Broken Brompton hinge

[lkingscott]

Hi everyone, only my second post to this forum.

I am repairing a Brompton for a friend. It has a broken main hinge, see picture.

I contacted Brompton for the cast part, but they only sell complete frame parts. Brompton seem to be the Apple of folding bikes, IMO!!!

Therefore I propose to rebuild it up with TIG weld or TIG brazed silicon bronze.

The silicon bronze would mean less heat and less susceptibility of cracking the original hinge metal, which looks like it's cast. It would also be much easier to drill. However it would wear quicker and be less strong. However I could add bushes, as on the titanium Bromptons, but I'd probably need to put a bit more meat on the hinge as it would need to be drilled to a larger diameter.

I'm thinking of not using the Brompton unmaintainable rivet pin and instead use a smooth bolt either threaded into one of the outer, still intact, hinge parts or use a nylock nut on the end. This could be greased regularly. To tap the hinge for the bolt, I'd fill the hole with braze, redrill and tap.

Anyone have any thoughts?

[rogerzilla]

Wouldn't it be better to find a donor bike with a rotted-out rear end?  That's an incredibly difficult repair, and safety-critical too.  Is it SWB (fillet brazed main hinge, a bit fishscale) or LWB (socketed main hinge, no visible fillet)?  SWB is very old, so you might find a really knackered L3.

[grams]

jeff_goldblum_scientists_quote.png

The silver extremities haven’t been offered for maybe 15 years or more. I’d say it’s had a good innings and your friend should be grateful they weren’t seriously injured the first time it snapped.

[LittleWheelsandBig]

That is a LWB hinge, which is cast.

[lkingscott]

Yes it's definitely cast, which is why I'm looking at brazing. However if I did that, I'd want to fit bushes which would mean building it up more.

Unless I F up, it won't be dangerous. To be fair, it did snap due to an accident.

People want crazy prices for old Brompton bits, so if I was to go down the replacement route a new rear main frame would be the way.

[LittleWheelsandBig]

I have all three sizes of hinge pins in my spares box. Wear is very, very slow, even on heavily-used Bromptons.

I wouldn’t bother with a repair of the broken hinge. Your hourly rate must be very low compared with a replacement frame component. I guess it would be fun to show off 1337 repair skills though.

[lkingscott]

I have all three sizes of hinge pins in my spares box. Wear is very, very slow, even on heavily-used Bromptons.

I wouldn’t bother with a repair of the broken hinge. Your hourly rate must be very low compared with a replacement frame component. I guess it would be fun to show off 1337 repair skills though.

My hourly rate is way higher, but if I considered my hourly rate whenever I did a hobby project, I'd never do any of them. Getting welding done is dirt cheap, a tiny fraction of my hourly rate, so I'd never have considered doing welding, but I've had a lot more fun and satisfaction playing with welding, woodwork, composites, etc... than working!

[lkingscott]

I forgot to mention, if it can be made to fit, the T-line main hinge nut, bolt and bushes would be a much better solution too, if I can make them fit.

Does anyone have any dimensions of the T-Line bolt and bushes?

I assume I'd need to put a bit more metal on, as the bushes would make the hole a bigger diameter.

[lkingscott]

In case anyone is interested, heres the first stage of rebuilding the hinge. The weld could be better, but I made sure it was well attached to the original casting and it is bigger than necessary. I added fillets at the ends, to distribute the attachment, but I'll probably need to grind a load away as it will probably interfere with the mating hinge part. I will probably find some inclusions, which I can fill with more weld. If I can I will add more metal inside, to increase the weld attachment area to the original.

Once I've done an initial basic grind, I'll drill it. I plan on using bushes and a shoulder bolt. The bushes will need it drilled to 8mm, so it may need more metal, if it will fit with the other part of the hinge.

On measuring it before welding, I discovered that the hinge pin is not parallel with the hinge plate, it's 3mm lower one end. I assume that was intentional and not crappy manufacturing. Makes things a bit more difficult as I can't use the face to square the ends, I'll have to grind them perpendicular to the drilling.

[Paul]

In case anyone is interested…

You can be sure of it.
When someone starts to go full Caractacus Potts, everyone pulls up a chair.

[lkingscott]

In case anyone is interested…

You can be sure of it.
When someone starts to go full Caractacus Potts, everyone pulls up a chair.

 

I checked the weld this morning. it's looking good. One end wasn't quite completely filled, so I improved that. I also wanted a bit more on the inside, as there were a couple of sharp corners, but as so often happens playing too much, the high current needed to melt the casting, which is quite thick relative to the tube, slightly melted the tube end (only visible when the hinge is open. Therefore I put a couple of tacks on the tube end, in case the braze has been weakened with all the heat.

