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flowboy Offline OP
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Evenin' all.

Due to the extreme shortage of skilled girder fork engineers, does anyone have any experience of truing, straightening or generally realigning bent / twisted girder forks? eek

I have a set of the ubiquitous BSA M20 girders. Among many other problems they can have (& mine do) is typically one side of the blades is no longer in line with the other (viewed from side) & the axle / spindle slots are then out of line. The front wheel would be pointing slightly to one side or the other when the forks were in line with the chassis (viewed from riding position) . Usually (but not always) it is the lower length of the tubes between the wheel spindle & bottom yoke; hitting rocks, hedges, kerbs or cars hard.
I have heard that some people pull these back straight, cold. No heating of tubes or brazed joints.
My top & bottom link tubes seem to be in line, so that's a start!

Technically I can see ways I could clamp the blades through the top & bottom link castings & other ways to hold one side solid while the other was tweaked. But... as much as I have a practical job, decent workspace & tools etc, I have never done this before.
I am interested to have pointers from anyone with experience of this kind of brutal shed work!
Heating castings to soften braze?
What about straightening any bent tubes individually within the blades?

I am also wary that one of my lower tubes may be close or vulnerable to being cracked through, right where it emerges from the bottom front damper link casting. I can see that if this happens (better in wkshop than on road!) then either re-tubing this side or sleeving internally would be reqd.
Re-tubing looks very tricky to me, involving heating all casting joints to dismantle, in which case the whole lot might as well be replaced eek

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Wrong end of the country but maybe he can offer advice/ do it for you?

https://jakerobbinsvintageengineering.co.uk/

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Looks like Jake has taken over where Elk Engineering used to be ?

"working with the late Steve Burniston, the man behind Elk".

https://gregwilliams.ca/girder-forks-and-jake-robbins-vintage-engineering/

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flowboy Offline OP
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Thanks for that, I have looked into it a bit, hence "shortage". My understanding is that Jake Robbins is at best, exceptionally busy. Tales of people waiting many many months for work to be done. Apparently Percival Brothers & Webb are also in the same situation with a long backlog. Very busy / slow. Other names have cropped up but it seems have all retired or ceased doing the work, as has my local go-to guy.
I once trusted a tank to a well known repairer; that took over 18 months of no communications & I got a different tank back, so I am quite reluctant to go down that path again.
I guess I can call & see if either are willing to advise so will try that.
I don't mind having a go at it (complete re-tubing excepted!) Just hoping for some tips from those who have maybe sorted some out previously.
They are not Girdraulics or any other high grade valuable & desirable girders, or have tapered tubes. (I know they've gone up quite a lot in recent years tho'!) The M20 is not known for it's fine handling & no-doubt many were ridden in pretty knocked about condition during WW2. So if I can get them closer to correct spec & a roadworthy state I'll be happy. The blades don't look so far out as to be un-rescuable if I am cautious. If I make a complete hash of it then I guess I will have to trust them to the back of the queue with an above named engineer & give up getting the bike on the road for the next 2 years or something...

Last edited by flowboy; 12/13/21 12:02 am.
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If you take them along, and get some advice, they are often quite helpful in person.

My local frame/forks guru is like that - busy to the eyeballs.
But if he likes what he sees (ie its easy) then you may jump the queue a bit.
Who is to know ...

I cut and replaced a tube in an old Triumph girder fork.
Oxy/black wire, easy peasy.
But the rust/damage wasn't near the lugs.
And it wasn't in danger of warping.
[Linked Image from i.postimg.cc]

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flowboy Offline OP
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Hi Rohan,
that looks like an even older blade. Also a simpler construction than the M20 Webb type blades.
I can see that you could just sweat or cut the tubes out & slide a new one in.
The BSA ones look a lot trickier to do that, with multiple tubes into the two cross / link tubes. Getting one out almost means getting them all out as far as I can see. Or de-brazing one side from the link tubes.
For me that would be a very hot forge/kiln with a lot of fire bricks & gas to keep the temp up. There are ways I could do it, although I would need a very much bigger heat source, as I only have a small Mapp gas torch which is only good for small brazing jobs. Plus there is the risk of embrittlement of the tubes if the heat is not kept correct & gets too high. Although its more likely not going to be hot enough, with a Heath Robinson shed-rigged "forge"!
I guess I could just cut the tubes, drill out the remains & save the brazing for the new assembly.

The M20 tubes are apparently, one piece top to bottom in heavy gauge - (which one?). Assembly wise, with the tubes being at angles to each other, it would make more sense to have a top section & bottom section brazed into the bottom link cross tube, sleeved internally if necessary, but whether this would be adequate, strength wise I don't know. Trying to remove & replace a full length single tube looks impossible for amateur shed work, although the front ones might be a little easier, still quite a job...

Possibly I could remove the axle lug & remove the tube that way then sleeve back into bottom link casting. Still, fiddly with the back tube coming in at an angle. Unless I cut both tubes (front & back) under the bottom link casting, replaced tube/s if reqd. & sleeve back into casting. I would also be needing to know the grade of steel-alloy tube used.
Anyway, at this stage, hopefully this will not be required!
Really it's more about the bending back to straight that I'm after, to start with.

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Hi Flowboy
DO NOT attempt to unbraze the tubes from the lugs!
First of all the tubes are pinned in place and the heat required to melt the braze is enough to damage the lugs when trying to remove the tube
I have straightened a couple of sets over the years and was able to straighten everything cold except a Webb top yoke that needed heat
I did as you mentioned, making matching angle brackets for the top and mid spindle positions, luckily my mill table is big enough to bolt the angle brackets to the bed, A large piece of channel iron could be substituted for the mill bed?
If the brackets do not sit flat will show up any twist in the top section
Clamp the centre and jack/ press the top parallel to the mid if needed
Establish a centre line along the 3 spindle positions,
When the top is straight clamp the brackets down solidly to the operating table.
Using the centre line look and measure where the wheel spindle fits for off centre and parallelism to the other spindles
The legs will easily bend to get the legs equal either side of the centre line
Then decide which leg needs moving to line up with the other (maybe both need adjustment ?)
Lever, jack /press the legs to correct parallelism then recheck the side to side
Now address the cracked tube, It might be necessary to cut the tube through and insert a sleeve
Then re braze the joint if the tube is found to be in good condition
Some people prefer silver solder to braze ?

John

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I agree with everything Chaterlea said.

On the subject of brazing, Paul Brodie has some good content on brazing and making repairs on his YT channel.

John.


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