What spins your wheels? Drivetrain technology explained

Mudfest 2013 Supercharged Range Rover 30

You know exactly what you want in your new car. You’ve chosen a powerful V8 engine, a peppy four-cylinder, an economical hybrid or a cutting-edge electric car, but there is still one more important question to consider: what kind of drive system should that engine be attached to? Rear-wheel drive? Front-wheel drive? All-wheel drive? 4X4? Whether your car is driven by its front wheels, rear wheels, all four wheels or something in between, there are advantages and drawbacks to each system.

First, some basics: a “drivetrain” is the combination of the engine, or in the case of an electric vehicle (“EV”), a “motor”, which powers the car, and a transmission, which uses the engine’s power to turn the wheels and put the car in motion. The engine and transmission are actually two separate and very different mechanical systems that are closely mated. Together they make up your car’s drivetrain, also referred to by some people as a “powertrain.”

This article is designed to give you a general idea of what to expect with each possible drivetrain configuration. Be aware that individual cars can perform very differently depending on how they are equipped and tuned. A Corvette and a Lincoln Town Car are both rear-wheel drive, but they obviously don’t perform the same way. Here’s why.

Power, defined

Each drivetrain configuration we’ll discuss here sends the engine’s power to the wheels in a different way, but how exactly do you measure a car’s power? When it comes to cars, power is generally measured in two ways: horsepower (measured in horsepower units) and torque (measured in pound-feet).

And yes, horsepower does have an equine origin. It’s actually an arbitrary figure that the inventor of the steam engine, 18th century Scottish engineer James Watt, devised when he wanted to compare his machine’s output to that of draft horses. It’s the equivalent of 746 watts (a unit of measurement named after the same guy) of electrical energy.

Horsepower is a form of energy. It’s what propels a car down the road, and what makes those heroic top speeds possible. However, it’s not what actually gets the wheels turning. For that, you need something called “torque.”

Torque is a twisting force; it’s the same force you use to open a pickle jar or loosen a screw. It’s also what gets a car moving from a standstill. That’s why you’ll often hear gearheads bragging about their cars’ “low-end torque,” and it’s why pickup trucks have so much of it: to accelerate quickly or get a heavy load moving, torque is what you need.

So a car’s ability to move is based on both horsepower and torque, but what do you do with it once you’ve got it? That’s where the drivetrain comes in.

Rear-wheel drive: The original way forward

The Ford Model T had its engine in the front while the drivetrain turned the wheels in back. At the time, the majority cars that followed did the same, with good reason. “Rear-wheel drive” is the simplest way to package a car’s drivetrain since the components which comprises the system that transfers the engine’s power to the wheels can be spread out across the length of the car’s underside. It’s also the best foundation for excellent handling. While that sounds like a pretty good deal, those same virtues can also be drawbacks.

Let’s start with the positives: Powering the rear wheels leaves the front wheels to deal with the steering and most of the braking. Asking the front wheels to also move the car – to do all three – can be very tricky, especially in powerful cars. Thus, rear wheel drive tends to be the favorite system for sports cars and their drivers.

This division of labor between front and rear wheels makes driving more fun. In very powerful cars with rear-wheel drive, a professional driver can actually use the gas pedal to help steer through a corner by spinning the rear wheels a little bit! This is known as “steering with the rear” of the car. Carefully modulating the power using the gas pedal affects the rear wheels’ traction, allowing the car to rotate slightly in the corner. It’s called “oversteer,” and it’s the magic behind “drifting” and all of the smoky sliding around in Hollywood car chases. But don’t try it at home.

The ability to lessen the wheels’ grip can be a bit of a problem if you’re not a stunt driver in a Hollywood movie. Since there is less typically less weight over the car’s rear axle, rear-wheel drive cars inherently have less traction than others (thus the ability to spin the wheels). That means when roads get slippery, wheels in a rear-drive system can more easily spin and the car can slide out of control. Modern safety systems like traction control help avoid this problem, but if you want to learn this advanced driving technique, take a professional driving class. Doing it wrong can cause a crash, collision or worse.

Of course, it might seem like putting the engine in back, over the rear axle, could solve that traction problem. Porsche has done it from the beginning, and the 911 is widely regarded as the world’s best all-around sports car. It’s also the only mainstream rear-engine car currently in production, because this layout creates its own handling issues. Remember the 1960s Chevrolet Corvair? The unusual handling dynamics created by its rear-mounted engine, among other things, inspired the ire of Ralph Nader, who wrote the book Unsafe At Any Speed and doomed the Corvair. But many people – including many skilled drivers – felt Nadar got it wrong and today the Corvair is a coveted classic car. Who was right? They both were, to some extent. Done wrong, a rear-engined car can be a handful to drive. Done right, as Porsche and Volkswagen have proven, can result in a car that is fun and safe to drive.

