Building Ultra Light-Weight Tubular Frame Vehicles, Part 1

DIY building of very light vehicles from steel tube

by Julian Edgar

Click on pics to view larger images

At a glance...

• Designing space frames
• Calculating relative tube strengths
• Chrome moly steel tube and plate
• Nickel bronze brazing
• Finishing processes
• Part 1 of 2 parts

This two-part series 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 the type of vehicles powered by human legs, a smallish electric motor or a small internal combustion engine, carrying only one person and perhaps 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.

Designing Tubular Frame Structures

For a non-engineer, by far the best way of designing tubular frame structures – and especially space-frames – is to build a model of the proposed design and then test it.

So how does the technique work? In short, you build a replica of the structure by soldering together copper wire, using a normal electronics soldering iron. You then input the forces in the direction that they occur in real life, and watch what fails.

This is a truly brilliant way of assessing different designs. By closely watching the failure mode (eg Close to a join? In the middle of an unsupported length of ‘tube’? In compression? In bending? Failure by a gentle radius bend or sharp kink?) you can very quickly optimise and refine designs.

Complex, multi-tube designs that would normally need Finite Element Analysis computer software or lots of engineering maths can be modelled in a short time – in some case, in minutes.

The modelling can be in two dimensions or in three. You can apply compression, extension, torsional and bending loads. You can model the affect of differing diameter (or wall thickness) tubes by altering the thickness of the wire, or by doubling the wire.

By photographing a full size mock-up, and then importing that pic into a PC and printing it out at much reduced size, you can take model tube angles and lengths directly from the print-out. In many cases, I actually build the model by laying the wire on the 2D print-out and soldering it in place in situ.

Forces can be applied by pliers with the model held appropriately by a vice or simple jig.

To see what you thought would be a strong design fail with the greatest of ease is an amazing wake-up call. To then make some (relatively) simple changes and develop a model that you physically can’t distort with the pliers is quite awesome, especially when you grab a piece of the bare wire and can bend it easily with just your fingers.

By juggling wire thickness and length, you can optimise the required relative strengths of the individual tubes: they need be only as strong as required.

By weighing the model (or measuring the length of its individual ‘tubes’ and working out how much it would weigh if built for real) it’s possible to quickly assess if weight is going up more quickly than tested strength. Alternatively, by setting yourself a weight limitation, you can to develop the strongest design possible with that weight.

By inputting forces with springs it’s possible to accurately model the different magnitude of input forces – for example, on a model of a vehicle frame, placing twice as much braking force on the suspension member than the static weight force.

Tube

Round section, thin-wall tube is structurally very strong. It’s strong in bending, in extension, in compression and in torsion. Therefore, if you cannot be certain of the magnitude of all the forces (and the direction of those forces) being applied in your design, round tube is best.

Using thin-wall, large diameter tube gives the best weight/strength ratio – much better than using thicker wall, small diameter tube. Let’s take a look.

If all that you are doing is comparing torsional stiffness of different tubes, the maths is surprisingly easy. There are two factors to take into account – the wall thickness of the tube and its diameter. Torsional stiffness is proportional to the wall thickness x diameter x diameter x diameter, that is, wall thickness multiplied by diameter cubed. Therefore, the stiffness of a 22.2mm x 1.2mm wall tube is 13,129 (units don’t matter in a ratio comparison). Increasing tube diameter to 35 x 1.2mm results in a stiffness factor of 51,450 – a stiffness that’s nearly four times as high!

Interestingly, the maths is also the same when comparing bending stiffness. Therefore, a good ‘feel’ can be gained for the relative stiffness of different tube diameters and wall thicknesses by doing just a simple calculation.

And it gets even better. Working out the relative mass of the different tube sizes can be found simply by multiplying the diameter by the wall thickness.

(Note: all these calculations are approximations that apply to only thin walled tubes. RS Edgar provided the engineering expertise.)

So what do the figures look like when a number of different tube diameters and wall thicknesses are compared to a 22.2mm x 1.2mm baseline?

 Tube Size (mm) Stiffness Stiffness Change over Baseline Weight Weight Change over Baseline Stiffness/Weight Ratio 22.2 x 1.2 (baseline) 13129 - 26.6 - 494 32 x 0.9 29491 2.3x 28.8 1.1x 1024 35 x 1.2 51450 3.9x 42 1.6x 1225 44.5 x 0.9 79309 6.0x 40.1 1.5x 1978

So in this comparison, the largest diameter, thinnest wall tube (44.5 x 0.9mm) is the stiffest of the tubes listed – and also has easily the best stiffness/weight ratio. In fact, it is four times better in stiffness/weight ratio than the baseline 22.2 x 1.2mm tube.

So, in general, go for the largest diameter, thinnest wall tube that can be used and obtained. The only caveat is that, to avoid wall buckling, the loads must be carefully fed into the whole tube, not just one part of the thin wall (we’ll come back to this point in Part 2 of this series).

Chrome Moly Steel

Normal run-of-the-mill mild steel tubing is cheap and widely available. It’s the sort of tube used in the making of furniture and other household goods; in larger sizes it is used to make vehicle exhausts. It’s available in different diameters and wall thicknesses. However, much stronger than mild steel tube is 4130 chrome-moly seamless tube. It’s also much more expensive and harder to source.

