Paul P. Mealing

Check out my book, ELVENE. Available as e-book and as paperback (print on demand, POD). 2 Reviews: here. Also this promotional Q&A on-line.

Saturday, 19 July 2014

Understeer, oversteer and steering in general

I promise that this will be my last post on this subject. Well, I shouldn’t promise, but I don’t want to change the fundamental nature of my blog. Driving is a basic life-skill in modern societies and people die from it, therefore it’s worth a bit of print. In fact, people receive horrendous injuries, comparable to what one might get in a war, but few of us ever think about that when we get into a car, otherwise we probably wouldn’t.

Airline pilots go through training drills regularly, which is why they can cope with most things that try to spill them out of the sky. In the case of driving, which most of us do above a certain age, the only training we get, beyond how to operate a vehicle and the local road rules, is what we learn ourselves. I’m a firm believer that we should be teaching kids how to drive in schools, and teaching them so-called advanced driving skills like how to brake and swerve at the same time. I know at least one driver who is convinced that if she swerves she will roll the vehicle, so she’s probably the norm rather than the exception. I know another driver who won’t drive on the shoulder because the prospect scares him to death. I’ve done both on more than one occasion and avoided certain disaster on each occasion.

So what can I possibly write on a blog that may help? Well I can explain the dynamics of a car when cornering because that’s the essence of driving in my view. I believe a car should be an extension of your mind and body, because in some ways it is. You think and act which produces effects that change direction and change speeds, often at the same time, in response to visual and sensory stimuli. The sensory stimuli include your sense of balance, the strain on the muscles of your neck, the weight of the steering wheel and the pressure you feel on the brake pedal. What’s more, you can’t see the extremities of your vehicle, let alone where the wheels touch the ground, yet you can place it on the road with centimeter precision, out of sheer practice.

Of course, some cars do this better than others, and, unfortunately, a lot of cars are designed and manufactured to do the opposite: isolate the driver from the driving experience as much as possible, so they can indulge in the illusion that the car does the driving for them. I guess this makes me old-fashioned; I even drive a manual.

Most cars are designed to understeer when pushed because that’s what most drivers expect and what they are comfortable dealing with. Understeer is technically when the front wheels slip more than the rear and oversteer is the opposite when the rear wheels slip more than the front. In real world terms, understeer is when the front of the car runs wide in a corner and oversteer is when it feels like the back is trying to overtake the front, and, in extremis, can lead to the car spinning. Spinning is not so bad an outcome, by the way, because the car loses its energy and doesn’t go anywhere, like sliding into a tree or another car. I’ve seen people spin cars, unintentionally, and they came out unscathed. It’s also why racing drivers spin their cars, intentionally, when they lose control, to try and lose as much kinetic energy (speed) as quickly as possible. These days, most cars have ESP (electronic stability programmes) or some such acronym, so spinning a car may be next to impossible. I don’t know, I haven’t tried recently.

Getting back to understeer, the antidote is pretty simple: you take your foot off the throttle or apply more steering lock or both, both of which are the opposite to what created it in the first place, so it’s easy and intuitive to do.

Some cars are designed to be neutral or well balanced, which means, that under ideal conditions, they let go at the front and rear simultaneously. This is my own personal preference, because you can change it from understeer to oversteer or vice versa. You may ask: what could possibly be the advantage of oversteer? Well, mild oversteer, as opposed to snap oversteer, can help to point the car into the corner, and well-balanced cars facilitate this in a very non-threatening and confidence-building manner. In most driving circumstances, there are only 2 inputs involved in cornering: steering and throttle. Throttle allows you to adjust the car and, in combination with steering, you can finesse it around a corner without lurid slides or screeching tyres, just fluid and efficient progress that impresses people rather than scaring them.

Now, it needs to be pointed out that front-wheel-drive cars are often particularly adept at this steering on the throttle, as it’s called, and lift-off oversteer is possible. In other words, you may get dramatic oversteer from simply lifting off the throttle sharply, though, with the electronic intervention that all modern cars have, this is unlikely. Rear-wheel-drive cars do the opposite and can be made to oversteer with extra application of the throttle, but again, modern electronics, makes this unlikely in today’s cars.

Finally, I wish to point out something that is not generally spoken or written about, and that is that steering is one of those skills that the brain delegates to the subconscious, as it does other skills like walking, or hitting a ball with a cricket bat or a tennis racket or a baseball bat. Fingering skills that musicians learn also fall into this category so they become automatic and we can do them without thinking about them. In fact, the brain does this, out of practice, so it can think about more important things.

