Front engine


* The large mass of an engine at the front of the car gives the driver protection in the event of a head on collision.
* Engine cooling is simpler to arrange
* In addition the cornering ability of a vehicle is normally better if the weight is concentrated at the front.




Rear engine


* It increases the load on the rear driving wheels, giving them better grip of the road. Most rear-engine layouts have been confined to comparatively small cars, because the heavy engine at the rear has an adverse effect on the ‘handling’ of the car by making it ‘tail-heavy’.

* Also it takes up good deal of space that would be used on a front-engine car for carrying luggage. Most of the space vacated by the engine at the front end can be used for luggage, but this space is usually less than that available at the rear.

The advantages of front-wheel drive (FWD) seem self evident: By avoiding the need for a driveshaft connecting the engine in front with the rear wheels, front-drive cars save space. The entire drivetrain can be packed into a neat compartment in the front, leaving the rest of the car’s volume for passengers and cargo. Plus, front-drive cars have better traction in slippery conditions (in part because the weight of the engine is on top of the wheels that are providing the power).

Why are rear-drive cars more fun? Every enthusiast may know the answer, but I didn’t. So I called up a helpful GM suspension expert, Vehicle Chief Engineer Ed Zellner. There are, I learned, five basic reasons:

1) “Balance”: The car rides on four patches of rubber, each about as big as your hand. An ideal car would distribute its weight evenly, so each tire had to bear the same load, and none would give way earlier than all the others. The ideal weight distribution, then, would be split about 50/50 between front and rear (actually, 48/52 to help with forward pitch during braking). “A rear-drive car can typically approach that,” says Zellner. Engineers can move the front wheels forward, so that the engine – which doesn’t have to be connected to those wheels — sits behind the front axle.

Meanwhile, the driveshaft and rear differential (necessary to send power to the rear tires) add weight in the rear. Front-drive cars, which must connect the engine and transmission to the front axle, typically have their engines mounted way forward and can’t do much better than a 60/40 front/rear weight distribution.

2) Center of Gravity: This is the point the car wants to “rotate around” in a turn. On a rear-drive car, it’s “about where the driver sits,” says Zellner. In a turn, in other words, the car seems to be rotating around you – you’re at the center. It’s a natural pleasant effect, suggesting you’re in control, the way you’re in control when you’re walking or running around a corner and your weight is centered inside you. (Analogy No. 2: It’s like wearing stereo headphones and having the sound centered between your ears!) A front-drive car, in contrast, with its massive front weight bias, wants to rotate around a point in front of the driver. So in a corner, the driver isn’t just rotating around his spine. He’s moving sideways, as if he were a tether ball on the end of a rope, or Linus being dragged when Snoopy gets hold of his blanket. Not such a pleasant feeling, or a feeling that gives you a sense of natural control.

3) “Torque Steer”: One of the most annoying habits of many powerful front-drive cars is that they don’t go straight when you step on the accelerator! Instead, they pull to one side, requiring you to steer in the other direction to compensate, like on a damn boat. This “torque steer” usually happens because the drive shafts that connect the engine to the front wheels aren’t the same length. Under power, the shafts wind up like springs. The longer shaft — typically on the right — winds up a bit more, while the shorter left shaft winds up less and transmits its power to the ground more quickly, which has the effect of pulling the car to the left. (This winding-up phenomenon occurs the moment you step on the pedal. After that, the wind-up relaxes, but “torque steer” can still be produced by the angles of the joints in the drive axles as the whole drivetrain twists on its rubber mounts.)

4) Weight Shift: Suppose you just want to go in a straight line. What’s the best way to get traction? Answer: Have as much weight over the driving wheels as possible. Front-drive cars start with an advantage — but when any car accelerates, the front end tips up, and the rear end squats down. This transfers weight to the rear wheels — away from the driving wheels in a FWD car but toward the driving wheels in a rear-drive car, where it adds to available traction. In effect, the laws of physics conspire to give RWD cars a bit more grip where they need it when they need it. (This salutary effect is more than canceled out in slippery, wet conditions, where you aren’t going to stomp on the accelerator. Then, FWD cars have the edge, in part, because they start out with so much more of their weight over both the driving and the turning wheels. Also, it’s simply more stable to pull a heavy wheeled object than to push it — as any hotel bellhop steering a loaded luggage cart knows. In snow, FWD cars have a third advantage in that they pull the car through the path the front tires create, instead of turning the front tires into mini-snowplows.)

