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Car handling

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Car handling and vehicle handling is a description of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. It also includes their stability when moving in a straight line. Handling and braking are the major components of a vehicle's "active" safety. The maximum lateral acceleration is sometimes discussed separately as "road holding". Handling is an esoteric performance area because rapid and violent manoeuvres are often only used in unforeseen circumstances. (This discussion is directed at road vehicles with at least three wheels, but some of it may apply to other ground vehicles.)

Cars, for use on public roads, whose engineering requirements emphasise handling above passenger space and comfort, are called sports cars.

Factors that affect a car's handling

Driver

Handling is a property of the car, but different characteristics will work well with different drivers.

Familiarity

A person learns to control a car much as he learns to control his body, so the more he has driven a car or type of car the better it will handle for him. One needs to take extra care for the first few thousand miles after buying a car, especially if it differs in design from those he is used to. Other things that a driver must adjust to include changes in tyres, tyre pressures and load. That is, handling is not just good or bad; it is also the same or different.

Weather

Weather affects handling by making the road slippery. Different tyres do best in different weather. Deep water is an exception to the rule that wider tyres improve road holding. (See aquaplaning under tyres, below.)

Road condition

Cars with relatively soft suspension and with low unsprung weight are least affected by uneven surfaces, while on flat smooth surfaces the stiffer the better. Unexpected water, ice, oil, etc. are hazards.

Weight distribution

Center of gravity height

The center (centre) of gravity height, relative to the track, determines load transfer, also called weight transfer, from side to side and causes body lean. Centrifugal force acts at the center of gravity to lean the car toward the outside of the curve, increasing downward force on the outside tyres.

The centre of gravity height, relative to the wheelbase, determines load transfer between front and rear. The car's momentum acts at its center of gravity to twist the car forward or backward, respectively during braking and acceleration. Since it is only the downward force that changes and not the location of the center of gravity, the effect on over/under steer is opposite to that of an actual change in the center of gravity. When a car is braking, the downward load on the front tyres increases and that on the rear decreases, with corresponding change in their ability to take sideways load, causing oversteer.

Lower center of gravity is the principle performance advantage of sports cars, compared to sedans and (especially) SUVs.

Body lean can also be controlled by the springs, anti-roll bars or the roll center heights.

Roll angular inertia

This increases the time it takes to settle down and follow the steering. It depends on the (square of) the height and width.

Greater width, then, though it counteracts center of gravity hight, hurts handling by increasing angular inertia.

Center of gravity forward or back

In steady-state cornering, front heavy cars tend to understeer and rear heavy cars to oversteer, all other things being equal. This can be compensated, at least mostly, by using wheels and tyres with size (width times diameter) proportional to the weight carried by each end.

When all four wheels and tyres are of equal size, as is most often the case with passenger cars, a weight distribution close to "50/50" (i.e. the centre of mass is mid-way between the front and rear axles) produces the preferred handling compromise. However, if unequal size tyres are acceptable, better handling is achieved by a rearward weight bias, using larger rear tyres to keep the steady-state cornering balance near neutral.

The rearward weight bias preferred by sports and racing cars results from handling effects during the transition from straight-ahead to cornering. During corner entry the front tyres, in addition to generating part of the lateral force required to accelerate the car's centre of mass into the turn, also generate a torque about the car's vertical axis that starts the car rotating into the turn. However, the lateral force being generated by the rear tyres is acting in the opposite torsional sense, trying to rotate the car out of the turn. For this reason, a car with "50/50" weight distribution will understeer on initial corner entry. To avoid this problem, sports and racing cars often have a more rearward weight distribution. In the case of pure racing cars, this is typically between "45/55" and "40/60." This gives the front tyres an advantage in overcoming the car's moment of inertia (yaw angular inertia), thus reducing corner-entry understeer.

Once a car is designed, weight distribution can be changed by using different diameter tyres or jacking the car up higher or lower at the suspension springs. Jacking is frequently done with screws or shims at the springs.

Yaw and pitch angular inertia (polar moment)

Unless the vehicle is very short, compared to its height or width, these are about equal. Angular inertia determines the rotational inertia of an object for a given rate of rotation.

The yaw angular inertia tends to keep the direction the car is pointing changing at a constant rate. This makes it slower to swerve or go into a tight curve, and it also makes it slower to turn straight again. The pitch angular inertia detracts from the ability of the suspension to keep front and back tyre loadings constant on uneven surfaces and therefore contributes to bump steer.

