As so often, I start with the pull from a post by Martin which inspired the thoughts below:
I think the chap that made and sold these Lord uplifts [for saddelbags] probably compensated for any possible material weakness by using thick plate and rods. The uplift and the support that fits to the saddlebag weigh over 400g, nearly as much as the Tubus Fly steel rack.
You're very likely right. Mr Lord had to work with the materials he could get at the time, the processes he could control. But today we have many more choices.
In such small members of a structure the stresses and strains of the load are mainly reacted in the outer fraction of the material, in other words near the circumference of the rods. It is therefore good engineering to use tubes rather than rods, to apply part of the material mass saved to choosing a thicker tube than the replaced rod, and to making triangulated structures or even to spend some of the weight saving in triangular flitch plates. Proper design in tubes rather than rods will save weight without any other sacrifice except to the expected vintage appearance, which in bicycle gear is often an excuse for incompetent engineering or absent skills. For instance small-diameter thin-wall tubes are a pain to weld without destroying them, and a proper high-strength weld requires precision curve grinding or laser cutting under computer control, and brazing requires even greater precision.*
DESIGN AND CONSTRUCTION OF
A MODERN MULTIFUNCTION SADDLEBAG SUPPORT
However, if I were to design a modern version of a saddle bag support, it would be cut out of a single flat sheet of quite thin stainless steel for durability and appearance. After cutting out the single shape which will be both the support and the fitting panel to the saddle rail, it will be folded, so that the main support will look like an upside down tray and the fitting part like a smaller tray folded upwards from the main tray.
The main part after cutting and folding will look from above like a flat rectangle or square with an X through the middle. All edges will be turned down, except the attachment section, which will be turned upwards only were it forms part of the main platform. That will be the last bend performed, the other edges of the attachment tray being turned down at the same time as those of the main tray.
The attachment tray too can have some of the inside cut away. The attachment tray will have the shape of the rails of the saddle from where they turn down at the back until they turn forward for attachment.
All of that is one piece of metal.
Now you need one or both the offcuts from beside the fixing tray, which you will cut and fold on all four sides to fit vertically inside the fixing tray and also inside the saddle rails, open side forwards so that you can get inside the folded sides, which are now doubled up as in the fixing tray and the fitting tray or trays.
The fitting tray folderovers and the fixing tray folderovers are drilled together and bolted together around the upright part of the saddle rails; no bolt or (horrors!) self-tapping screws visible or, worse, in contact with your expensive saddle bag.
All interior corners from cutting away metal for weight saving must be rounded with an appropriate large radius; these are the places where cracks will start if they are sharp.
The centre of the X on the main platform and on the fitting tray can be cut to a decorative pattern if you like, as long as you don't take away too much metal.
Both trays and also the bolted inside fixtures can be strengthened by cutting up the offcuts, bending them into angle and spot welding or bolting sections of the angle inside the trays where the flanges meet.
Finally, at last a slightly difficult brace to make. By now both the bearing tray and the fitting tray are independently stiff enough for purpose everywhere that matters, except where they meet in a right angle. To provide a buttress brace on top would steal loading space. So a buttress brace will have to be supplied on the back. This means that at least one vertical edge of the metal in every brace must be at ninety degrees to the surface being braced.
The last thing to make is the nearest to a precision job in the whole assembly. Start with two large L-shaped pieces of metal with letters fat enough on the long arm to make a substantial angle, and on the short arm a lesser width of angle. The long side of the L must be longer than width of the platform, and the short end must be longer than the height of the fitting plate. The excess length in each case should be enough to bend another angle so that the support L can be fitted to the existing fold-downs of visible plates without any screws or rivet visible on the carrying surface or otherwise touching the saddle bag.
The flat bearing angles must all be bent from the inside of the L.
On the long part of the L, the angle will be bent outwards to make this crucial additional support as wide as possible. On the short part of the L, there won't be space to bend the angle outward, so bend it inwards towards the centre of the bike.
Now comes the difficult bit. Where you want to arrive is each L-shaped arm supporting the carrier plate on top of the angle of the long arm with a close fit but without any fixings at this stage. Simultaneously the inside corner of each L shape must be as tight as possible at the underside corner at the bend of the main plate where it becomes the fitting plate.
We want at both ends of each L to just touch with the four ends, without pre-stressing, the fold-down we put into the flat load bearing and fitting plates, so that the angle on the short arm of the L lies flush to the fitting plate.
Now trim the arms of the Ls to length, then offer up to the construction and adjust for a tight fit at the fold and along the angle bearing. Do the same for the other side.
Strengthen each L-plate end-to-flange junction with an angle bracket, drill and rivet or bond if you're using bondable metal.
Still no bolt-heads in contact with your saddlebag.
lf built in aluminum or laser-laid in engineering plastic, appropriately thicker sections will be needed.
I'll be happy to provide dimensioned drawings if anyone wants to pay for them.
*A. In probably 1980, we returned from introducing our child to the grandparents in Australia, and settled in a house with extensive garages on Hayling Island, between Chichester and the Isle of Wight, because I wanted to supervise the shortening of a Bentley chassis I was turning into a sports car. Imagine my surprise when I arrived to meet the primarius of hotrod metal work, Geoff Jago, the first thing I saw as I walked in the door of the metal workshop (there was also a GRP workshop in a separate building) was a computer with on the screen a design by me for hanging Ted Knight's exemplary Jaguar XJ independent rear suspension very neatly and nearly invisibly, with below it some kind of a gas cutter under the control of the computer. As soon as I recovered from my amazement at seeing such sophistication at a hot rod welder, I held out my hand, palm up, as Geoff was trying to shake my hand, and said, 'I don't recollect giving you a license for using my design. How many of these adaptor plates and crossbars have you sold? Where are my royalties?' Not a small tube example, but today a small laser cutter that you can control with your phone costs a couple to a few hundred quid (Geoff's pathfinder setup back then cost in the tens of thousands), and you can buy engineering grades of plastic on a reel to have a laser under computer control build you anything you can program.
B. Innovation also helps. When I designed my small-tube bike, which I decided on aesthetic grounds should have lugs rather than visible welding, I chose 1mm wall stainless aircraft tubing, and made lugs and internal and external butts by layering inside and outside with graduated short sections of the same 1mm wall tubing in different diameters brazed into the ends of pre-cut lengths of the main tube, with the last, shortest, outside section fully decorated and the other sections decorated at the showing edge, the whole assembly then to be laser cut in three dimensions to within a tenth of a millimetre, and brazed together with other similarly built up sections. The whole thing would have been remarkably light even in stainless, but my limit is neat soldering joints on large valve-era electronics, with tiny brazing being entirely beyond my eyes or my skills. I spent a couple of years talking to bicycle braziers who promised everything as long as it could be built with tubes off their shelves; they were basically not selling innovation in bicycles unless you wanted a bike in wood or bamboo, which I didn't need them for (a 68ft sailing yacht I designed and built of laminated veneer squares and then thrashed in the toughest oceans around the world is still operating year round in both China Seas over half a century later). Eventually a guy who wanted me to give a series of lectures at an art college to advanced students made some calls for me and I had a choice of art school welding instructors, all but one never having braced anything in their lives (and the one who could braze was a lush), at which point a manufacturing jeweler who had one of my designs in her catalogue suggested that her silver brazier could do the job... It was a question I should have asked first: Who will build it? I gave up the idea of a bike of strictly rational design, proven in Finite Element Analysis, and bought a proven design in production (then) for nearly 70 years, which, ironically, uses non-standard Columbus custom tubes with custom lugs made in the same method I described above.
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