Broken Brompton hinge

[alfapete]

Excellent stuff, thanks. Looking forward to the next installment.

[lkingscott]

Excellent stuff, thanks. Looking forward to the next installment.

 

I spent the afternoon in the workshop, not much in photos to show as it all look similar to the last ones, but after grinding out some inclusions and what looked like a crack then adding new weld, I've been grinding and filing it all back. I've built it up a lot more than it was pre break, on the basis that if there are any defects, more metal is better than less. If I uses bushes, I'd like more metal too

I also did a lot of filing of the hinge pin hole, to adjust the alignment. It's not yet 6mm diameter, but I can fit M6 bolts into the ends, through the other part of the hinge and it all aligns pretty well now. The previous pics were with M5, so accommodated a lot of slop. To get the ends perpendicular to the pin, it's ended up a bit shorter than before. However if I use bushes, this will be accommodated, or I can add shim washers.

[lkingscott]

I'm still waiting for some parts for the main hinge repair (namely the stainless steel tube).

In the mean time, the rear hinge needed renovation. I started talking about this on the Brompton rear hinge thread, but I thought it would be better to put everything on the one thread, as I do it and the parts arrived today for the rear hinge.

Basically I had the common problem dismantling the rear hinge in that one bolt came out fine and the other not. I could have just bought a hinge repair kit, but I don't like the design and the parts I got are cheaper.

So in place of the countersunk bolts and tube, I'm using countersunk cylinder nuts and a piece of threaded rod, which will mean it's much easier to disassemble. Furthermore, due to available metric sizes, I have used metric polyamide (Nylon) top hat bushes. I could use similar bushes to the original, but I think that Nylon will be as good. If not it will be easy to replace with similar metric versions of the original bushes and still use my modification. Another advantage with Nylon bushes is that the cylinder nuts should never wear, the only wearable part will be the bushes.

For anyone interested, the cylinder nuts are M6, 8mm diameter and 16mm long. The bushes are 8mm inside diameter, 12mm outside diameter, 10mm long plus 2mm thick x 16mm diameter flange.

When they arrived, I initially thought that the bushes inside diameter would be too big. They are slightly oversize (moulding tolerance I guess) at 8.2 to 8.3mm and the cylinder nuts are 7.9mm diameter. However the inside diameter of the frame tube is 11.9mm. It's not particularly nice as they leave the powder coat on before fitting the original bushes, but it has rusted too. Anyway, I pressed one of the Nylon bushes in and it went in fine and the inside diameter of the bush closed up to 7.8mm, making it a really nice fit with the cylinder nuts. I will polish the nuts using the lathe before I complete the assembly, to minimise wear on the bushes.

Although I should wait until I've touched up the powder coating, I'm impatient to see how the hinge performs, so here are some initial pics.

[rogerzilla]

How do you ensure the rear triangle isn't pinching the bushes?  This is normally done by making the sleeve fractionally longer than the frame tube plus bush flanges.

[Paul]

First tests after initial drilling, some grinding and filing, looking good.

I found a few inclusions and a possible crack, so since these photos, I've done more grinding and will add more weld.

Top notch fettling. There ought to be some sort of honour.
Maybe a CBE (competent Brompton engineering)?
Or an OBE (outstanding Brompton engineering)?
Or an MBE (magnificent Brompton engineering)?
Or even a BEM (Brompton engineering madness)?

[lkingscott]

Top notch fettling. There ought to be some sort of honour.
Maybe a CBE (competent Brompton engineering)?
Or an OBE (outstanding Brompton engineering)?
Or an MBE (magnificent Brompton engineering)?
Or even a BEM (Brompton engineering madness)?

 

[lkingscott]

How do you ensure the rear triangle isn't pinching the bushes?  This is normally done by making the sleeve fractionally longer than the frame tube plus bush flanges.

Actually the tube is made to a length such that there is a very small amount of preload on the washers, which are there to reduce dirt ingress into the bush and act as endfloat bearings.

The nuts will be tightened until the rotation of the rear frame is just giving very slight resistance, i.e. the bushes are tight but not overtight. The nuts on the threaded rod will be thread locked.

The main issue with my design is rotation of the nuts + threaded rod in the countersink holes, rather than rotating in the bushes. I'm thinking that a small amount of thread lock should minimise the likelihood of this. In fact, enough thread lock to seal around the countersink would also be good, to minimise the possibility of dirt and water ingress.

