Main frame hinge 6.35mm pin

[neilrichardson55]

Hi all silly question if you hinge is over the 6.35 mm biggest pin. Is this the frame scrap can you not braze on a new hinge ? And start again ?

Anyone had this

 

[wafter]

Hi all silly question if you hinge is over the 6.35 mm biggest pin. Is this the frame scrap can you not braze on a new hinge ? And start again ?

Anyone had this

I assume that yes; once the largest pin is sloppy the frame is non-viable economically to repair in the way you suggest; which would require a whole new hinge assembly (which I assume aren't readily available) plus the time and grief of fitting, then re-finishing... far easlier and cheaper to bin the frame and start again.

After the hinge misery mine's currently subjecting me to I've been looking alternatives. The T-line has a much more sensible arrangement with an easily removable shoulder bolt and bushes in the frame - meaning the frame itself should never wear.

In theory it would be possible to recreate this setup on a normal steel Brompton, but it would require the frame to be machined to take the bearings, possibly sleeving in areas of wear and would reduce the wall thickness at the hinge significantly.

The nominal Of the hinges is around 13mm so with a 6.35mm pin fitted have a wall of around 3.3mm. I assume the T-line pin is 6mm and the bushes 8mm OD, so machining out the hinge to take these would reduce the wall to around 2.5mm. It seems that the Ti frame of the T-line has a similar geometry (the hinge looks pretty minimal from the pin replacement video on the Brompton site) however I'm not sure of the material properties of the steel hinge assys - I assume they're cast which might make them more prone to brittle failure.

Another alternative might be to forgo the bushes and fit a shoulder bolt that runs in a next-size-up reamed hole (maybe 6.5mm) directly in the frame; which would obviously subject the frame to wear but I suspect this might last a lot longer than the standard Brompton design which just looks shite.

If I had access to an otherwise scrap frame I'd be tempted to give it a go. On my three year old example; not so much!

Has your hinge got sloppy? If so I'd be interested to read your observations in my thread on the subject

 

[neilrichardson55]

This is a do upper bike. There was little play when pulling pin 🧷 it was a 6.35 and it’s got slop was wondering if you could sleeve it but with the Ti frame it would be stronger ?

You are right it really is a rubbish idea they say if you keep the clamps tight it should not ware I don’t see that !

Welding up the item and re drilling and sleeving using the Ti pin arrangement ?

 

[neilrichardson55]

This is a do upper bike. There was little play when pulling pin 🧷 it was a 6.35 and it’s got slop was wondering if you could sleeve it but with the Ti frame it would be stronger ?

You are right it really is a rubbish idea they say if you keep the clamps tight it should not ware I don’t see that !

Welding up the item and re drilling and sleeving using the Ti pin arrangement ?

As they did the rear triangle with bearings and pins why not the main hinge ?

 

[wafter]

This is a do upper bike. There was little play when pulling pin 🧷 it was a 6.35 and it’s got slop was wondering if you could sleeve it but with the Ti frame it would be stronger ?

You are right it really is a rubbish idea they say if you keep the clamps tight it should not ware I don’t see that !

Welding up the item and re drilling and sleeving using the Ti pin arrangement ?

That sounds unfortunate!

I don't know the dims of the hinge on the Ti frame, although it looks ballpark similar to the steel item.

Ti is typically less strong (its low density and corresponding specific strength is what makes it special), so dims etc being equal there should, in theory be no issue modding the steel frame to reflect the geomtry of the Ti job... although this would be uncharted territory and again I'm not sure about the properties of the cast hinge material.

In theory you could weld it up and rebore it; but again I don't think this would be easy - getting down full inside the the holes, ensuring they're clean to prevent porousity, the potential for heat to bugger up the brazing, possibly create hard spots depending on the carbon content of the casting.. burn all the paint off and hence require re-finishing. Plus You're faced with starting from scratch getting the holes re-cut and in the right place; which I imagine in itself would be non-trivial.

You could maybe look at drilling oversize to say 7mm in the centre and outside sections, pressing / bonding in some turned brass sleeves then reaming in situ to take the Ti pin (assuming the length is also correct - no idea if this is the case). This of course would still reduce the wall of the hinge and rely on a thin wall on the sleeve.

Tbh it's probably all heavily over-engineered, however given the complexity of the shape it's difficult to get an idea of stresses involved. If only I still had access FEA software..

As they did the rear triangle with bearings and pins why not the main hinge ?

Blatantly cost, sadly. It's clear from the T-line that they're savvy enough to do the job properly- they've just chosen not to

 

[wafter]

Did you notice in which part of the hinge the slop was? Could you see the pin moving relative to the outside sections of the hinge? How does the spline look on the end of the pin, and it's corresponding bore inside the top part of the hinge?

 

[neilrichardson55]

The larger inner part not the top or bottom. When the frame was unhinged it was “ratting” the tangs were fine where the spline was and were it was flared over the pin it’s self shows ware but not silly amounts. Just wondering if the t line is done as the metal is not a strong ?

