The Drivetrain - Front, Rear, All-Wheel, 4x4 - It's a Jungle Out There

Drivetrains Defined

By definition the drivetrain of a vehicle (car, truck, crossover, SUV) consists of the engine, transmission, universal (if needed), control arms, driveshaft and wheels.

That's it. No mystery at all, right? It does not matter whether the drivetrain pushes the car (rear-drive), pulls the car (front-drive) or uses all four wheels (all-wheel or four-wheel) some of the time or full-time, intelligently or dumbly.

This is the drivetrain and each type has its own handling characteristics. Here's a look at each type of drivetrain and a few thoughts for each. The information is sourced through the author's more than 30 years of auto writing and reviewing and Wikipedia.com.

• Front-Wheel-Drive (FWD): Actually developed more as a people-packaging device than as a real control device, FWD drives the front wheels through a transaxle. The transaxle combines the features of an axle and transmission. The transaxle drives the wheels through two half-shafts and constant velocity joints that act in place of the universal you find in a rear-drive vehicle. Although most folks rave about their front-drive vehicles, now, it took Detroit more than a decade to dial out all of the nasties that FWD was prone to. For instance, FWD exhibits oversteer, trailing throttle oversteer and tends to jump to the right on hard acceleration. Oversteer is a function of the fact that more than 50 percent of the vehicle's weight is over the driving wheels so that the wheels not only have to drive the vehicle, with this weight on them, they also have to steer and stop the vehicle. This puts lots of linear and lateral strain on the drivetrain. (Oversteer, by the way, is the tendency of a vehicle to tuck in during a turn where the front wheels follow the line you choose through the corner - it is like dropping line with a weight attached, the line, because it is front-weighted drops straight. If you try to turn it, you will not succeed. Similarly, in FWD, where 69 percent of the weight is over the driving wheels it is similar to the weighted line. The front end follows the line in which you entered the turn and if you try to change it, you risk having the rear wheels break free and tend to slide out. It's a situation that can cause a spin. Trailing throttle oversteer is caused by quickly lifting your foot off the accelerator during a turn. This, in turn, causes the rear wheels to try to snap free while the front wheels follow the line you are trying to drive. This, too, can cause a spin or skid and all you can do is countersteer out of it, which is made tougher by the weight over the driving wheels. FWD does have an advantage in starting up in snow, but, don't make any sudden moves or it's "oversteer time." By the way, FWD cars tend to jump to the right when you hit the gas. It's a function of lateral and driving forces on the front wheels. (FWD was developed by the auto industry as a way to maintain interior space while shrinking exterior size. FWD combined the transmission, front wheels and engine into one package under the hood and eliminated the need for a transmission/driveshaft tunnel and hump that ran the length of the car. This is why it is called a packaging concept.)

• Rear-Wheel-Drive (RWD): For years, the standard drive layout in domestics, RWD uses an engine that can be a four-, six- or eight-cylinder. It mates to a transmission bellhousing at the firewall and introduces a large hump into the vehicle. This hump contains the transmission and driveshaft and does take away from available interior space. The transmission can be a manual, multi-gear automatic or continuously variable automatic. The manual transmission adds a pedal to the pedals a driver uses, the clutch, as well as a real gearshift that controls the gear you are in. Most manuals, especially on high-line cars today, tend toward six-speed, short-throw. The most popular transmission (with the manufacturers that is because the tooling is in place) is the automatic. For years, the industry made do with three speeds and then began to add a fourth for economy. It then added a fifth speed as an overdrive gear for better mileage, as it has now added a second overdrive gear for the same reason so that you can have six-speed automatics. The continuously variable transmission (CVT) is also an automatic that stays in the optimal drive setting for a given speed and traction conditions. Unlike a standard automatic, the CVT relies on a series of planetary gears internally to maintain the correct drive settings. Interestingly, the RWD vehicle with its engine, transmission, driveshaft (needed to get the power to the universal joint in the rear where it is split between the rear wheels) and the rear axle itself tend to make RWD vehicles quite good handlers and are preferred by performance drivers as you can actually use the RWD to steer through corners and turns. You'll find that BMWs still use RWD, as do Mercedes-Benz models, both of which are known for their great handling and ride.