5) “Oversteer” and the Semi-Orgasmic Lock-In Effect: In a rear-drive car, there’s a division of labor — the front tires basically steer the car, and the rear tires push the car down the road. In a FWD car, the front tires do all the work – both steering and applying the power to the road – while the rears are largely along for the ride. That, it turns out, is asking a lot of the front tires. Since the driving wheels tend to lose traction first, the front tires of front-drive cars invariably start slipping in a corner before the lightly loaded rear tires do — a phenomenon known as “understeer.” If you go too fast into a curve — I mean really too fast — the car will plow off the road front end first. In rear-drive cars, the rear wheels tend to lose traction first, and the rear of the car threatens to swing around and pass the front end — “oversteer.” If you go too fast into a corner in an oversteering car, the car will tend to spin and fly off the road rear end first.

What’s the best way to fly off the road? Safety types prefer frontwards — understeer. Why? To control an oversteering skid, where the rear wheels are heading for the weeds, you have to both slow down and counterintuitively turn the wheel in the opposite of the direction you’re turning. In a front-drive car, with the front wheels slipping, you slow down and keep turning the way you’d been turning to get around the corner in the first place — a more natural maneuver, since you’re pointing the car in the direction you want to go. This is why, for safety reasons, even rear-drive cars sold to average consumers tend to have their springs and other suspension bits set up to make them understeer — to make the front tires slip first, despite the car’s innate oversteering tendency. Only by applying lots of power in a corner can you actually break the rear end of a bread-and-butter rear-drive car like the Mustang loose — a maneuver favored by sports car freaks, but one you try at your own peril.

I’m not saying that any rear-wheel-drive car is better than any front-wheel-drive car, the way, say, any car with plain black tires looks better than any car with whitewalls. But it’s close! Front-drive cars can be fun.

Rear-wheel drive
With rear-wheel drive the rear wheels drive the vehicle. For decades, rear-wheel drive was the system of choice, primarily because it is easy to manufacture, simple, inherently robust and reliable. The typical rear-wheel drive layout consisted of an engine in front, connected to a transmission, then the power went through a driveshaft to the rear-axle gears and then the rear wheels.

Almost all trucks—except a few light-duty models—have been rear-wheel drive. If you look under the rear of a typical pickup truck you will see the rear axle housing, which has a big lump, or bulge, in the center, roughly the size of a pumpkin or basketball. Inside that bulge will be found the ring-and-pinion gears—which transfer power from the driveshaft to the wheels, provide the appropriate gear ratio and also allow the power to make the right-angle turn from the driveshaft to the wheels—and the gears and assembly that make up the differential. By the way, the ring-and-pinion gears—also known as the final drive—are not the differential, and you can have one without the other.With rear-wheel drive the rear wheels move the vehicle and the fronts provide steering.

Thus, there is somewhat of a division of labor. The advantages of rear-wheel drive are based upon the application. For trucks and heavy-duty vehicles, rear-wheel drive offers rugged durability and, as the load is increased, the traction also increases, because that load pushes down on the driving wheels. For passenger cars, rear-wheel drive offers the capability to deal more effectively with higher engine outputs and higher vehicle weights. Luxury cars, for example, tend to have rear-wheel drive. All true sports cars have rear-wheel drive, and all purpose-built race cars, such as those raced in Formula One Grand Prix racing, or in NASCAR, have rear-wheel drive. For performance applications, a primary advantage of rear-wheel drive is that weight transfer causes traction to be increased with acceleration—the more acceleration, the more weight transferred to the rear wheels and the more available traction, all of which enhances acceleration.
A rear-wheel drive vehicle also has a more equitable balance of the vehicle’s weight front-to-rear, so each tire carries a more equal share of the load, which leads to improved cornering response and higher potential cornering limits. Finally, a rear-wheel drive vehicle can offer potentially superior braking performance because, when the brakes are applied, the weight is more equitably allotted among all four wheels.

By the way, in case you’re interested, the ideal weight distribution, as it is known, for maximum performance—as with a pure race car—is to be rear-wheel drive and tail-heavy. This helps acceleration, because there is more weight on the rear, driving wheels, and also helps braking because, under the extreme weight transfer that occurs during braking in racing conditions, the vehicle’s weight then becomes more evenly distributed among all four wheels, so each of the four wheels can make a maximum contribution to stopping power. A high-powered, pure race car, such as those that race at Indianapolis or in Formula One, will have roughly 35 percent of its total weight on the front wheels and 65 percent on the rear wheels.