Angular inertia is an integral over the square of the distance from the centre of gravity, so it favors small cars even though the lever arms (wheelbase and track) also increase with scale. (Since cars have reasonable symmetrical shapes, the off-diagonal terms of the angular inertia tensor can usually be ignored.)

Suspension

Automobile suspensions have many variable characteristics, which are generally different in the front and rear and all of which affect handling. Some of these are: spring rate, damping, straight ahead camber angle, camber change with wheel travel, roll centre height and the flexibility and vibration modes of the suspension elements. Suspension also affects unsprung weight.

Many cars have suspension that connects the wheels on the two sides, either by a sway bar and/or by a solid axle. The Citroën 2CV has interaction between the front and rear suspension.

The flexing of the frame interacts with the suspension. (See below.)

Tyres and wheels

In general, larger tyres, softer rubber, higher hysteresis rubber and stiffer cord configurations increase road holding and improve handling. On most types of poor surfaces, large diameter wheels perform better than lower wider wheels. The fact that larger tyres, relative to weight, stick better is the main reason that front heavy cars tend to understeer and rear heavy to oversteer. The depth of tread remaining greatly affects aquaplaning (riding over deep water without reaching the road surface). Increasing tyre pressures reduces their slip angle, but (for given road conditions and loading) there is an optimum pressure for road holding.

Track and wheelbase

The track provides the resistance to sideways weight transfer and body lean. The wheelbase provides resistance to front/back weight transfer and provides the torque lever arm to rotate the car when swerving. The wheelbase, however, is less important than angular inertia (polar moment) to the vehicle's ability to swerve quickly.

Unsprung weight

Car diagram.jpg

Ignoring the flexing of other components, a car can be modelled as the sprung weight, carried by the springs, carried by the unsprung weight, carried by the tyres, carried by the road. Without the unsprung weight, the force of a tyre on the road would come from the vehicle weight and motion, transmitted by the spring. But the unsprung weight is cushioned from uneven road surfaces only by the springiness of the tyres (and wire wheels if fitted). To aggravate this (for fuel economy and to avoid overheating at high speed) tyres have limited internal damping. So the "wheel bounce" or resonant motion of the unsprung weight moving up and down on the springiness of the tyre is only poorly damped, mainly by the dampers or Shock absorbers of the suspension. For these reasons, high unsprung weight reduces road holding and increases unpredictable changes in direction on rough surfaces (as well as degrading ride comfort and increasing mechanical loads).

This unsprung weight includes the wheels and tyres, usually the brakes, plus some percentage of the suspension, depending on how much of the suspension moves with the body and how much with the wheels; for instance a solid axle is completely unsprung. The main factors that improve unsprung weight are a sprung differential (as opposed to live axle) and inboard brakes. (The De Dion tube suspension operates much as a live axle does, but represents an improvement because it is lighter, thereby reducing the unsprung weight.) Aluminium wheels also help. Magnesium wheels are even lighter but corrode easily.

Since only the brakes on the driving wheels can easily be inboard, the Citroën 2CV had additional dampers on its rear wheel hubs to damp only wheel bounce.

Aerodynamics

Aerodynamic forces are generally proportional to the square of the air speed, therefore car aerodynamics become rapidly more important as speed increases. Like darts, aeroplanes, etc., cars can be stabilised by fins and other rear aerodynamic devices. However, in addition to this cars also use downforce or "negative lift" to improve road holding. This is prominent on many types of racing cars, but is also used on most passenger cars to some degree, if only to counteract the tendency for the car to otherwise produce positive lift.

In addition to providing increased adhesion, car aerodynamics are frequently designed to compensate for the inherent increase in oversteer as cornering speed increases. When a car corners, it must rotate about its vertical axis as well as translate its centre of mass in an arch. However, in a tight-radius (lower speed) corner the angular velocity of the car is high, while in a longer-radius (higher speed) corner the angular velocity is much lower. Therefore, the front tyres have a more difficult time overcoming the car's moment of inertia during corner entry at low speed, and much less difficulty as the cornering speed increases. So the natural tendency of any car is to understeer on entry to low-speed corners and oversteer on entry to high-speed corners. To compensate for this unavoidable effect, car designers often bias the car's handling toward less corner-entry understeer (such as by lowering the front roll center), and add rearward bias to the aerodynamic downforce to compensate in higher-speed corners. The rearward aerodynamic bias may be achieved by an airfoil or "spoiler" mounted near the rear of the car, but a useful effect can also be achieved by careful shaping of the body as a whole, particularly the aft areas.

