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CATEGORIES (articles) > Steering, Suspension, brakes & drivetrain > Technical > Car Handling explained

Car Handling explained


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, 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.

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

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.

The centre of gravity height, relative to the track, determines load transfer (side to side and front/rear) and causes body lean.

Track width affects load transfer too. Body lean can be controlled by the springs, anti-roll bars or the roll centre height.

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

Suspension

Automobile suspension has 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, 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 an anti-roll 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

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 the tyre on the road would be the vehicle weight 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 centre), 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.

Yaw and pitch angular inertia (polar moment)

Unless the vehicle is very short these are about the same. The yaw angular inertia tends to keep the rate of change in pointing direction constant. 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 to keep front and back tyre loadings constant on uneven surfaces.

Angular inertia is an integral over the square of the distance from the centre of gravity, so it favours small cars even though the lever arms also increase with scale.

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.

Position and support for the driver

Having to take up lateral "g forces" in his/her arms interferes with a driver's precise steering.

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 recognisance 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.

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.)

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 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.
  • 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. It is likely to contribute to understeer or oversteer. 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.
  • 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.

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 center 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.

Cars with unusual handling problems

  • Porsche 911 — the inside front wheel leaves the road during hard cornering on dry pavement. This causes increasing oversteer, but it is still considered to have acceptable handling, even for a sports car. The roll stiffnesses are apparently set to compensate for the rear-heaviness and give 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 active suspension.
  • 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, but that appears to have been based more on political aspirations than on engineering knowledge. Since it was designed in the 1930s as the minimum family car that would be reliable at motorway speeds, it is not surprising that it was top-heavy and somewhat rear-heavy (42/58). (Oversteer appears not to have been considered a disadvantage in Germany at that time.) 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 initial testing by Consumer Reports, due to an observed tendency to display an uncontrollable oscillating yaw from side to side under certain steering inputs. While Chrysler's denials of this behaviour were countered by a persistent trickle of independent reports of this behaviour, production of the cars was altered to equip them with both a lighter weight steering wheel and a steering damper, and no further reports of this problem were heard.
  • The Suzuki Samurai — was similarly reported by Consumer Reports to exhibit a propensity to tipping over onto two wheels, to the point where they were afraid to continue testing the vehicle without the attachment of outrigger wheels to catch it from completely rolling over; once again, they rated it as "Unacceptable", and once again the manufacturer denied that it was any sort of problem "in the real world", while reports by owners who had experienced such rollovers steadily trickled in. The vehicle was eventually taken off the market before any changes were made to the handling. As SUVs became popular, however, it became evident that their high centre of mass made them more likely to tip over than passenger cars, and some even did so during Consumer Reports' testing; but none other than the Samurai showed such a readiness to roll over that they were rated unacceptable. This was in all probability due to the Samurai's being exceptionally short and narrow. See http://www.safercar.gov/Rollover.
  • Ford SUVs — then were cited as having a dangerous tendency to blow a rear tyre and flip over. Ford and Firestone, the makers of the tyres, pointed fingers at each other, with the final blame being assigned to quality control practices at a Firestone plant which was undergoing a strike; it was widely surmised, however, that at least part of the problem was caused by Ford specifying lower than optimum pressures in the tyres in order to induce them to lose traction and slide under sideways forces rather than to grip and force the vehicle to roll over. An internal document dated 1989 states
Engineering has recommended use of tyre pressures below maximum allowable inflation levels for all UN46 tyres. As described previously, the reduced tyre pressures increase understeer and reduce maximum cornering capacity (both 'stabilising' influences). This practice has been used routinely in heavy duty pick-up truck and car station wagon applications to assure adequate understeer under all loading conditions. Nissan (Pathfinder), Toyota, Chevrolet, and Dodge also reduce tyre pressures for selected applications. While we cannot be sure of their reasons, similarities in vehicle loading suggest that maintaining a minimal level of understeer under rear-loaded conditions may be the compelling factor. http://www.citizen.org/autosafety/articles.cfm?ID=5336
This contributed to build-up of heat and tyre deterioration under sustained high speed use, and eventual failure of the most highly stressed tyre. Of course, the possibility that slightly substandard tyre construction and slightly higher than average tyre stress, neither of which would be problematic in themselves, would in combination result in tyre failure is quite likely. The controversy continues without unequivocal conclusions, but it also brought public attention to a generally high incidence of rollover accidents involving SUVs, which the manufacturers continue to address in various ways.

(One of the handling advantages of sports cars is that their very lack of carrying capacity allows their standard tyre pressures, as well as sizes, to be optimised for light load.)

  • The Jensen Healey hatchback coop — was introduced in attempt to broaden the sales base of the Jensen Healey, which had up to that time been a roadster or convertible. Its road test report in Motor Magazine and a very similar one, soon after, in Road & Track concluded that it was no longer fun enough to drive to be worth that much money. They blamed it on minor suspension changes. Much more likely, the change in weight distribution was at fault. The Jensen Healey was a rather low and wide fairly expensive sports car, but the specifications of its suspension were not particularly impressive, having a solid rear axle. Unlike the AC Aceca, with its double transverse leaf rear suspension and aluminium body, the Jensen Healey could not stand the weight of that high up metal and glass and still earn a premium price for its handling. The changes also included a cast iron exhaust manifold replacing the aluminium one, probably to partly balance the high and far back weight of the top. The car had also suffered reliability problems with engines that Jensen bought from Lotus. The factory building was used to build multi-tub truck frames.




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