Full time 4 wheel drive turbo charged car dynamics

The dynamics

All full time 4 wheel drive cars share some common characteristics in their handling and road holding abilities. A car's handling ability is most easily judged when cornering at high speeds. There are mainly three types of cornering behavior:

• Understeer (which denotes a car's tendency to exit the curve by following a trajectory whose radius is longer than the corner's)

• Oversteer (which is characterized by the tendency of a car to follow a radius that is shorter than the corner's)

• Neutral (a behavior in which a car follows naturally a curve's radius)

Ideally all cars should of course display a neutral cornering characteristic. Then again we are not living in an ideal world are we? In real life most full time 4 wheel drive cars display a changing cornering character. The car has a tendency to understeer when entering the corner, a neutral behavior in mid-corner and an oversteering tendency when exiting the corner. This, of course, depends greatly on the car's power output and is mostly true for cars with an output in excess of 200Bhp. Usually, the more power a car disposes the more the above handling pattern is true.

Note that all handling behaviors described herein are applicable in "close to the limit" situations which is to say close to the limit of grip and are mainly valid on high friction surfaces i.e. tarmac. Additionally these handling characters are valid when no major driver intervention or artifacts are used i.e. no hand brake use or manual differential locking.

Each type of car has a natural tendency for one of the above mentioned road holding characteristics depending on its architecture (mass distribution, engine position, driven wheels, inertia, overhangs, turbo lag time, ...). Below you will find the major characteristics which best describe each type of car's behavior on dry tarmac surfaces.

Front wheel drive cars

Most front wheel drive cars display a heavy understeering behavior while cornering (at road holding limit speeds). This is due to the fact that in front wheel drive cars most of the car's weight is distributed in the front part of the car. This induces great inertia forces while in a corner. Since no opposite force is present to spin the car around its vertical axis (i.e. no rear wheel traction) the car understeers. These cars have to be driven hard in order to obtain fast cornering. You have to "break late" and I mean really late. Braking late "charges" the front wheels making them "stick" to the road and therefore adds traction while, at the same time, discharges the rear axle making the rear wheels more subject to sliding. When the brake pedal is released the rear axle is charged again helping the car out of the corner. The usual way to drive fast this kind of car is what is known as left foot braking. This technique consists in using your left foot to brake while using the right foot to accelerate at the same time. Left foot braking is a demanding technique and takes a long time to master. It can, therefore, be dangerous to novices. The usual effect of braking with the left foot is to lock the rear wheels of the car thus inducing an oversteering character to the handling.

As you might have guessed these are not the most "fun" cars to drive. Their market predominance is mainly due to the fact that they feel "reassuring" to inexperienced, everyday drivers. I mean by that that if a front wheel drive car enters a corner too fast, by driver misjudgment or inexperience, the usual driver's reaction is to brake while inside the corner and subsequently lift his right foot from the gas pedal. A front wheel drive car in this situation will reduce its understeering character and enroll the corner more easily thus "forgiving" its driver's optimism. A second reason for the front wheel driven car's predominance is their reduced cost of manufacturing compared to other types of cars..

Rear wheel drive cars

This type of car used to dominate the market until the early 80s. They still prevail in the US but they do so for reasons that have nothing to do with sporty driving.

Evidently in these cars the rear wheels are driven by the engine. This allows for a better mass distribution inducing less inertia [Some of these cars such as the Porsche 924, 944, 968 and the 1980's Alfa Romeo GTV and GTV6 had the gear box mounted on the rear axle to further improve mass distribution]. Rear wheel drive cars have a natural tendency to oversteer. When one of these cars enters a corner fast (with the gas pedal down) its rear wheels will lose traction (will have a tendency to spin) and the gyroscopic force they produce will lead them off the curve. This road holding characteristic, although spectacular, is very far from efficient. BMWs and Mercedes-Benz cars are still made this way, mainly for tradition's sake.

You may note that these cars are much more fun to drive than front wheel drive cars are. Their oversteering can be easily modulated by "dosing" the gas pedal. Rear wheel drive vehicles, especially the most powerful ones, tend to "surprise" inexperienced drivers.

An extreme example of rear wheel drive car is the Porsche 911 series. In these cars not only the traction is applied to the rear wheels but the engine is also mounted at the rear of the car in an overhang position (behind the rear wheels axle). The Porsche 911 series has been known as very difficult to drive at the limit since, when cornered, the inertia induced by the overhanging engine will easily have the car spinning around its vertical axis.

The progressive elimination of this breed of car is due to the higher manufacturing costs they induce. Additionally the oversteering they generate is sometimes excessive and can lead to a total loss of control of the vehicle.

All wheel drive cars

The road holding characteristics of these cars are a bit more complex. They display, usually, a combination of both of the above road holding characters and are very far from neutral. Usually 4 wheel drive cars are based on a front or rear wheel drive version of the same model. This fact greatly influences their handling (i.e. cars that are based on rear wheel drive models display a more oversteering type of handling whereas cars that are based on front wheel drive models display an understeering type of handling). This is just a rule of thumb however and, depending on engine power, one or the other type of handling may prevail.

Generally this type of vehicle changes its handling characteristics while in a corner. The car has a tendency to understeer when entering the corner and oversteer when exiting. One can modulate this characteristic by using the throttle and brake pedals in a way to induce a neutral type of handling. Like in front wheel drive cars the driver has to "brake late" when entering the corner and push the gas pedal while inside the corner in order to allow a "torque transfer" (operated by self locking differentials) to the rear wheels. The torque transfer is most noticeable in turbocharged vehicles. When the turbocharger reaches its full throttle speed, at around 100,000-150,000 RPM depending on the turbo's model, and the car's engine develops its maximum torque. When this happens the front wheels will have a tendency to spin due to the excess torque applied to them. Their spin is detected by the center differential which will progressively lock and send engine torque to the rear wheels thus inducing oversteer.

Although these cars are not such "fun" to drive as are rear wheel drive cars they are so much faster and efficient than the other types that unless one drives them he is unable to imagine the cornering speeds 4wd turbocharged cars are able to reach.

The major drawbacks of 4wd turbocharged cars when compared with their 2wd counterparts are:

• Increased mechanical complexity affecting their price and weight

• Higher mechanical losses (friction) due to the number of gears to drive (differentials, axles, ...)

• The famous "turbo lag" effect which forces the driver to anticipate the car's reactions

• The "nervous" reactions the cars might have when their handling characteristic changes from understeer to oversteer

As you might have guessed these cars are a lot more demanding when driven to the limit. A certain time is necessary for a novice before he can master and anticipate the car's reactions. Most examples of full time 4WD turbocharged cars where developed in the sole purpose of competing in the World Rally Championship.

One can only regret the progressive disappearance of four wheel drive turbocharged models from the vendors' catalogues. The days when a car enthusiast could opt for this breed are unfortunately (almost) over.

Please remember to drive safely...