• Driving Physics

    • An instructor grabs the wheel, tugging it tighter into the turn while bellowing "More gas, give it more gas!" as he coaxes students out of their comfort zones.
    • The advanced driving course includes wet driving
    • High-speed cornering and emergency braking.
    Date:31 March 2009 Tags:

    Driving Understanding the science of car control could save your life

    We’re standing on Midrand’s Kyalami race track as a 200 kW BMW 330i bullets towards us at 120 km/h – that’s 33 metres every second. Hearts pounding, we watch it approach the mark that signals the driver to stomp on the brakes. Our job: to line up opposite where we think it will screech to a juddering halt.

    Boom. We’re all out. Way, way out. The car thunders past us, speed bleeding away almost imperceptibly until, with a last violent dip of the nose and chirp of the tyres, it finally comes to rest maybe 40 metres away.

    How could we be so wrong? We’re not alone in our miscalculations, either. Every day, Gauteng drivers clock up 330 collisions. Many of them are fatal. Estimated cost to the State’s coffers: R8,5 billion.

    We’re not suggesting there aren’t other factors at play. Drivers with fake licences, road rage and poor road surfaces add to the problem.

    But learn to know your car’s dynamics, and you'll tilt the odds in your favour.

    “Every motorist should be aware of the laws of physics applicable to their cars, and experience these laws under the right kind of guidance,” says Abdul Dangor, general manager for BMW’s Driver Training Academy. Dangor and his team of internationally accredited instructors are on a mission to create safer roads.

    I wish I’d been taught what these guys know when I was learning how to drive. The advantages of attending one of their training courses cannot be understated. Each course lasts a day; the range includes defensive, anti-hijacking and advanced driving (the one I picked). Our advanced driving course was broken up into segments: theory in the morning, time on the race track, and emergency braking before lunch, followed by an afternoon skidpan session.

    From the vehicle to the road

    A car’s driving behaviour hinges on four contact points about as big as the palm of your hand. Yep, we’re talking tyres. Also known as “grip area”, these contact points are where the circumferential forces act lengthwise on the car when braking and accelerating, and the lateral forces infl uence the sides of the car when cornering. Under normal conditions, forces are conveyed by positive friction, with the tyres and the road surface getting along just fi ne. However, when the dynamic driving forces exceed a certain level, the tyres lose their grip and start to slip – causing the car to slide and possibly swerve out of control.

    Kamm’s Circle is a model that illustrates the distribution of forces on a wheel and the interaction between dynamic driving forces and friction. Basically, it represents traction (the bigger the circle the more traction and vice versa). All four wheels are represented. As your vehicle turns, the shift in the car’s weight distribution determines the size of all four circles. When a wheel moves out of its circle, it’s skidding. Whether you’re driving a front-wheel drive or rear-wheel drive makes no difference.

    Simply put, when accelerating the vehicle’s weight effectively “shifts” to the rear. The circles representing the rear wheels grow, and those representing the front shrink.

    Under braking, the vehicle’s weight shifts to the front wheels, providing more traction up front.

    In a bend, centrifugal forces work on the vehicle’s lateral plane to shift its weight to the outside wheels, giving them more traction than the inner two. Wet and icy roads also influence traction, as do tyre condition and construction.

    Cornering principles The grip on our tyres performs three functions rolled into one – steering, braking and acceleration. During a turn, 60 per cent of the grip on the front wheels goes into the turn; this leaves us with only 40 per cent for acceleration or braking. If you are already committed to the turn and then hit the gas hard, you’re going to go one way – and that’s straight.

    The golden rule of successful cornering is to keep it smooth. Having chosen your cornering line, before actually starting to turn, ensure that you’re at the right speed to negotiate the corner. You’ll probably need to brake, increasing braking force until you reach your chosen “turning point”.

    Now is when you take your foot off the brakes. This allows your vehicle’s suspension to settle, giving your wheels more traction, before you turn the wheel.

    There are two basic cornering methods:

    Constant radius. This is the one we’re the most familiar with. You enter and leave the bend at the same speed, following the curve exactly.

    Variable radius. You move to the outside of your lane as you approach the bend. On reaching your turning point (and releasing the brakes, naturally) you turn across towards the inside of the corner, to your “clipping point”. The straighter driving line allows you to accelerate out of the corner.

    Understeer and oversteer

    When cornering, lateral acceleration causes a build up of centrifugal forces that “push” the car to the outside of its lane. Generally, these forces are balanced by the lateral stability maintained by the wheels. As soon as the centrifugal forces exceed the car’s lateral stability, or a bend tightens in response to the steering, the front wheels are no longer able to follow the steering angle set by the driver.

