In automotive speak, handling is usually defined as that aspect of vehicle control which begins where roadholding leaves off. In its widest sense, the word embraces every aspect of car control, including directional stability at high speed on a straight road and steering instability at any speed. Virtually all modern cars (we will exclude those made in China and India, as we have not had the pleasure of trying these) are designed well enough for them to be able to undertake most manoeuvres at reasonable speed and acceleration, without the driver even thinking about how the car is "handling".
These days we learn of a cars handling from print or TV journalists, who have the opportunity to test cars at much greater speeds than the motorist can ever dream of achieving - legally that is. So the question remains, why do motoring journalists spend so much time pushing a car to its limits to determine the handling characteristics, and any peculiarities?
The answer is because, at the limit, cars do behave very differently. And that difference can make a car dangerous or safe. One of the first things motoring journalists will do is drive at a certain speed through a corner and, when half-way round, sink the boot. The car being tested may begin to run wide, while another may turn its nose in towards the centre of the corner and begin to follow a curve of tighter radius. Yet another car may do the same thing if, instead of pressing down on the accelerator, the driver releases it completely; if you do the same thing in that same car on that same course but travelling at a higher speed, the tail of the car might swing out, while the car as a whole runs wide.
All these are examples of the most complex variable to be encountered in handling, which is steering response. All the other variables have the effect of modifying this one, which is what makes it so complex; and, yet it is the one with which the driver is most concerned when coming, perhaps literally, to grips with the handling of their car. Even such superficially unrelated problems as keeping the vehicle pointing and travelling straight under heavy braking, or avoiding the potentially dire consequences of becoming airborne over the crest of a hump-backed bridge in the middle of an S bend, are fairly extreme cases of the same basic problem, which is of persuading the car to pursue the desired course when the various forces acting upon it conspire to deflect it.
These forces are numerous, and the factors influencing them even more so. The location of the centre of gravity, the size and type and inflation of the tyres, the steering geometry, the suspension geometry, the wheelbase and track dimensions, the spring rates and damper capacities, the amounts of tractive or braking torques applied to the driving wheels and the moments of inertia about the three axes of the car, are the principle factors. In certain circumstances, though, others can become significant, such as the behaviour of the airflow around the body or around the tyres
(if they are exposed) at high speeds, or the presence of a self-locking differential or certain kinds of automatic transmission in circumstances where wheel-spin might be possible.
The first investigation of how some of these factors might affect the handling characteristics of a car was made in the early 1930s by Maurice Olley, an Englishman who went to work for General Motors
in the USA. Untill GM's work, the engineering put into a cars handling had been largely intuitive, with designers figuring that a tail-heavy car would be tail-happy - and it was Olley more than anybody else who, by seeking to substitute reason, laid the foundations of the scientific study that handling was to become. Olleys work studied polar moments of inertia, which indicated the alacrity with which a car could be deflected from its path.
Yaw, Roll and Pitch Axes
There are three such moments - the yaw, roll and pitch axes - the first being the most important. The car with its main masses situated towards the ends has a high polar moment in yaw, which means that it will not readily change direction, whereas if the masses are concentrated near the centre of gravity the moment will be low and the car will respond very quickly to the steering. But, a low-moment car can easily be made to spin, and as easily checked; while a high-moment car will be slower to spin and, once spinning, more difficult to stop. The car with a high polar moment of inertia in yaw will also have a high polar moment in pitch so that the effects of any inclination to squat or nose-dive under the influence of acceleration or braking will remain apparent for longer than in a low-moment car, though their effect will be less sudden.
A Fiat 128 demonstrates the effects of handling.
The Polar Moment of Inertia
The polar moment of inertia can be significant in the rolling plane, too: racing-car designers found that wide-spread pannier fuel tanks could make a car feel much more ponderous than one that carried all its weight within the confines of a narrow body. The interaction possible between the forces liberated in these three planes is obviously complex. The fact that the axes about which the car pitches, rolls and yaws do not mutually intersect makes their interaction more complicated still. The car rolls about an axis passing through the instantaneous roll centres at front and rear, which are determined by the suspension geometry, but its pitching moment must be measured around the centre of gravity of the sprung mass, and its yawing moment about the centre of gravity of the entire car.