Rear-wheel drive also comes with some packaging issues. To give the driveshaft and rear differential (a gear mechanism that transfers power from the driveshaft to the wheels) enough clearance, a tall transmission tunnel running down the middle of the car is necessary, and it eats up some interior and trunk space. That’s where that hump in the middle of a rear-drive car’s interior comes from.

In essence, rear-wheel drive is the most fun but least practical way to go. Lack of traction may make driving difficult if you live in an area that has a lot of rain, snow or ice, but most of the best driver’s cars are rear-drive. It’s hard to argue with speed demons like the Ferrari 458 Italia and BMW M5, both of which are rear-wheel drive.


Front-wheel drive: The mainstream choice

The limits of rear-wheel drive led carmakers to look for an alternative, and they came up with the most popular configuration on the market today: front-wheel drive. The setup favored by the Honda Civic and Toyota Camry is the opposite of rear-wheel drive, and not just in the obvious way. While rear-drive offers driving pleasure for many performance cars at the expense of practicality, front-drive puts practicality first, and fun second.

An obvious benefit of front-wheel drive is better traction: since the engine sits over the driven wheels, there is more weight pushing them down onto the road. That makes front-drive cars easier to drive in winter or slick conditions.

Front-wheel drive is also more forgiving. It’s characteristic handling trait is understeer, the sensation that the front wheels aren’t turning as a car goes around a corner. When most people feel that they are losing control of their car, they instinctively let off the gas and steer. That knee-jerk reaction works best with front-wheel drive, but it could cause a spin in an oversteering rear-wheel drive car.

Powering the front wheels may be be safer, but it is also less enjoyable. Asking the front wheels to do the driving, braking, and steering is a tall order; people aren’t very good at multitasking, and neither are cars. Powerful front-drive cars have “torque steer,” in which the wheels are actually tugged in different directions by the engine’s power as the car accelerates. That is definitely not fun, and it’s why most sporty front-wheel drive cars are small, moderately powerful hatchbacks like the Ford Focus ST, Mazdaspeed3 and Volkswagen GTI.

Front-wheel drive is, after all, more about packaging than performance. Most front-wheel drive cars have a “transaxle,” which combines the transmission and front axle into one piece (hence the name), but functions the same as normal rear-wheel drive system. In addition to being more compact than a separate transmission and axle, a transaxle also allows front-drive cars to have their engines mounted sideways. That allows for a smaller engine bay, leaving more room for the passenger cabin. The lack of a transmission tunnel and rear differential also increases interior and trunk space – there’s no hump in the middle of the car’s interior.

2013 Audi R8 front three-quarter viewAll-wheel drive: The best of both worlds?

Powering just two wheels, whether in front or in back, clearly has limitations along with their advantages. So what about powering all four?

There is more than one way to do this. Carmakers generally refer to “four-wheel drive” and “all-wheel drive” as different things, and there are, in fact, two different systems. The first and original system, four-wheel drive, widely known as “4X4,” involves redirecting power from the transmission to both the rear and the front wheels via a mechanical device called a “transfer case.” This is the preferred system for off-road vehicles, such as Jeeps, the type that usually come with “4×4” decals. This is what people are usually talking about when they use the phrase “four-wheel drive.”

From the driver’s seat, four-wheel drive vehicles feel decidedly old school. Most 4X4-enabled vehicles require drivers to shift into four-wheel drive manually with a lever mounted next to the shifter, or on luxury off-road vehicles, there’s a special button to push. Most of the time, vehicles with a 4X4 system only use two-wheel drive to get around, until the going gets tough. Since they’re designed primarily for off-road driving, 4×4 vehicles also rely on low gearing to improve and manage traction. If you’ve ever ridden a mountain bike over varying terrain, you know that being able to change gears can really make things easier. It’s the same deal with 4x4s: their low gearing can limit acceleration and dull handling, but they are also able to surmount obstacles that would strand vehicles without a 4X4 system.

Once out of the rough terrain, most vehicles with 4X4 ability can switch back to two-wheel drive and be driven normally. Here’s some 4X4 action in a Jeep and a Range Rover:

The second and more popular option is a variation on four-wheel drive managed by electronics commonly referred to as “all-wheel drive” or AWD. Again, a transaxle powers the front wheels with a second output shaft sending power to the rear wheels. Because it doesn’t require a low-hanging front differential, this is the preferred setup for road-going cars and crossovers like a Subaru Forester.