    When a car in such a scenario tends to want to run wide, it’s known as understeer. Front-wheel drive cars tend to understeer. Not only are they front-heavy, but in addition to that transmitting power through the front wheels reduces the grip available for cornering. Whether you’re driving a front-wheel drive or rearwheel drive, the remedy for understeer is the same – simply tap off and straighten the wheel.

    Oversteer is when the vehicle turns too sharply and the rear wheels lose frictional contact with the road surface. Correct it in a front-wheel drive, by applying more power in combination with countersteering into the slide (turning the wheel towards the direction of the slide). In rear-wheel drives, disengage the clutch to remove power from the rear wheels, and counter-steer.

    Hitting the brakes

    Contrary to popular belief, cars don’t stop very well. Ironically, at speeds up to 60 km/h cars without anti-lock (ABS) brakes stop quicker than those fitted with ABS. Above 60 km/h, a car without ABS cannot steer while braking; a vehicle fitted with ABS can. ABS also makes stopping much safer in wet conditions.

    When driving at 120 km/h, you cover 33 metres every second and it takes about 45 metres to stop. At 160, your speed is 44 metres per second and your stopping distance is about 100 metres. You do the maths. Oh, and that’s not even taking into account driving at night, when you can’t see further than your headlights. Even in congested traffic, when typical speed is about 30 km/h, there are risks. With a reaction time of one second (distance travelled: 9 m) plus stopping distance (4 m), you’re expecting too much with a following distance of only one car-length (5 m). Still wonder why there are so many fenderbenders during rush hour? Driver position It should go without saying that seating position is crucial to “feel” the grip on the wheels and reacting appropriately

    Seat semi-reclined and arms outstretched in a racer-boy pose (1) may benefit your centre of gravity, but that’s about all. In a head-on collision, chances are you’ll “submarine” under the seatbelt. This position also doesn’t give you enough control of the vehicle during violent manoeuvres, and when swerving you can get flung out of the seat. Plus, your airbag becomes ineffective. Sitting too close to the wheel (2) is also bad news. You can’t exert sufficient leverage on the steering wheel in a crisis, and you can’t turn the wheel through 180 degrees to avoid nasty surprises.

    Here’s how to sit correctly. (3)

    Your clutch pedal leg should be slightly bent. This prevents kickback from the pedals during a collision from shattering all the bones from your foot to your hip.

    •  The backrest should be as straight as is comfortably possible.
    •  Stretch your arms out and rest your wrists on top of the steering wheel. Then slide them down to the 3 o’clock and 9 o’clock positions.
    •  Line up the middle of your head restraint level with your ears and eyes.

    Besides helping you control the car better, seating position ensures that airbags and seatbelts work properly.

    Which is as good a point as any to reflect that at the end of the day, we all walked away safer drivers. We’d also learnt how to drive in a way that minimises wear and tear on our own vehicles.

    We did learn one other thing: pushing a 330i hard around Kyalami is a hell of a lot of fun…

    Based at the Kyalami racetrack in Midrand, the BMW Driver Training Academy has its own dedicated skidpan and uses the track for high speed, performance vehicle instruction. Providing training in both front- and rear-wheel drive vehicles, they have a wide variety of vehicles for attendees to chose from – namely sedan, hatchback, coupé, roadster and compact. For more information, contact the BMW Driver Training Academy on 011-466 2710 or visit www.bmwdrivertraining. co.za

    Newton’s laws of motion are three physical laws that form the basis of classic mechanics, directly relating the forces acting on a body to the motion of the body. And yes, they are as applicable to moving vehicles as they are to how the planets in our solar system revolve around the sun or how satellites make it into space, and stay there.

    Newton’s first law

    Newton’s First Law (a.k.a. the law of inertia) states that an object in motion stays in motion, and an object at rest stays at rest, unless acted upon by an outside force. Inertia is the resistance an object has to a change in its state of motion. All cars have inertia because mass is the measure of inertia, and all cars have mass. The same goes for us. If you crash your car into something, your body will resist the change in motion, and unless something stops it first, your body will crash into the object at the same speed that the car did. Seatbelts and airbags help protect us from this one.

    Newton’s second law

    The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is the same direction as the net force, and is inversely proportional to the mass of the object. In other words, net force divided by mass is equal to acceleration. Meaning: small vehicles have less inertia, so they can accelerate easily. Large trucks have more inertia and require more energy to move – so they need more force to experience the same acceleration or stopping power.

    Newton’s third law

    Also called the law of reciprocal actions, it tells us that when you push against something it pushes back on you with an equal and opposite force. A force is a push or a pull, and friction is a force – it acts to resist the relative motion of objects or materials that are in contact. It’s difficult to drive on wet or icy roads because the tyres cannot grip the road, as there is little or no friction. If the car can’t push on the road, the road can’t push the car. As a result, the car slides.
     

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