An article on how front wheel drive versus rear wheel drive versus all wheel drive affects the Indian motorist.

Vijay A. Nebhrajani is an electronics engineer working in the Silicon Valley. His primary job function is to design Application Specific Integrated Circuits (ASICs) for Fortune 500 companies in the Silicon Valley. he has designed several chips for companies like Philips, Maxim Semiconductors, Advanced Fiber Communications and Digital Reflection. Prior to working in California, US, he was working for Centre for Development of Advanced Computing, or C-DAC, in Pune, India. In C-DAC, Vijay was on the design team of the Param series of supercomputers. Here, he also set up the country's first commercial, and highly successful training program on the design of Very Large Scale Integrated Circuits or VLSI circuits. This course is still taught at C-DAC Pune. In his spare time, Vijay does several activities - he is an avid amateur photographer; he writes technical articles for the silicon industry; he hikes in the California mountains; and builds audio circuits. Above all, he loves cars, spends time researching and test driving them. He is an off-road enthusiast, having been on several off-road excursions in his Mitsubishi Pajero (Montero in the United States). He is 28 years old.

In most cars the world over, the front wheels are used for steering the car - that is for determining the direction of motion. There are few exceptions to this, and in those exceptions too, all four wheels were used to steer. I have not heard of a car available for consumer use that has its steering controlled solely by the rear wheels. The notable exceptions to the front wheel steering norm are the pre 1997 Honda Prelude and the pre 2000 Mitsubishi 3000 GT. Both these cars had four wheel steering.

In most cars the world over today, the front wheels are used for propulsion. There are several advantages to this arrangement; however, there are quite a few cars in which the driving wheels are the rear wheels or all the four wheels. Each drive wheel arrangement has certain characteristics - some are good, some not so good. As an informed buyer, it is important to know and understand these characteristics. And dare I say, someday this understanding may save your life. It has mine.

In front wheel drive cars, the engine drives the front wheels. The steering also connects to the front wheels. This leads to a common phenomenon called understeer. Imagine that you are taking a curve at some high speed, and that this curve is enough to cause your wheels to lose adhesion. Since the rear wheels are just rolling along, the car's tendency is to proceed on its inertial path, or continue in a straight line. Thus, the front wheels skid first - usually accompanied by the front tires rolling on their sidewalls. This is the physics - in practice, the car does not lose all steering ability (and proceed on a tangent). Rather, its angular velocity changes less than what should have happened for that angle of the front wheels. This called understeer, or in the parlance, plowing.

In rear wheel drive, the situation is more complex - since the rear wheels drive but the front wheels steer. On a curve, the centripetal force vector is provided by the front wheels, which try to pull the car into the curve. However, the rear wheels are pointed away from the curve, and this causes a torque to develop which tends to spin the car in the direction of the curve. Think of it this way - a stick lying on the ground is pushed from its rear end. It is very difficult to get the stick to move straight because if the front end encounters the slightest obstruction, the rear end moves out. Application of more force on the rear end only worsens the situation, causing the stick to rotate further. This same phenomenon exists in cars - the car turns more than what it should have for a certain angle of the front wheels. Of course this is called oversteer or in common usage, fishtailing.

From the discussion above, it should be fairly obvious that in a straight line, front wheel drive cars tend to be more stable. This is true in general for front wheel driven cars, but that is not to imply that rear wheel drive cars are not stable in straight lines. In fact, there are many rear wheel drive cars that are stabler in a straight line than front wheel drivers.

There is another characteristic of front wheel drive cars that needs elucidation. In a front wheel drive car, when you accelerate from a stop, one front wheel may have less traction than the other. This could be because of various reasons - one wheel is on an oily patch or on water. In that case, the wheel which has better traction will pull the car forward with more force as compared to the other wheel. This causes the car's nose to lurch sideways on hard starts from a stop and is called torque steer. A similar situation exists for rear wheel drive cars where the tail spins out on hard acceleration from a stop, but this is rarer.

Have you ever stopped to wonder why during braking the nose of the car dives down and during acceleration, the nose of the car rises and the rear end of the car pushes down? This is because of dynamic mass distribution. When any car accelerates, the intertial reaction combined with the weight creates a force vector directed from the center of gravity of the car to the rear wheels. During braking this force is in a straight line from the center of gravity to the front wheels. This is like redistributing the mass of the car - under acceleration, the mass seems to "shift" to the rear end, and on braking, mass seems to "shift" to the front end, causing what is known as squat and dive respectively. Thus, front wheel drive cars fight a losing battle every time you accelerate - the faster you accelerate the more mass shifts to the rear wheels, which can't respond, since they are not driven. The phenomenon of dive causes a very notable design to exist - that of the front brakes being bigger and more powerful than the rear ones. This should be fairly obvious since they have to handle more mass during deceleration. Similarly, rear wheel drive cars need smaller engines to achieve the same acceleration as front wheel drive cars. You may also observe, if you read technical specifications, that the static mass distribution of front drive cars is to the order of 65 front 35 rear, whereas that of rear drive cars is to the order of 45 front, 55 rear. Naturally this affects tyre pressures, which must compensate for this type of loading.