I will use non permanent thread lock, i.e. the type which does not require heating to remove. Normal rotational forces are small.

[lkingscott]

Here's a pic before I assemble.

[rogerzilla]

It looks neat.  I'm not sure threadlock alone will be sufficient without being able to tighten against a hard stop (Brompton bolts use a pretty low torque but the threadlock is quite tenacious; Moultons use 30Nm and a dry threadlock).

Will you be able to tighten it if it loosens on the road?  The chainring usually covers the RH allen head.

[lkingscott]

It looks neat.  I'm not sure threadlock alone will be sufficient without being able to tighten against a hard stop (Brompton bolts use a pretty low torque but the threadlock is quite tenacious; Moultons use 30Nm and a dry threadlock).

Will you be able to tighten it if it loosens on the road?  The chainring usually covers the RH allen head.

The turning force (torque) in normal use really is tiny, unless something rusted solid. Just consider where are the forces and how they are applied. The diameter of the countersink head is the biggest diameter in the assembly, with a radius of around 8mm and the 2 ends would need to rotate in different directions for it to change tension. These sill be locked to both the rear frame, via the countersink and the threaded bar. They rotate in polyimide which will but relatively small rotational forces on them at a radius of 4mm.

I've used all stainless, so rusting is unlikely!

Yes, adjustment just needs a metric allen key. Adjustment would only be necessary when the ends of the bushes wear. Non permanent thread lock has the advantage that it tends to relock, or provide similar resistance to rotation, after nuts and bolts have been moved.

[LittleWheelsandBig]

Pedalling forces might induce precession in the countersunk hinge bolts.

[lkingscott]

Here it is assembled to try for size. It feels rock solid with smooth rear frame rotation. Even without thread lock the nuts don't move at all.

I'm very pleased with it, considering how loose the bushes were on arrival. I didn't ream the frame tube, but I cleaned the rust out. Once fitted, the bushes required a slight reaming, but 8mm is a standard size. My reamers are slightly undersize, so once reamed the fit of the nuts on the bushes was tight, but not too tight.

Shame it will all need dismantling for the powder coating. I also hadn't realised the rear frame is looking a bit shabby, so will also need stripping and renovating!

[rogerzilla]

Try to check the inside of the open rear triangle tubes.  They rust out as (whatever Brompton claim) they are virtually unprotected.  The powdercoat on the outside can look shiny until there is no metal left and it cracks

When it's been painted, spray Dynax S50 or similar into the open tubes.

[lkingscott]

Try to check the inside of the open rear triangle tubes.  They rust out as (whatever Brompton claim) they are virtually unprotected.  The powdercoat on the outside can look shiny until there is no metal left and it cracks

When it's been painted, spray Dynax S50 or similar into the open tubes.

Yes, the tube ends and the rubbing faces on the rear frame are both rusted. I intend to re-powder coat all the damaged paint areas, so these areas will be attacked. Powder coat is pretty good, but if rust ever gets underneath it just keeps on going. I'm not sure if powder coating would stick to phosphated steel. The phosphate tends to inhibit further corrosion. I intend to try a wet powder process, so touching up and getting into nooks and crannies works better than electrostatic powder coating. Air brushing is even possible.

I had to try it all out before committing to this, but it's a shame to have to dismantle it now, to complete the job properly!

The parts cost nothing compared to Brompton's rear hinge kit. The bushes were €3.25 for 10, the post cost more than that!!! The Stainless cylinder nuts were €9.26 for 5 inc post and the M6 stainless threaded rod works out to under €0.25. So about €5 for all of it for one bike.

[lkingscott]

Ooh that's good... phosphate conversion coating helps powder coating adhere...

https://www.besttechnologyinc.com/industrial-parts-washers-cleaning-systems/phosphate-lines-powder-coat-prep/

Looks like I need some phosphoric acid!!!

[lkingscott]

I've ordered the phosphoric acid. I need to check the recipe to get a conversion coat on the fresh steel. I know it will convert the rust though.

Here's the 3 frame parts assembled to check alignment, all looking good. The rear frame hinge is as final assembly, but front hinge still waiting for the stainless tube so that I can complete the reaming, although I'm still intending to add bushes, which will require opening another 2mm diameter. I'm also going to use braze to fill any more defects before powder coating.