It is the height of the centre of the total mass, in relation to the track dimension, that determines the total lateral weight transfer in roll, and its position in relation to the wheelbase which determines the longitudinal weight transfer in acceleration and braking; but it is the centre of gravity of the sprung mass which determines, by its height above the roll axis, how the lateral transfer will be apportioned between the front and rear tyres. The effect of variations in load on the tyres
is fundamental to how a car handles because, if it were not for the elastic properties of these vital components, handling problems simply would not exist.
The Slip Angle
subjected to a lateral force will deflect, as would any other elastic body and, if it is rolled along a flat surface while thus deflected, its path of contact with that surface will diverge from what it would have been without the lateral force. The angle between the plane of the wheel and this divergent path is called the slip angle - which has nothing to do with sliding - and it will vary according to vertical, lateral and longitudinal loads on the tyre, as well as according to its inflation pressure and its construction, the dimensions of the wheel rim upon which it is mounted and any camber or tilting deflection of the wheel introduced by the suspension or steering geometry to make it other than upright in relation to the road surface.
If the front and rear tyres
are deflected by the same amount (in other words, if the slip angles are equal), the resulting path will be a straight line diverging from the original straight course before the application of the deflecting force; but if the front and rear slip angles differ, the path will be curved. It is the introduction of such curvature which creates the various anomalies of handling, but already it is clear that, however numerous the factors responsible, there can only be two kinds of resulting aberration: one where the front slip angle is the greater, the other where the excess is at the rear. So non-linear are the responses of a tyre
to the various loads to which it may be subjected, that the net difference between those at the front and at the rear-and, for that matter, between those on the right and those on the left-may vary rapidly and frequently in the course of one simple manoeuvre.
Up to a certain point the tyre's cornering power will increase at the same time, but beyond a certain point it will not, and further increases in load may reduce its cornering power. If the increase in load is as the result of roll in cornering, so that the outer tyre
has more weight than the inner, the cornering power of the two together can decrease as the difference between the two individual loads increases, for the cornering power of the inner tyre
may fall off more rapidly with unloading than that of the outer tyre
rises with increasing load. This is typical of non-linear response and, if the roll-couple distribution is such that the proportion of load transfer is different between the front tyres
and the back tyres, one end of the car will be left with more cornering power than the other, which will then tend to drift outwards or operate at a greater slip angle. Which end it will be depends on the relative magnitudes of each of the effects just discussed.
Since load transfer may take place longitudinally - under the influence of acceleration, or braking, or aerodynamically
induced forces any discrepancy between the net cornering powers of the front and rear pairs of tyres
may be exaggerated or negated by this additional imposition of forces. When all these things are considered, it becomes clear that no reliance can be placed in any of the popular generalisations about the influence of weight distribution and suspension design on handling characteristics. A front stabiliser spring, such as the conventional anti-roll torsion bar, may often decrease the cornering power of the front tyres
relative to those at the rear, as it is always supposed to do. However, in some circumstances, it could have the opposite effect, increasing the cornering power at the front.
The Anti-Roll Bar
Apart from its usefulness in providing roll stiffness and load transfer without the bump stiffness of the springs having to be undesirably increased, the anti-roll bar
can be useful as a means of inhibiting undesirable excesses of camber variation with independent suspension. According to the geometry of the suspension, this may be more or less severe, but so correspondingly may the tyres
be less or more sensitive to it. It is commonplace to say that a tyre's maximum cornering power can only be realised when it is perpendicular to the road surface, but this is by no means always true; the cornering power may with tyres
of some kinds increase with negative camber up to 2 or 3 degrees and decrease again if the camber exceeds that critical angle, while other kinds of tyres
may suffer a drastic diminution of cornering power if subjected to more than half a degree of negative camber.
