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PostPosted: Mon Nov 30, 2020 7:06 pm 
retrobike rider
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Joined: Tue Jan 29, 2008 11:04 pm
Posts: 2606
Location: A wretched hive of scum and villainy...
Right then...on to the bent seatpost.

Here's the problem:
Attachment:
IMAG0825.jpg
IMAG0825.jpg [ 119.46 KiB | Viewed 333 times ]
About 9mm out of plane, with a distinct tight bend over a short distance where the post stuck out of the frame.
I don't have a frame that I would want to use to replicate the original bending force, just in the opposite direction, and that would be really hard to hold and load safely anyway.
If I support the ends ot the tube and push in the middle, because the tube is of equal stiffness all the way along, it will want to bend into a U-shape along its length.
This would leave the tube in a W-shape, rather than re-straighten just the short bent section in the middle :|

But first, to mark on the plane of the bend, and the start and finish points.
Hold the bent tube flat on the surface table and run a square along a length of masking tape on the side:
Attachment:
IMAG0826.jpg
IMAG0826.jpg [ 338.64 KiB | Viewed 333 times ]
And mark the bend position by eye off the surface table.

So, how to make the tube bend preferentially at the middle?.. :|
I need to make the straight sections on the ends stiffer than the bent section, more like a nunchuk than a fishing rod :lol:
Luckily, I have some bending compound in my toolkit. Traditionally this was a low temperature melting metal alloys such as Cerrobend.
Attachment:
vintage-cerro-bend-cerrobend-alloy_1_02ecd8b9d6691736153f209399ae34fb.jpg
vintage-cerro-bend-cerrobend-alloy_1_02ecd8b9d6691736153f209399ae34fb.jpg [ 56.45 KiB | Viewed 333 times ]


However, while mostly made from reasonably benign bismuth and tin, it also has some nasty stuff in it that's best not heated (Lead and Cadmium :shock: ), so is generally discouraged these days, unless absolutely necessary.
It has been replaced with resins such as WS8, which, in the words of the UK manufacturer is "a new technology alternative to high density fusible alloys for tube bending operations. Resin WS8 is a low density solid (~1) with a melting point of about 70DegC which can be used to fill tubes prior to bending operations. It offers excellent dimensional and wall support during bending, can be simply removed with heating and a special advantage any residue can be removed with plain water." :P

It's a white sparkly crystalline product, best not smuggled through customs:
Attachment:
IMAG0828.jpg
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which when gently heated (on top of my workshop stove):
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IMAG0827.jpg
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melts into a slightly viscous clear liquid:
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IMAG0830.jpg
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While the resin was melting, I made 2 dipsticks to test the depth of fill, because it's dark down there:
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IMAG0829.jpg
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After filling to the first level mark (start of the bend) and leaving to cool, I inserted a plug of plastic foam to stop the resin filling the bent section, but still let the tube bend.
Then checked its position and length with the shorter dipstick:
Attachment:
IMAG0831.jpg
IMAG0831.jpg [ 261.92 KiB | Viewed 333 times ]


To be continued.

All the best,


Last edited by danson67 on Mon Nov 30, 2020 8:45 pm, edited 4 times in total.

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 Post subject: Re:
PostPosted: Mon Nov 30, 2020 7:30 pm 
retrobike rider
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Joined: Tue Jan 29, 2008 11:04 pm
Posts: 2606
Location: A wretched hive of scum and villainy...
Continued:

Fill the second, lower section of seat tube with molten resin. You can see where it has cooled and quickly set where in contact with the colder aluminium:
Attachment:
IMAG0832.jpg
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Leave to cool overnight. Completely solid:
Attachment:
IMAG0838.jpg
IMAG0838.jpg [ 192.04 KiB | Viewed 327 times ]

Mount the seatpost on two rubber lined tube blocks in my hydraulic press, wih the marked plane of bend facing upwards.
Another rubber lined block acts protects the tube from the piston and spread the load; I don't want a flat spot:
Attachment:
IMAG0840.jpg
IMAG0840.jpg [ 295.53 KiB | Viewed 327 times ]

Set the piston to the top of the block and count pulls of the pump lever (so it can be repeated if necessary) as the piston loads the tube and the tube looks loaded enough past straight.
This is by eye and experience, as it will need to be bent past the straight point, as the material will always spring back when the load is removed.
Remove the piston load and check the straightness.
Ideally it's one shot only, and I certainly don't want to go past straight, and have to re-re-bend back in the original direction.
Thankfully done in just 2 attempts...nice and straight again:
Attachment:
IMAG0841.jpg
IMAG0841.jpg [ 112.86 KiB | Viewed 327 times ]

Place the post on top of the stove to warm up enough to melt out most of the resin into the pan for re-use.
Rinse out any residue with boiling water.

All the best,


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 Post subject: Re:
PostPosted: Mon Nov 30, 2020 7:40 pm 
Gold Trader / MacRetro rider
Gold Trader / MacRetro rider
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Joined: Mon Aug 27, 2012 9:56 pm
Posts: 2517
Location: Eilean Siar
Wow! :shock:

Superb stuff Mr D 8)


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PostPosted: Mon Nov 30, 2020 8:00 pm 
retrobike rider
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Joined: Sat Jul 28, 2007 1:03 pm
Posts: 305
Location: Hartlepool - North East
Dan this update is absolutely amazing and showcases the fantastic attention to detail you apply to repairing components. I can’t thank you enough for all of your expertise you have given to this restoration


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 Post subject: Re:
PostPosted: Mon Nov 30, 2020 8:03 pm 
Retro Guru
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Joined: Sun Feb 01, 2009 9:27 pm
Posts: 1296
Location: Abingdon
Great work Dan :D

As I read the prep, I was thinking surely he’ll have to insert a solid bar into the tube somehow - but I didn’t expect a solidifying solution to be the answer :shock: Very cool 8)


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 Post subject: Re:
PostPosted: Mon Nov 30, 2020 8:46 pm 
BoTM Winner / retrobike rider
BoTM Winner / retrobike rider
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Joined: Wed Nov 23, 2005 2:34 pm
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Location: Wellington, New Zealand
Wow,

Awesome work Dan that’s spectacular

Another piece of the puzzle done David :)


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 Post subject: Re:
PostPosted: Mon Nov 30, 2020 8:52 pm 
Old School Grand Master
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Joined: Sat Jul 13, 2013 8:01 pm
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Excellent work as always Dan. Love these types of threads 8)


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 Post subject: Re:
PostPosted: Mon Nov 30, 2020 10:29 pm 
BoTM Winner
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Joined: Thu Aug 14, 2014 8:44 am
Posts: 1128
Location: Sweden
Super interesting to see 8)


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PostPosted: Mon Nov 30, 2020 11:06 pm 
Old School GrandMaster | Rider
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Joined: Thu Nov 26, 2015 7:29 pm
Posts: 7734
Location: peak district
Very interesting, every day is a school day on here 8)


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PostPosted: Mon Nov 30, 2020 11:36 pm 
BoTM | rBoTM | rider
BoTM | rBoTM | rider
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Joined: Wed Jan 17, 2018 8:42 pm
Posts: 4900
Location: VCC Member 5558 East Riding of Yorkshire
Excellent work Mr Dan.... :xmas-cool:


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