Geometry undressed.. Explanation part 2 chassis frequency (by Wim)

In part 1 we spoke about “us…humans”, our ability to drive the car and absorb our surroundings. Before we can fully appreciate what Geometry is, we need to understand what a chassis is. During the development of the chassis many factors need to be ascertained and expressed. At this stage we are only interested in two areas 1> Dynamic indexing 2> Natural frequency Dynamic Index is the output of Inertia Match Theory Inertia Match theory describes the relationship between a vehicles Centre Of Gravity position (COG), the vehicles wheel base and then it’s yaw inertia (mass squared). There is no voodoo, the hard truth of it is that a 50/50 weight distribution will always give the best Dynamic Index figure for a given yaw inertia. There’s no doubt about it that Inertia Match Theory is much more important for a vehicle design engineer who is working for a OEM manufacturer than it is to us working in the after market, this is because vehicle wheelbase will always be pre-defined and indeed set in stone by the time we’re actually dealing with a car. However simply by being aware of Inertia Match Theory and its various mechanisms we can gain valuable knowledge about a chassis by calculating it’s dynamic index. The dynamic index or DI gives us a unique insight into the way the chassis will behave when maneuvered on a ground plane at speed, armed with this knowledge we are better equipped to then go forward with tuning the chassis to suit any specific needs. Natural Frequency When we talk about chassis dynamics we often talk about a car chassis as though it’s a singular thing, but actually when trying to think about technical aspects of either driving a chassis or it’s design that’s actually incorrect and unhelpful. The car chassis is in reality just a collective term for a group of independent mechanical sub-systems. Most people are of course just interested in the driving so are only concerned with the net result of all of the sub-systems performing together as one. The unfortunate effect is that this makes it quite hard for these people to explain themselves clearly when talking to technical engineers who are in place to change the chassis set-up to suit them. Understanding each and every one of the individual sub systems certainly does go a very long way to help in understanding exactly how a car chassis works, but understanding how all of these sub-systems interact with one another really is the Holy Grail of chassis dynamics. So if we begin to think of the chassis in fact just as a complicated mechanical system we can take a look at other mechanical systems in general and start to apply the universal sciences and truths involved in systems engineering. To avoid going into a great deal of depth talking about harmonics I think it’s best now if we can just except for the time being that systems have a natural frequency (actually they have more than one, that’s a discussion for another time) and that these natural frequencies of the system are determined by the systems stiffness. The primary frequency of a car chassis will normally be very low and measured in Hertz or Hz for short, this description is historically used in science to indicate cycles per second. Ok now for the bit we actually care about, the effect of the chassis’s primary natural frequency. In real life this frequency dictates the minimum level of chassis correction or inputs per second required from the driver in order for the car to stay on its intended path. For example: A Rolls Royce might typically be set-up with a natural frequency of around 0.65hz, meaning that you can be very lazy when driving this car and it will still follow it’s prescribed path. A Mitsubishi Lancer EVO which is fairly typical of the aggressive sports saloon car market, might have a natural frequency of around 1.8hz meaning that the driver must give inputs at a rate of just under 2 adjustments per second, that’s quite a work rate. In the extreme a formula 1 car is typically set at 4.5-4.8hz frequency, that’s a MINIMUM of almost 5 adjustments per second! Humans and Hz Our perception of the chassis frequency leads to the topic covered in part 1. The Human template and the chassis is not regarded as absolute law, nor will any manufacturer satisfy all abilities/deficiency’s within our varied Hz tolerance. Example Customer one: I need coilovers because the car feels nervous and twitchy and I want it to feel planted and more stable. Customer two: I need coilovers because the car feels lazy and soft changing direction, and I want it sharper especially at the grip limit. Customer one probably doesn’t want to hear it but coilovers will make his situation worse, the reason his car feels nervous to him is quite likely because he’s struggling to keep up with the chassis. Any off-the-shelf coilover suspension kits tend to have stiffer spring rates and so will raise the natural frequency of his chassis. What this customer could really do with is a toned down Geometry set-up to start with. Customer two is absolutely barking up the right tree, a properly organized stiffer set-up will give him exactly the chassis change he’s looking for. Interesting Points: • Formula 1 car chassis are tuned right up the to the discomfort threshold of the human driver. At frequency’s around or over 5Hz the ligatures holding many of our internal organs in place simply lose control and with high enough g-forces acting at these frequency’s our organs can collide with the inside of our skeletons causing serious long term damage, not to mention a great deal of pain. • Why do you think most sports road cars like the Mitsubishi Lancer Evo’s are released to the public market with a chassis tuned to no higher frequency than 1.8Hz?