Re: [CR] Classicrendezvous Digest, Vol 80, Issue 107

(Example: Framebuilders:Brian Baylis)

From: "Andrew R Stewart" <onetenth@earthlink.net>
To: <classicrendezvous@bikelist.org>
References: <mailman.15912.1251205866.344.classicrendezvous@bikelist.org>
In-Reply-To:
Date: Tue, 25 Aug 2009 10:02:10 -0700
Subject: Re: [CR] Classicrendezvous Digest, Vol 80, Issue 107


Harvey- Thicker walls don't reduce the heat level achieved during brazing (if so the filler wouldn't wet out). They allow for a strength loss due to the needed heat (and duration of heat) and not have the tube's finishing strength fall bellow a minimum. Or put the other way: When a wall is chosen for the after brazing strength, butting allows the nonheat effected portion of the tube to be lighter and still strong enough. I think any ride benefits are secondary to the weight savings (just look at the current use of Alu. for proof of this).

As to lug points "can openering" into the tube: this is usually the result of serious bending forces, like the classic head end accident bending the top and down tubes. During brazing the usual problem is that the lug tip lifts off the tube making the tip (and not the tube) over heat very quickly. I don't think I've ever seen a tube deformed during brazing if the lug's fit was any where near well done. Now cracks (or tears) can occur at the stress riser points over time and with typical riding forces. Like sharp lug ends or the corners of a shift boss. It's easy to reduce this likelihood with low heat filler (silver), good shape (rounded corners, tapering socket walls) and adequate tube wall. Most frame failures are due to too little filler, overheated joints or design that didn't meet the use (especially with the MTB world).
> ------------------------------
>
> Message: 2
> Date: Mon, 24 Aug 2009 20:57:38 -0400
> From: Harvey Sachs <hmsachs@verizon.net>
> Subject: [CR] re butted tubing...
> To: <gillies@ece.ubc.ca>, Classic Rendezvous
> <classicrendezvous@bikelist.org>
> Message-ID: <4A933702.6050408@verizon.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
>
> In a generally spot-on message from Don Gillies, I have doubts about two
> statements, so I'm going to <snip> the rest to comment on them:
>
> <snip>
>
> Don: The main purpose of butting is that (a) it's designed to allow thin
> and springy tubes but also (b) it's designed to be thick at the ends
> for the joinery of the tubes. So, the thick ends of the tubes should
> have a long enough butt (typically 75-100mm or 3-4") so that the lug
> points do not dig into the ultra-thin part of the tubing. So an
> italian long-point lugged frame should probably have longer butts than
> a bocoma pro (short-point) lug set.
>
> Harvey: I've never heard the assertion about lug points digging into
> thin tubing; it's all very soft at temperature. I think the real point,
> which you allude to, is that the thicker butt was supposed to increase
> the heat capacity and thus limit the local temperature rise when doing
> hand-held (or hearth) brazing.
>
> Don: The benefits of quad-butting are imho doubtful. On some bikes like
> Miyata, I believe they put the downtube shifter bosses after the first
> (but not second) butt. Tearing at the shifter bosses is a very common
> downtube failure mode. So I can perhaps understand triple butting on
> a downtube, but I seriously doubt if it's needed when you could use
> low-temperature silver-solder for installing downtube shifter bosses,
> bottle bosses, and pump pegs, etc., to minimize heating and weakening
> of the tubes at the thinnest points.
>
> Harvey: Agreed on the silver solder for tiddly bits, but I've never seen
> a frame with tearing at the shifter bosses. Accidents? Real monster
> abuse like kicking the thing? I have seen frames that failed with
> transverse cracks behind the head tube, as though heat had been too
> localized (but could have had other causes), and there was some talk
> that Nishiki Competition frames tended to suffer downtube failures.
> Below the shifter bosses, as I recall, but this is veyr fuzzy.
>
> Harvey sachs
> mcLean va
>
>

Andrew R Stewart
Raleigh, NC