Pros and cons of all-wheel drive, front-wheel drive, etc.

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Which wheels should do the driving is a surprisingly contentious topic among engineers and enthusiasts. People who treat their cars as appliances don't seem to care so much as long as the weather is good.

Rather than going through strengths and weaknesses for each type, I will focus on a few critical areas that are the biggest areas of difference between these different technologies.

Weight and space efficiency: these two are inter-related, since the most important factor for most cars is the amount of interior space. In this area, front-wheel drive cars reign supreme. The original Mini made the point that if the engine and transmission are crammed into the smallest possible area, you can fit a surprising amount of room into a very small car. A rear-wheel drive car with the same amount of interior room will be larger and heavier than a well-designed front-wheel drive car. An all-wheel drive car will be heavier yet all that extra equipment generally adds at least two hundred pounds to a car versus a two-wheel drive equivalent.

Fuel efficiency: Front-wheel drive and rear-wheel drive have similar mechanical efficiencies. However, front-wheel drive becomes the technology of choice for delivering fuel efficiency due to its inherent space efficiency. All-wheel drive is not a good choice for fuel efficiency. In addition to the additional weight, all-wheel drive is less efficient mechanically. This means an AWD vehicle will need more power to deliver the same performance, and will therefore deliver lower fuel efficiency (generally by 1-2 MPG, based on ratings for FWD and AWD Audis).

Traction: All-wheel drive is the technology of choice for maximizing traction under power. However, for most drivers most of the time, this doesn't matter. Why? Because with the way most drivers use their cars, they are not traction limited under acceleration very often. High powered front wheel drive cars, however, can benefit from adopting all-wheel drive because it will improve the steering feel to not have so much power being delivered through the front wheels.

Rear-wheel drive vehicles tend to have better weight distribution than a front-wheel drive car, and the physics of acceleration (the car's weight will shift towards the rear wheels under acceleration) also help maximize rear wheel drive traction.

Poor-weather traction: In poor weather, all-wheel drive becomes a clear favorite. Front-wheel drive has some advantages, mostly because the weight of the engine and transmission sit right over the drive wheels, and in poor traction conditions weight transfer won't matter as much. Rear-wheel drive can be quite good as well, especially if the car has a limited slip differential or traction control. If you are trying to climb a steep, slippery hill from a standing start, rear-wheel drive will often outperform even an all-wheel drive vehicle, because so much of the car's weight will be on the rear tires. A final point the tires may be more important than which wheels are doing the driving. Snow tires are vastly superior to most all-season tires in snowy and icy conditions so whichever wheels are doing the driving, make sure you have the appropriate tires for the conditions.

Braking: traction under power is not the only concern, though. All-wheel drive will not help with braking. Period. Under normal conditions, rear-wheel drive cars tend to have an advantage, again due to weight distribution. However, modern technology such as the latest ABS systems are minimizing these differences. Under low traction conditions, the tires and how good the ABS system is become the most important variables.

Handling: This is where the differences between the drive systems become most controversial. In broad terms, rear-wheel drive will usually deliver better handling than front-wheel drive, and the higher the power the more this is true. However, things are rarely this simple. For example in the early 1990's, Lotus had 2 cars a 300 hp Espirit, with rear-wheel drive and a 160hp Elan, with front-wheel drive. Very few drivers were able to drive the Espirit faster than the Elan around a typical race track because most drivers are more comfortable with front-wheel drive handling dynamics at the limit. Very few of us are comfortable using the throttle to push a high powered rear-wheel drive car right to the limits of traction while turning. However, in the right hands the Espirit was clearly capable of going faster. So which is better?

One area where rear-wheel drive is clearly better than front-wheel drive is in steering feel, and once again the more power involved the more this tends to be true. An all-wheel drive car also usually has superior steering feel to a two-wheel drive car in dry conditions. However, the extra weight of AWD generally more than offsets any benefits in the dry. Throw in some rain, snow, ice or gravel roads, and the situation changes. A well-setup all-wheel drive car can be driven under power through turns in these conditions at speeds that are just unachievable by two-wheel drive cars.
Between front-wheel and rear-wheel drive in poor weather conditions, front-wheel drive tends to have more forgiving handling characteristics. In a skid, you need to steer into the skid with both types of cars. With front-wheel drive, though, getting back on the power will tend to pull you out of the skid. In a rear-wheel drive, this would usually be a recipe for a spin.

Now that we understand the different strengths and weaknesses of each type of drive technology, it is easier to understand why different cars use the different types. For fuel efficient, roomy cars front-wheel drive is the way to go. The fact that the handling is generally more forgiving is also a benefit in this market segment.

For higher powered cars, whether sedans or sportier cars, rear-wheel drive is the favored technology. Some of this is image-driven. Can you imagine a FWD BMW? Neither could BMW it became the new MINI. To take a front-wheel drive car to its maximum potential in a high horsepower situation, more and more car makers are turning to all-wheel drive to balance out the weaknesses in front-wheel drive handling.