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Marketing, physics and busting the 50:50 weight balance myth

14 Nov 2016

Robert Pepper 11

Mazda, BMW, Jaguar and others like to talk about the ‘perfect’ 50:50 weight distribution. You shouldn’t believe them.

A 50:50 WEIGHT distribution means that half the car’s weight is on the front axle, and half on the rear. According to some manufacturers, that’s perfection:

And BMW agree:

It’s a simple idea, and it seems logical as that way each axle has to do half the work, and the car is balanced front to rear. Certainly ‘balance’ is a good marketing word for sports cars, it evokes all the right feelings, and you can also throw ‘equilibrium’ around too for a bit more pseudo-tech feelgood which is marketing defined, never mind the facts, get the right emotions flowing.

But like most simple ideas, it’s wrong, even if it’s dumbly parroted by journalists copy-pasting from yet another cliché-filled and technically inaccurate press release.

To back that statement up here’s an explanation, starting with a definition:  

It’s mass distribution, not weight balance…

Mass is the amount of matter in an object. Weight is the force that mass exerts. Here on earth, a “1000kg car” has 1000kg of mass and 1000kg of weight (approximately). On the moon, still 1000kg of mass but only about 165kg of weight.

When that 1000kg car corners left there is a weight shift from left to right, and when it brakes, from rear to front – say it was balanced 500/500kg rear/front, that may change to 400/600kg rear/front. The total mass remains the same, the weight remains the same too but the weight distribution changes. So what we need to talk about is mass not weight.

Now look at this image:

Yeah, not the finest graphics but that’s why I’m a tech ed not a marketer. Anyway, both cars have the same mass, and have a 50/50 front/rear mass balance. All else being equal, one would handle well and the other would be close to unmanageable, like trying to drive a pendulum.

If you doubt this, there’s a simple test. Grab two bottles of something, say wine, one in each hand. Extend your arms fully horizontally, then spin 360 degrees and stop, fast as you can. Then, pull your hands close to your chest and do the same thing. That will demonstrate quite nicely how mass distribution is more important than mass balance. 

And if you dare, go to Bunnings, grab some concrete and try driving these two around:

Both have a 50:50 weight balance, but it’s a practical demonstration of why it’s better to have mass centrally distributed than precisely equally distributed. Mass balance is about whether the vehicle is equal front/rear, mass distribution is where the mass is relative to the vehicle’s dimensions. Then we have this:

The two axles are not equal…

If a car steers and drives with both front and rear wheels, and spends all its time turning around a circle of constant radius, then the front and rear axles will do the same work and there would be an argument for a 50:50 mass balance.

But life isn’t like that. Cars will accelerate, brake, corner and transfer weight from left to right, fore to aft. The front wheels must steer, and possibly also drive. The rear wheels don’t steer, or don’t steer as much as the fronts. They may also drive.

In short, the work the back and front axles do varies constantly, and also varies according to the design of the car. Therefore, the idea that both axles need the same mass over them is invalid. If anything, an ‘ideal’ figure would probably be rear-biased for a rear-drive car, maybe 40:60 front/rear, as indeed most open-wheel racecars tend to be. Racecars and the highest performance roadcars and also tend to be mid-engined, so as to place the mass centrally.

When you brake you get a weight transfer to the front, so with a rear mass bias you don’t stress the front axle as much under braking. And when accelerating you’ve got that weight transfer to the rear, and already have some useful mass where you want it.

So far, we’ve talked about front/rear. But there is also:

Left/right mass distribution…

Unless you’re an oval racer that only turns left, an equal left/right mass balance is important. Arguably, left/right balance is more important than front/rear. If there’s more mass on one side of the car than the other then the suspension will need to be different, and there’s differing traction left and right under brakes and acceleration – not at all good for handling or dynamics. Yet you rarely hear much about left/right mass distribution. Subaru talk about it a bit, to their credit, with their symmetrical AWD system.

So what is important?

For a sports car, lightness is important above all else. There’s then simply less mass to accelerate and brake, so you can accelerate faster and brake quicker. The car goes around corners better and changes direction. And a central mass distribution, both front and rear and left and right is important. 

Then you have the suspension tune and alignment, a factor which has a huge impact on handling. The Mazda MX-5 gets this right – the fact I’ve used it as an example of a 50:50 distribution doesn’t take away from what a superb car it is, and certainly there’s nothing wrong with a 50:50 distribution – the point is that a 50:50 isn’t some sort of automatic route to perfection. And BMW make some of the finest sports cars on the planet. It’s just a shame those two feel the need to resort to this level of marketing to sell their cars.

Do not buy a sports car because it is touted to have a 50:50 weight balance. The performance of a car is dictated by lots of criteria of which mass distribution is just one, and performance is different to enjoyment. Just read honest, expert reviews and make your mind up based on those rather than tired old marketing claims based on misleading interpretation of physics.

Further reading

• When should I change up for maximum acceleration?
• Don’t bother with the power and torque figures