Without exception, all tyres
suffer degradation of their cornering ability when subjected to positive camber - but it is important to recognise that camber variations can be introduced not only by the suspension but also by the steering. A large castor angle, intended to endow a tyre
with strong self-centring, will produce a marked increase in the negative camber of the outer front wheel when steered normally around a corner, whereas opposite lock applied to correct a tail-slide during the corner will result in a diminution of that camber, and that creates an unstable condition when the grip of the rear tyres
The design engineers job is a difficult one, particularly as these days, a car comes under the microscope far more than it did 40 or even 20 years ago. But when you simplify it, there are two main issues they need to address - the first being where the front slip angle exceeds the rear, the other where the opposite is the case. At this point, there is a further crucial principle that must be mastered: any change in the curvature of the car's path results in a change in the centrifugal force to which it is subjected, and this in turn changes the deflection of the tyres, which changes the curvature, and so on. In other words, the phenomena of handling are not steady-state conditions but events. Only when this is understood and accepted is it safe to introduce the terms 'oversteer' and 'understeer' which are so often bandied about in discussion of car handling.
Neutral steer is a safe enough proposition: front and rear tyres' cornering powers are equal, and the car can be driven around a curve of a given radius at any steady speed without need for corrective alterations to steering or accelerator. However, if a change in slip angles is greater in the front than in the rear, because the cornering power of the front tyres
is less, the curve described by the car grows larger, reducing the centrifugal force and thus bringing the car back into a state of equilibrium. Such a car is self-compensating in that it will seek that radius which results in a balance of forces independent of the driver's reactions. This course of events we call understeer, and it is often considered safe because of this self-compensating nature.
In fact, the radius where equilibrium is restored may be far larger than the space permits, meaning that the car might go head-first off the road. The essential feature of this understeering syndrome is that there is a negative servo effect on the steering. The opposite characterises true oversteer, in which there is a positive servo effect: something happens to increase the cornering power or reduce the slip angles of the front tyres, which accordingly steer the car into a curve of smaller radius. This increases the centrifugal force and modifies the loadings and attitudes of the tyres
again so as either to increase the oversteer still further - in which case the self-servo effect becomes self-multiplying until the car goes into an uncontrollable spin - or to decrease it, in which case the car hence the overcorrection that often upsets a driver's parrying of a tail skid.
Oversteer is often, but not necessarily, destabilising: it all depends on the non-linear responses. If each incremental increase in centrifugal force produces successively smaller increments of radius reduction, equilibrium can be achieved. The same applies to a handling pattern that is often called oversteer but is not really either that or understeer. This is what happens when something occurs to reduce the cornering power or increase the slip angles of the rear tyres. In this event (these are events, not conditions), the nose of the car does not swing into the corner as in oversteer, but the same yaw effect is produced by the tail swinging out. The result is completely different, as the path of the car is deflected towards the outside of the curve, just as in understeer.
Tail Slide and Drift
As the radius of curvature increases, the centrifugal force diminishes, but it does not necessarily follow that the car will recover its equilibrium, as the rate of yaw could introduce inertia effects or others tending to aggravate the rear tyres' loss of cornering power faster than the diminution of centrifugal force can mitigate it. In such a case, the car would spin off, and the event could be described as neither understeer nor oversteer but tail slide. If, instead, each incremental decrease in centrifugal force produces successively smaller increments of attitude change and/or radius enlargement, the car will eventually reach a curious equilibrium in which it is describing a curve of constant radius at a constant and considerable yaw angle.
Any one of these events - understeer, oversteer, tail slide or drift - can be exacerbated, controlled, or corrected, by steering and/or by accelerating or decelerating. Which of these, and how much, is beyond calculation by anything other than the computer in the driver's head, whether it be programmed by experience or by genius. There are no rules - at least, none that are better than generalities. Even thus equipped, drivers can still go wrong. It is difficult for a normal driver to detect changes of less than 10% in the steering forces they describe as 'feel' through the steering wheel. No matter how good the driver, there is a threshold of sensitivity (although the actual quantity varies from driver to driver and probably from time to time in a given driver). The effect of this threshold is unexpected: when the tyres
of a car are replaced by others of greater cornering power but otherwise identical, the driver's stress (easily measured) in emergency situations is greatly reduced, yet on the other hand it is somewhat increased in normal manoeuvres.
