Weight distribution

Weight distribution /wayt dis-tri-byoo-shuhn/ noun (uncountable)

Weight distribution is the apportioning of a vehicle's total mass between its front and rear axles, commonly expressed as a front-to-rear percentage ratio. This is the dark art of balancing a car so it does not handle like a supermarket trolley with a wobbly wheel. It is arguably the single most important factor in determining how a car feels to drive, far more so than headline horsepower figures. The perfect, almost mythical goal for a performance car is a 50/50 split, achieving a natural, neutral balance. Many classic British cars, with their heavy iron engines at the front and virtually nothing at the back, often had a weight distribution closer to that of a thrown hammer, which made for some interesting moments in wet corners.

The Full Story of Weight Distribution

The physics of how a car behaves is dictated by where its weight is placed. The whole business of handling is a game of managing how the four small contact patches of the tyres interact with the road. The distribution of the car's mass determines how those tyres are loaded, and therefore how much grip they have. A car with too much weight at the front will overwork its front tyres, causing it to plough straight on in a corner in a dull, safe, and deeply frustrating phenomenon known as understeer. A car with too much weight at the rear can be even more treacherous, as the heavy tail can lose grip suddenly and swing around in a spin.

For most of motoring history, cars have had their engines at the front, which has made achieving good weight distribution a constant headache for engineers. The classic British sports car from the 1950s and 60s was a prime example of this compromise. With a heavy iron engine and gearbox up front, and little more than a flimsy fibreglass or aluminium tail at the back, these cars were inherently nose-heavy. The only way to counteract this was to place the driver and passenger as far back as possible, often with their legs stretched out alongside the gearbox. Engineers would also resort to clever tricks, such as moving the battery to the boot, in a desperate attempt to shift a few precious kilograms to the rear.

The purist's solution to this problem is the mid-engined layout. By placing the heavy engine and gearbox between the driver and the rear axle, the car's mass is centralised, achieving the coveted 50/50 balance. This concentration of mass also results in a low "polar moment of inertia," which means the car feels incredibly agile and eager to change direction. It is the layout used by almost every serious racing car and supercar, from the British-pioneered Cooper Formula One cars to a modern McLaren.

Some British manufacturers were particularly obsessed with the finer points of weight distribution. Bristol, a company run by engineers with an aircraft background, went to extraordinary lengths. On their cars from the 1950s, they famously placed not only the battery but also the spare wheel into compartments hidden in the front wings, just behind the front axle line. This was a classic piece of logical, if eccentric, engineering to "centralise the mass" and improve the car's balance and ride comfort, a world away from the cruder efforts of their rivals. It is this obsessive attention to the physics of feel that separates a truly great-handling car from a merely fast one.

For The Record

What is the ideal weight distribution?

For a performance car, a 50/50 front/rear split is considered the theoretical ideal as it provides the most neutral handling balance. However, a slight rearward bias, for example 48/52, can be beneficial for a rear-wheel-drive car as it improves traction under acceleration.

Does weight distribution change?

Yes, constantly. The static weight distribution is just the starting point. When a car accelerates, weight transfers to the rear. When it brakes, weight transfers to the front. When it corners, weight transfers to the outside wheels. How the car manages these dynamic weight transfers is key to its handling.

How does front-wheel drive affect things?

Front-wheel-drive cars are almost always front-heavy, as the entire engine and transmission unit is located over the front axle. This is actually a benefit for traction in slippery conditions, as the weight of the engine is pressing down on the driven wheels, helping them to grip.

What is "polar moment of inertia"?

It's a measure of how resistant an object is to being rotated. A car with its weight concentrated in the middle, like a mid-engined car, has a low polar moment and feels very agile, like a short stick that's easy to spin. A car with its weight at the ends has a high polar moment and feels more stable but less willing to turn, like a long barbell.

Why did Bristol put the spare wheel in the front wing?

To improve weight distribution. By moving heavy items like the spare wheel and the battery from the extremities of the car to a position inside the wheelbase, they were "centralising the mass." This reduces the polar moment of inertia and improves the car's agility and ride comfort.