snapped brake boss/pivot on fork

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Brucey
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Re: snapped brake boss/pivot on fork

Postby Brucey » 28 Oct 2018, 9:43am

re drilling and tapping the boss so that a threaded insert can be used; it depends entirely on how the boss is constructed, as to whether there will be enough 'meat' to screw the insert into once you have done the repair. This is far from guaranteed to be the case.

Thinking about it I bet there is someone with a MIG welding set not far way and I reckon you could make a serviceable repair quite easily. I quite fancy a repair along the following lines:

- Get some unplated steel M6 studding, 8.8 strength grade should be OK (NB under no circumstances use a plated piece or a 12.9 grade bolt).
- run an M6 tap into the remains of the old boss base, (such that you have at least half a dozen good threads inside the boss base)
- run the M6 tap through the piece that broke off too
- clean everything with solvent, having removed all corrosion/plating from all parts
- shorten the piece that broke off by about 2mm, at the fracture end
- cut the M6 studding to length so that it bottoms in the boss base, and sticks out beyond the original boss length by at least 10mm (i.e. enough to use a nut to retain the brake arm once the repair is complete)
- screw the piece that broke off over the M6 studding, so that it is set to the correct length
- you should now have a 'gap' in the boss, bridged by the studding.
- MIG weld the 'gap' so that the weld penetrates into the studding all the way around. You will probably need to do the weld in short bursts to avoid overheating the part.
- carefully grind the weld repair flush with the original surface, or at least close enough that the brake will slip over the repaired area (some brakes with integral pivot bushings have a larger centre bore than you might expect)

if done well this repair will be at least as strong as the original part was. If you can TIG weld the part all the better.

NB if there is no vent hole in the base of the boss (typically ~2mm diameter, through the side) it isn't a bad idea to drill one; this will prevent any trapped gas inside the boss from trying to burp through the final part of the weldment.

Given that (for unknown reasons) one boss broke, I would also fit a nut/length of studding inside the other boss, such that the studding extends all the way into the base. As a general rule these fastenings need to be set at low torque and then retained by threadlock or similar, not by being tightened within an inch of their lives.

cheers
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SimonCelsa
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Re: snapped brake boss/pivot on fork

Postby SimonCelsa » 28 Oct 2018, 10:20am

Are you really sure that boss won't come out by unscrewing? Try tightening the bolt up, giving it a couple of sharp raps with a hammer, chisel/score off the paint where the boss penetrates the fork and try and unscrew using some decent mole/vice grips. I don't think pliers would afford good purchase. I only suggest this as I had a similar but 'opposite' thing happen on a Specialized MTB frame; the boss came out with the bolt. There were no flats on that boss either. Nothing to lose?

gxaustin
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Re: snapped brake boss/pivot on fork

Postby gxaustin » 28 Oct 2018, 3:34pm

A new fork is 80 £, at this stage I'd rather get a new fork.


Have you contacted the manufacturer and discussed this? They may have upgraded the forks? Maybe you could ask for a discount in view of the unusual failure after such a short time? Failing that £80 = peace of mind :?:
I think I'd make sure I could fit a caliper brake (as suggested above) just in case you need a hack to get you out of trouble when on tour.

9494arnold
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Joined: 21 Jan 2007, 3:13pm

Re: snapped brake boss/pivot on fork

Postby 9494arnold » 28 Oct 2018, 4:45pm

Even more so now you have said you intend to put leave on a long tour I would personally suggest you forget a diy repair,
Bosses for screw in pivots are slightly more substantial than pure braze on ones where the pivot screws in. So even if you flush and thread, you aren't going to have a lot of internal thread to play with. And the boss is already compromised, so it may be completely defective. And you don't want to start making holes in the fork blades that would not do at all.
I run a couple of Trikes (which only have front brakes) plus a Jackson Touring Bike all of which run braze on Cantilever brakes so I do appreciate what's happening.

lvabd
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Joined: 3 Mar 2018, 4:44pm

Re: snapped brake boss/pivot on fork

Postby lvabd » 28 Oct 2018, 5:49pm

Brucey wrote:re drilling and tapping the boss so that a threaded insert can be used; it depends entirely on how the boss is constructed, as to whether there will be enough 'meat' to screw the insert into once you have done the repair. This is far from guaranteed to be the case.

cheers


on the other side of the boss (looking from the rear of the bike to the front), I can see the threading of the boss almost all the way. Do you think that if I manage to drill and tap a M10 into the boss, so that a removable boss with M10 threading would be threaded for all the length of its thread, it would be a sturdy repair?

