The Dutch Fiets magazine subjects all the frames they test in the magazine
to three different tests of rigidity - head tube torsional resistence and
bottom bracket deflection and I cannot remember what the last one is. These
results can be at sometimes throw up some quite interesting results - in a
test in the mid 1990s of an identical model Gazelle frame in different sizes
built from Reynolds 531 they found contrary to expectation that the largest
frames were stiffer. Gazelle had been very clever using thicker gauges on
the larger frames and on the largest frames an OS top tube so building in a
greater degree of stiffness for the heavier and more powerful riders... Very
clever but almost totally unseen.
What must be remembered though that lugs, tube mitreing, etc play virtually
no part in the stiffness of a frame nor whether a tube has been overheated
or not. Steel has a modulus of elasticity that does not vary to any
significant extent, lugs/frame joints do not move unless they are in the
process of failing and the stiffness of a frame is simply determined by the
the diamer and wall thicknesses of the tubing and the length of the tubes.
Tube butt length will play a part but even that is not especially
significant except in the case of the seat tube where most loads are bending
ones. How well a frame is joined and whether the tubing has been overheated
will however play a major factor in the length of service a frame will
provide before failing.
Hilary Stone, Bristol, England
> From: "Norris Lockley" <norris@norrislockley.wanadoo.co.uk>
> Date: Fri, 16 Jun 2006 11:25:24 +0100
> To: <classicrendezvous@bikelist.org>
> Subject: [CR]Oversize top-tube v. oversize down tube
>
> I refer to the various contris that have been sent to the List in recent
> days on this subject and would point the interested parties to the
> extended discussion of very similar matters of only two or three weeks
> ago.
>
> However.. in a more positive frame of mind, and in response to the
> suggestions of building three frames for testing, I would just suggest
> that if three builders were to each build three identical frames, that
> none of the frames would be similar to any other except for dimensions,
> materials, etc due the variables introduced into the equation such as
> the skill of the builder, the heat source used, the size of nozzle
> chosen, the manner of brazing/bronze-welding, the choice of dia. of rod,
> the number of passes made over th joint assuming that the joints were
> bronze-welded/fillet brazed etc etc. All of these factors have more than
> a little bearing on the integrity and mechanical qualities of the joints
> and hence of the frame.
>
> Three frames built by the same builder would be interesting, but the
> results could not be taken as empirical facts, because another builder
> using different methods, heat sources etc would probably not have
> identical reults. Arguably finite analysis is a valuable tool when all
> variables can be controlled...but in frame-building they cannot be..and
> equally arguably no frame-builder coulod ever produce two "identical"
> frames ie materials, sizes etc that turned out to be identical in their
> ride qualities and response. in the cyclists' tea-rooms of Yorkshire, a
> region where you are likely, or were likely some years ago, to find an
> abundance of Bob Jackson frames, a regular topic of conversation was
> whether one had a "good Jackson " or one of the "poor Jacksons". These
> discussions had nothing to do with appearance or even external signs of
> build-quality...just about how the frames rode. I have heard similar
> discussions about Mercians too.
>
> I think that it was just inside the time frame of the List that I
> visited Kirk...the maker of the blast-moulded magnesium girder section
> frames of the early..possibly mid-80s, at the time that he had just set
> up his own very small factory, after having developed the concept frames
> by using sub-contracted foundries.and it is possibly a little known fact
> that the Kirks that were ridden by Pro riders and were used by some
> Dutch riders in the T-d_F were pressure cast in aluminium alloy , not
> magnesium.
>
> However on the day in question a very very enthusiastic Mr Kirk talked
> me through his design , told me about the one-metric cube of sea water
> that provided enough magnesium for each frame, and waxed eloquent about
> the .8 of a second that it took to blast the molten "atomised" magnesium
> under tremendous pressure up a tube and into the waiting
> mould..Bingo!...just a blink of the eye and another Kirk was blasted
> into life.
>
> All this was fascinating stuff..bujt Mr Kirk then went on to make the
> claim that only his frame and his frame alone could provide the proof of
> just who, in the peloton, was the best rider. Claiming that no two
> brazed frames however well and carefully constructed possessed the
> identical mechanical and ride qualities but that each and every Kirk was
> 100% homogeneous in its construction and that each frame was identical
> in every respect. I think that this was his amketing justification for
> insisting that every rider in the "Grand Bucle" should ride one of his
> frames...the only true test of the champion.
>
> Trying to be of some assisitance in resolving the query about the choice
> of oversize down or top tubes..I venture to suggest that some indication
> could be obtained by a much easier experiment. It would be a simple
> matter for a frame builder to build two partial "main triangles"..of
> identical dimensions, one with an oversized top-tube but no down tube,
> and the other woth an oversized down tube but no top tube..only the head
> tube and seat tube being constant factors. the construction would be
> bronze-welded/fillet brazed becuse the integrity of the joint would be
> more visibly obvious than would that of lugged joints ie penetration of
> braze material etc etc.
>
> The structures coulod be clamped in some way to insure absolute
> rigidity..with only the "front ends " being able to move. An accurately
> machined bar, in terms of diameter to ensure a snug play-free
> interference fit with the head tube, would be inserted, with the same
> length protruding..and a torsional "force" applied to the bar..and the
> deflection and resistance to the applied force measured in some way. I
> am not an engineer, only a frame-builder, so I leave the data collecting
> bit to the experts.
>
> To make this experiment even easier, I suggest that the mitres of the
> tubes at the point where they are to be welded to the seat tube, be
> accurately cut a few degrees off true alignment..a milling machine or
> lathe could be used for this degree of accuracy..so that the tubes when
> bronze=welded into position would be out of track with the seat tube
> by the same amount. It would then be a simple matter of "elbow power"
> for someone, pulling and twisting the bar inserted through the
> head-tubes to attempt to "cold set" the tubes back into alignment... the
> number of beads of sweat forming on his brow as he attempts to do so
> during an accurately timed period, being the determining measurement as
> to which oversized tube provided the greater resistance to whatever it
> was hoped to measure. Or words to that effect...
>
> Norris Lockley..my legs tell me more about a frame than any dial gauge
> can...Settle UK