The problem is, you've got a structure (the "traditional high-quality lugged steel frameset") that has an expected minimum fatigue life of maybe a hundred years if properly maintained (OK, maybe 200 for a '70s Schwinn Paramount...). You'd have to wait an awful long time, and follow an awful lot of framesets through their lives to collect any meaningful data on this issue.
>From an Engineering standpoint, think of the difference between bending raw fork blades over a mandrel in a controlled fashion (something the blades were designed for), spreading the stress (and strain) over a large surface area, and spreading the rear triangle on an existing frame (something it wasn't designed for), which tends to localize the stress and strain.
When you spread a rear triangle, where does the yielding occur? Often all in one small area on each of the four stays, in or near heat-affected zones of brazed joints. That *may* not create a fatigue life problem down the road, but it sure could. That is potentially a lot of strain in a small, localized area of a thin-walled tube that has already been somewhat weakened by brazing.
I tend to agree that, depending on the gauge of the tubes, four or five mm is relatively low-risk if done carefully, (I had a frame pop a chainstay bridge once when doing that operation years ago, though - repair and repaint time!). Cold-setting from 120 to 130 or 135 is potentially a lot of yielding/bending of some pretty thin tubes in some cases I think. It's certainly possible that the reduction in fatigue life for the heavier-gauge classic tubesets (say, 531C or heavier) wouldn't really show up for most folks, or would be so far in the future that they would never know why the frame failed at that point vs. ten or twenty years later. However, I think it only prudent to minimize the amount of cold-setting done to an already 20, 30, or 40-year-old lightweight steel frameset.
OK, I'm going back into the shop to bend some frames now....
Greg Parker
chief nerd
Bicycle Classics
Dexter, MI
Date: Wed, 21 Jul 2004 14:21:14 -0400 From: Peter Jon White <lists@peterwhitecycles.com> To: "richardsachs@juno.com" <richardsachs@juno.com> Cc: classicrendezvous@bikelist.org Subject: Re: [CR]Frame integrity Message: 16
richardsachs@juno.com wrote:
> i feel differently.
> fork blades are designed in gauge, taper, and overall length
> to be bent. chainstays are not.
To which PJW replied:
Are you concerned about failure? Or is it an aesthetic issue? If it's failure, have you ever seen one fail as a result of cold setting? I haven't, and I've been doing it since the mid 70s, making 120 frames into 126mm for 6 speeds. I've been spreading old 120 frames to 135 for loaded touring since the late eighties without a single problem.
--
Peter Jon White
Peter White Cycles
24 Hall Rd.
Hillsborough, NH 03244
603 478 0900 Phone