• All-Wheel-Drive (AWD); 4X4: AWD comes in several varieties. There's the standard brute 4X4 with its locking hubs, huge nubbly wheels and a body style that says it can take all comers in a bare-knuckle brawl. You will usually find this type of vehicle in a low-end work pickup truck that has to answer the call of the work site. You'll usually find a separate shifter on the floor for this style of AWD and you will have to turn the hubs about a quarter-turn to get them to lock into place. It's also best used sparingly as it will take a normally two-wheel-drive pickup that, when lightly loaded, tends to skip all over the road, and make it into a vehicle that will ride hard and fight you. If you want to tow with it, as most of these rigs usually do, you will have to add a skid plate, larger intercooler for the transmission and larger capacity cooling system, as well as a heavier duty battery and larger alternator. This is not the vehicle for the faint of heart. A step up from this is the more intelligent or automatic 4X4 system that does away with the need to lock the hubs and lets the engine computer system decide whether it wants to be in four-wheel-drive with an "automatic" setting. This pickup, normally seen in mid-level and higher-level pickups, also offers you automatic or manual 4X4, as well as two-wheel-drive capability (RWD), as well as low 4X4 settings. You'll find this on Chevy's Silverado, GMC models, and Ford pickups, for instance. You'll also find it on vehicles such as the SUV Tahoe or Suburban from Chevrolet and the other GM divisions. The more genteel crowd prefers the AWD systems you find in vehicles like the Lexus 300 Series, Nissan Murano, Ford Edge or Mazda CX-9 (actually the same platform). These platforms use intelligence built into multiplexed microcomputer systems and viscous coupling-style universal transfer systems to take the power generated by the engine and send to the automatic transmission and then on to the front driveshaft, viscous coupling universal and then on to the rear driveshaft and standard U-joint and determine which wheels need the extra traction. That this can be done is thanks to the wheel and transmission sensors that watch engine rpm and wheel slippage to move the power to the wheels that need it. A variant of this system is used on Honda's CR-V and Toyota's RAV-4. Both are essentially front-drive vehicles, that, when sensors tell the vehicle that the rear driveshaft has to be activated then turn into AWD vehicles. Both of these vehicles come with four-cylinders as standard and can be equipped with automatic or manual transmissions.

If you look closely at each description, it would seem that there are actually six or seven different drive systems available, but, really, there are only three. The complication comes when you add AWD (Ford offers a series of AWD Fusion sedans that are interesting handlers, and the same in the Taurus or Mercury Sable). At this point for the beleagured buyer it begins to seem as if there's a world of drive systems available when there really is not.

It simply boils down to the three types of drivetrain systems available, however, this simple equation becomes much more complex when you throw your basic salesperson into the mix. They, of course, would like to sell you the most expensive system they can and that usually includes not only AWD, but also GPS, climate control, heated seats and an upscale entertainment system. This is great stuff, but do you need it? Ultimately it depends on the type of driving you do.

Some Advice

In order to retain your sanity and keep your marriage in one piece - imagine planning to spend $18,800 for a nice car and coming home with one that costs about $10,000 more and explaining it to your wife. In this case, the best thing for you to do well before you get to the showroom is look at your total driving needs.

For instance, you will find that FWD systems have flatter floors and easier access than RWD vehicles . They also tend to be SUVs or larger family cars that can carry you and your kids and the family pooch on any trip you take. You'll also find they tend to be driven by four- or six-cylinder engines and have automatic transmissions. They do ride nicely, but firmly.

Do you really need all of this capability if: 1. you stay home on lousy days and telecommute via cable modem and cellphone, and 2. you just drive back and forth to work 10 miles from the house. You'd probably be better off with a standard FWD model.

If you want an upscale vehicle that can handle, you'll find that RWD is the ticket. Given the improvements in engine technology, you will find the mileage is very reasonable. Also, the standard package of your basic BMW or Mercedes or Lexus includes things like climate control, power windows, doors, locks, steering and brakes, as well as heated seats, GPS and more. The idea behind this type of vehicle is not only luxury but handling and they have also added AWD models that overcome some of the objections folks have had about RWD vehicles.

AWD is one of those "it depends," items. If you have a family you take to hockey practice or soccer practice and the weather is lousy, you'll probably want an AWD model like the Ford Edge or Mazda CX-9 or maybe even a Lexus or Land Rover. That choice is yours.

The key here is to do your homework and the rest will follow.