Ohhh, Jawine! You've stirred up a potential hornet's nest of theoretical discussion with this topic -- thank you!
By coincidence, after a couple hours' sleep, I was re-reading this very article a few hours ago. For more information tangentially related to the topic, Google-search "What is a planing bike?"
Not that it affects any Thorns -- and especially not TiG-welded ones -- a dirty little secret of many production bikes in the 1980s and 1990s is the tubes would sometimes come through unmitered and two key joints (where the seat tube and downtubes entered the bottom bracket) often had really skimpy amounts of braze.
How do I know this, firsthand?
A friend of mine for 35 years owned a busy secondhand bike shop for 25 of those years. Knowing my interest in hobby-framebuilding, he culled all the damaged frames and set them aside for me to cut up, rebraze, repair, or rebuild into other creations; tens of frames numbering very close to a hundred over the years. Those I didn't want went to the local steel recycler's where he was paid by the pound (usually covered the fuel cost to haul them there in his pickup truck) A good part of my Dan-built full-sus Folder came from this source...Ishiwata seatstays, Tange Prestige chanistays, True Temper seat tube, Reynolds steering mast, and a pretty rare Phil Wood elliptical tandem keel tube set on edge for the main frame spar (new from another source, but accidentally cut too short for its intended use; I got it cheap). All the tubing past the HAZ (heat-affected zone) was good, provided it had not been crash-damaged (mostly head-on curb strikes and crashes into parked cars that wrinkled the undersides of the top- and downtubes) and good cheap fun with the torch (full tubesets are very not-cheap). I used to be very active on the early iBOB listserv shortly after it was founded by Piaw Naw (this in the days following the paper version of Grant Petersen's Bridgestone Owner's Bunch), and a few members graciously mailed me their cracked and crash-damaged frames so I/we could find out what made them tick internally that might account for some of their noted ride qualities.
Sadly,
some of the more supple frames -- the ones that might be described in current online literature as "planing frames" -- came by that characteristic because they were barely glued together with braze and/or the tubes had unmitered or crudely mitered ends! I won't name names, but I could cite examples nearly equally from all major marques sold in America over two decades (Centurions and Miyatas were notable exceptions, as were early Nishikis; all the ones I examined were made with great care and extremely close miter-tolerances and had their full fill of brass). Occasionally there would be a tube-butting flaw resulting in a break at the end of a butt, though most of these surprisingly "bad" frames were held together only by the lugs or in some cases by the brazing pins used to locate the joint. Some tube ends weren't mitered at all, being cut-off straight. Others were only crudely mitered, still showing ragged ends in the cut-open lugs where aircraft tinner's snips had hacked crude half-circles that didn't even reach the head tube.
It was an education and a revelation all in one, especially for someone like myself who had always valued the functional artistry of nicely done lugwork. Some semi-custom (small-run production) bikes with high-zoot tubing and silver brazing were not immune from lug flaws either; in those cases, the problems were largely caused by failing to get the joints really clean before brazing. Being a lower-temperature process with different flux, silver-brazed joints just aren't as tolerant of grease and contamination as brass. silver doesn't fill gaps as well as brass, so if the tolerances were generous, the joint was weak. One joint had pulled loose in use, causing a shimmy that made the bike unridable. A number of brass-brazed bikes I cut up had cracked seat tubes, consistently about 1-2cm above the BB. Many would last forever under the lower stresses of spinners, while mashers could expect more problems caused by the cyclic loading of heavy pedaling. In each of the cases I saw, the seat tube had broken just above its lower butt due to inadequate brazing of the BB shell resulting in excess flex with every hard pedal; a classic fatigue failure secondary to poor brazing. The BB is a big heat sink, and it is hard to get the shell hot enough for brass to flow adequately without cooking the ends of the seat-and downtubes. Under the pressures of high-production, some just spent too little time in the autobraze fixtures or too little time under the brazer's hand torch before the next example came down the line.
One late American brand of overseas-sourced randonneur/porteur bikes suffered from tube-butting defects that caused cracks or had cracks that initiated secondary to an interference-fit of the fork crown-downtube. They were designed so the custom front racks bolted to threaded sockets atop the fork crown. When the front wheel turned, the allen bolts holding the racks dented the downtube right below the top butt; cracks followed about 18-24 months After. Oops. Owners oozed rhapsodic over their bikes' ride qualities...till the bikes literally fell apart.
After some teething problems, production TiG welding did solve a lot of these problems because it required good miters for each joint and there were no lugs to braze. So long as the weld beads were nicely lapped and not undercut, the joints generally held well (exception: right-rear dropout; Thorn's socketed, brazed dropouts plus some touring models' additional 'stay bracing neatly address this issue from the start). Hard to go too far wrong with fillet-brazing unless the tubes are cooked.
The lesson in all this is
sometimes Bike Feel is/was not the result of obvious things like tube diameter or wall thickness, geometry or design, but came down to workmanship and the presence or absence of errors relate thereto. A bicycle is a complex system of factors and components that all work together -- or not. Pays to have a good designer and good workmanship first. Once that is established, there is a level playing field for comparison.
Oh! What a great topic for the propeller-beanied gear-heads among us...
Best,
Dan. (...who actually
has a propeller beanie*)
*note the snazzy Aussie product labels shirt as well: Australasian Jam, Uncle Toby's Oats, Arnott's Biscuits, Billy Tea and good ol' Vegemite.