The Threshold of Sensitivity
This is explicable by reference to the threshold of sensitivity: in the gentler manoeuvres, by the time he is aware of what he has done, he has gone too far, and the classic over-correction syndrome may be diagnosed in the events that follow, with the motions of the car getting more and more out of phase with the driver's steering. The remedy is to increase not only the cornering power of the tyres
but also their sidewall stiffness or resistance to lateral distortion. In this way, the steering aberrations are damped out, and by thus giving the driver a better feel of the situation, the laterally less compliant tyre
allows them the same relaxation of stress in normal conditions as in emergencies.
There are more complications yet in tyre
behaviour which govern vehicle handling. In addition to the constantly varying relationship between slip angle and cornering force and load, there is the tyre's own self-aligning torque (a product of tyre
design that can be felt making steering heavier as you corner harder); there is also a varying response as road speed increases. All these different reactions have to be added to each other before the car designer can even begin to approach knowledge of how their vehicle will behave in a given situation and, since road conditions are changing almost constantly, complexity is further compounded and the whole proposition becomes virtually incalculable.
To many, vehicle handling is both a science and an art. Knowing that a driver may accelerate or brake in mid-corner, and introduce at least three variables for each, means there is rarely a perfect answer. Right up until the mid 1970s it was thought that braking and traction alike reduced the cornering power of a car, but research was to prove that this was not true, and that although traction will almost always reduce the cornering power, braking will slightly increase it. Needless to say, this is dependent on the amount of either force applied: if it becomes considerable, then the cornering force will be greatly reduced until the point is reached where the wheel is either spinning or has locked.
Weight transfer is a modifying factor ... as the tyre
generates higher cornering force as the vertical load on it is increased (up to a certain point, at any rate), it is possible that at some speeds the amount of variation in cornering power induced by additional load on a tyre, which in turn has been affected by transfer due to braking or acceleration, may partially or entirely overcome any degradation in cornering power induced by the acceleration or braking involved. A car of different characteristics, the same car on different tyres
or the same tyres
inflated to different pressures, might behave very differently in the same manoeuvres. Despite the advances in Traction and Stability Control, at Unique Cars and Parts
we believe that the best computer you should use is installed in your head.
Engine Location And Its Effect On Handling
In the beginning it didn't much matter where the engine was, so long as it was near the driving wheels. The important thing was whether or not a car actually moved under its own power, not how fast it travelled or how well it went around corners. But as cars became more sophisticated and people became more sophisticated about cars, controversy arose concerning the inherent superiority of one design over the others. One famous manufacturer of front-wheel-drive cars argued that since no one in his right mind would put the cart before the horse, an engine should be used to pull the car instead of pushing it.
To this day, these and similar arguments continue, and though automotive design practice has evolved into four basic categories - front engine/rear drive, rear engine/rear drive, front engine/front drive, and mid-engine/rear drive - there is still a general lack of agreement as to which is best. One area in particular - vehicle stability and handling - has been the subject of some of the most heated debates. Each manufacturer can list any number of reasons why its particular design is superior. That some carmakers have engines and drivetrains in more than one location in similar cars and can present equally convincing arguments for each with a straight face leads to further confusion. Better to say that each has its own unique characteristics, advantages and disadvantages.
Front Engine / Rear Drive
Descriptions of the first automobiles as horseless carriages were really quite appropriate. They were literally carriages - short wheelbase, enormous ground clearance, large-diameter wheels and all with a small engine fitted under the seat. At that time this was adequate for low-speed motoring with two or three passengers. But cars began to grow larger, especially in America, and space for six or seven passengers was demanded. Engines had to be increased in size to pull (or push) these loads and it soon became apparent that locating the engine under the seat was no longer practical.