I am less incline to the welding work you suggest: I don't have access to welding kit, and that sounds rather complicated. The drilling and taping does sound easy enough ...

EDIT: I am not sure I used the correct denomination here: I think the boss is intact (the steel bit that is, or at least used to be, painted the same color as the frame), but it's the pivot that snapped. I think the second picture attached shows that.
Last edited by lvabd on 28 Oct 2018, 6:04pm, edited 1 time in total.

lvabd
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Re: snapped brake boss/pivot on fork

Postby lvabd » 28 Oct 2018, 5:57pm

Brucey wrote:re drilling and tapping the boss so that a threaded insert can be used; it depends entirely on how the boss is constructed, as to whether there will be enough 'meat' to screw the insert into once you have done the repair. This is far from guaranteed to be the case.
fastenings need to be set at low torque and then retained by threadlock or similar, not by being tightened within an inch of their lives.

cheers



here are some additional pics of "the problem"

I would say that there is thread between 0.5 to 1cm in the steel bit (once what's left of the pivot would be cut off). Is that the "meat"you are talking about?

(thanks a lot for your help!)
Image Attachments
photo 2 (1).JPG
photo 1 (1).JPG

lvabd
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Re: snapped brake boss/pivot on fork

Postby lvabd » 28 Oct 2018, 6:02pm

9494arnold wrote:Even more so now you have said you intend to put leave on a long tour I would personally suggest you forget a diy repair,
Bosses for screw in pivots are slightly more substantial than pure braze on ones where the pivot screws in. So even if you flush and thread, you aren't going to have a lot of internal thread to play with. And the boss is already compromised, so it may be completely defective. And you don't want to start making holes in the fork blades that would not do at all.
I run a couple of Trikes (which only have front brakes) plus a Jackson Touring Bike all of which run braze on Cantilever brakes so I do appreciate what's happening.



mmmh I didn't quite understand: are you saying that the screw in pivots systems are less sturdy than the non-removable set up?
about not having a lot of internal thread to play with: if I drill and tap so that a removable pivot (such as one one the pic attached) can be screwed in, wouldn't that be sufficient?
Image Attachments
41Ke5mociLL._SX425_.jpg

peetee
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Re: snapped brake boss/pivot on fork

Postby peetee » 28 Oct 2018, 6:11pm

Given that your scenario is an unusual one have you checked the other boss for any signs of strain or possible failure?
Current status report:
Latter side of fifty and feeling less than nifty.
Too many bikes on pegs and too few miles in the legs.

AndyA
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Location: Edinburgh

Re: snapped brake boss/pivot on fork

Postby AndyA » 28 Oct 2018, 6:13pm

I'm in Edinburgh, I have a TIG welder and know how to use it. I'd be happy to do the Brucey repair for some beer

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foxyrider
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Re: snapped brake boss/pivot on fork

Postby foxyrider » 28 Oct 2018, 9:53pm

Eman Resu wrote:Another option would be to use a 75mm drop nutted brake caliper such as the Dia Compe 806 .. available for a paltry 16.99 from SJS ... fixed via the central mudguard hole. I have used this on a steel touring fork and it works well.

But would of course require removal of the 'good' boss on the other fork leg!
Convention? what's that then?
Airnimal Chameleon touring, Orbit Pro hack, Orbit Photon audax, Focus Mares AX tour, Peugeot Carbon sportive, Owen Blower vintage race - all running Tulio's finest!

Brucey
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Re: snapped brake boss/pivot on fork

Postby Brucey » 28 Oct 2018, 11:53pm

lvabd wrote:
I would say that there is thread between 0.5 to 1cm in the steel bit (once what's left of the pivot would be cut off). Is that the "meat"you are talking about?


yes it is; however the problem is that the base of the boss (the part that is welded to the fork) is almost never solid. It is a tube of some kind, commonly with a wall thickness of 2 to 2.5mm (in modern frames) and less than that in some older ones. It is not clear to me that there would be enough material there to tap good thread into.

