Last week in Part 1 we looked at the fundamentals of constructing an ultra light-weight tubular frame vehicle. In short, that included modelling the space frame design, using chrome moly seamless tubing that was nickel-bronze brazed together, and finishing with sandblasting and powder-coating.
This week we’re going to look at some tricks and tips for keeping strength up and weight down.
Again, this story is about building vehicles that, when unloaded, typically weigh less than 25kg - and perhaps a maximum of 50kg. When loaded, their total mass can vary from 100 – 150kg. So they’re vehicles powered by human legs, a smallish electric motor or a small internal combustion engine, carrying only one person and minimal luggage.
The same techniques can be applied - to a greater or lesser degree - to all light-weight tubular construction vehicles, but as the vehicle mass and power increase, so techniques will need to be altered.
Feeding Loads into Tubes
One of the most important ideas to get your head around when working with thin-walled tubular structures is that the strength of a tube relies in its full cross-section working at resisting the forces.
A tube placed in bending has one side in compression and the other side in extension – pretty well all the metal is 'working'. The same applies for a tube in torsion or compression – all the metal is doing something to resist the forces.
But contrast that with this diagram. The smaller vertical tube connects only to the lower wall of the larger diameter tube. Forces in the directions of the arrows will cause the wall of the larger tube to buckle. That’s because the loads from the smaller tube are not being fed into both walls of the larger tube.
However, by extending the small tube right through the large tube, and then brazing on both sides of the large tube, the loads are fed into both walls of the large tube.
That’s what’s been done here.
Note that a small internal tube can be used in some cases for the ‘wall to wall joining’.
However, what if the tubes meeting at right-angles are the same diameter? In that case, the larger ‘fishmouth’ that is made in the end of one tube automatically feeds loads into half the wall diameter of the adjoining tube.
If, however, even greater strength is required, a sheet metal gusset can be placed around the join like this.
Gussets are also useful in other locations, primarily because they spread the load along the wall of the tube. The load is still being fed into just one wall, but because the force isn’t concentrated in one spot, the tube’s wall can better resist buckling.
Pivot points are often needed for suspension, steering or frame folding. I think the best approach is to use sealed ball bearings – they’re available very cheaply, in a wide range of sizes, have high load capability, are light and are easily mounted. The latter often causes concern, but there’s an easy way to do it.
If 30 x 9mm (diameter x width) ball bearings are used that have a 10mm through-hole, it will be found that these fit very well into 32 x 0.9mm wall chrome moly tube. The bearings can be held in place by Loctite or, more often after the mounting tube has been brazed, it will be found that the bearings are a tight, push-in fit. A 10mm through-bolt can be used to connect to the pivoting assembly.
If greater strength is needed in the mounting tube than provided by 0.9mm wall of the tube, a sheet steel gusset can be used around it (as shown here), or a second wall thickness can be gained by slipping a tight-fitting tube over the top and then brazing the two tubes together.
The bearings are cheap enough (especially if bought on eBay) that some can be sacrificed by being kept in situ while brazing is done – this wrecks the bearing but keeps the tube round!
In many cases two bearings will need to be in-line, but relatively widely spaced. For example, this is typical on a suspension arm mounting. A single, long through-bolt can be used to pass through both bearings – but it often adds excessive weight. Another approach is much lighter and still very strong.
A long piece of threaded rod is selected. A nut is screwed a long way onto the rod, and then a section of small diameter chrome moly tube follows it. A second nut is then screwed onto the rod and the two nuts snugged up against the opposite ends of the tube. The nuts are then brazed to the tube and the threaded rod unscrewed from the assembly.
What we have then are two nuts, perfectly in line and mounted on opposite ends of the tube. This assembly can be located between the two bearings and then, rather than using a long through-bolt, two much shorter bolts can be used, one screwed each end into a nut.
If every gram counts, grind off the flats of the nuts.
