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