Re: [CR]Frame integrity


Example: Events:Cirque du Cyclisme:2002
From: <gpvb1@comcast.net>
To: lists@peterwhitecycles.com
Subject: Re: [CR]Frame integrity
Date: Wed, 21 Jul 2004 21:56:27 +0000
cc: classicrendezvous@bikelist.org

PJ: I can easily turn that right around on you: If the tubing is already weakened in the area of the chain stay indent, then weakened further by brazing the c/s bridge very near that area, should one weaken it still further by bending it again to increase rear spacing? It depends.... It's a calculated risk. Has everyone tracked every frame they've re-spaced for 50-100 years afterwards? Most traditional, non-exotic steels are very forgiving (i.e. malleable). That's why steel is real! When the load is distributed over a reasonably-sized area, and the metal is pushed around carefully, often there will be no immediate or long-term problem. Damage can have a way of stacking up in a particular area over time, though, as your example would illustrate. And we haven't even touched on re-spacing a frame that was over-heated when initially brazed up. Bending a complete assembly (frame) is more unpredictable (distribution-of-stress-and-strain-wise) than bending (raking) loose fork blades using a mandrel to distribute the load and apply it over a specific, large area. We aren't really talking about immediate catastrophic failure in this thread, we're talking about reduced fatigue life, followed by a (usually somewhat controlled) failure far down the road, IMO. We are assuming that the frame doesn't fail catastrophically during the cold-setting process, or even soon thereafter. All that said, everyone has their comfort zone with regards to this procedure, it would appear. Some tolerate zero re-spacing, some a few mm, some a lot. C'est la vie. (Allez train bleu!). (Sorry, got caught up in the spirit of the Francophone moment there). Cheers, Le Greg Parker Dexter, MI, Etats Unis (there it is again! Sacre bleu!)


-------------- Original message --------------


> gpvb1@comcast.net wrote:

\r?\n> > The problem is, you've got a structure (the "traditional high-quality lugged

\r?\n> steel frameset") that has an expected minimum fatigue life of maybe a hundred

\r?\n> years if properly maintained (OK, maybe 200 for a '70s Schwinn Paramount...).

\r?\n> You'd have to wait an awful long time, and follow an awful lot of framesets

\r?\n> through their lives to collect any meaningful data on this issue.

\r?\n> >>From an Engineering standpoint, think of the difference between bending raw

\r?\n> fork blades over a mandrel in a controlled fashion (something the blades were

\r?\n> designed for), spreading the stress (and strain) over a large surface area, and

\r?\n> spreading the rear triangle on an existing frame (something it wasn't designed

\r?\n> for), which tends to localize the stress and strain.

\r?\n> > When you spread a rear triangle, where does the yielding occur? Often all in

\r?\n> one small area on each of the four stays, in or near heat-affected zones of

\r?\n> brazed joints. That *may* not create a fatigue life problem down the road, but

\r?\n> it sure could. That is potentially a lot of strain in a small, localized area of

\r?\n> a thin-walled tube that has already been somewhat weakened by brazing.

\r?\n> > I tend to agree that, depending on the gauge of the tubes, four or five mm is

\r?\n> relatively low-risk if done carefully, (I had a frame pop a chainstay bridge

\r?\n> once when doing that operation years ago, though - repair and repaint time!).

\r?\n> Cold-setting from 120 to 130 or 135 is potentially a lot of yielding/bending of

\r?\n> some pretty thin tubes in some cases I think. It's certainly possible that the

\r?\n> reduction in fatigue life for the heavier-gauge classic tubesets (say, 531C or

\r?\n> heavier) wouldn't really show up for most folks, or would be so far in the

\r?\n> future that they would never know why the frame failed at that point vs. ten or

\r?\n> twenty years later. However, I think it only prudent to minimize the amount of

\r?\n> cold-setting done to an already 20, 30, or 40-year-old lightweight steel

\r?\n> frameset.

\r?\n> > OK, I'm going back into the shop to bend some frames now....

\r?\n> > Greg Parker

\r?\n>

\r?\n> Greg,

\r?\n>

\r?\n> That sounds interesting. But what frame builder hasn't indented

\r?\n> chainstays to increase tire clearance, particularly on Columbus and True

\r?\n> Temper stays? How many decades has that been done? And isn't the

\r?\n> indenting being done in exactly the same place where most of the bending

\r?\n> from cold setting to a wider spec would take place? And isn't the tubing

\r?\n> exactly the same thickness?

\r?\n>

\r?\n> And when was the last time you saw a frame fail at one of those indents?

\r?\n> I've only seen problems with heat treated tubing in that area. I split a

\r?\n> True Temper stay for a touring frame I was building several years ago.

\r?\n> A bit of annealing on another tube before indenting eliminated the problem.

\r?\n>

\r?\n>

\r?\n> --

\r?\n> Peter Jon White

\r?\n> Peter White Cycles

\r?\n> 24 Hall Rd.

\r?\n> Hillsborough, NH 03244

\r?\n> 603 478 0900 Phone

\r?\n> 603 478 0902 Phax

\r?\n> http://www.PeterWhiteCycles.com