"Relaxing" a rear triangle

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woolwich
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Joined: Sun Dec 19, 2010 17:24 pm

"Relaxing" a rear triangle

Postby woolwich » Tue Mar 12, 2013 17:02 pm

Over on another thread a member has mentioned re-setting the rear triangle on an old frame to accept a wider hub.
This is something I have done previously by either something similar to Sheldon Browns approach. Or by threaded bar and washers. I think both these methods are called cold setting broadly speaking.
However he states that Bob Jacksons have informed him that this is not the ideal approach and his frame should be "relaxed".
Im always interested to learn best practice. Could someone please explain what "relaxing" a frame is? And why its beneficial. I figure it must be something to do with grain of metal or application of heat but am unclear.

Cheers
Mud to Mudguards. The Art of framebuilding.
http://locksidebikes.co.uk/

Yossie
Posts: 2589
Joined: Thu Jul 15, 2010 10:23 am

Re: "Relaxing" a rear triangle

Postby Yossie » Tue Mar 12, 2013 19:19 pm

Deep breathing, an open log fire, some nice red wine and a bit of Portishead always does it at chez Y/TDV.
This is only a joke by the way, I am in no way implying directly or otherwise that there is any kind of link or similarity between BR and North Korea :)

RPD Steve
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Joined: Mon Feb 13, 2012 22:08 pm

Re: "Relaxing" a rear triangle

Postby RPD Steve » Tue Mar 12, 2013 22:25 pm

Basically when you 'bend' metal, you are causing what is called strain hardening or work hardening. You are spot on with the metal grain idea... As you 'work' the metal you brake the grain structure of the metal and it increases in hardness and normally tensile strength, but reduces in toughness (its ability to resist crack propagation) and energy absorption ability - i.e. you make it brittle. Think of bending a clear plastic ruler until it snaps - exact same think.

I am assuming that by 'relaxing' he means stress reliving. This is when you heat a frame (or any metal) to over a given temp dependent on the material and chemical comp, and hold it there for a while. When allowed to cool, the metal grains will start to re-grow and re form to their 'natural' state.

If you were to rapidly cool the object the grains get very little chance to grow, remain small and you have a 'hardened' material, if you allow it to cool very slowly they will grow very large and you will have a soft and ductile material, sometimes called annealed.

A bit of a rough description but should give you the idea. I can see no 'need' for it when expanding a frame as you have explained. The level of stress induced is minimal, and I can see no way that it would impact of the ride or safety of the frame.

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dennisn
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Re: "Relaxing" a rear triangle

Postby dennisn » Wed Mar 13, 2013 01:31 am

Not an answer to your question but your question did raise another one.
So what happens if you do nothing but put the wheel on and ride it, even though the frame hasn't been "relaxed"? I have an older 5-6 speed lugged steel frame that I ran a DA 9 speed on for a year or so. It's now been switched back to a 5-6 and is mounted on my trainer with no discernable problems. I'm guessing that the frame simply opened up a bit, without damage, and went back to it's norm(5-6) when the wheel was removed. There seemed to be enough flex to fairly easily open it up the few mm's required for the wider hud without damage. I could be all wrong but that's how it went for me.

woolwich
Posts: 280
Joined: Sun Dec 19, 2010 17:24 pm

Re: "Relaxing" a rear triangle

Postby woolwich » Wed Mar 13, 2013 08:06 am

Steve

Thanks for explanation. Totally clear. So I figure Bob Jacksons heat the tube before re-setting it, then let it cool in controlled way, sand possibly? I imagine this involves both extra time and the need to at least respray the chainstays. Good to know that there are guys still prepared to do the job right.


Dennisn

I wouldnt worry at all. None of the frames I've messed about with seem to have suffered any harm. I found it necessary to reset in order to squeeze 135mm hubs in but I know you can get away with just pulling them apart sometimes. Some manufacturers space hubs at 132.5mm to allow you to spring them both in and out.
I think there is a slight risk of bearing damage, or at least accelerated wear, if the dropouts do not remain parallel but it could be many miles before you hit a problem.
Mud to Mudguards. The Art of framebuilding.
http://locksidebikes.co.uk/

mikenetic
Posts: 441
Joined: Thu Aug 18, 2011 20:28 pm

Re: "Relaxing" a rear triangle

Postby mikenetic » Wed Mar 13, 2013 09:42 am

Woolich is spot on for steel frames.

For example Surly deliberately set the rear triangle at 132.5mm on the Cross Check to allow them to be run geared, hubbed, or single speed. I think in their case the design of the relatively long, semi-horizontal, rear dropout means they tend to pull into parallel alignment under the clamping load of the axle without too much bother.

Surprised that Bob Jackson are a bit down on cold-setting, as lots of people do it. Is it an unusual steel in the frame or something?

woolwich
Posts: 280
Joined: Sun Dec 19, 2010 17:24 pm

Re: "Relaxing" a rear triangle

Postby woolwich » Wed Mar 13, 2013 14:50 pm

The frame in question was a 1985 Colnago Mexico Super. Maybe Bob Jackson has some experience here? Perhaps as you say they use a particular treated steel.
I wish I had asked on the original thread, I just didnt want to turn the op's thread about groupsets of at a tangent.
Weird though, as you mention I thought cold setting was common practice in framebuilding.
Mud to Mudguards. The Art of framebuilding.
http://locksidebikes.co.uk/


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