PAT SYMONDS
STIFFNESS: THE ANSWER IS IN THE RIGGING
We have discussed in this column previously the importance of ride quality even in a stiffly sprung racing car, but the current Formula 1 aerodynamic regulations have led to teams getting maximum performance by running the cars very close to the ground. This very limited ground clearance leads to needing extremely stiff springs to maintain the low ride heights under the immense downforce that’s trying to compress the springs and tyres, and push the plank and the skids into the track.
These very stiff springs lead to a very harsh ride. Now the total vertical stiffness of a car is not just a function of the suspension springs. Any vertical load, whether it comes from the aerodynamic downforce or bumps in the road, also has to pass through the tyre – and the tyre is in itself a spring. You’ll have noticed in your own cars how, if you park on a kerb, the tyre is compressed. This is because more load is passing through it. There is very little you can do to alter the spring rate of the tyre – in fact the only thing under the control of the teams is the tyre pressure and they want to run this as low as the prescriptions from Pirelli will allow, so they can optimise the contact patch.
So how does a team go about deciding how stiff to make the suspension springs to find the correct compromise between supporting the car close to the ground and yet giving it enough suppleness to absorb at least some of the bumps?
The answer lies first in modelling the suspension system and then in testing it on a sophisticated rig. The modelling is done by using a computer to solve the equations of motion of the system. Many will remember from school physics Newton’s second law of motion: that the acceleration of an object depends on the mass of the object and the force applied to it. This is an equation of motion, and, for a suspension system, we’re able to write a much more complex equation which sums all the forces and resistances from the inputs to the system and the suspension elements such as the springs and dampers. The equations are slightly complicated by the fact that the stiffness of the suspension is different depending on how much it’s deflected. Engineers call this non-linearity, but computers deal with this relatively easily.