Delivery of power to the wheels and brakes

The coefficient of friction of rubber on the road limits the magnitude of the vector sum of the transverse and longitudinal force. So the driven wheels or those supplying the most braking tend to slip sideways. This phenomenon is often explained by use of the circle of forces model.

One reason that sports cars are usually rear wheel drive is that power induced oversteer is useful, to a skilled driver, for tight curves. The weight transfer under acceleration has the opposite effect and either may dominate, depending on the conditions. Inducing understeer by applying power in a front wheel drive car is less useful. In any case, this is not an important safety issue, because power is not normally used in emergency situations. Using low gears down steep hills may cause some oversteer.

The effect of braking on handling is complicated by load transfer, which is proportional to the (negative) acceleration times the ratio of the centre of gravity height to the wheelbase. The difficulty is that the acceleration at the limit of adhesion depends on the road surface, so with the same ratio of front to back braking force, a car will understeer under braking on slick surfaces and oversteer under hard braking on solid surfaces. Most modern cars combat this by varying the distribution of braking in some way. This is important with a high centre of gravity, but it is also done on low centre of gravity cars, from which a higher level of performance is expected.

Position and support for the driver

Having to take up "g forces" in his/her arms interferes with a driver's precise steering. In a similar manner, a lack of support for the seating position of the driver may cause them to move around as the car undergoes rapid acceleration (through cornering, taking off or braking). This interferes with precise control inputs, making the car more difficult to control.

Being able to reach the controls easily is also an important consideration, especially if a car is being driven hard.

Driver position and support is also an important safety consideration, as during an accident considerable forces are applied to the driver, who must be restrained as much as possible from hitting hard objects (such as the steering wheel, windscreen, side windows or B-pillar). A supportive seat and good seat belt contibute to holding the driver in place, although these may be augmented with further active safety devices.

Steering

Depending on the driver, steering force and transmission of road forces back to the steering wheel and the steering ratio of turns of the steering wheel to tuns of the road wheels affect control and awareness. Play — free rotation of the steering wheel before the wheels rotate — is a common problem, especially in older model and worn cars. Another is friction. Rack and pinion steering is generally considered the best type of mechanism for control effectiveness. The linkage also contributes play and friction. Caster — offset of the steering axis from the contact patch — provides some of the self centring tendency.

Precision of the steering is particularly important on ice or hard packed snow where the slip angle at the limit of adhesion is smaller than on dry roads.

The steering effort depends on the downward force on the steering tyres and on the radius of the contact patch. So for constant tyre pressure, it goes like the 1.5 power of the vehicle's weight. The driver's ability to exert torque on the wheel scales similarly with her size. The wheels must be rotated farther on a longer car to turn with a given radius. Power steering reduces the required force at the expense of feel. It is useful, mostly in parking, when the weight of a front-heavy vehicle exceeds about ten or fifteen times the driver's weight, for physically impaired drivers and when there is much friction in the steering mechanism.

Four-wheel steering has begun to be used on road cars (Some WW II reconnaissance vehicles had it). It relieves the effect of angular inertia by starting the whole car moving before it rotates toward the desired direction. It can also be used, in the other direction, to reduce the turning radius. Some cars will do one or the other, depending on the speed.

Steering geometry changes due to bumps in the road may cause the front wheels to steer in a different directions together or independent of each other. The steering linkage should be designed to minimise this effect.

Suspension travel

The severe handling vice of the TR3 and related cars was caused by running out of suspension travel. (See below.) Other vehicles will run out of suspension travel with some combination of bumps and turns, with similarly catastrophic effect. Excessively modified cars also may encounter this problem.

Electronic stability control

Since automobile safety is mainly a control issue, one should expect a largely electronic solution. Apparently there has already been some advance in this direction.

On the other hand, since stability control works by reducing sudden manoeuvres, until the electronics helps to detect the danger sooner, it can never take the place of a low centre of gravity, which provides both stability and fast avoidance. (See Wireless vehicle safety communications.)

