The dynamics
Have you ever noticed that no one will ever admit he/she is a bad driver? You might have met people that admit being bad tennis players, bad skiers or football players or even bad losers. Never will anyone admit he’s a bad driver. There must be some psychological reason behind this but that is not really our subject here. Note that I do not pretend, myself, being a better driver than anyone else, my mere purpose is to present the physics behind a car’s road holding character.
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. When a car understeers the driver has to steer more than he’d normally have to in track the corner’s radius to follow the corner
- Oversteer, is characterized by the tendency of a car to follow a radius that is shorter than the corner’s. When a car oversteers the driver has to steer less than he’d have to, and sometimes counter-steer, in order to track the corner’s radius
- Neutral, a behavior in which a car follows naturally a curve’s radius
Ideally all cars should 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 cornering character that varies while inside the corner. 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, is greatly dependent on parameters such as the car’s power output, chassis rigidity, suspension design and dimensioning, torque distribution between axles and is mostly noticed on cars with a power 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. dry tarmac. Additionally these handling characters are valid when no major driver intervention or artifacts are used i.e. no hand brake use, lift-off, braking 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. To make things worse the engine is almost always mounted transversally further hurting an even mass distribution. This unbalanced weight distribution 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 has a natural tendency to understeer. These cars have to be driven hard in order to obtain fast, efficient cornering. You have to “brake late” and provoke them. Braking late “charges/loads” the front wheels, by what is known as weight transfer, making them “stick” to the road and therefore adds traction while, at the same time, removes load from the rear axle making the rear wheels more subject to sliding. When the brake pedal is released and the accelerator pressed again a load transfer to the rear axle takes place helping the car out of the corner. Weight transfers occur when load is transferred from one of the car’s axles to the other in instances such as braking or accelerating
One of the most efficient ways to drive this kind of car fast is known as left foot braking. The 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 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. The main disadvantage of left foot braking is the fact that no gear change can be performed during the braking phase hence the driver has to select the gear to be used at the corner exit prior to initiating the maneuver.
Understeer
As you might have guessed these are not the most “fun” cars to drive. Not being fun to drive does not make front wheel drive cars less efficient or quick. Their market predominance is mainly due to the fact that they feel “reassuring” to inexperienced, everyday drivers. If a front wheel drive car enters a corner too fast, due to driver misjudgment, inexperience or optimism, 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 operate a weight tranfer to the front wheels making them “stick” better and thus reducing its understeering character and enroll the corner more easily “forgiving” its driver’s error. A second reason for the front wheel driven car’s predominance is their reduced manufacturing costs 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 and polar moment. Generally the engine is mounted in a longitudinal position further favoring mass distribution. 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. Most BMWs and Mercedes-Benz cars are rear wheel drive cars, mainly for old times sake.
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 centrifugal force the car is subject to combined with the reduced friction coefficient to the rear wheels will lead their rear end off the curve. Releasing the gas pedal in these circumstances or, worse, braking will almost certainly lead the car off its ideal trajectory into oversteer. In fact what the driver has to do when a rear wheel drive car oversteers is in opposition to the natural driver’s reaction. The driver has to countersteer i.e. turn the steering wheel to the opposite side of the turn and dose the gas pedal in order to avoid sudden weight transfers to the front axle that would result in terminal oversteer. This road holding characteristic, although spectacular, is very far from being efficient.
Oversteer
You may note that rear wheel drive cars are much more fun to drive than front wheel drive cars are. Their oversteering character can be usually 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 is the power 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 polar moment 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 that may lead to law suits against manufacturers.
All wheel drive cars
The road holding characteristics of these cars are a bit more complex. They usually display a combination of both of the above road holding characters and are very far from being neutral. Usually full time 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 will initially display a more oversteering type of handling whereas cars that are based on front wheel drive models will initially display an understeering type of handling. The above description is just a rule of thumb however and, depending on the engine output, weight and torque distribution, 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. The driver can modulate this characteristic by using the throttle and brake pedals in a way to induce a neutral type of handling. Like in the case of front wheel drive cars the driver has to “brake late” when entering the corner, to operate a weigh transfer, and push the gas pedal while inside the corner in order to allow a “torque transfer” (usually 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 type, and the car’s engine develops its maximum torque the front wheels are overloaded and will have a tendency to spin due to the excess torque applied to them. The spin is detected by the center differential which will progressively lock and bias engine torque sending the excess torque to the rear wheels thus inducing oversteer.
Most people have the wrong impression that 4WD is only useful on slippery roads and conditions. The best argument against this impression is to drive all three types of vehicle (front or rear wheel drive) on a dry tortuous road. In the first tight corner try extracting the car at the limit of its tires’ grip and see what happens. Wheel spin is simply unavoidable.
A front wheel drive car will have its inside wheel spinning and will almost stall in the absence of a self locking differential. The only choice left to its driver is to lift off.
A rear wheel drive car will also have its inside wheel spinning and will get into oversteer. In the absence of a self locking differential the driver, apart from counter-steering, will have to ease on the gas pedal if he is to avoid exiting the road.
A full time 4 wheel drive car can negotiate the same corner with the gas pedal almost floored…
Although full time 4 wheel drive cars are not as much “fun” to drive as are rear wheel drive ones 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 cars are able to reach safely.
The major drawbacks of 4wd cars when compared with their 2wd counterparts are:
- Increased mechanical complexity affecting their price, weight and reliability
- Higher mechanical losses (friction) due to the number of gears to drive (differentials, axles, driveshafts, …)
- In turbocharged versions the famous “turbo lag” effect which forces the driver to anticipate the car’s reactions
- The, sometimes, sudden character of the transition from understeer to oversteer mid-corner
As you might have guessed these cars are a lot more demanding when driven to the limit. A certain familiarization 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 manufacturer’s’ lineups. The days when a car enthusiast could opt for this breed are unfortunately (almost) over.
Please remember to drive safely…
