliver. And therefore they would judge differently and not claim wrong things. But judging ignorantly on things w/o even trying to understand them first seems to be a plague of the modern age.

Last edited by berlinonaut; 08-11-19 at 10:44 AM.
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Old 08-11-19, 11:15 AM
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So let's see how it's handled on other similar bikes, the Curl, Mezzo, Birdy etc.. I assume they use a modern system with regular bearings, and are much easier to service? And probably don't require servicing often? I've not owned any so I don't know.
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Old 08-11-19, 11:41 AM
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Originally Posted by berlinonaut View Post
And you are a brave and wise man by not doing so. The construction is as follows:

- inside the hinge part of the main frame there are bronze bearings.
- through the hinge part goes a steel spindle, that sits on those bearings. The spindle is slightly longer than the hinge part of the main frame
- the rear frame is sitting on this steel spindle, so the load goes onto that spindle, not onto the bolts.
- between the rear frame and the main frame there sit slim nylon shims on each side
- the bolts are just there to keep the rear frame on the spindle, very much like the nuts on a bike wheel's axle. They press from the side and do not get any vertical load. Those bolts are conical and they are glued in.

Over time the bronze bushings wear which leads to side play in the rear frame. In this case the bushings have to be exchanged. This is an ordinary, well described maintenance job, the parts are cheap and so is the work. It is however nothing for the ordinary hobbyist or fainted hearted as the bolts have either to be drilled out from it's heads or cut in the gap between main frame and rear frame as they are glued in. The bronze bushings have to be removed, the whole thing has to be reamed with an exactly fitting reamer, then the new bushings come into place along with a new spindle and the whole thing goes back together. If the job is not done properly the new bushings will wear quickly or will not stick, the play will not be gone, the whole thing will not fold properly or you'll loose the bolts. Thus it is a good idea to give this work to an experienced workshop that knows what he is doing because attention to detail is relevant during this operation. Many Brompton dealers do not know how to do it properly and delegate this work to someone who knows (which is a good idea) while others in the same boat deliver bad work. Ignoring massive play for too long can lead to a state of unrepairability due to overwear and a damaged main frame.

When a bolt sheers this is not only very, very rare but due to the fact that the glue that holds it in place was either not sufficient, had not enough time to harden or was dispersed. The latter happens if the owner of the Brompton has i.e. the idea to lubricate the thing with oil. Longterm it will loose the bolts and there you go. If you leave it alone as you are supposed to nothing will happen. When the bolt comes loose the rear frame can move sideways on the spindle and give load to the bolts that they are not designed and intended to handle and so it sheers. This is however a process and it needs a bit of an ignorant rider not to recognize while riding that something is seriously wrong when a bolt has started loosening. So in most cases nothing sheers, you glue the bolt back in and tighten it, job done. And even that is a more than rare event.

Same goes with the bushings btw. - if you lubricate those (and possibly overlubricate) they will come loose and the trouble starts.

My question was if there was not something longer lasting and more sophisticated than the bronze bushings but I was told by an material expert for those kind of things that this way of handling the thing is still state of the art and perfectly fine for the application.

The people who judge and claim that the whole construction would bad because the bolts are so tiny just missed to understand how the whole construction works. If they had they would know that the bold does not carry transverse load and is perfectly fine for the purpose that it is intended to deliver. And therefore they would judge differently and not claim wrong things. But judging ignorantly on things w/o even trying to understand them first seems to be a plague of the modern age.
Yeah! I got the pin out last night. I can say for sure the rear triangle is not resting on the 3/8" diameter pin. The length of the pin matches the inner width of the rear triangle exactly. Rider weight is supported solely by the two 1/4" x 28tpi ybn bolts. The counter sunk bolt holes in the rear triangle are too small to allow the pin to fit through. If they were 3/8", removing the pin with a broken off bolt would have been a trivial task. The 'good' bolt was held quite securely by a greyish threadlock. No sign of the bolts loosening. The spacers seem in good shape. The bronze bushings were lacking in lube but seem to be useable. This may be another of the Brompton changes over time if the later models' rear triangles do rest on the pin. The remains of the broken bolt refuse my efforts to remove so a new pin, spacers and bolts are on order. tick tock. I hope the parts arrive in time for the ABCE in September. I've been assured from several sources that the bolts breaking is an extremely rare occurrence so I ordered those of the same design. Next time it breaks, if ever, I'll drill out the rear triangle arms and install a 3/8" solid shoulder bolt and lock nut.

I did ask Brompton and got the typical email reply. "Take it to a dealer." That's one corporate approach to customer service that is designed to keep the dealers happy.

rick
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Old 08-11-19, 11:52 AM
  #20  
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Originally Posted by berlinonaut View Post

Same goes with the bushings btw. - if you lubricate those (and possibly overlubricate) they will come loose and the trouble starts.
Manual call for hinge to be oiled, though infrequently.
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Old 08-11-19, 06:59 PM
  #21  
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Originally Posted by Schwinnsta View Post
Manual call for hinge to be oiled, though infrequently.
My manuals (2018) dont speak to that under “cleaning and lubrication”. They DO warn against getting grease on the seatpost or wheel rims though. No warning on the triangle hinge....
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Old 08-12-19, 07:45 AM
  #22  
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This is from the 2010 manual.
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Old 08-12-19, 08:36 AM
  #23  
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It would be interesting to contact Brompton and ask if there have been any material changes to that recommendation 2010 +....
(maybe I will try to contact them)
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Old 08-12-19, 07:50 PM
  #24  
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Originally Posted by berlinonaut View Post
Surprise: This has consequences.
You jerk, now I am going to spend the next 20,000km worrying about failure.
buzzkiller

Only consolation is a great review of the elements that will invoke my eventual demise
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Old 08-14-19, 03:50 PM
  #25  
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Originally Posted by FolderBeholder View Post
It would be interesting to contact Brompton and ask if there have been any material changes to that recommendation 2010 +....
(maybe I will try to contact them)
Update (and replying to my own post...strange)

I Did contact Brompton regarding any updates to the rear hinge pivot 2010+ and here is the response straight from the horse's mouth:

Thank you for contacting us here, at the Brompton factory.

The manual for the 2018 model bikes has indeed changed from the manual from 2010. This is due to the constant development of the bike and consequently, all manuals we provide with a newly built bike will be specific to its build specifications and manufacturing year. Regarding your enquiry about the rear hinge, the bearings of this part have changed since 2010 and nowadays this part is made from self-lubricating material. Please follow the instructions of the manual which was provided with your bikes as both bikes are new models built in 2018.


So, apparently lubricating the newer hinge points isn't needed/recommended/necessary.
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