Re: [CR]More about Silver vs Brass

(Example: History:Ted Ernst)

Date: Tue, 27 Sep 2005 11:59:34 -0700
From: "Kurt Sperry" <>
Subject: Re: [CR]More about Silver vs Brass
In-Reply-To: <>
References: <>

Wonderful stuff there Bob, thanks. While the methodologies might be quibble d with, the results are still highly instructive. There's a TON of mythology related to our beloved lightweight bikes, some myths will withstand empirical testing; some won't. Kurt Sperry Bellingham WA

> OK - I spoke with Bill Davidson about his recollections of this test and
> as
> usual, he remembers a lot more detail than I do. I had most of it right
> but
> I hate to say Brian is correct and I remembered wrong on one point. None
> of
> the silver joints pulled away from the tubes in this particular test. Mor e
> about that in a second.
> The device we used was borrowed from Charlie Cunningham. Just as I said,
> it
> held the front triangle rigidly while a lever was used much as a fork
> would
> be in a front end crash, to wrinkle the front end, using a hydraulic jack
> as
> the power. A strain gauge was hooked up to this to measure the force
> required, which would tell you how resistant the frame was against an
> impact. The
> absolute amount of force required is irrelevant due to the leverage
> involved,
> only the relative differences are significant. Here are the results that
> Bill remembers: With Silver, the frame wrinkled right near the points of
> the
> lugs. With brass, it wrinkled about 1 cm down from the lugs. This didn't
> change with the different tubing or lugs, just with silver vs brass. This
> is as
> you might expect, since brass requires a higher temperature than silver
> and
> so the heat affected zone is further out. Bill says this is significant
> because it means that the weakest point of the tube, the margin of the
> heat-affected zone, is right near those nasty stress risers - the points
> of the lugs -
> in a silver brazed frame. In a lifetime of flex cycles, if it is going to
> break from fatigue, that is where it will go first. We have seen a lot of
> frames go like that. The brass-brazed frame which wrinkles farther down,
> would
> at least be stronger or at least more uniform where the points are and
> less
> likely to be taken out by fatigue cycles.
> Another significant thing Bill mentioned: Remember I told you about
> brazing
> with a big rosebud torch to preheat the joints so you can do it faster? W e
> did some that way and some the usual way with only a small torch. Bill
> said
> the preheated joints withstood a lot more force according to the strain
> gauge, although ultimately they failed in the same manner. That means the
> frame
> should be able to survive a somewhat harder impact. Since we only braze
> this
> way with brass (too much heat for silver) that is a point in brass's
> favor.
> Now back to those silver joints coming apart. I didn't make this up. It
> just didn't happen during this test. We get a lot of crashed bikes in for
> repairs, and Bill says he has seen many frames brazed with silver pull
> apart
> during a crash. Not just ours, but from other very well known builders
> too. I
> should point out that we have used Silver to braze also for many years on
> very
> light tubing. I have seen a number of these too, and somehow remembered
> them
> as being in the test. Some particular lugs are more prone to this than
> others. The Otsuya lugs that we used back the early 80s had very small
> surface
> area behind the head tube, and these were more prone to coming apart, as
> you
> might expect. I crashed my Davidson with these lugs in a high speed front
> ender in 1988, and while it did wrinkle the frame pretty good, it didn't
> come
> apart. Luck of the draw I think. I straightened and rode it a bit more,
> and
> it eventually the .7/.4 Prestige downtube cracked so I rebuilt it. Bill
> has
> seen a number of times where when the joint pulled apart, it left a thin
> layer of steel on the brazing material. He suggests that this has
> something to
> do with how the metal cooled in the investment casting process, leaving a
> weak
> surface layer. He started shining up the inside of lugs when he noticed
> this to remove that shear layer.
> Conclusions? Draw your own. It still seems to me that a frame can be
> stronger when brazed with brass and using a preheat torch in the hands of
> a good
> brazer who can do it quickly, but there is nothing wrong with silver as
> long
> as you don't plan to crash it. And if you crash, well, the frame is
> probably
> the least of your worries.
> Ride safe
> Bob Freeman
> Elliott Bay Bicycles
> 2116 Western Ave
> Seattle, WA 98121
> 206-441-8144
> Home of Davidson Handbuilt Bicycles