Why mid-engined cars are special and the 50/50 myth
One of the most important fundamentals for car handling is weight, and distribution of that weight.
GET THOSE TWO right, and you’re well on the way to designing a great performing and handling vehicle. Get them wrong, and all you can do is mask the inherent problems, these days with electronics, and hope nobody notices.
And actually, it’s mass distribution, not weight. Mass is the amount of matter in an object, and weight is the force exerted on mass. So we talk about a car’s weight shift from side to side around a corner, but the mass of the car remains the same.
Given two identical cars other than mass, the lighter car will outperform the heavier in any performance test you care to name. It’ll accelerate and stop quicker, change direction more readily, hold a higher cornering speed, and even use less fuel.
Lightness is a virtuous circle. Energy is required to change velocity, so for the same performance, a lighter car can use a smaller, lighter engine than the heavier car, get away with lighter, less powerful brakes, a smaller fuel tank, and pretty much everything else. It can even run softer suspension as it won’t body-roll as much, and that means a softer, gripper ride compared to harshly-sprung heavier vehicles.
That is why mass is so critical for sportscars, and why big, heavy cars like Mustangs are at an inherent handling disadvantage compared to Toyota 86s or Mazda MX-5s.Imagine pushing an empty shopping trolley around a supermarket as fast as you can compared to one loaded with a fortnight’s shopping. It’s the same deal for cars.
Incidentally, acceleration from 0-100km/h is all about power to mass, so light cars with small engines do quite well. However, mass is almost irrelevant to top speed where you just need vast amounts of power to punch through the rapidly thickening wall of air as speed builds, so heavy, powerful cars often have high top speeds compared to light cars with better power-to-mass ratios.
Now a quick word on a myth. It is absolutely true that the heavier the car the greater the traction. If you want to pull say a 60-tonne trailer, then you may need to add 5 tonnes of mass to your 4WD towcar, otherwise, the tyres will just spin uselessly, which is why when carmakers use some 4WD or other to tow an airliner for a stunt they have to overload the car for traction.
However…that doesn’t work for handling, because the extra grip you get from adding mass isn’t offset by the amount of extra grip you need due to the weight. For example, when cornering, a heavier car will indeed have more grip than a lighter car, say 20% more, but it needs maybe 30% more grip to compensate for its extra mass.
So, light mass is the way to go and there’s no downside for performance cars. But having praised the virtues of lightness, here’s the caveat – mass distribution. Just as important as total mass is where that mass is distributed across the car.
Here’s a practical experiment. Go find two wine bottles, preferably full. Hold one in each hand, stretch your arms out straight and horizontal. Then, quick as you can, perform a 180-degree spin on the spot and stop. Then, clutch the bottles close to your chest and spin 180 again.
What you’ll find is that the second 180 will be a lot easier than the first. Yet, the total mass is the same. The reason is of course that you’ve shifted the mass closer in toward the centre of rotation, so it travels less distance and is easier to stop and start.
Now you see why mass distribution is important. You can have your 50/50 balance, but if the mass is at opposite ends of the car it won’t handle well, just like towing a trailer with heavy masses at either end as opposed to centrally located. You can even try that with your shopping trolley, too. And that leads us to the three main engine layouts:
The heaviest part of a car is the engine, so the more central you can locate the engine, the better. The car is able to change direction more readily (think back to the wine bottle experiment), and the centre of gravity (CoG) is typically such that there’s less weight on the front axle than the rear. That’s a good thing, because when a car brakes you get a weight shift to the front axle away from the rear, which means the front brakes do a lot of the work – that’s why they’re larger on the front than the rear.
An aft CoG with a mid-engined car means the weight distribution under braking is more even, and therefore all four wheels can contribute better to braking. And as mid-engined cars are rear-drive, then the mass is right next to the rear axle – the more mass on a tyre, the greater the grip. There are very few rear-engined vehicles and perhaps the most famous is the Porsche 911…and Porsche had to do a lot of work to make the 911 handle effectively as having mass behind the rear axle is not, from a dynamics perspective, an inherently good idea.
But mid-engined cars suffer significant disadvantages compared to the common front-drive layout. The flip side of the mid-engined car’s greater agility is less directional stablity (see right side of the diagram above), they can’t easily be configured with a second row of seats, and there’s not much storage space. The reason for the lack of stability and greater agility is because of that central mass which places the centre of gravity further away from the front axles, known as a smaller moment of interia.
Here’s the 4C Spider again on a weighbridge:
There’s no way you’d get a front-engined sportscar to have 60% of its weight over the rear axle. Incidentally, that’s why mid-engined cars tend to run ‘staggered’ tyres, where the front wheels are smaller diameter and/or width than the rear, because the front of the car is now quite light so why not save some mass and use smaller wheels – in the case of the 4C, the rear tyres are 25mm greater diameter and 30mm wider than the front tyres. And of course, the staggered layout looks racecar-good! Wider tyres at the rear also help with handling all the power the mid-engined car can deliver thanks to the mass nicely over the drive wheels.
Now does a mid-engined layout make a difference on public roads? Well, some people wouldn’t notice whether the car is front, mid, rear-engined, or which set of wheels it drives…but yes, there are noticeable differences. The mid-engined car is better able to change direction, less likely to wheelspin, and brakes better. Plus, they look cool and sound great!
So there’s your answer – lightness is glorious for handling, a central mass distribution means agility and you get that with a mid-engined layout; the 50/50 ‘perfect’ mass balance is a marketing myth and why small and light cars don’t have high top speeds. Now get out there and put the theory into practice!
Read our full Alfa Romeo 4C road test review.
- Busting the 50/50 weight balance myth
- When should you change gear for maximum acceleration?
- Triple the power but only 40% faster?