If you are familiar with only mild steel tube, seamless chrome moly is a revolution. Its strength is simply incredible. Furthermore, because it is seamless (ie there’s no welded join running along its length) it doesn’t fail by splitting - as most cheaper tubes will do. In addition, the metallurgy means that the tube has a lot more ‘spring’ in it – it can be distorted further and then still return to its original shape when the load is released. This makes it much more forgiving of temporary overloads that may occur in use.

The cheapest way of obtaining chrome moly tube is to buy good quality (but unwanted) bike frames and chop them up. However, this limits the length of straight tubes you will obtain, and the wall thicknesses become a bit of a lucky dip. Buying the tube new from specialist suppliers allows you to specify diameter, wall thickness – and of course, buy as much as you like (or can afford!).

Bending?

I have found that a small bench-mounted hand bender works quite successfully with chrome moly tube up to about 19mm (3/4 inch) x 0.9mm wall. A cheap bender may place a little dimple or two in this size tube (these imperfections can be filled by brazing) and will slightly flatten the tube at the bend. However, 12.7mm (1/2 inch) x 0.9mm wall tube bends perfectly.

Joining the Tube

As a home constructor, I have found the best way of joining chrome moly steel tube is to braze it with flux-coated, nickel-bronze rods. To do this you’ll need an oxy-acetylene welding set-up.

The most expensive part of oxy-acetylene welding is the on-going hire of the cylinders – these cannot be bought outright. However, in addition to brazing, oxy-acetylene equipment allows you to:

- Fusion weld - where you melt together metals of the same sort (eg steel), usually with the addition of small amounts of a filler rod made from the same material as the metals.

- Silver solder – similar to brazing but done at a lower temp and can be used on stainless steel, brass and steel

- Heat metals to allow them to be formed and bent, and also annealed and hardened

- Cut materials, including quite thick plate

So while the set-up and on-going costs are not minor, having oxy-acetylene welding gear allows you do a lot of things. But back to brazing chrome moly tube.

Most bicycles use lugs to join the tubes. These are female fittings into which the tubes push, and are then brazed or silver soldered into place. However, in a custom vehicle, it’s impracticable to make lugs for each tube join. Instead, I ‘fishmouth’ the tubes (that is, make a semi-circular cut-out of the end of the tube) so that the end of one tube fits over the wall of another. I then braze the join with the nickel-bronze rods.

In effect, this isn’t true brazing (where the braze fills a very small gap, often by capillary action) but instead ‘braze welding’.

Braze welding allows the joining metal (the braze) to be built up into a fillet, so the braze material can be made much thicker than the wall thickness of the tube. Good quality nickel-bronze braze has a tensile strength of about 58 ksi, while chrome moly steel has a tensile strength of 90 ksi. Thus, to put it in simple terms, if the thickness of braze is twice as thick as the wall thickness of the tube, the braze will be stronger than the parent material.

When I first started nickel bronze brazing 4130 chrome moly tube, I brazed two bits of scrap tube together, placed the assembly in a vice, and used a hammer to try break the weld. It proved very strong.

In testing of home-constructed human powered vehicles, I have suffered failures including chrome moly tube buckling (pictured), tube cracking and tube breakage. But I have never had even a single nickel-bronze brazed weld fail.

Nickel-bronze flux coated rods are a bit like the chrome-moly tube – not widely available. Prices also vary a great deal, so shop around.

Finally on the advantages of taking a brazing approach, it’s an easy welding technique to learn, and because it uses lower heat than fusion welding techniques, does not dramatically alter the metallurgy of the parent material. When prototyping, brazed tacks can also be relatively easily ‘undone’.

Removing the Flux

The flux that remains on the surface of the weld is difficult to remove. If you leave it there long enough, it will turn to a white-ish powder than can be wire-brushed away. In the short-term, you can use a pin punch and a hammer to shatter it off (it’s like glass – brittle and hard... so always wear safety glasses). The final traces should be removed by sandblasting (see below).

Sheet

In addition to thin wall tube, I often use 1.6mm chrome moly steel sheet. Like the tube, this is very expensive but again like the tube, it is immensely strong. It’s malleable enough to be able to be folded without cracking and is ideal for making brackets, plating over the mouths of open tubes and forming gussets.

Finishing

I choose not, as a matter of course, to go over all the welds with a sanding disc or file. (If the weld has a specific lump on it, I may just file that off.) Instead, when the construction is finished, I get the frame sand-blasted by a specialist blaster. Don’t get any old sandblaster to do this work; if the wall thickness is only 0.9mm then it would be quite easy for the blasting to reduce this to a critical thinness. Sandblasters are often familiar with the idea of ‘aircraft quality thin wall tubing’, so use that phrase when selecting a blaster.

I then get the frame painted in primer (the sand blaster can sometimes do this job as well) and then have that followed by powder-coating.

These ‘finishing’ steps can be quite expensive, so don’t get them done until you’re absolutely certain no more changes to the vehicle are needed. During testing and development, just spray the frame with a can of paint to prevent surface corrosion. The sandblasting will easily remove this, and – furthermore – act as a marker coat during the blasting.

Conclusion

Equip yourself with oxy acetylene gear, nickel bronze flux-coated brazing rods, a workbench, some hand tools and a vice, and you’ve got all the required gear. Add some large diameter, thin-wall chrome-moly tube and with good design, you can make a strong, stiff and light-weight vehicle.

Next week: we’ll look at some of the tricks and tips than can be employed to keep strength up and weight down