snapped brake boss/pivot on fork

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foxyrider
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Location: Sheffield, South Yorkshire

Re: snapped brake boss/pivot on fork

Postby foxyrider » 29 Oct 2018, 9:20am

Eman Resu wrote:
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


You assume too much - I have been there and done it and mark my words, the bosses fouled the caliper when I tried it.
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!

lvabd
Posts: 39
Joined: 3 Mar 2018, 4:44pm

Re: snapped brake boss/pivot on fork

Postby lvabd » 29 Oct 2018, 9:33am

Brucey wrote: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.


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.


cheers



it does look like it was the problem: the cantis that were stock, and that I removed, had long bolts, but the brand new shnimamo deore vbrakes I fitted had surprisingly short bolts (and no long bolts were provided). I did put 3 of them (the 2 rear and 1 front) without any problem though, the 4th one snapped the pivot. Well I will now replaced all the other bolts with the previous ones from the cantis
it does look like I will get a new fork though, and fix this fork DIY and put it on my commuter. Doubt Surly/dealer will contribute to it. oh well.
also gonna invest in a torque wrench...

lvabd
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Joined: 3 Mar 2018, 4:44pm

Re: snapped brake boss/pivot on fork

Postby lvabd » 29 Oct 2018, 10:02am

AndyA wrote: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



Super sweet of you: I think it looks like now I will buy a new fork, as we are leaving in 2 months and still have tons of things to sort out, and later on will fix the fork that's damaged. So I keep your offer in mind, and will message you next time I am in Edinburgh if that's possible...!?

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NATURAL ANKLING
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Re: snapped brake boss/pivot on fork

Postby NATURAL ANKLING » 29 Oct 2018, 10:30am

Hi,
What Brucey said last post, before reading it, the snapped post has come about because eccess torque Has been applied to the fragile section of post, Due to the fact the head of the bolt is in contact with the end of the post, Tension is not being applied along the whole length of the post, and at least a portion of the bolt thread, Bad engineering application.
As said a correct bolt that goeus all the way into the boss Base Would reduce the torque considerably Applied to the exposed threaded section of post.

If you don’t have the experience at least that of a mechanical fitter then repair is probably best left with someone with more skill to be safe. Or buy another Fork.
As said by Brucey applies to non-internal Pivots, where head of Bolt comes into contact with the end of post.

Just remember to use the washers under the head of the bolt pivots, where head of bolt comes into contact with end of post.

And more importantly remember to use medium strength thread lock.
Unless the brake Pivot seizes no turning force will try to loosen the bolt thus they do not need to be very tight!

(Text by voice) excuse bad grammar.
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AndyA
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Joined: 21 Mar 2009, 9:16pm
Location: Edinburgh

Re: snapped brake boss/pivot on fork

Postby AndyA » 29 Oct 2018, 8:03pm

lvabd wrote:
AndyA wrote: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



Super sweet of you: I think it looks like now I will buy a new fork, as we are leaving in 2 months and still have tons of things to sort out, and later on will fix the fork that's damaged. So I keep your offer in mind, and will message you next time I am in Edinburgh if that's possible...!?


Sure thing, hope you have a good tour!