[lkingscott]

Wehey... the stainless hinge tube has arrived, now I can complete the main hinge bore. Then I'll need to cut and tap the tube. It is bang on 6mm diameter with close to 4mm bore. It's pretty smooth, but I'll polish it too, to ensure the bushes last.

I'm not looking forward to tapping it. I love using stainless parts as they are harder wearing and never rust on bikes. However stainless is more brittle and a PIA to work with average tools as it is harder and it work hardens. E.g. when drilling do the opposite of mild steel, punch through with as much pressure on the drill bit as you dare, but whatever happens, don't let it get too hot. This works fine with HSS on a drill press, but with a hand drill I've melted the ends of drill bits on SS, because I didn't press hard enough and let the workpiece heat up and harden.

I looked for off the shelf tapped tube, but couldn't find anything long enough.

For this tube I had to get a longer narrow shaft tap as the tube will be longer than standard taps can go. Standard long shaft taps have a shaft with wider diameter than the M5 thread...!!! I know this makes the one I got much weaker, so I'll probably go as far as I can with my standard tap, before using the longer shaft. I will bore first to ensure there's nothing to make the tapping hard going and I'll be backing the tap more than I'll be going forwards with plenty of cutting paste/fluid.

I did think of using tapped 8mm or even 10mm tube for the rear hinge with M6 or M8 CSK bolts, rather than the cylinder nuts, but as cylinder nuts were available off the shelf, it was the simpler option.

[lkingscott]

I've started thread cutting, it's going very slowly and keeps sticking, although I have used loads of cutting compound, both liquid and paste. Stainless is horrible to work with.

About 20mm so far, another 30mm to go. I really hope this long thin tap doesn't snap!

[Paul]

Ooh, jeopardy!

Popcorn!

[lkingscott]

Ooh, jeopardy!

Popcorn!

 

[rogerzilla]

I once converted a front brake to a rear brake by extending the M6 thread and sawing off the excess.  It's not much more fun on chromed steel.

[lkingscott]

Well my tap and I won, sort of...

My plan was to tap the whole 57mm long tube and use 2 x 25mm bolts. The hinge faces are about 10mm from either end, so when I got far enough to fit a 20mm bolt, I decided that's enough, cut the tube, then threaded from the other end.

It's a very tight fit in the original holes, even after a slight reaming, but that's good. I think the front frame part shown in the photos below was slightly bent. As I reamed, the reamer didn't initially line up through the holes. They were undersize, less than 5.9mm, although they are specified to be 6mm. Maybe from dirt and corrosion, but now they are reamed to 6mm. The tube needs a bit of polishing so that it doesn't wear the middle pivot joint.

I'm looking for a long enough (min 25mm) end tap, as I think I'll fit lock washers to stop the tube and bolts rotating (the bolts will be thread locked in the tube). They would add around 1mm, so I just need to end tap to 24mm. My long tap has an extremely shallow lead angle, so a long enough end tap should work better.
 
I now need to ream the repaired hinge part, although once reamed it's likely I'll bore to 8mm for bushes. If it was mine I'd probably just leave without bushes, but it needs to be easily maintainable, so bushes make much more sense. On the other hand, I wouldn't be surprised that the hinge tube/bolts might bend over time, so maybe I'm over thinking and it is just a wearable part. Although, even if it does bend, it will still be easier with bushes as the pivot hole will not wear and an original size hinge tube can be used.

BTW, those aren't the final bolts I'll use, I've got some flat head types.

[lkingscott]

FYI, I'm planning to use 'Iglidur' top hat bushes. These are self lubricating plastic/fibre composite 1mm thick, with better wear and pressure rating than Nylon.

The Titanium Brompton uses coated metal bushes. As they are used by Brompton already they were my initial thought. However when the coating is worn, then the hinge pin will wear, as happens on the rear hinge. The Iglidur bushes may last a bit less time (maybe they will last longer???), but they are very inexpensive. If the wear is too fast, they can be replaced with the coated metal bushes.

[rogerzilla]

Igildur's bushes are more robust than Isildur's, at least if you plan on using the bike in Mordor.

[lkingscott]

Igildur's bushes are more robust than Isildur's, at least if you plan on using the bike in Mordor.