The computer-controlled all-wheel drive systems on new cars and SUVs allow them to be driven like any other car: you just get in and go. There are usually no buttons to push or levers to pull, the AWD system is “on” all the time. Computers monitor wheel speeds and can send power to wheels that have the most grip on the fly. Consequently, all-wheel drive cars and SUVs tend to handle more like their two-wheel drive counterparts on the road. Engineers can vary the front-rear power split, providing different characteristics for different driving situations. Open road driving might favor more power to the front wheels for easy cruising and better gas mileage, while driving in snow will have all the wheels working to keep traction, all without any changes being made by the driver. Different cars use AWD systems in different ways.  The family crossover SUV might do fine sending most of its power to the front wheels, but for supercars like the Audi R8 or Lamborghini Gallardo, more power to the rear wheels results in better acceleration.  It is a flexible type of system.

Are there any all-wheel drive drawbacks? All-wheel drive systems do add weight and complexity to a car, and the engine needs to work harder to turn all four wheels as needed. That means fuel economy takes a hit, and base prices for AWD versions of cars that have it as an option are higher than their two-wheel drive versions. Buyers also have to be in the market for the right kind of car: other than SUVs, some luxury sedans and most every Subaru, the choices are also limited but you might be surprised to see what kind of cars you can now get with AWD.

But wait, there’s more.

Electric motors and a more simplified approach

Hybrids and electric vehicles (EVs) are bringing a new type of all-wheel drive to the market: they can use electric motors to directly power individual wheels within the wheel itself, no driveshafts, transfer cases or complicated transmissions required.

The Mercedes-Benz SLS AMG Electric Drive is a battery-powered version of Mercedes’ top sports car. It replaces the gasoline version’s single V8 engine and conventional drivetrain with four electric motors. Not only do they give this electrified sports car the balance and grip of all-wheel drive, the four in-wheel motors can also be used to brake individual wheels in corners, tugging the car into the proper cornering line. Here’s a video of the SLS AMG EV in action showing what’s possible with electric motors in each wheel.

The multiple motor approach can also work well with a hybrid car. The Porsche 918 Spyder is definitely the most outrageous plug-in hybrid ever made, and it uses two electric motors; one to drive the front wheels and another attached to its 4.6-liter V8 engine at the back of the car. The electric motors are computer controlled and also allow the car to be driven purely on electricity, giving the 918 amazing gas mileage for such a high-performance car. The upcoming Acura NSX (below) uses three motors: one for each front wheel, and one tag-teaming the rear wheels with a 3.5-liter V6 gas engine. The future is wide open when it comes to how many ways electric motors can be utilized in cars.

Brits allowed to place pre-orders for the up-coming U.S.-built Acura NSX

In the popular Tesla Model S electric car, a single watermelon-sized electric motor is positioned between the rear wheels. Because electric motors produce power over a much larger range than gas engines, most electric cars do not have a transmission in the usual sense. That is, there are no gears to change because the electric motor is connected to the drive wheels nearly directly, with perhaps a simple reduction gear between the motor and wheel. To go fast, the electric motor simply spins faster, no extra gearing is required. The ability to produce cars without a complicated transmission reduces weight, cost and complexity for automakers and also results in a very quiet and smooth ride, something most electric cars are known for. The drivetrain in most electric cars is really very simple in comparison to gas-powered cars and could lead to enhanced reliability and reduced repair costs for owners.

This begs the question: With so many motors and engines crammed into one vehicle, how do you figure out the horsepower of a hybrid or electric car? The easiest way is to add the output of the engine and each of the electric motors; carmakers call this “total system output.” Take the Porsche 918 Spyder mentioned above: it’s gasoline engine produces 608 hp, it’s front electric motor produces 127 hp, and its rear motor produces 154 hp. That brings the grand total to an amazing 887 hp!

Of course, the power output of electric motors is usually measured in kilowatts (kW), not horsepower. How do you convert kilowatts into horsepower? Simply multiply the number of kilowatts by 1.341 (i.e.: 100kw x 1.341 = 134 horsepower).

It should also be noted that, in hybrids, the electric motors can only provide power when their batteries are charged, so all of that power may not be available at all times. Hybrids often have different drive modes that prioritize all-electric driving, fuel economy, or performance by varying the combination of gasoline and electric power being fed to the wheels. These systems vary from model to model.

If you have a question about drivetrains, let us know in comments.

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