Driving Wheels: Introduction and Rear-Wheel Drive
By Carey Russ
In a motor vehicle, getting power to the ground is the name of the game. Traction caused by friction between the driven tires and road surface is what makes a car move. There are many ways to get the engine's power to the ground. Front-wheel drive, rear-wheel drive, all-wheel drive, four-wheel drive...what are they? Which is best? And what's the difference between all- wheel drive and four-wheel drive, anyway?
Let's take these questions one at a time. First, some basic definitions. Front- wheel drive (FWD) and rear-wheel drive (RWD) are simple enough to define. Front-wheel drive vehicles transmit power through the front wheels. Rear-wheel drive vehicles transmit power through the rear wheels.
All-wheel drive (AWD) and four-wheel drive (4WD or 4x4 - four (wheels) by four (driven wheels) are a little trickier to define. AWD and 4WD systems direct power to all four wheels - some of the time or all of the time. Broadly speaking, "four-wheel drive" refers to systems with a two-speed transfer case, designed primarily for low-speed, off-road driving. Often, only the rear wheels are driven in normal operation, with four-wheel drive selected only for extreme conditions. "All-wheel drive" systems are designed for full-time, all-speed, all-weather driving, primarily on paved surfaces, and so have no transfer case. There are many ways to do each, and, with the popularity of sport-utility vehicles and pickup trucks for everyday use, an increasing amount of overlap between the two. I will discuss AWD and 4WD more in a future column.
Question two: Which system is best? That depends on the purpose and use of the vehicle, and the available technology. Competition vehicles are also usually subject to design rules. A family sedan, a street-legal sports car, an open-wheeled road-racing car, and a rally car all have different missions in life and are designed for different conditions. What works for one does not necessarily work for another. Each type of drive system has advantages and disadvantages, which we will explore here and in future articles. But, briefly: In a family sedan or minivan, passenger space is the primary design consideration. An engine placement and driveline that maximize interior space are important. In the design of street-legal sports cars, coupes, and sedans, passenger space and comfort are of lesser importance than in a family car, and acceleration and handling are of greater importance. Engine placement and driveline reflect this. Competition vehicles are designed to win races. The type of race and regulations that the vehicle is subject to determine which wheels are driven and the engine placement.
All parts of a car are inter-related. Chassis and suspension design, engine placement, and consequent weight distribution are critical to the handling of a car, no matter which wheels are driven. Put simplistically, a car with more weight towards the front will be likely to understeer, or go off at a tangent at its limits of cornering. A car with rearwards weight bias will be likely to oversteer, or lose the rear end, at its limit. Understeering vehicles are easier to control - if the front starts to "push" outwards in a corner because of excessive speed, gently slowing will usually bring it back into line. Slowing down when control is lost is a driver's natural reaction. Doing that in an oversteering car can make the situation worse. Although weight distribution is important in determining oversteer or understeer, suspension design and placement of mass in the vehicle also are critical. These topics will be covered in other columns. For now, understand that all passenger cars are designed to understeer for reasons of stability and safety. Many racing cars also are set up to understeer, but to a far lesser amount.
The dominant system for both street and competition automobiles used to be front engine, rear-wheel drive. There were a number of reasons for this, including tradition, cost, and limitations in chassis and suspension knowledge and technology. The heavy engine was placed in front for reasons of stability, as explained above. The rear wheels were driven because it was easier and less expensive to do so, and also because rear-wheel drive offers the best traction under acceleration. With rear-wheel drive, the front suspension and steering do not have to deal with power delivery. For a passenger car developed on a tight budget, this means less cost. Advantages for more performance-oriented machinery include better steering and suspension response.
Before the advent of reliable, high torque capacity constant-velocity (CV) joints, independent suspension on the driven axles of a high-powered vehicle was an expensive and not particularly reliable proposition if done with U- joints and sliding, splined axles. Simpler swing axles had serious camber- change and hence handling problems. A solid rear axle is not the best form of rear suspension, but can work reasonably well if it is suspended and located correctly. Non-independent suspension on the steering (front) axle has many drawbacks, including poor reaction to bumps and consequent difficulty in control. For those and other reasons, early front-wheel drive cars were either relatively low-powered or specialized racing machines. (Front-wheel drive will be the next installment in this series.)
Rear or mid-engine rear-wheel drive layouts were rare. Most early cars of such design had reputations for treacherous handling, especially those with large, powerful (heavy) engines. The rearward weight bias of a mid- or rear- engined car can give it serious oversteer problems. (See the explanation of oversteer above. Read historical accounts of the pre-World War II rear- engined Auto Union Grand Prix cars for the best example of early mid- engined technology. )
Today, nearly all purpose-designed racing cars are rear-wheel drive, and most are mid-engined. Why? As I've stated, rear-wheel drive offers the best traction for acceleration. Under acceleration, weight is transferred backwards toward the rear wheels and away from the front wheels. Placement of the engine and transmission behind the driver adds increases the rearward weight bias, and, hence, traction. Careful suspension design compensates for oversteer. (Also note that most "rear-engined" cars are actually mid-engined, with the engine placed ahead of the rear axle. The Porsche 911 is one of the few cars that is truly rear-engined, and its' engine is very light in weight. A heavy mass behind the rear axle is a very bad thing for reasons of stability.) Placement of the engine behind the driver also can reduce frontal area and improve aerodynamics, particularly in relatively small single-seat, open- wheeled racing cars.
So. Rear-wheel drive has advantages for traction under acceleration for all vehicles, and packaging for some competition vehicles. Also, and most importantly, with rear-wheel drive, the steering function is separated from power delivery. The front wheels steer, the rear wheels are driven, and the suspension of each may be optimized for each specialized purpose. Are there any disadvantages to rear-wheel drive? Absolutely. Mid- and rear-engined passenger cars are few and far between because of packaging problems. Most such street-legal vehicles are two-seat sports cars. (and no, the parcel shelf in a Porsche 911 doesn't count as a seat for real people regardless of how Trans Am rules were once interpreted.) In order to maximize passenger space, the engine can be placed in the extreme front or extreme rear of a car. Most commonly, it has been placed in front. This puts more weight on the front wheels. With such as design, traction may not be a problem in good conditions. But in slippery situations, wheelspin can occur.
Rear-wheel-drive pickup trucks have extreme front weight bias, especially unloaded. I own a small, rear-wheel drive (two-wheel drive or 2WD in truck parlance) pickup. Soon after I bought it, I stopped at a stop sign on a wet, drizzly day. I slowly inched out into the intersection, checking for traffic. Seeing that it was clear, I put the truck in gear, put my foot on the gas, and promptly went nowhere thanks to the rear wheels spinning on the slippery paint of a crosswalk. A more gently application of power (low-tech, do-it- yourself traction control) solved that problem. A camper shell on over the pickup bed helped even more, adding weight over the drive wheels.
On the other hand, I once owned a Volkswagen Beetle. The Beetle's rear- engine, rear-wheel drive layout gives it excellent traction in poor conditions. Reasonable ground clearance, even in standard form, also helps the Bug's off- road ability. I drove my Beetle down many dirt fire roads and nominal 4- wheel drive trails to get to good fishing spots in the Sierra Nevada mountains. Years later, I've been on 4-wheel drive SUV introductions and realized that the road was familiar. Weight over the driving wheels allowed the Beetle to get through situations that might otherwise be impassable. The Beetle didn't have much power, but it did get it to the ground.