When taking this approach, the threaded rod will normally need to be unscrewed while the assembly is still hot - practice in assembly and brazing will give best results. Pick the wall thickness and diameter of the tube to match the required strength, and in cases where higher strength is required, use the braze to build up fillets betwen the nuts and the tube.
Off the Shelf Fittings
Many bicycle-style frame fittings are available off the shelf. For example, ‘drop outs’ (the bits of steel that bike wheels bolt to) are available, as are steering headstocks, forks, folding frame couplings and so on. In some cases, this parts availability can save a considerable amount of work.
If nuts and bolts need to be attached to the frame, the easiest way is to use off the shelf nuts and bolts, brazing one component permanently in place.
For example, I needed a threaded spigot onto which a Firestone airbag spring would mount. (The spring has a threaded recess in it.) I sourced the appropriately threaded high tensile bolt, cut off the head, then brazed the threaded portion into a chrome moly tube of similar diameter to the bolt.
Similarly, nuts can be brazed into place over holes, allowing bolts to be screwed into them, with the threaded part of the bolt passing right through the nut.
A nut or a bolt can be attached in line at the end of a tube if a chrome-moly blanking plate is first cut out in a circle to match the internal diameter of the tube. A hole is drilled through the centre of the blanking plate and then the nut or bolt brazed to one side of the plate.
The plate is inserted into the end of the tube and brazed in place. This is the approach I take for suspension arms.
Clearly, if the loads are high, don’t just braze a nut to one wall of a tube and then expect the resulting assembly to be very strong - but if the loads are fed into both walls of the tube, quite good strength can result.
Brazing Thick to Thin
Brazing a thick piece of steel to thin tube can be problematic. The trick is to get the thick steel hot first – often the flame will not be directed on the thin tube at all. With the thick metal a dull red, apply the flux-coated brazing rod at the join. The braze will normally flow over both the thick and thin metals – the thin metals having been sufficiently heated by proximity alone. A good braze weld will appear the same in appearance, no matter what the relative thickness of the metals being joined.
Tube Mouth Caps
If, near its end, a tube is subject to loads that might cause it to crush or change shape (eg from circular to oval), a metal blanking plate must be brazed into place over the tube end. In effect, this acts as an internal gusset.
If, however, no such forces are present, it is lighter (and easier) to use a plastic plug to fill the hole. The difference in strength is major, so ensure you know what happening at each tube opening!
If a bolt passes through a sheet metal bracket, or the wall of a tube, you may want to strengthen the thin metal at that point. The easiest and lightest approach is to braze a washer over the hole.
If a bolt passes through a hole and its location must be accurately maintained, even when the bolt is removed and replaced, you can drill a hole and then ream it to size to suit the shank of the bolt. However, doing it in the following way is easier and also better spreads the load.
Select a piece of tube whose internal diameter matches the bolt. Select a sacrificial bolt with a tapered seat. Use the oxy to heat the end of the tube red-hot, then drop the bolt into the tube. Working quickly, use a hammer to force the bolt further into the tube.
The mouth of the tube will expand to have a female taper than matches the taper of the bolt. With practice, this procedure results in a perfectly symmetrical, well-formed recess.
Cut the tube to the required length and place it through the hole in the bracket or tube wall. Braze into place. Each time the bolt is inserted into the collar, it will line up on the same axis.
Don’t try to save weight by using the braze weld sparingly. As described last week, the braze should be at least twice as thick as the thinnest wall thickness of the material being welded. However, weight can be saved by:
Cutting all bolts to the minimum required length
Drilling holes in sheet metal gussets
Cutting off all tubes to the minimum required length (often best done for some tubes when the frame is near finished)
Specifying large diameter, thin wall tubes wherever possible
Ingenuity and a little bit of thought can go a long way in making durable, light-weight and strong structures. But the two most important aspects in the design and build of such vehicles are to model the structure (and then test the model to destruction), and ensure that all major loads are being borne by the complete cross-section of the tubes.
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