Making sense of the Regulations...

Now, I know that I'm supposed to make sense of regulations, or at least try and find all the useful loopholes in them, but the RACMSA 'Blue Book' makes about as much sense as a bucket of chocolate frogs. I'm trying to work out how small I can make the tubing for the spaceframe chassis, and depending on which interpretation of the rules you like. Section Q covers all the safety features you could or should fit on a car and paragraph 1.3 mandates CDS steel tubing, with a minimum yield strength of 350 MPa and either 45x2.5 mm or 50x2mm diameter and wall thicknesses. Yet paragraph 1.5.2 for Sports racing cars says a minimum of 48.3mm with a wall thickness of 2.6mm and 1.5.4 (b) for non-standard cars says 32x1.5mm. Which is right? I think we should be a Sports racing car rather than a 'non-standard' car, but there are significant weight penalties for using thicker than necessary tubing.

So if we look at the cross-sectional areas of the four options we get values of 333mm², 301mm², 373mm² and 143mm². The maximum compressive force that these members can take before starting to permanently deform (for those who remember A-level physics, the equation is of course Force = Stress x Area), varies between 50 kN for the lowest member and 130kN for the largest area. So which is adequate and which will give you a few short microseconds of terror before a basilar skull fracture puts you out of your misery for good?

The human resistance to acceleration was found out in nicely empirical fashion by Dr John Stapp of the USAF who strapped himself to a rocket sled and pulled 46.2G in a frontal deceleration. Using Newton's second law (Force = Mass x Acceleration), if we get our car down to the minimum weight of 560kg, we get an impact force of 254 kN. If that impact force is suddenly applied (and you can bet it is), the initial impact stress is actually double that (so effectively we can only use half the limiting impact force if we want no margin of safety). I'm vastly oversimplifying the reality, but SolidWorks doesn't come with a copy of DYNA3D or PAMCRASH, which is what the automotive industry use for this sort of thing.

Anyway, if we have two roll hoops, with extra front and rear stays, we'll have 8 struts resisting an impact force.
If we use the smallest diameter tubing the cage will yield under such loading, whereas if we use the largest tubing we have a factor of safety of around 2. Is it really worth saving weight if you get one of those once in a lifetime accidents - and you want to be around to tell the grandchildren all about it? I'd be inclined to use the lightest of the large tubing (which has other advantages, such as being less likely to buckle than the thinner diameter - so that's 50mmx2mm thickness round tubing.

For frontal impacts we can trade deformation for force - i.e. limiting the force and hoping the car stops before the metal tubing makes contact with our delicate feet. F1 expects a peak rate of deceleration of 10g for the first 150mm of deformation, and 20g for the first 60kJ or energy absorption. For our car 20g and assuming that our non-CDS metal tubing (so a yield strength that could be as low as 275 MPa) will yield under such an impact, we need an impact force of 110 kN and thus a cross sectional area of about 400mm². There will probably be 4 frontal force members here (and a nice big chuck of energy absorbing honeycomb to keep the force at those levels for at least the initial phase of the impact). So each tube needs a cross-sectional area of 100mm². So it's a case of running through the sizes to work out what tubing will work in these areas. A quick bit of maths lets me know that I need tubing that weighs around 780 g/m. In circular tubing that means 7/8"x16g, in square that means ¾"x16g and in rectangular we can use 1"x½"x16g.

I finally got out to measure someone's chassis and bodywork at the weekend and the picture at the top is a quick revision of the chassis, less the back end and using thinner tubing. With a 50mm diameter cage on the top, but using only ½" round tubing, the whole lot weighs around 34kg. The chassis was using 1" tubing and had a lot of extra tubes knocking around (it was originally a prototype chassis), so weighed considerably more. Even with a ramping up in tube size, I reckon we can get significant weight savings in place to get the car close to (or even below) minimum weight.