Which is quickest around a corner – a single seat race car or a double-decker London bus?

The answer is pretty obvious. Leaving aside such details as aerodynamics and power-to-weight ratios, the bus just looks as though it would tip over at anything much above walking pace. It doesn’t take a degree in physics to know the tall, narrow bus is less stable than the low, wide race car. But explaining why requires a grasp on the concept of “center of gravity.” Here's a quick overview from DAS Demo.

The center of gravity can be thought of as the balance point of an object. In physics terms, it’s the point about which forces can be considered to act, and if you want to know how to calculate it, there’s some guidance on NASA’s website. Gravity is one such force, but it isn’t the only one that acts on a car.

Take a corner at speed and you’ll feel a force pushing you sideways in your seat. The same force acts on the car’s center of gravity. If you were driving on ice it would make the car continue in a straight line, and perhaps into a ditch, rather than going around the corner. In normal conditions though, the grip from the tires resists the sideways force and you get home safely.

The problem is the center of gravity is usually higher than the wheels, so as the sideways turning force pushes one way, the grip from the tires pushes back lower down. This creates a turning “moment” or torque that wants to tip the car over towards the outside of the curve.

Torque is force applied at a distance. As the distance from the ground gets bigger, so too does the torque. When we look at the London bus we instinctively estimate the position of the center of gravity, and because the bus is so tall we assume it’s far above the road. As a result, we visualize lots of torque tipping the bus over as it takes a curve.

Much to the relief of Londoners, this is a very rare occurrence, and the reason is London buses are designed with a very low center of gravity. Because most of the weight, particularly the engine and transmission, is very low in the chassis, the bus has to lean a long way – 28 degrees, according to one rather old movie that shows bus testing -- before it will topple over.

Race cars are designed on similar lines. For maximum speed through corners, designers place all the weight as low as possible. Then, to make it even harder to tip, they spread the wheels as far apart as the rules allow.

Few of us drive London buses or single seat racing machines, but the point about a low center of gravity holds true for regular sedans, trucks, and SUVs. The lower the center of gravity, the better the vehicle resists cornering forces that want to tip it over. So if you’re driving a relatively tall vehicle, bear in mind it’s more inclined to roll than a low sports car. Physics dictates the forces, but your safe driving can keep you from ever having to worry about tipping!