In wet weather, oversteer is difficult to control, and if a car has begun spinning, there is little that can be done to correct it unless you straighten the wheel and accelerate. This however is not the normal reaction of most people - they stomp on the brakes. This reaction causes the wheels to lock and lose adhesion which worsens the oversteer problem. On front wheel drive cars, braking actually slows the car down, and the car can begin to regain control again since the plowing front wheels are now trying to straighten out. It is generally considered that front wheel driven cars are stabler on wet roads, but that does not imply that all rear wheel drive cars are not.

Experienced drivers often use a little oversteer on purpose to tackle curves or corners at higher speeds. This is not possible with front wheel driven cars unless you get very creative with the parking brake (which usually affects only the rear wheels), but is natural for rear drive cars. You will see that many sports cars are rear drive. It is possible to design sports cars that are front wheel driven, but in absolute performance, rear wheel drive seems to have an edge.

It is generally considered easier and cheaper to design and manufacture front wheel drive cars. This is because the drivetrain can be simpler and localized to the front part of the car. The links between engine and wheels are simpler and you don't need complex suspension and chassis designs to keep the car planted on the ground. Its rather difficult for designers to plant more weight on the rear of a car since most of the heavy stuff (engine for instance) is usually in the front - this naturally aids the design of front wheel drive cars, but detracts from the design of rear drive cars. Putting the shoe on the other foot, a known issue with front wheel drive cars is that of CV joints, or constant velocity joints. This is the solution to the rather complex problem of having the front wheels turning from side to side during steering and moving up and down during suspension movement while still having to drive them from the engine. A CV joint allows this, but is prone to failure more easily - rear drive cars do not use CV joints.

Part of the reason of the popularity of front wheel drive cars is because most people do not drive their cars like race cars. The other characteristics of front wheel drive cars like straight line stability and wet weather safety also appeal. Costs can be lower, ending up in a lower price for the consumer. This makes a lot of economic sense in India, and you can see that almost every car starting from the Maruti 800 has been front wheel drive.

All this discussion begs the question - why can't you have a vehicle which has the advantages of both types of drive? Certainly you can, and such cars, called all wheel drive do exist. Do not confuse all wheel drive with four wheel drive. There are subtle differences between the two. Four wheel drive is a topic for another article. All wheel drive is becoming more popular, and it has a drivetrain that can drive all four wheels. It usually starts out by having a primary set of drive wheels - this could be either front or rear. Electronic sensors detect wheelspin, loss of traction and several parameters and then an onboard computer directs power to the wheels with most traction. Thus, if the primary drive wheels lose traction, the other wheels power the car out of the situation. This can be combined with traction control, which works by applying brakes till the car regains control, to create cars that track true on virtually any surface conditions. Two notable examples are cars from Subaru and the Quattro cars from Audi. However, it is generally more expensive to design, manufacture and maintain all wheel drive cars. Besides, these cars are less fuel efficient because they have more moving parts (read more friction). Naturally they are heavier too. All wheel drive cars do retain some oversteer and understeer characteristics, but these are considerably better controlled - you will find people living in the snow belt regions of the world having this type of car most often.

In India, where the driving conditions are like nowhere else in the world, we have certain design features proliferating. The first of them is front wheel drive. Rarely do we see snowy or icy conditions on Indian streets, and front drive cars handle wet roads well. Few people race cars professionally, so its hard to justify the extra expense for rear drive cars. Nor can we justify the extra expense and fuel cost (the petrol prices being as they are) for all wheel drive cars. And so, the market dictates front wheel drive and so it is.

Driving along in a front wheel driven car, you can do a lot to enhance your own safety by knowing your car's limitations and capabilities. First, don't indulge in jackrabbit starts. This means, don't accelerate too hard from a stop. Your shiny black 100 HP car may thrill you with a burst of power and a screech, but is also likely to exhibit some torque steer which takes you right into the vehicle beside you or onto oncoming traffic. Second, When you drive in the mountains or on curvy roads, remember, if your car were to "skid", it would probably understeer - proceed straight off the mountain. Third, don't go by the old adage that the rear tyres should have greater pressure. This is not true for many front wheel drive cars, and you may overinflate the rear tyres, causing the car's rear end to bounce. Follow the manufacturers recommended pressures. Fourth, if you have to replace tyres, consider replacing them in pairs. If you have a mismatched pair up front, you may have more torque steer and balance problems than you bargained for. Fifth, regularly inspect the CV joints in your car. There are usually four CV joints in a front drive car, and you can see these as rubber wrapped points where the drive shafts exit the engine and are where they connect to the wheels. Damage to CV joints will be apparent by either a visual inspection or by a clunking or clicking noise from the front wheels during turning. In India, with rocks and other obstructions on the road, you can end up with quite a bit of expense if you do not realize this problem, prevent it or catch it early.

With these new insights, I certainly hope that you will have a long and rewarding experience with your car.