The geography of the U.S. had an influence on engine placement for American car manufacturers. Long, flat stretches in the midwest and the high mountains of the west required cars with greater horsepower and as metallurgy was in its infancy, increasing power meant larger and larger engines. Displacements of 500 and 600 cu in. weren't uncommon - and at this size it was simply impossible to put the engine under the rear seat. The effect of racing on automotive design was also strong. Cars that were successful in racing sold well to the public and early racers were big, brutish affairs with enormous engines located up front. But even then designers knew something about wheel loadings and the effect of weight distribution on handling, so many of these cars were actually forward mid-engine designs with the engine several inches behind the front axle for better weight distribution.
Over the years a great wealth of know-how about designing cars with front engines and rear drive was built up, and it was cheap and convenient to incorporate a high proportion of well-tried components in each succeeding new model. So the conservative auto industry, especially in the U.S. and Britain, was reluctant to change from the established front engine and rear drive. There were advantages other than cost and design experience to a front engine/rear drive layout. Size was a major factor. Packaging five or six passengers plus luggage is a more difficult engineering task with a rear engine design and to date it hasn't been done with an amidships engine. A front engine/rear drive car lends itself to an inexpensive independent front suspension
for improvements in ride and steering
A solid axle at the rear cannot offer the first-class ride of independent designs but does have its advantages. One is low cost; the other is the elimination of camber change with its possible extra tyre
wear and abrupt reversals in handling characteristics. The effect of ram air through the front grille and forward-mounted radiator makes a front engine relatively easy to cool and accessibility of commonly serviced components is generally better than with mid- or rear-engine designs. Today, safety is an obvious consideration and most designers see in the front-engine layout the only solution to providing adequate crush space (for protecting passengers in a front-end crash) at reasonable cost.
Then there's handling. A nose-heavy front-engine car understeers under most conditions and engineers view this as an asset for the average driver. A driver is less likely to get into serious trouble upon entering a corner too fast if the car's front end runs wide than if its tail has a tendency to come around. There are minuses to a front engine/rear drive layout as well. The driveshaft is a necessary evil of any front engine/rear drive car; it encroaches on passenger space and makes for uncomfortable seating for the middle passengers. An aerodynamic
body shape is more difficult to achieve when a designer has to work around a bulky engine up front since it won't let the nose taper as sharply as it might.
For racing applications the front-engine car is also at a disadvantage as far as driver comfort is concerned: cockpit heat is a serious concern in most big front-engine race cars. Weight transfer toward the rear when accelerating is an asset in a rear-drive car, but wheel-spin can still pose a problem if the proportion of weight on the rear wheels is low. Conversely, weight transfer toward the front during braking unloads the rear wheels too much in a very nose-heavy car to fully use the four tires' friction capabilities.
for road vehicles goes back long before the invention of the internal-combustion engine
, the most famous ancestor being Cugnot's 3-wheel steamer of 1770. The first car to actually use front-wheel-drive
was patented in 1904 by an American, Walter Christie. He mounted the 4-cyinderl engine transversely across the frame, a disposition that went on to gain widespread acceptance, particularly in Europe, during the 1970s. After Christie's racing car, there was a long time before any company used the principle in a series production car. In Germany during the Thirties the low-priced Adler
proved popular as did the front-wheel-drive DKW
, and Europeans developed an interest in front-wheel-drive
built nothing but front-wheel-drive
cars since 1936 and in England front-wheel-drive
was given a boost by various Austin
models starting with the Mini
. By the mid 1970s companies such as Fiat
cars with technical and commercial success. The first pre-war US built car that featured a front-wheel-drive
configuration was the relatively short-lived coffin-nose Cord
, discontinued in 1937. The first post-war American front-wheel-drive
design of note was the Oldsmobile Toronado
, which appeared in 1966
and was followed one year later by the related 4th Generation Cadillac Eldorado
. Though technically interesting, these cars were successful more for their luxury appeal than the esoteric aspects of front-wheel-drive
As time went on there was a trend toward smaller cars, particularly in Europe and Japan. Leaders were the Peugeot 104
, Renault 5
, Honda Civic
models to name just a few. So why the interest in front-wheel-drive
? A front-wheel-drive
layout gives maximum interior passenger and luggage space with minimum outside dimensions, particularly when the engine is transverse in the chassis. It's easy to see why. Combining the engine, transmission and final drive into one unit makes for a compact power package. The driveshaft is eliminated, making a flat floor possible, and even a simple beam axle at the rear keeps boot intrusion to a minimum. There is a drawback to this design sophistication: added cost. Contributing factors include the more costly gearing on a front-wheel-drive
car and the rather tricky front axle system.