So I don’t believe that anyone thinks about steering when they drive a car around a corner – I know I don’t – I just do it. What I think about when approaching a corner is what gear I should be in, whether I brake or just lift off the throttle, so I’m only thinking about things that affect my speed of entry. I never think about where I should put my hands on the wheel or when I should turn in or even where I should apex the corner – I do all of that automatically. But speaking about speed, one of the worse things you can do is look at the speedo when you’re entering a corner – I’m sure a lot of accidents have occurred because of that – yet no one ever tells you. It’s like taking your eye off the ball. If you want to know what speed you’re doing around a corner, then look at the speedo on exit, not on entry. Also, possibly the worst thing you can do is enter a corner with a preconceived speed in your head, and have it on the speedo before you commit. You should be able to judge what speed to do around a corner without looking at the speedo – in fact, I think that’s fundamental.

Lastly (I know I’ve already said finally) a lot of ink has been used and many words spoken on the technique you should use for steering. There are 2 favoured methods: feeding the hand and the racing driver technique. I think there’s a place for both of them, but I have another which I evolved myself without any instruction. When I was learning to drive (in Oz) driving instructors were teaching what I call the shuffle technique, whereby student drivers were shuffling the wheel in short strokes in order to keep their hands on opposite sides of the wheel at all times. I was reminded of this recently when I was a passenger in a car where a woman of my vintage was doing a 3 point turn using this very technique. Now, it’s not her fault – it’s what she was taught, and because the brain delegates this to the subconscious she’s condemned to do it for the rest of her driving life.

What I believe these instructors were trying to teach was the ‘feed-the-hand’ technique, whereby we turn one hand over the top of the wheel - left hand for right turning and right hand for left turning – into the opposing hand which remains stationary. Thus, when we have applied the correct lock, our opposing hand is in the correct position to control the car. By correct position, I mean it’s ideally placed for maximum leverage and control which is on the side of the wheel. In fact, this is the best position to have both hands when we are driving straight ahead as well. In some cars there are little indents for the thumbs that facilitate this position when the steering wheel is in the straight ahead position.

And this is the position that’s advocated in the so-called racing driver technique, only they don’t change their position when they turn the wheel. I'm a firm believer in adopting the racing driver technique as a default position because it’s the best place to have your hands if you need to swerve. When you swerve, it’s always a reflex action and you don’t have time to change positions or move your hands on the wheel.

However, when approaching a corner, I move one of my hands over the wheel (depending which way I need to turn) so it automatically applies the right amount of lock when it returns to the default position (on the side) with wheel in hand. In other words, instead of feeding my hand, I grab the amount of wheel I think I’ll need. The difference, in practice, is that with feeding-the-hand, one hand ‘hands over’ to the other at some point in the process; whereas, with my technique, the handover occurs before you actually turn the wheel.  Now, I’m not the only one who does this, but no one taught me: it just evolved and I do it without thinking. It has the advantage that subconsciously I must intuit how tight the corner is as I judge how much lock I need before I enter the corner. Once I’m in the corner, my hands (both of them) are on opposing sides of the wheel which gives me best control. The only time I use the feed-the-hand technique is when I know I need more than one handful of lock, and I have to reach one hand over the other, which is the case for most suburban intersections.

There is one other advantage in a well-balanced or neutral car and that is that if it slides it will correct itself due to the underlying physics – the car will intrinsically seek neutrality. In other words, in an oversteer slide I will simply let go of the steering wheel and the car will correct itself. So why should a car slide? Well, it depends on the conditions, like mud or snow or slush, so I’m not talking high speeds. Even with electronic intervention, slides are possible if the conditions are diabolical enough.

I haven’t mentioned how important good tyres are – they are your lifeline – and how equally important it is to maintain their air pressure. I put air in mine about every 1,000km (600 miles) or every second time I fill up with petrol. They lose around 2-3psi in that time, so I put in an extra 2psi more than what is recommended by the manufacturer. Imagine how much you would care about your tyres and air pressure if you only had 2 wheels instead of 4.

