I went over a bump and....

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NATURAL ANKLING
Posts: 13780
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: I went over a bump and....

Post by NATURAL ANKLING »

Hi,
But a conventional brazed front fork should be good for quite a long time should'nt it.
My old raleigh randonneur (18 years old) has had a reasonably tough life, I spent one winter green laneing on it and the rest of the year exploring bridle paths disused rail tracks etc, even been on the moors with camping gear.
Still time for it to let me down yet I supose...........
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
Brucey
Posts: 44513
Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

I don't disagree. I have quite a few forks that are much older and have done many more miles than the one that failed.

Hence I can only suppose that this one may have been manufactured badly or perhaps it had taken a bad knock before I ever saw it ( it was a little bit out of track).

But something like this does make you think, right enough. I am now even less inclined to take the integrity of any bike for granted....

cheers
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breakwellmz
Posts: 1982
Joined: 8 May 2012, 9:33pm

Re: I went over a bump and....

Post by breakwellmz »

Is it a Pashley fork?
mig
Posts: 2701
Joined: 19 Oct 2011, 9:39pm

Re: I went over a bump and....

Post by mig »

are there many sources of new, well built forks? i looked for 1 inch, threadless, with mudguard eyes last year and found very few. i don't think that there are that many threaded ones either. which points you at the second hand market & thereby introduces uncertainty of provenance. it's pricey to have a set built too, in my case, unjustifiable for my commuter hack.
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

breakwellmz wrote:Is it a Pashley fork?


nope

mig wrote:are there many sources of new, well built forks? ....


I don't think so. I am told that right now, you can't easily get all kinds of quite normal forks; for example if you prang your touring-ish bike with 27"/700C wheels (to take a side pull or centre pull brake) then you are a bit stuffed.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
mig
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Joined: 19 Oct 2011, 9:39pm

Re: I went over a bump and....

Post by mig »

so is there a plan to replace said item?
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

well, in the interests of scientific enquiry, and given that the fork has a load of slightly peculiar fittings on it which will be difficult to replicate, I might repair the old ones....

My plan (loosely, subject to change) is to separate the blades from the crown entirely, clean up carefully, then weld the blades back on. If I do this right, I can't see how it is going to be any worse than it was before, or other welded forks of various kinds.

In view of the fact that I won't get all the braze metal out of the old joint, I expect on the crown to have to build up, grind back, then build up again before welding the blades on. The blades themselves are a reasonable wall thickness. I also intend to add stiffening tangs to the crown, that reach down the blade for a couple of inches. These will be welded to the blade. All the weld beads will be toe-ground and radiused (which takes ages and is very fiddly to do) and the repaired area will be painted a light colour. I think the result should be stronger than it was to start with, and if it ever does start to crack, I shall be able to see the cracks very easily.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
mig
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Joined: 19 Oct 2011, 9:39pm

Re: I went over a bump and....

Post by mig »

hope it works & good luck :!:
Stewart H
Posts: 126
Joined: 9 Jun 2014, 9:47pm

Re: I went over a bump and....

Post by Stewart H »

As long as you can clean the braze completely from the weld area that should be as strong as a new un !
This is another situation where being able to wheelie for extended distance would be handy :wink:
Brucey
Posts: 44513
Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

well I've spent a few hours with the welding set/ grinder and my repair is now complete.

I even reassembled the notoriously fiddly dynamo wiring (which runs through the steerer tube and the stem on this bike) only to find there is a bad connection in the wires that run to the rear light... ho hum...

Anyway I had a chance to inspect the failed joints in more detail. One blade was almost completely broken and the other was about half-way gone. Quite a lot of the more failed joint looked as if it might never have been brazed properly, in that the failure included about 1/3 of the original brazed interface. When I looked at it, both surfaces were covered in braze metal, and that looked as it if hadn't cracked through.

This confused me until I took a section through the other joint and I realised that the fork had been a bad fit in the crown and the braze metal simply hadn't bridged the gap everywhere, leaving parts that were wetted with braze metal, but unfused. In actual fact the shore lines would have (superficially anyway) looked pretty good, with the braze metal wetting the blades about 2mm all round the joint. Just no fill, and no fillet.

Both joints would have had similar defects in, and both were cracked. Interestingly the bike has a 90mm hub brake, but the side with the reaction arm (which should see more force) was less cracked, and in fact the cracks that had formed were at the back of the crown thus as a result of road loadings (i.e. bending the forks forwards) rather than brake loadings (bending the forks backwards). So it is pretty clear that the brake loadings were not contributing greatly to the failure, anyway.

Now the bike is back together I've been for a little test ride and it is pretty clear that the fork is a fair bit stiffer than before. I reckon it must have been cracked for months, just gradually, insidiously, getting worse all that time. I went with one reinforcing tang on each fork blade in the end; hopefully the repair will last well...

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
mig
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Joined: 19 Oct 2011, 9:39pm

Re: I went over a bump and....

Post by mig »

sounds good. did you paint it brucey?
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

mig wrote:sounds good. did you paint it brucey?


yeah, badly.... but the welded area also has a coat of rattle-can silver as topcoat and I reckon that ought to let me see any cracks... well that is the theory, anyway.... :wink:

I use that bike every day and whilst I have others, they don't do the exact same job, so I had to get on and fix it really. At least it isn't going to sit around for months now....

BTW I have some photos of the cracks etc which I will post once I resize them etc.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

as promised, some resized photos;
Image00039.jpg

Image00040.jpg

Image00041.jpg

Image00042.jpg

Image00043.jpg


in the final image you can see how the joint has failed on one side; it appears to have failed in the joint itself but in reality these parts were never stuck together in the right way. I think.

You can also (just) see that there looks to be a paint defect where the braze-metal fillet should be on the inside of the other fork blade; in actual fact there is a defect in the brazed joint, and this is all that would be seen prior to the joint moving wholesale somehow.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Samuel D
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Re: I went over a bump and....

Post by Samuel D »

Unexpected brand.
Brucey
Posts: 44513
Joined: 4 Jan 2012, 6:25pm

Re: I went over a bump and....

Post by Brucey »

Samuel D wrote:Unexpected brand.


yeah, I wasn't expecting that. Mind you it is old and has had a hard life... but even so....

Here is the repair;

Image00047p.jpg


The fork crown has been extended downwards and dressed (you can see a couple of tiny marks where the fork crown used to finish), then the clean fork blade has been welded to the new added material. Then the weld bead has been dressed to mitigate toe defects and give a profile that mimics the original fork crown. Just to go belt and braces, I've added a triangular tang inside both fork blades; that is welded all round. The lap weld between the tang and the blade is undressed, but has received a surface stress-relief treatment. [Also, all the welds have been done using a 'burst' technique which means that every part of the weld received several thermal cycles, the last of which should temper the weldment, and render it less hard and brittle than it might otherwise be.]

Some cheap silver paint over the weld areas means (hopefully) that any cracks that might develop will be visible early on.

If I've done this repair well, it should be fine. Even if I've not done it that well, it can't be worse than what was there to start with, and if it fails in a similar way it shouldn't get a chance to kill me.....I hope!

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