The Hidden Virtues of Front Wheel Drive



The Hidden Virtues of Front Wheel Drive

There's a lot more to front-wheel drive in an automobile than most people think.

Just about every automaker agrees that front-wheel drive is the best way to maximize utility in an economy car or restore traditional comfort to a down-sized family sedan. Call almost any automaker's answering service Los Angeles to New York and they'll steer you towards front-wheel drive. As a bonus, it also provides better traction on slippery roads. But that's only part of the story, says Saab.

The Swedish automaker has been using front-wheel drive since 1949 not only for utility and comfort, but also for sportiness. That's why Saab engineers have a different light to shine on the subject.

A Warning System for the Driver

Take for example drivetrain elasticity. This is not some- thing that gets mentioned in automotive advertising. It should, because it's important.

Saab Engineer Gunnar Larsson explains why: "One of the most important performance and handling characteristics of a car is the way it provides the driver with the correct information. This is how he can determine his next action." On a front-wheel drive car the short distance between the engine and the driven wheels makes the transmission of power crisp and direct. There is no long propeller shaft, and drivetrain elasticity is minimal.

As a result, the front assembly -- including the steering gear -- creates a warning system for the driver: because the front wheels are driven, the driver is immediately warned through the steering wheel when there is any loss of traction. "Man is very sensitive in this respect," Larsson says. "He will perceive differences of one-hundredth of a degree."

This builds a confident relationship between the car, the driver, and the road. The front-wheel-drive Saab 900 has what is called "true steering wheel response." This means that the car quickly notifies the driver of how it has understood his various actions.

Another desirable characteristic of front-wheel-drive cars is their ability to forgive a careless maneuver of the driver.

When a rear-wheel-drive car enters a curve too fast, it slides out from the rear. A non-expert driver reacts by backing off. Unfortunately, this only compounds the problem and forces the car into a dangerous spin.

Front-Wheel Drive Makes Instincts Correct

On the other hand, with a front-wheel-drive car, the front wheels lose adhesion first. This is safer because in this case the driver's instincts are correct: by backing off he restores traction to the front wheels and regains control. This makes front-wheel drive a lot safer in accident-avoidance or other sudden maneuvers. "Front-wheel drive cars have an exceptional ability to stay on course," says Saab's Larsson.

Rallyist Erik Carlsson, two-time winner of the Monte Carlo Rally, makes a related point: "Because rear-driven cars tend to slide from the rear, if you're going to crash, you'll probably hit the obstacle sideways. It's better to crash head on, you're better protected."

Equally important is the consistent behavior of a front- wheel-drive car, no matter how it is loaded.

"The Saab 900 has a weight distribution of 60 percent on the front wheels and 40 percent on the rear wheels," Larsson says. "Fully laden, the weight distribution changes to 51 percent front and 49 percent rear, but the car retains its fundamental front-to-rear balance. The center of gravity stays towards the front of the car and the handling characteristics are the same."

As opposed to this, a typical front-engine/rear-drive car has a weight distribution of 55 percent on the front wheels and 45 percent on the rear wheels. This is fine until the car is fully loaded. The center of gravity shifts to the rear and the road behavior changes unpredictably, with obvious risks

Insensitive to Crosswinds

Directional stability is an inherent advantage of front- wheel-drive cars. "They are relatively insensitive to cross- winds and the camber of the road," Larsson says. "This is because the directional wheels are the same that pull the car forward, striving to keep the car going in the direction the driver aimed it." According to Larsson, a Saab maintains its course even when accelerating on ice.

Front-wheel drive also means that the rear axle is without heavy and bulky transmission components. On the Saab 900 the rear axle is a lightweight tube. It therefore has very little inertia. Engineers call this "low unsprung weight." In practi- cal terms, this is the rear wheels' ability to follow irreg- ularities of the road without transmitting them to the body. The result is a smoother ride and better roadholding. "The tires don't bounce, they maintain a constant grip on the pave- ment," Larsson says.

Many Interacting Factors

But good road behavior is not the result of front-wheel drive alone. A car's behavior depends on a host of interacting factors -- not all front-wheel-drive cars were created equal.

At moderate speeds on good roads, all cars are well dis- ciplined. It's when conditions become difficult that a car shows its worth. Indeed, what makes the Saab 900 so good for drivers who like to maintain high average speeds is its ideal combination of front-wheel drive, favorable weight distribution, suspension, rack-and-pinion steering and the geometry of the steering, and the front and rear assemblies in general. Saab was among the first to claim that front-wheel drive was worthy of further development. The test of time proved Saab engineers right. Perhaps it is due to their long experience that they now know more than most about front-wheel drive.


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