I was surprised to see that the hinge hole didn't completely close up with weld. This should help a lot to drill it in the correct alignment. I can put a straight piece of wire through, clamp it in the drill press vice so the wire is vertical... job done!

It's taken about an hour of welding so far, so if this was a professional, not expensive... so far!

I checked the clearance with the mating part of the hinge and there's plenty of space for an extra mm of metal, so I will definitely aim to use bushes and a shoulder bolt. I.e. 8mm drilling, bushed down to 6mm. Unfortunately, all the off the shelf shoulder bolts I can find have a thread size 1mm less than the shaft diameter, this means either a nut on the end, or I need to fill one end of the mating hinge part in order to drill and tap. As the hinge should only rotate on the broken/welded hinge side, filling the other hinge part with braze then drilling and tapping would seem best. Some sort of thread lock compound will keep the bolt from rotating.

The paint appears to be a pretty thick powder coat. It could be epoxy, because it's got higher tolerance to heat than I'd expect from powder coat. I'm not sure how to tell the difference. It probably gives me the chance to play with home powder coating, rather than a cheap rattle can!

When completed, if this is ever taken to a Brompton shop for maintenance, they will be scratching their heats!!!

[Sergeant Pluck]

In case anyone is interested…

You can be sure of it.
When someone starts to go full Caractacus Potts, everyone pulls up a chair.

Oh yes.

[rogerzilla]

This reminds me of the guy who fixed his broken head tube with epoxy bandage.

[lkingscott]

This reminds me of the guy who fixed his broken head tube with epoxy bandage.

I ground one end and centre punched it for drilling. I can't remember what grade steel the welding filler rods are but standard HSS drill bits failed completely.

This is no epoxy!

I've ordered some cobalt drill bits.

[rogerzilla]

The epoxy actually worked perfectly - it was just ugly.

[lkingscott]

The epoxy actually worked perfectly - it was just ugly.

 

Well, carbon fibre composite is really carbon fibre + epoxy.

I'm guessing it was a glass fibre / epoxy bandage.

[lkingscott]

Progress!

So having tried with some standard HSS drill bits to drill through the weld on the hinge and failed, I ordered some cobalt drill bits from Ebay.

They arrived yesterday and obviously not cobalt just from the colour. When I tried one of them I found they are obviously made from finest Chinesium. The cutting edge even bent! I would hardly expect that from HSS much less HSS-Cobalt bits, which are much more brittle and would snap.



I know it's overheated, but I was spraying it with cutting oil and it didn't even get 1mm into the hinge.

I then discovered the local DIY store has Cobalt bits for not too much money. I bought a few of them, initially drilling undersize. I will probably ream the hole to size. I'm going to try going for 6mm to start with, but I'm still thinking I'll probably bush it, so it would need the ends drilling to 8mm.

I don't know what grade welding wire I have and what grade steel the casting is made from, but even with a cobalt bit, it's hard going. I've had to wait for it to cool a few times and the bit expands when it gets hot, so makes going more difficult through the hole. The drill cutout has operated a few times. It really needs a much higher coolant flow rate, but then I'd need a suds tray too. Something else on my wish list!!!

Probably the most difficult was to clamp the frame and hinge in the correct position to drill it. As the clamp is not perfect, I also needed to pack directly underneath in line with the drilling to have something to press against. I pondered welding pieces on, to fix it in the right position for drilling. I hope it's accurate enough! The hinge pin must slope relative to the hinge face, so that one end is 3mm lower than the other, but in another plane, it should be parallel with the clamp side of the hinge. If it's off, then this Brompton will be bent!!! 

[rogerzilla]

eBay is a minefield of fakes and lies 

[lkingscott]

eBay is a minefield of fakes and lies 

I've had mostly good experience. The amount I've saved over the years has been worth the odd piece of s't!!!

[lkingscott]

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.

[Adam]

This is absolutely fascinating - well done!

[lkingscott]

This is absolutely fascinating - well done!

 

[Goldcrank]

Me too. I've been following along. Really interesting to see.

I would be very interested in a periodic update once it is finished too. If there has been any deformation over time etc.

Thanks.

[lkingscott]

Me too. I've been following along. Really interesting to see.

I would be very interested in a periodic update once it is finished too. If there has been any deformation over time etc.

Thanks.

No prob!

[lkingscott]

FYI, I've been looking at powder coating as the old coating has melted and burnt somewhat near the welding. Elsewhere it's obviously had wear and tear.

As I have no powder coating equipment, but I have spray equipment, I've discovered some water based liquid that can be mixed with the powder so that it can be applied like paint, then when dry, baked as normal powder coat, or heated with a heat gun.

I'll be hopefully trying this.