Even if you have only a passing interest in cars, you know the fast ones usually have their engines behind the driver. When handling is a priority, putting the engine behind the passenger compartment, but ahead of the rear axle, is ideal. A mid-mounted engine has been a defining element of an exotic road car since Lamborghini installed a V-12 sideways just behind the passenger compartment of the Miura in 1966. But plenty of attainable sports cars are mid-engined, too, from early examples like the Fiat X1/9 to today’s everyman supercar, the C8 Corvette. To explain why mid-engine cars handle so well, we need to talk physics. MOTOR101 Welcome to Motor101, where we explain the burning car questions you’ve been afraid to ask. From all the different types of hybrid systems to, yes, why enthusiasts and engineers go crazy over mid-engine cars, we’ll cover it all. Tell us what you want to learn about in the comments. First, A Primer On Weight Transfer If you ever go to racing school, one of the first things you learn about is weight transfer. When a car is sitting parked or moving in a straight line at a constant speed, its weight is static. But that weight doesn’t stay static, because, well, your car doesn’t stay static. When a car accelerates, brakes, and turns—or does some combination of accelerating/braking and turning—that weight shifts around. Under acceleration, the weight shifts rearwards; under braking, it shifts forwards; when cornering left, it shifts rightwards; when cornering right, it shifts leftwards. This is called weight transfer. We all understand weight transfer implicitly because we experience it all the time. But it’s something you need to know deeply to be fast. The tire is your and your car’s only interface with the road beneath you, and the more weight—or vertical load—you put onto a tire, the more grip it has, up to a point. For an extreme example, imagine a dragster doing a wheelie off the line. The wheelie is the result of all that weight shifting violently rearwards under hard acceleration. Now imagine trying to turn the steering wheel when the front tires are off the ground. The car isn’t going to turn. That’s an extreme example of understeer, where the rear tires have more grip than the front tires; oversteer is the opposite, where the front tires have more grip than the rears; neutral steering is, of course, all four tires with an equal amount of grip. So what does all this have to do with mid-engine cars? Handling Balance: Oversteer, Understeer, Neutrality, & You Good drivers understand weight transfer, and so do good engineers. Moving a car’s major weights around changes the static and dynamic weight distribution, and thus, the car’s handling balance. The heaviest component in a car—well, an internal-combustion car—is the engine. I probably don’t need to tell you that most cars have their engines up front because long ago, we decided that it made the most sense to put the powerplant in a small box up front, and the people and cargo in one or two larger boxes out back. Depending on where you put the engine up front, you can end up with a lot of weight over the front tires. A basic hatchback might have 65 percent of its static weight over the front, while a BMW with its engine tucked behind the front axle proudly touts a 50:50 front-rear weight distribution. Some front-engine performance cars move the transmission to the back for a slight rear weight bias, too. With a front-engine car, very generally speaking, you’ll have a more understeer-biased handling balance, because with more weight over those front tires, it’s easy to overwhelm them. And in a rear-wheel drive car, you have less weight transfer to the driven wheels under acceleration. Moving a car's major weights around changes the static and dynamic weight distribution, and thus, the car’s handling balance. I’m speaking in generalities, but it’s clear to see the advantage of a mid-engine car. With a mid-engine, rear-drive configuration, you’ve got lots of nice weight over the driven wheels for acceleration, but good stability under braking and in cornering. Which is why it’s a popular layout for high-performance and racing cars. (A note on rear-engine cars, like the Porsche 911: You get great traction under acceleration and stability under braking, but the significant rear weight bias promotes understeer, while the pendulum effect of the heavy engine at the back can lead to snap oversteer. Over time, Porsche has shifted the 911’s weight balance more forward, making it feel a bit more like a mid-engine car.) While there was some pre-war experimentation with mid-engine cars, most notably with Ferdinand Porsche’s Auto Union grand prix cars, the layout didn’t really catch on until the 1950s. Porsche had the 550 Spyder in sports-car racing, while English constructors Lotus, Cooper, BRM, and the like pioneered mid-engine open-wheel cars in Formula 1. The "rear-engine" revolution came to Indianapolis in the early 1960s, and now, you’ll very rarely see a purpose-built, front-engine race car anywhere. Polar Moment of Inertia: Now With Dumbbells There’s another factor at play here—polar moment of inertia. In a car, you want to get all the mass as centralized as possible because that makes it easier to turn. I’m going to shamelessly crib an example from Ross Bentley’s Ultimate Speed Secrets, which is a book you should own. Imagine holding up a barbell with its weights out on the extreme ends, versus one with weights close to your hand. The latter is a lot easier to turn. Because the masses are closer to the middle, it has a lower polar moment of inertia. You can get a low polar moment of inertia with a front-engine car, of course, and there are cars that have relatively high polar moments of inertia that handle well. Think Honda Civic Type R or Porsche 911, for example. But if you’re starting from a clean sheet of paper, you want that mass centralized. And the easiest way to do that is by putting the engine just behind the driver. In a car, you want to get all the mass as centralized as possible because that makes it easier to turn. This, in turn, explains the particular brilliance of the Porsche Boxster and Cayman. Not only are they mid-engine, but their boxer engines are both short in height and length, which promotes a super low center of gravity and a low polar moment of inertia. It’s also why some mid-engine cars with long engines, especially V-12s, can be tricky to drive. There’s also the pendulum effect of a big, heavy engine in the back. Now, there is considerable and ongoing debate about whether a car that oversteers, understeers, or is perfectly neutral is fastest. The debate won’t end because different drivers prefer different things. Really fast drivers might want a car that’s loose (oversteer-y) on corner entry because it helps rotate the car into the turn, but neutrality to a little bit of understeer in mid-corner and on exit is easier to control as you add power. Most road cars, even mid-engine road cars, are tuned to be somewhat understeer-y because it’s safer and easier to control for the average driver. This might sound like a bad thing to an enthusiast, but most drivers will be quickest and safest in a car that’s just a little pushy. Weight Distribution Vs Engine Location Now, there are so many complicating factors. A four-cylinder Porsche 718 Boxster, for example, has a 45:55 front: rear weight distribution, which isn’t all that different than a Mercedes SLS AMG’s 47:53, thanks to that car’s front-engine, rear-transaxle layout. The mid-engine Lotus Emira V-6 has a weight distribution of 39: 61….which is more rear-biased than a current 911 GT3’s 40: 60. Consequently, the mid-engine Lotus handles like an older 911. The engine is often the heaviest component in a car… but there are other heavy parts, and engineers cleverly move stuff around to get whatever weight distribution they’re targeting. And while weight distribution, both static and dynamic, is so, so important, there are so many other things that affect vehicle handling: Tires, suspension components, driven wheels, differentials, sprung vs unsprung masses, center of gravity, etc. Oh, and the driver. A ham-handed leadfoot can push a car into terminal under- or oversteer, no matter where you put the engine; a good driver knows how to work all four tires to their maximum at all points. All of this is to say, mid-engine cars are not the exclusive purveyors of good handling, nor will getting into a mid-engine car automatically make you a better driver. But putting the engine behind the driver and ahead of the wheels has a lot of benefits, which is why so many cars use this layout when handling is a priority.