We use a metal fabrication company at work. I’m going to talk to them in the morning see what they say

 

[neilrichardson55]

well had a day with a very nice person the hinge was toast we tried to sleeve it and it cracked as it got to thin.
we talked about making and welding a new hinge on, worked out at a 600 quid job not really worth it.
good advice was the clips need to be replaced every 2 years the ware casing, the hinge to move which is why the pin wares more under load. the little clip should not bottom out when you tighten it up fully. hope this helps little i have chanced the clamps on bike as i was bottoming out.

 

[wafter]

well had a day with a very nice person the hinge was toast we tried to sleeve it and it cracked as it got to thin.
we talked about making and welding a new hinge on, worked out at a 600 quid job not really worth it.
good advice was the clips need to be replaced every 2 years the ware casing, the hinge to move which is why the pin wares more under load. the little clip should not bottom out when you tighten it up fully. hope this helps little i have chanced the clamps on bike as i was bottoming out.

Bugger - so the frame is scrap?

What sort of OD were you trying to sleeve it to? I assume you were going for an interference fit and it cracked when attempting to press the sleeve in...?

Pretty much confirms my thoughts about significant remedial action on the hinges, sadly. So bloody avoidable too if they'd have gone with a competent solution in the first place

The situation with the clamps is a known thing; as you say they should be replaced before they bottom out on the frame (or I think I've heard when they get down to less than 1mm of clearance between the inside of the clamp and the frame).

 

[neilrichardson55]

Bugger - so the frame is scrap?

What sort of OD were you trying to sleeve it to? I assume you were going for an interference fit and it cracked when attempting to press the sleeve in...?

Pretty much confirms my thoughts about significant remedial action on the hinges, sadly. So bloody avoidable too if they'd have gone with a competent solution in the first place

The situation with the clamps is a known thing; as you say they should be replaced before they bottom out on the frame (or I think I've heard when they get down to less than 1mm of clearance between the inside of the clamp and the frame).

Yes we were trying to press them in we drilled out to 6.5 as big as we though the we was to drill it out to 6 mm but the pressing in cracked the cast, yes frame is scrap, there is things that can be done but the cost is far to much . we did think about brazing on a tube and drilling it out like with the rear tringle. but run out of time, might send a email to brompton see what they say.

 

[wafter]

Yes we were trying to press them in we drilled out to 6.5 as big as we though the we was to drill it out to 6 mm but the pressing in cracked the cast, yes frame is scrap, there is things that can be done but the cost is far to much . we did think about brazing on a tube and drilling it out like with the rear tringle. but run out of time, might send a email to brompton see what they say.

Thanks - speaks volumes about the quality of the casting if it's capable of such brittle failure. I saw another online (that had admittedly been involved in an accident) that looked like it'd failed in a similar way.

I'd be interested to hear what Brompton say; I imagine it'll go along the lines of "they'll last for long enough with the range of oversize hinge pins available, once this point has been surpassed the frame must be scrapped". Along with a disclaimer of "you've buggered that and attempting to fix it will be dangerous".

It's really starting to look like this lashed up hinge assy is the bike's Achilles heel

 

[neilrichardson55]

Suppose the bike was a 2004 so suppose they will say 20 years with the 3 pins is good enough. But I have seen bikes a year old with wobble from the main hinge. The bikes do get hard use. Where it broke it was clean so it was not a previous crack.
Like you say a lot could be from
Bad handing or accident damage.

 

[rogerzilla]

I change the clamps on any used Brompton.

Brompton have often claimed the hinges are very hard to get right and are where most clones fail

 

[neilrichardson55]

I change the clamps on any used Brompton.

Brompton have often claimed the hinges are very hard to get right and are where most clones fail

They are very easy to mess up reaming them to much but not straight all sorts Lolol

 

[wafter]

Suppose the bike was a 2004 so suppose they will say 20 years with the 3 pins is good enough. But I have seen bikes a year old with wobble from the main hinge. The bikes do get hard use. Where it broke it was clean so it was not a previous crack.
Like you say a lot could be from
Bad handing or accident damage.

Yeah, as much as the setup makes my inner engineer sick in his mouth you can't argue with that innings. Sadly mine appears like it will last nowhere near as long as that, or that it's failed in the way one might expect through legitimate wear.

That's interesting about the failure mode; I've since read that cast steel "isn't brittle" however I can't see a piece of ductile "normal" steel failing in the way you describe. I assume a bit actually fractured and fell off..?

I'm not necessarily suggesting it had already been weakened; more that the example I saw elsewhere was resulting from accident damage so not necessarily a suggestion that it's excessively weak; just prone to a questionable failure mode.

They are very easy to mess up reaming them to much but not straight all sorts Lolol

Yes, I'd expect the factory to have some sort of jig that attaches to the frame / clamp somewhere to ensure everything lines up during this process.