I am pretty sure that I have some bosses of that type that I cut off an old frame, intending to braze (or weld) them to another frame. If I can find them I will take a photo or two and post it here, to show you what I mean.

cheers
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tim-b
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Re: snapped brake boss/pivot on fork

Postby tim-b » 29 Oct 2018, 5:47am

Hi
With the greatest of respect to all those that have contributed, for £80 (and the possibility of a contribution from the manufacturer) a home repair truly isn't worth the faff. The consequences of a failure of the repair are serious, and at best would merely ruin a hard-earned holiday
I don't know where in Scotland you are, but you also have two well-regarded frame builders: Ben Cooper in Glasgow and Shand Cycles at Livingston. It's possibly a day there and back, but...
Regards
tim-b
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9494arnold
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Re: snapped brake boss/pivot on fork

Postby 9494arnold » 29 Oct 2018, 7:53am

I was trying to say the opposite to what you read.
Boss for a screw in pivot will have enough "meat" to take the internal thread and stand up to the the stresses involved.
Pure 'braze on" boss will almost certainly be less substantial behind where the pivot stops (it doesn't need to be more substantial) so although it might take a thread and you could screw the pivot in (there might not actually be enough room for the screw to go all the way back too ) but even if you managed to tap the thread and it screwed all the way back , it wouldn't be as strong and may fail again under use.
Effectively, if you tap a thread in, you are taking metal out.
Like Bruce said.
I wouldn't take the risk.

Eman Resu
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Re: snapped brake boss/pivot on fork

Postby Eman Resu » 29 Oct 2018, 8:08am

foxyrider wrote:
Eman Resu wrote:Another option would be to use a 75mm drop nutted brake caliper such as the Dia Compe 806 .. available for a paltry 16.99 from SJS ... fixed via the central mudguard hole. I have used this on a steel touring fork and it works well.

But would of course require removal of the 'good' boss on the other fork leg!

I can see that someone who has not done this type of brake arrangement would think this, but you don't have to remove any of the canti bosses. The caliper brake pads will be above the canti bosses. There are a couple of pictures in this thread:
viewtopic.php?t=15869

Brucey
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Re: snapped brake boss/pivot on fork

Postby Brucey » 29 Oct 2018, 8:15am

Re Tim b's concerns; as I mentioned upthread, the repair scheme I have outlined will give a boss that is at least as strong as it was originally.

This may not be apparent, but these bosses are not that strong to start with; they are 7mm OD and have a 6mm thread tapped inside them. This means that they are nominally 0.5mm wall thickness above the thread tops, i.e. the total cross-section is about 10 sq mm. This is about as strong (in tension) as the shank of an M4 bolt, i.e. it is not very strong. The working wall thickness is less than most of the tubes in a lightweight steel frame.

There are several things that you can take away from this;

1) that you need to be fairly careful when tightening canti boss bolts. If you have brakes with integral pivot bushings the bolt exerts a tension force on the boss (beyond the bolt length if it is a short bolt) and failure will (given a few givens) occur if the tension force gets to about half a tonne (*). Half a tonne is easy with an M6 bolt (this is about the load in a headset top cap that damages headset bearings; think how common that is..... :roll: ); you can get to about three times that before a strong M6 bolt will break, and most people's idea of 'a tight M6 bolt' develops nearer one tonne tension load in the bolt.

2) On the front brakes especially, it isn't a bad idea to use bolts that are long enough to extend into the (thicker) base part of the boss, if the threads are tapped deep enough to allow this. This takes much of the tension load off the thin-walled boss itself. Plenty of brakes come with a pair of short bolts and a pair of long bolts so that the long bolts can be used in the front bosses.

3) The repair scheme I have outlined is (in tension) at least three times stronger than the original boss (fitted with a short bolt) would have been.

Edit; (*) If (on a brake with an integral pivot) you overtighten the bolt and it is short enough to load the boss in tension and it fails, the failures look identical to the OP's. Here https://www.parktool.com/assets/img/repairhelp/torque.pdf it suggests 8Nm for these bolts (compare with ~14Nm for M6 threaded hollowtech II crank pinch bolts). 8Nm is about half the load that a strong M6 bolt can be tightened to. For example to get to 75% of the proof stress of a (dry) M6 12.9 grade bolt, the torque required is about 16Nm. Even with an 8.8 grade bolt you will cause the pivot to fail well before the bolt breaks.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~