More money must also be spent to keep noise and vibration from the power unit out of the passenger compartment. But there are offsetting cost savings as well. A beam axle is simpler to suspend than the solid live axle of a rear-drive car (although some front-wheel-drive
cars have independent rear suspension
). The driveshaft is also eliminated - another saving. Front engine/front drive cars are very nose-heavy, 60% or more of the total weight on the front end being common, so excessive front tyre
wear can be a problem. On large front-wheel-drive
cars a further disadvantage is incurred from the extreme forward weight bias; power steering
becomes a necessity rather than a nicety.
Weight bias and the forward driven wheels combine to explain the driving characteristics that are unique to a front-wheel-drive
design. On a steep gradient, for example, weight transfer from the front to the rear wheels helps traction with rear-wheel drive and reduces it with front-wheel-drive
. But the disproportionate successes of front-wheel-drive
cars in rallies and ice racing provide the real answer to suggestions that they lack traction in difficult conditions. With rear-wheel drive the driven wheels try to propel the car along a straight line, resisting efforts to deflect it from its path. Driven front wheels apply their tractive effort in the direction in which they are steered and on slippery roads this is a definite advantage.
In a front-wheel-drive
car weight transfer to the rear when accelerating reduces traction, and this wheel-spin problem led many designers to postulate that front-wheel-drive
was only suitable with low-powered cars. But the Eldorado
disproved this contention. Under braking the opposite effect occurs - weight is transferred to the front. The same overloading of front brakes
and locking at the rear mentioned for front engine/rear drive cars applies, only more so. Modern disc brakes and proportioning systems at least minimize this disadvantage.
A criticism of fwd cars is that it is necessary to corner with power on, and that if power is taken off the car becomes unstable and oversteers. Front-wheel-drive
cars do corner differently but it's hardly a truism with today's designs that instability when cornering is their basic nature. The reverse is closer to the truth. With almost any car there is some change in cornering behaviour when the driver accelerates because a tyre
which is transmitting power cannot generate as much side force as the same tire when it is rolling freely. So a cornering tyre
runs at a larger slip angle when also delivering power. With rear-wheel drive, acceleration when cornering increases the slip angles of the rear tyres
, increasing oversteer (or in the case of most front engine/rear drive cars, reducing understeer).
it is the front tires which run at larger slip angles when cornering under power, so the front end tends to run wide in an understeering attitude with power applied. However, if power is suddenly released the front tires are relieved of their double duty and the car understeers less, assuming the tucked-in nose characteristic of front-wheel-drive
cars under these conditions. Depending upon how much the designers of the particular car have tried to suppress fwd understeer, the result will be simply less understeer or - rarely - some oversteer. But many front engine/rear drive cars also oversteer under the same conditions.
Many manufacturers mention the "arrow principle" in explaining inherently superior straight-line or crosswind stability of front-wheel-drive
. Their reasoning is simple; did you ever try to throw a dart, feathered end first, and hit a target with any degree of accuracy? There is a bit of truth of this "logic," we must admit, as evidenced by the unusually stable behavior most nose heavy front-wheel-drive
cars exhibit in a crosswind. But the reasons are a bit more complex than the simple arrow principle implies.