Addendum: I need to say something about cruise control. In Australia, cruise control is very popular, partly because people use it to avoid breaking speed limits. Australia has the lowest tolerance to exceeding speed limits of probably anywhere in the world. Having said all that, I never use cruise control, because I have a psychological problem with giving up that aspect of the car’s control – I like to know I’m controlling the car’s speed all the time. I know that makes me an outstanding exception. The problem with cruise control, as I see it, is that we give up our sense of speed - we delegate it to the car - though I consider it to be essential to driving. By sense of speed, I mean we know longer make judgements about how fast we should be going, because we no longer are allowed to.

Addendum: Can I just say that probably the best book on this subject is How to Drive by Ben Collins, aka The Stig (from Top Gear). Unlike me, he's a professional driver. He was also a stunt driver for at least one James Bond movie.

Sunday, 13 July 2014

The Physics of Motorcycling

Since I wrote a post on the Physics of Driving (March 2014), it seems only logical and fair to write one on the physics of motorcycle riding. The physics is more complex and counter-intuitive, but it’s also more intriguing.

In both cases the driving force (excuse the pun) is gyroscopic dynamics, though, in the case of a motorcycle, it’s both more central and more controlling. I can still remember the first time I went round a decent corner (as opposed to a street intersection) on a motorcycle and felt the inherent weightlessness it generates. This is the appeal of riding a bike and what separates the experience viscerally from driving a car.

As I’ve already explained in my previous post on driving, it’s the muscle strain on our necks that tells us how hard we are cornering, whether we are in a car or on a bike, though the effect is reversed from one to the other. In the case of a car we lean our heads into the corner to balance the semi-circular canals in our ears, and our neck muscles subconsciously tell us what the lateral force is in a subjective sensory manner. In the case of a bike we lean our bodies and keep our heads upright - because we feel effectively weightless - but the strain on our neck muscles is exactly the same, even though it is reversed.

So that explains how it feels but it doesn’t explain how it all works. The physics is not easy to grasp, but the effect is relatively easy to explain, even if one doesn’t understand the dynamics behind it, so please persevere with me. There is a second law of angular momentum, which effectively says that if you apply a torque around an axis perpendicular to the rotating axis, you will get a rotation around the third axis, called precession. One usually draws diagrams at this stage to demonstrate this, but I can do better: I will give you an example that you may be able to perform at home.

A surveyor’s plumb bob works best to demonstrate this, but a bicycle wheel can work as well. Take a plumb bob with its string wrapped around it, hold it horizontally so the wound string is vertical, then let it go while holding the end of the string. As it falls the unwinding string makes the plumb bob spin about its horizontal axis, but when it gets to the end of the string, it doesn’t fall over.  It precesses, giving the impression of weightlessness. This YouTube video demonstrates what I’m talking about rather dramatically with a heavy flywheel, and its sequel demonstrates it even better, and explains the so-called weightless effect. And this video explains the physics concerning the 3 axes using an ordinary bicycle wheel on the end of a rope (which you may be able to do yourself) .

So what has all this physics got to do with riding a motorcycle? It’s what gets you around a corner – as simple as that – but the way it does it is completely counter-intuitive. To get the bike to lean over we apply a torque, via the handlebars, perpendicular to the rotational axis, only we apply it in the opposite direction to what we might think. Basically, if you push on the bar in the direction you want to turn, it will lean over in that direction. By ‘push’ I mean you push on the left bar to lean left and on the right bar to lean right. This is the counter-intuitive part, because we would think that if we pushed on the left bar the wheel would turn right. In fact, I’ve argued about this with people who ride motorbikes, but I know it’s true because, I not only understand the physics behind it, I put it into practice in over a decade of riding.

Now, when the bike leans over, it behaves exactly the same as the fly-wheel in the videos, and, under the force of gravity, the bike precesses around the corner, generating a feeling of weightlessness at the same time.

So that’s the core of the physics of riding a motorcycle but there’s more. In a car you can swerve and brake at the same time, as any advanced driving course will teach you. But on a bike you can do one or the other but not both. If you brake in a corner, the bike will ‘stand up’ and there is nothing you can do about it. This is different to simply closing the throttle, when the bike will tighten its line (turn tighter). Now, why this quirk of physics may seem catastrophic, it’s what allows you to brake in a corner at all. You see the bike will still follow the same curved trajectory while it’s slowing down, and it does it without any intervention from you except for the application of brakes.

The other laws of physics I explained in my last post, regarding the inverse law of speed versus rate-of-change of direction, and the braking distance following the speed squared law still apply. In other words, it takes twice as long to change direction at double the speed, and it takes 4 times the distance to brake at double the speed.