By Allen Penticoff
I’ve noticed that there are a great many more cars on the road lately with all-wheel-drive. I recently tested the Mazda CX-5 AWD version, and that got me to thinking I need to expound upon my experiences with all the various drivetrains I’ve lived with over the years.
A drivetrain is not the engineer who pilots a locomotive — no, it is the mechanical apparatus that mates with a vehicle’s engine and connects it to the tires to make it go. In the last 100-plus years, there have been quite a few drivetrains invented, and oddly, most of them are still around, albeit in much improved versions.
In the dawn of automotive transportation, what we call cars had tiny engines mounted in the back and were driven by chains — much like a go-kart, but not nearly as fast. This established the rear axle as the place to connect the engine to the road. The front wheels did the steering, much like a wagon of the then-current horse-drawn era of transportation. To do anything else would have complicated things immensely. Henry Ford’s first homemade car was of the horseless carriage variety with this sort of engine/drivetrain combination.
By the time Ford began producing the Model T in 1908 (of which 15 million would be built), it was pretty well established that the engine would normally be in the front of the vehicle, a transmission would attach to the rear of the engine, a rotating drive shaft would connect the transmission to a rear-mounted differential with two axles that would deliver the power to the wheels. It is a system that still appears in many new vehicles built today. We call it rear-wheel-drive (RWD or 2WD, I’ll use RWD).
There are essentially four different drivetrains, with some sub-variations among those. I’ve owned every sort, and still have all but one of them, only recently having sold my all-wheel-drive Subaru. In subsequent columns, I’ll spell out the nature of each system and its benefits and detractions. We’ll start here with RWD, then move on to front-wheel-drive (FWD), four-wheel-drive (4×4), then lastly, all-wheel-drive (AWD).
I am intimately acquainted with all these, so can speak from personal experience rather than just research, other opinions or test drives. In the case of RWD, I have a 1994 Mazda MX-5 Miata sports car and a 1990 GMC Suburban. With FWD I’ve had many, mostly Honda Civics. For 4×4, I have a 1979 Chevy Suburban and a 1980 Dodge Powerwagon. The AWD I’ve had were the 1997 Subaru Outback and a 1981 AMC Eagle. Three VW Campers have rounded out the rear engine — RWD contingent, including the present 1987 Vanagon camper. If you are considering buying a vehicle, new or used, what drivetrain system it uses is a very important consideration in that purchase. I’ll explain the economics of each as well.
Until it was challenged by the big wave of new imports in the 1980s, RWD was the dominant drivetrain system in the United States — little different from the Model T, except for the development of automatic transmissions and general overall improvements in components. Essential to all vehicles, no matter what drivetrain they have, is the inclusion of a “differential.” This component, using a special set of gears, allows one wheel to turn faster than the other without causing any binding. In a turn, the wheel on the inside is traveling less distance than the one on the outside of the curve — so one is turning faster than the other. The differential allows for this — but in a pure differential (and these are still quite common), if one wheel is on something slippery like ice, it will spin while the other wheel that may have good dry pavement does nothing. It is not broken, it is just its nature. A tip you can use if this happens is to lightly press on the brake while pressing on the accelerator. This stops the spinning wheel, while transferring power to the other wheel to get out of a bad spot. It works. And essentially this technique is used by electronic traction control (more about this later).
Most RWD vehicles are configured with the engine in front, and the axle in the rear — the problem being that the weight distribution puts most of the weight up front with the engine, and not over the drive wheels in back. The lack of weight over the wheels means less traction when things get slippery. It also means that the front will have traction when the back does not. This can result in a spin-out, when the back is going faster than the front. This is also called “oversteer” — when a vehicle turns more than you intend it to. It can be controlled by counter-steering, but this is on-the-edge driving.
For winter driving, the simple expedient is to put some weight in the bed of the truck, or trunk of a car and install good snow tires. Sandbags purchased at a home center work great. This will work on nearly any RWD vehicle. But over the years, some have harkened back to the very earliest days and put the engine over the rear wheels. Volkswagen and Porsche are probably the best-known makers of this style of drivetrain — and old VW Beetles have legendary good traction in snow. Porsche has been able to make this drivetrain work very well in high-performance vehicles as well. Rear-wheel-drive in general is the choice combination for high-performance and sport vehicles. BMW and others insist on RWD for their performance cars. It is a refined system that can be tuned very exactly to ride and handling demands. The Chevrolet Corvette is RWD.
Rear-wheel-drive is still the choice in most lines of trucks, from the smallest to the largest. RWD is the preferred system for towing and heavy load hauling. The weight shift rearward in all vehicles under acceleration aids in creating more traction, especially for towing. In many cases, whether for sport driving or heavy-duty, an option is to have a limited slip differential — sometimes called “positraction.” Both my 1990 Suburban and Miata have this feature. This will allow some of the engine torque to transmit to the non-spinning wheel and, therefore, keep you moving in limited traction situations. In most systems where this is present, it happens automatically, without driver input. It is a very worthwhile option or feature if considering a RWD vehicle — although there is some additional expense in maintaining it properly. Even most race cars now feature this as it reduces wheel spin that can induce a loss of control. What is called “traction control” on many new vehicles is found most often on front-wheel-drive, although it may be on some RWD as well. I’ll get into traction control with the FWD column next.
Rear-wheel-drive can be inexpensive to build, yet it weighs more than front-wheel-drive. It tends to be found in heavier luxury cars, SUVs and trucks as a result. It is not necessarily less efficient. Small cars with RWD can be quite efficient. However, most smaller new vehicles have moved to FWD for other reasons — leaving the less efficient larger vehicles with RWD.
In the end, environmentally, if you have RWD, you are polluting a bit more, because you are likely driving a larger car or truck; and whether you really need that, is, well, a personal choice.
From the Feb. 6-12, 2013, issue