Rear Engine / Rear Drive
Although rear-engine cars such as the Julian in the U.S. and the little German Hanomag were produced in the 1920s, it was probably the success of the Auto Union Grand Prix cars in the 1930s that laid the foundation for future mid-engine and rear-engine designs and encouraged Dr Porsche
to apply the principle to his VW
design. The Auto Union was technically a mid-engine design but the car was so large and the enormous engine positioned so far rearward that a distinction between the two engine locations was hardly ever made. The Porsche 356
which made its debut at the Geneva Show in 1948 broke completely with traditional sports-car design. The conventional front engine sports car of the day had its engine up front and drive wheels in the back, a ladder-type frame with low torsional rigidity, rock-hard suspension
with little roadholding on anything but the smoothest of surfaces, and little or no protection from the elements.
The objective of the Porsche
was more along the lines of a high-speed touring GT with due consideration for fast, safe touring in relative comfort. To achieve this goal Porsche
used what were basically his VW
components - a lightweight, air-cooled
engine at the rear, a rigid platform chassis, independent suspension at all four wheels - in an aerodynamic
body shape. Compared to conventional sports cars the Porsche
had light, responsive steering
, smooth ride on all surfaces, a roomy passenger compartment and adequate luggage space, and was incredibly quiet at speed. It also oversteered to an excessive degree because of its rear weight bias, swing-axle rear suspension
and skinny tyres
. And from these early examples grew the notion that all rear-engine cars oversteer in dramatic fashion.
More than 10 years after the introduction of the 356
, the first and last modern rear-engine American car appeared: the Corvair
. Early examples suffered many of the symptoms of classic oversteer but later models, tamed by better design, were enthusiast's delights. While several companies have built rear-engine cars of modern design, only the smallest Fiats, France's Alpine-Renault A310, Czechoslovakia's Tatra and various VW models stuck with the rear engine placement - but that was decades ago now. There were several advantages to a rear engine location. As with front drive, there was no driveshaft tunneling through the cabin. With a natural weight bias on the driven wheels, wheel-spin should be reduced and traction improved in slippery conditions, again much as with front-wheel-drive
Weight transfer toward the rear when accelerating increases the load on the rear wheels and further improves traction; weight transfer to the front during braking tends to equalize wheel loads for more even braking. Clean aerodynamics
, as important for fuel savings and quiet cruising with a road car as for top speed and stability in a race car, are easier to achieve with a rear-engine layout. Cockpit overheating, an important consideration with a race car, is less likely to arise when the engine is behind the driver. Offsetting these advantages are the problems with adapting this configuration to more than a 2-place or 2 + 2 design. A full sedan design puts an inordinate amount of weight on the rear end when the back seat is occupied and a station-wagon load in the rear compounds the imbalance.
Though a sloping front end is fine aerodynamically
, it invariably compromises the front luggage compartment, so rear-engine cars can't always take advantage of the lack of an engine
up front in this way. Air is the logical cooling medium for rear engines, although water cooling has been successfully used. Thus the problem of cooling the engine
in a confined location out of the direct airstream arises. A front radiator
is a possibility but this solution is costly, complex and wasteful of luggage space. The obvious solution is a small, alloy engine
which can be adequately cooled by ambient air, fitted to a small car. Sounds like a VW
, doesn't it? The abrupt oversteer characteristic of early rear-engine cars is not necessarily a problem with later designs: it is possible by judicious juggling of suspension, tires, tire pressures, geometry, etc, to considerably tame oversteering tendencies, but the problem of sidewind stability is more difficult.
Mid-Engine / Rear Drive
Though mid-engine designs are typically thought of as products of modern racing technology, Gottlieb Daimler
placed the engine
behind the driver, offset to the left, on his 1886 motor carriage. This was more for convenience than an attempt at design superiority. However, convenience wasn't the reason for the mid-ship engine layout of the Benz Tropfenwagen, also known as the "teardrop car" because of its uncompromisingly streamlined shape. Raced with moderate success in several versions in the 1920s, the Tropfenwagen was overshadowed by the more glamorous supercharged Mercedes and later Mercedes-Benz cars.