The stability control of some cars may not be compatible with some driving techniques, such as power induced over-steer. It is therefore, at least from a sporting point of view, preferable that it can be disabled.

Alignment of the wheels

Of course things should be the same, left and right. Toe in affects steering because a tyre tends to move in the direction the top of it is leaning.

Rigidity of the frame

The frame may flex with load, especially twisting on bumps. Rigidity is considered to help handling. At least it simplifies the suspension engineers work. Some cars, such as the Mercedes-Benz 300SL have had high doors to allow a stiffer frame.

Common handling problems

When any wheel leaves contact with the road there is a change in handling, so the suspension should keep all four (or three) wheels on the road in spite of hard cornering, swerving and bumps in the road. It is very important for handling, as well as other reasons, not to run out of suspension travel and "bottom" or "top".

It is usually most desirable to have the car adjusted for neutral steer, so that it responds predictably to a turn of the steering wheel and the rear wheels have the same slip angle as the front wheels. However this may not be achievable for all loading, road and weather conditions, speed ranges, or while turning under acceleration or braking. Ideally, a car should carry passengers and baggage near its centre of gravity and have similar tyre loading, camber angle and roll stiffness in front and back to minimise the variation in handling characteristics. A driver can learn to deal with oversteer or understeer, but not if it varies greatly.

The most important common handling failings are;

  • Understeer - the front wheels tend to crawl slightly or even slip and drift towards the outside of the turn. The driver can compensate by turning a little more tightly, but road-holding is reduced, the car's behaviour is less predictable and the tyres are liable to wear more quickly.
  • Oversteer - the rear wheels tend to crawl or slip towards the outside of the turn more than the front. The driver must correct by steering away from the corner, otherwise the car is liable to spin, if pushed to its limit. Oversteer is sometimes useful, to assist in steering, especially if it occurs only when the driver chooses it by applying power.
  • Bump steer – is the tendency for unevenness of a road surface to affect the yaw angle (heading) of the car. This will always happen under some conditions but depends on suspension, steering linkage, unsprung weight, angular inertia, differential type, frame rigidity, tyres and tyre pressures. Extreme bump steer may results when a vehicle's suspension travel is exhausted so that a wheel either bottoms or leaves the road. As with hard turning on flat roads, it is better if the wheel picks up by the spring reaching its neutral shape, rather than by suddenly contacting a limiting structure of the suspension.
  • Body roll - the car leans towards the outside of the curve. This interferes with the driver's control, because he must wait for the car to finish leaning before he can fully judge the effect of his steering change. It also adds to the delay before the car moves in the desired direction.
  • Weight transfer - the wheels on the outside of a curve are more heavily loaded than those on the inside. This tends to overload the tyres on the outside and therefore reduce road holding. Weight transfer (sum of front and back), in steady cornering, is determined by the ratio of the height of a car's centre of gravity to its track. Differences between the weight transfer in front and back are determined by the relative roll stiffness and contribute to the over or under-steer characteristics.
    • When the weight transfer equals half the vehicle's loaded weight, it will start to roll over. This can be avoided by manually or automatically reducing the turn rate, but this causes further reduction in road-holding. (A collision may be preferable to a rollover.)
  • Slow response - sideways acceleration does not start immediately when the steering is turned and may not stop immediately when it is returned to centre. This is partly caused by body roll. Other causes include tyres with high slip angle, and yaw and roll angular inertia. Roll angular inertia aggravates body roll by delaying it. Soft tyres aggravate yaw angular inertia by waiting for the car to reach their slip angle before turning the car.

Compromises

For ordinary production cars, manufactures err towards deliberate understeer as this is safer for inexperienced or inattentive drivers than is oversteer. Other compromises involve comfort and utility, such as preference for a softer smoother ride or more seating capacity. High levels of comfort are incompatible with a low centre of gravity, body roll resistance, low angular inertia, support for the driver, steering feel and other characteristics that make a car handle well. Inboard brakes improve both handling and comfort but take up space and are harder to cool. Large engines, tend to make cars front or rear heavy. In tyres, fuel economy, staying cool at high speeds, ride comfort and long wear all tend to conflict with road holding, while wet, dry, deep water and snow road holding are not exactly compatible. A-arm or wishbone front suspension tends to give better handling, because it provides the engineers more freedom to choose the geometry, and more road holding, because the camber is better suited to radial tyres, than MacPherson strut, but it takes more space. Live solid axle rear suspension is mainly used to reduce cost, but, in general, cost is a relatively less important factor.

