> 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
I get asked quite often to change the spacing on older frames. My stock reply is as follows: "If you lent your favorite bike to a friend, and he came back and said he crashed and bent your fork. Is that a good thing or a BAD thing??? But, if you call it "cold setting" it is just fine. I have seen chainstays ripple, bridges pop apart, and dropouts crack. This doesn't always happen, but it can, and I just don't think it is worth the risk.
Just my opinion.
Bruce Gordon
http://www.bgcycles.com