Dynamic Instability. A scientific explanation of driving ‘fun’

by | Jun 4, 2018 | Motoring

Reading time: 5 minutes

Dynamic stability is a very interesting principle, one that applies to moving objects and analyses their behaviour when their motion is disturbed. The principle essentially states that a moving object is dynamically stable if it spontaneously goes back to its original status after modifications from a third party force.

Too heavy duty before your coffee? Okay, let’s look at a specific example.

Take a supermarket trolley. If you pull it back towards you and someone bangs into the side of it, the rear wheels will oscillate, but so long as you continue the pulling, the oscillations will diminish and you’ll continue moving in a straight line. If, however, you push the trolley in a conventional fashion and the same action is deployed, the oscillations will increase until the trolley starts to spin, forcing you to counter-act this direction-change through the handle bar.

Push rather than pull a trolley then, and you have what is called dynamic instability, laws of physics that the motor industry is very familiar with. Indeed, it’s one of the basics engineers and project managers consider when the sheet is still blank, given that the choices they make will deeply affect the final result.

Automotive dynamics 101: while front-wheel drive cars send power from the engine only to the front wheels to increase stability, rear-wheel drive examples send power rear-wards, meaning the front wheels need only control direction change.

The difference between front-wheel and rear-wheel drive cars

Typically when a manufacturer is designing a family car for instance, and deciding to which wheels power will be sent from the engine, they will normally consider front rather than rear-wheel drive. A FWD car, after all, is pulled along by its front wheels. It’s a setup that gives the machine an innate stability, even if external forces disturb its momentum. Should the driver temporarily lose control of the back end of the car, pressure on the gas pedal instantly brings the car back into a straight line, much like pulling the shopping trolley back towards you brings it back into a straighline.

“Dynamic stability is one of the basics engineers and project managers consider when the sheet is still blank, given that the choices they make will deeply affect the final result.”

Is it fun? Sure, it can be. Front-wheel drive cars give their drivers confidence, playability and safety in equal measure, but others say that rear wheel drive is a much more entertaining architecture. Formula 1 cars for instance (and in fact all single seaters) are rear wheel drive, likewise most sports and super cars, the added thrill being that the driver has to ‘catch’ a RWD car should they lose control, in much the same way you have to counter-act the shopping trolley when you push it. If the back end of your rear wheel drive machine looses grip and starts drifting, it requires a balanced mix of gas and counter-steer to correct the temporary dynamic instability. Stand on the accelerator and you only increase the oscillation, and more than likely, end up in a complete spin.

This, inevitably, makes dynamic instability more entertaining, and, quite honestly, more fun.

An example of ‘the dance’ you have with a rear-wheel drive sports car. Oh, and did we mention we also drove the brand new Aston Martin Vantage recently? [Cough]

Why is dynamic instability more fun?

If you have ever driven a rear wheel drive car before, chances are you will have experienced what happens when the back tries to turn around you. You will have learnt how to ‘dance’ with it – not without shivers and thrills – because your reaction is paramount to the final result: react too slowly and you’ll end up in a spin (and that’s the best case scenario), but counter-steer just enough to correct the drift, bringing the car back onto your desired line, and you’ll be rewarded with glorious controlled oversteer. Your skill will be proven and tested, and thrills will be had.

“Front-wheel drive cars give their drivers confidence, playability and safety in equal measure, but others say that rear wheel drive is a much more entertaining architecture.”

And that makes sense. A part of human nature is living with the need for a constant adrenaline rush. Some get it with extreme skiing, flying, diving, and/or gambling. Some are fascinated by, and draw pleasure from, riding motorbikes to their limits, or finding a sports car’s sweet spot, and when you have this sort of passion – mine is very much the latter – you choose your car by the level of fun and satisfaction ‘she’ could bring you. You compromise on fuel consumption, practicability, price, space and sometimes looks simply because you crave ‘the dance’ with her. Yes, it will challenging, but it will also be fun and extremely rewarding.

It’s like attracting the best-looking girl in the class: you know she’s a bit spoilt and has a difficult character, but you also know that if you can tame her, the rewards will be massive, unforgettable, and an inebriant. She is not balanced, she is not stable, but her qualities are second-to-none, and you’re not looking for a boring relationship.

…seriously, check out our review of the brand new Aston Martin Vantage!

The effect stability controls have had on driving ‘thrills’

Such a thrill is not possible in a dynamically stable front wheel drive car, and with the advent of Electronic Stability Control Systems, it’s getting increasingly more difficult to find ‘the thrill’ from dynamically instable cars. Unless they’re turned completely off, stability controls for instance will monitor the car’s weight shifting, the rotation of the wheels, the throttle position, and the steering angle hundreds of times-per-second to determine the dynamic stability of the car and intervene to keep you safe if/when you’re about to step over that line. It’s a specific decision taken by manufacturers to create safer cars, and this can take a lot of fun out of the rear-wheel drive experience. True, these technological marvels have probably saved more lives than a good medicine, but stability is not fun, and science knows very well about it. Dynamic Instability has the DNA to create fun but only because it requires more skilled intervention from the driver. You need only look at drifting competitions, or at the incredible maneuverers perfected over the years by international rally drivers for proof of that.

At the end of the day, it’s about personal taste. Thrill is subjective. The good thing is that if you are passionate about driving and the thrill it can give you, science has decided to give you a precise choice: you can ‘go safe’ with front-wheel drive stability, or choose the rear wheel drive ‘fun’ option and learn how to tame it. Push or pull the shopping trolley. It really is as simple as that.

If you enjoyed this Dynamic Stability article, you can check out more Motoring stories HERE