Step 3 – Boundary conditions
The boundary conditions for the tub are quite straightforward. You may recall that the entire back end of the car - comprising the engine, gear box and so on – is attached solidly to the rear bulkhead of the tub. Other bits and pieces such as electrical wiring, controls, and water pipes we can forget about.
Download this video clip.Video player: Video 8
Transcript: Video 8
Dr. Keith Martin, The Open University
The engine itself forms a structural member. So it’s the front of the engine which bolts firmly to the tub using six threaded fasteners. No rubber anti-vibration mountings on racing cars. The engine, of course, is hugely stiff, almost a solid lump, in fact. Thus, we can say that the chassis tub connects to an infinitely stiff structure at six mounting points.
We say that, under any load condition on the tub, the back end mountings are going nowhere. We assign them a boundary condition restraint of zero displacement in all three directions, x, y, and z. That’s restraining the tub.
The load is applied at the front end as equal and opposite moment arms, a couple in other words, acting through the suspension pick up points. The suspension itself is assumed to be very stiff, no spring resilience for this bit of the exercise. So, the chassis tub experiences a pure torsion due to the applied couple.
Video 8
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