In fact, cost may sometimes be negatively correlated with handling, because small size, though it makes little difference in the cost of the car itself, improves both handling and fuel economy (as well as braking, parking, etc.). This may have been true in the US in the late 1950s when many of the European imports undersold the Detroit "dinosaurs". It may again be true in the 2000s, now that large cars, called SUVs or styled as pickups, have regained popularity.

Aftermarket modifications and adjustments to affect handling

Component Reduce Under-steer Reduce Over-steer
Weight distribution centre of gravity towards rear centre of gravity towards front
Front shock absorber softer stiffer
Rear shock absorber stiffer softer
Front sway bar softer stiffer
Rear sway bar stiffer softer
Front tyre selection1 larger contact area2 smaller contact area
Rear tyre selection smaller contact area larger contact area2
Front wheel rim width or diameter larger2 smaller
Rear wheel rim width or diameter smaller larger2
Front tyre pressure higher pressure lower pressure
Rear tyre pressure lower pressure higher pressure
Front wheel camber increase negative camber reduce negative camber
Rear wheel camber reduce negative camber increase negative camber
Rear spoiler smaller larger
Front height (because these usually
  affect camber and roll resistance)
lower front end raise front end
Rear height raise rear end lower rear end
Front toe in increase decrease
Rear toe in decrease increase
1) tyre contact area can be increased by using wider tyres, or tyres with fewer grooves in the tread pattern. Of course fewer grooves has the opposite effect in wet weather or other poor road conditions.

2) These also improve road holding, under most conditions.

In addition, lowering the centre of gravity will always help the handling (as well as reduce the chance of roll-over). This can be done to some extent by using plastic windows (or none) and light roof, hood (bonnet) and boot (trunk) lid materials, by reducing the ground clearance, etc. Increasing the track with "reversed" wheels will have a similar effect, but remember that the wider the car the less spare room it has on the road and the farther you may have to swerve to miss an obstacle. Stiffer springs and/or shocks, both front and rear, will generally improve handling, at the expense of comfort on small bumps. Performance suspension kits are available. Light alloy (mostly aluminium or magnesium) wheels improve handling and ride as well as appearance.

Moment of inertia can be reduced by using lighter bumpers and wings (fenders), or none at all.

Cars with unusual handling problems

  • Porsche 911 — the inside front wheel leaves the road during hard cornering on dry pavement. This causes increasing understeer, but it is still considered to have acceptable handling, even for a sports car. The roll bar stiffness at the front is set to compensate for the rear-heaviness and gives neutral handling in ordinary driving. This compensation starts to give out when the wheel lifts. Later model 911s have had increasingly sophisticated rear suspensions and larger rear tyres.
  • Triumph TR2, TR3 and TR4 — began to oversteer more suddenly when their inside rear wheel lifted.
  • Mercedes-Benz A-Class — early models showed excessive body roll during sharp swerving manoeuvres, most particularly during the Swedish moose test. This was later corrected using Electronic Stability Control and retrofitted at great expense to earlier cars.
  • Volkswagen Beetle — (original Beetle, for young people) The limitations of the Beetle's handling and roll stability were blamed, by Ralph Nader, on the swing axle suspension. Its design was based closely on Porsche's Auto Union grand prix car in the 1930s, it is surprising for many to hear that it was neither top heavy (as its appearance would suggest) nor particularly rear-heavy (in fact a well ballanced 42/58). Since they were produced for so long, with stickier tyres and more powerful engines, people who drove them hard fitted reversed wheels and bigger rear tyres and rims.
  • The gaudy 1950s American "full size" "dinosaurs" — responded very slowly to steering changes, because of their very large angular inertia, soft but simple suspension and comfort oriented cross bias tyres. Auto Motor und Sport reported on one of these that they lacked the courage to test it for top speed. Contact with Europe and the 1970s energy crisis have gradually relieved this problem. (Large trucks, also, cannot be made to respond quickly because of their angular inertia.)
  • Dodge Omni and Plymouth Horizon — these early American responses to the Volkswagen Rabbit were found "Unacceptable" in their initi