With the exception of some classes of karts and dragsters, just about every racing vehicle imaginable uses a form of suspension, and over the years a whole 'black art' of suspension geometry and set-up has been developed around two basically very simple components and the various methods of attaching them to the wheels and chassis. Firstly, some form of springing soaks up bumps and irregularities in the surface by allowing the wheel to which it is attached to move up and down vertically, while a damper or shock absorber controls the wheel rebound thus ensuring that uncontrolled oscillations don't take over and convert the car into a form of automotive kangaroo.
In recent years, understanding this complexity became even more important as engineers sought a stable platform that would not upset their carefully crafted aerodynamic flows over and under the car. As you will know from previous issues of Mach 1 News this is exactly the case with ThrustSSC and was one of the major challenges faced by Glynne Bowsher as he sought a suspension design that would meet Ron Ayers' aerodynamic requirements. It should come as no surprise therefore, that ThrustSSC features active suspension, although only on the rear.
Why? - well as Glynne Bowsher, the man behind ThrustSSC's mechanical engineering design explains, 'The Black Rock desert is very flat and smooth. When Thrust 2 ran there, vertical g forces of only 1/4g were the order of the day. Richard Noble likened it to driving a Rolls-Royce and in fact, the car suffered more on the transporter than it did while running!' The whole upper surface of the SSC will act as one big wing creating just enough downforce to maintain optimum aerodynamic trim. Glynne further explains, 'with a car this long and heavy with the centre of gravity so far forward, the non-steering front wheels can be fairly stiffly sprung using relatively simple, conventional components while the more complex active rear does the clever bit by keeping the angle of attack within Ron Ayers' stated parameters.'
As Glynne says, all very clever, but how does active suspension actually, work? Enter Jeremy Bliss, who has been involved with active ride since the pioneering days at Lotus when the likes of Ayrton Senna were getting to grips (literally!) with the benefits it could offer. His experience makes him the ideal candidate to ensure that it works on ThrustSSC and to explain to you how it does it. Jeremy takes up the story....
Active Suspension has become a buzz phrase in motor sport in retent years and is generally seen as either a panacea for all wrongs or an evil device which takes away the skill of the driver and makes racing dull. Unfortunately these opinions have been formed without really understanding what active suspension is or does. This is a pity, because active suspension is really very simple. In essence it can be summed up in one sentence. Sensors tell a computer about the world and the computer tells a series of hydraulic actuators where to put the wheels.
Now I'm sure that most people wouldn't be
happy with this rather glib explanation and would like a
little more. For instance, what the heck is a hydraulic
actuator. Well, since I am told a picture says a thousand
words, figure 1 shows a simple drawing of a hydraulic
actuator, thus saving me a thousand words on my trusty
PC.
When looking at the drawing, you will notice that as
the hydraulic fluid is forced in, the piston will move
downwards. This reduces the volume in the lower
chamber.
Reducing the volume of the lower chamber means fluid
will be forced out, hence the need for a return tank to
store this fluid. It is also fairly obvious from looking at the
picture that it is possible to apply pressure to the lower
chamber and open the top chamber to the return tank. In
this case the piston would be driven upwards. This kind of
actuator is called a double acting actuator because it can
be driven in both directions and is the type found in most
active suspension systems. The pressures used in the
actuators are very high and thus the actuators are very
powerful. In the case of ThrustSSC, the actuator can
move even when well over a tonne of force is applied.
Now that the mystery of actuators has been solved we must move on to look at how an active system finds out what is going on in the world. It achieves this miracle with sensors or as they are more commonly known in active suspension circles, transducers. There are several types of transducer used to measure a variety of inputs needed by the active system. To name but three, there are; LVDT's for measuring movement, Load Cells for measuring loads and Thermocouples for measuring temperture. All these devices give out an electrical signal which is proportional to the thing it is measuring and this signal is passed on to the computer, which then uses the information to determine what's going on in the world and move the actuator accordingly.
In order to describe this more clearly, it is probably best to go through one of an active suspension system's favourite tricks - stopping a vehicle rolling while cornering, but still maintaining a good ride. Again, things will be clearer if you begin by looking at a drawing. Figure 2 demonstrates what I am about to describe.
I'm sure you will agree that because of the
particularly high price of motor vehicle repairs, it is
probably best to follow the road around a corner when you
meet one! During this act of cornering you will notice a
side force acting on you, particularly when travelling
around sharp bends at speed. This force increases the
load on the outside wheels and reduces the load on the
inside wheels. In a normal car this would cause the
vehicle to roll a little, but in an active vehicle this can be
prevented. For those of you well acquainted with passive
vehicle suspension systems, yes I do know all about anti-
roll bars but for the sake of simplicity here, let's just forget
'em, OK? In an active system the transducers known as
load cells detect the load increase on the outside wheel
and the load decrease on the inside wheel. However, a
second transducer also detects the side or lateral force as
well. This transducer is called a lateral accelerometer.
The reason why this transducer is called an
accelerometer is really rather boring but trust me, it knows
about cornering loads. Both these bits of information end
up in the computer (called a controller if you want to use
the jargon). Now the computer looks at both these bits of
information and says, I can see an increase to the
load on my outside wheels and decrease of load on my
inside wheels. But, it also says, my lateral accelerometer
has detected cornering loads so this load I am seeing is
due to the vehicle going around a corner.
Therefore, I
won't move the actuators and so the vehicle will stay level.
Now this is well and good, but what about bumps or pot
holes in the road I hear you cry. Well, that's the clever bit.
The lateral accelerometer only sees sideways load, not
vertical loads so only cornering loads are removed. Any
other load seen by the load cell must be a bump or pot
hole so the computer orders the actuator to move
accordingly.
Well there you have it, active suspension in a nutshell. Hopefully by now, my rather short description should make a little more sense of it. If it still isn't entirely clear, try to think of an active suspension system in terms of your own body. The actuators would be the muscles, the sensors, or transducers, the nervous system, while the computer (controller) is your brain.
Right, now you can all go off and build you own systems and put me out of the job!
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