The significance of this car wasn't lost on Dr Porsche
, who made the midship engine
/transaxle configuration work successfully in the Auto Union Grand Prix cars. Later, Porsche
designed the mid-engine 550 Spyder, the first racing Porsche not derived from the 356 series, which enjoyed a fantastic competition record and was a forerunner of such successful Porsche racing models as the 904 coupe, the 908 and the turbocharged
917/10. Cooper pioneered the first truly modern mid-engine Grand Prix cars in the mid-1950s and built one of the earliest sports-racing cars, the Monaco. Following Cooper came a succession of successful mid-engine designs - the indecently quick Lotus 23, Lola 70-Chevrolet, Ford GT40
, Ferrari 250LM
and the much-copied McLaren designs of the late 1960s and early 1970s.
Production mid-engine cars have lagged behind their racing counterparts. One of the first was the Rene Bonnet Djet, later called the Matra Djet, introduced in the early 1960s. The Djet was a somewhat ungainly-looking 2-seater sports car - an inauspicious beginning for mid-engine designs to follow - expensive 2-seat luxury GTs like the DeTomaso Mangusta
and its replacement the Pantera
, the Ferrari Dino
, the Lamborghini Miura
(first with a transverse mid-engine) and the Maserati Bora
. These were cars that ushered in a new era in automotive design, and for those with more down-to-earth budgets there was the Lotus Europa
, Porsche 914
- the first mass produced mid-engine car - and the Fiat X1/9
The almost complete takeover of racing by cars with a mid-engine configuration can be explained by the term polar moment of inertia, which was explained earlier in this article. For illustration, consider two bowling balls attached at the ends of a weightless bar. If you lift the bar in the middle and try to turn it you will find that the weight of the balls at each end make movement difficult to initiate and stop. Now, however, if the bar is shortened so that the two balls are placed next to each other the system is much easier to rotate, without a decrease in total weight. The polar moment of inertia has been decreased. When the driver, engine, transmission, fuel tank, etc, are all placed between the wheels, the polar moment of inertia of the car about the centre of gravity is low; thus the tires can more easily alter the course of the car.
Steering is usually responsive and sensitive too. There are other benefits as well: the majority of weight is again on the driving wheels but the distribution isn't so extreme as with the rear engine or front drive; cornering power can be at a maximum; handling characteristics can be tailored with few compromises. Aerodynamic considerations are an important factor. Unhindered by an unwieldy chunk of cast iron at the front end, designers have far more latitude in shaping the nose and overall body shape for minimum drag with maximum down force. But if the driver can easily move the car out of a straight line, so can other kinds of forces, such as a bump or a sidewind. So the mid -engine car gives but it also demands. In exchange for higher cornering power, the mid-engine design asks for a driver with a high degree of competence.
Mid-engine cars don't break away easily but when they go, they go suddenly. Drivers who are accustomed to tyre squeal or body roll as signs of imminent danger often have difficulty in predicting the breakaway of mid-engine cars because they approach their limits so un-dramatically. In racing, where lower lap times alone are the telling factor, a designer may often compromise some areas of the car to reach this goal. Road cars, however, must meet more complex automotive needs. So there are several problems which have kept the mid-engine car from achieving the popularity one might expect. Cost is a primary consideration. Innovation usually costs money, at least until economical solutions to basic problems are achieved.
Because the engine sits right behind the driver in a mid-engine car, such a layout is really only adaptable to 2-seat sports and GT cars. Attempts that have been made to provide extra seats, as in the Lamborghini Urraco
, were incomplete solutions at best. For a mass-produced car serviceability is a definite consideration, and accessibility is not a strong point of most mid-engine designs. Add to this heat and noise from an enclosed engine compartment close to the passengers, and you have problems that make even strong engineers cringe. Finally there's the question of rearward vision. No mid-engine car yet with the possible exception of the Porsche 914
has come up with a successful solution to that problem.