How should we make our roll hoops?
Now that filler & streamlining are being banned, there is suddenly an unpleasant compromise to be made between aerodynamics and safety.
It seems inevitable that the rules will eventually stipulate exactly what tube must be used (diameter, wall thickness, material etc).
Independently of the current furore over this, CAUC have been setting up a series of material tests for rollbars (we should be ready to do some violently destructive testing this coming week :) but as the subject is of current interest it seemed like a good idea to get ideas from the forum before destroying all our test structures.
First up - what do the professionals say....?? here's a quote from FIA for sports/rally car roll structures.
FIA Appendix J 2009, Article 253, p14
8.3.3 Material specifications
Only tubes with a circular section are authorised.
• Minimum Material: Cold Drawn Seamless Carbon Steel
• Minimum Yield Strength: 350 N/mm2
• Minimum Dimensions (Ø mm):
•45x2.5 or 50x2.0 (main rollbar or lateral rollbars)
•38x2.5 or 40x2.0 (lateral half-rollbars and other parts of the safety cage)
The tubing must be bent by a cold working process and the centreline bend radius
must be at least three times the tube diameter. If the tubing is ovalised during bending, the ratio of minor to major diameter must be 0.9 or greater.
The above is for cars exceeding a tonne at speeds of over 100mph, and which might fly down ravines (any number of these incidents on youtube) so our requirements are considerably less. The structure generally has to withstand a car rolling over, not landing upside down from a great height.
Furthermore, I remember the old MSA blue book specifying that the roll structure inside an amateur rally car should withstand the weight of the vehicle on any node without taking a permanent 'set' - that's the weight of the vehicle, not 2 or 3 times it.
OK, back to details. FIA, MSA both say round tube. not square. Both say use cold drawn seamless carbon steel (that's DOM in USA).
The 'impact' loading on a structure can be described in 3 ways:
1) static loading. For us, let's say the car weighs 150kg. Nobody is going to argue against static load capability of 1500N
2) Impulse test; collision imparts momentum. M*v = force integrated wrt time. I'm going to assume a free fall of 300mm of the complete 150kg car, that's 364Ns
3) energy of collision: in our 300mm case that's m*v^2/2 = m*g*h = 441J
As this is clearly a low speed but high momentum impact, our test is designed to get as close as we can by dropping a large weight from a low height. We have procured an old 56lb weight (that's about 25kg) and I've made a release mechanism to drop it in a controlled way. We've made a massively strong jig into which sample rollbars can be fixed by wedges, and we'll do the tests on the concrete base of some fire escape steps round the back of the school.
If we drop the weight 1m, this would be an impulse of 111Ns and energy = 246J
If we drop the weight 2m, this would be an impulse of 157Ns and energy = 492J
I expect we should be able to do the drops accurately and consistently at 2m.
We have 4 test rollbars made from pieces of tube I was able to beg/borrow/steal, I'll give details of these next installment (one of then is electrical conduit - I don't expect the results to be pretty)
Please give me your thoughts on the validity of what we're trying to do & anything you would change. And if you want a test doing on a sample rollbar PLEASE send us one!!!
I might make a couple of solid rollbars just to see how they behave... that must be best for aero for a given level of strength...
Final thought - I actually want the rollbar to deflect to some degree, I want it to absorb the energy of the impact, rather than pass it directly to the chassis. Less than 50mm though.... :)