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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".
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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 that drive on public roads, whose engineering requirements emphasize handling above passenger space and comfort, are called sports cars.
Factors that affect a car's handling
Weight distribution
Center of gravity height
The center of gravity height, relative to the track, determines load transfer, (related to, but not exactly 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 tires.
Height of the center of gravity relative to the wheelbase determines load transfer between front and rear. The car's momentum acts at its center of gravity to tilt 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 tires 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 principal performance advantage of sports cars, compared to sedans and (especially) SUVs. Some cars have light materials in their roofs, partly for this reason. It is also part of the reason that traditional sports cars are open or convertible.
Body lean can also be controlled by the springs, anti-roll bars or the roll center heights.
Center of gravity forward or back
In steady-state cornering, because of the center of gravity, front-heavy cars tend to understeer and rear-heavy cars to oversteer, all other things being equal. The mid-engine design offers the ideal center of gravity.
When all four wheels and tires are of equal size, as is most often the case with passenger cars, a weight distribution close to "50/50" (i.e. the center of mass is mid-way between the front and rear axles) produces the preferred handling compromise.
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 tires, in addition to generating part of the lateral force required to accelerate the car's center 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 tires 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 "40/60" and "35/65." This gives the front tires an advantage in overcoming the car's moment of inertia (yaw angular inertia), thus reducing corner-entry understeer.
Using wheels and tires of different sizes (proportional to the weight carried by each end) is a lever automakers can use to fine tune the resulting over/understeer characteristics.
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, and (for a uniform mass distribution) can be approximately calculated by the equation: .
Greater width, then, though it counteracts center of gravity height, hurts handling by increasing angular inertia. Some high performance cars have light materials in their fenders and roofs partly for this reason.
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 tire loadings constant on uneven surfaces and therefore contributes to bump steer.
Angular inertia is an integral over the square of the distance from the center 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.)
Mass near the ends of a car can be avoided, without re-designing it to be shorter, by the use of light materials for bumpers and fenders or by deleting them entirely.
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 center 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.)
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.
Tires and wheels
In general, larger tires, 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 tires, 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 tire 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 to pitch angular inertia, 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 modeled as the sprung weight, carried by the springs, carried by the unsprung weight, carried by the tires, carried by the road. Without the unsprung weight, the force of a tire 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 tires (and wire wheels if fitted). To alleviate this (for fuel economy and to avoid overheating at high speed) tires have limited internal damping. So the "wheel bounce" or resonant motion of the unsprung weight moving up and down on the springiness of the tire 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 tires, 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 the diff is mounted to the body, thereby reducing the unsprung weight.) Aluminum wheels also help. Magnesium alloy wheels are even lighter but corrode easily.
Since only the brakes on the driving wheels can easily be inboard, the Citroën 2CV had inertial 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 center of mass in an arc. 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 tires 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 useful via proper use of "Left-foot braking." 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 center 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 center of gravity, but it is also done on low center of gravity cars, from which a higher level of performance is expected.
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 turns 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-centering 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 tires and on the radius of the contact patch. So for constant tire 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 his 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 different directions together or independent of each other. The steering linkage should be designed to minimize this effect.
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 center 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.
Static alignment of the wheels
Of course things should be the same, left and right, for road cars. Camber affects steering because a tire generates a force towards the side that the top is leaning towards. This is called camber thrust. Additional front negative camber is used to improve the cornering ability of cars with insufficient camber gain.
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.
Driver handling the car
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 them. One needs to take extra care for the first few months after buying a car, especially if it differs in design from those they are used to. Other things that a driver must adjust to include changes in tires, tire pressures and load. That is, handling is not just good or bad; it is also the same or different.
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.
In some circumstances, good support may allow a driver to retain some control, even after a minor accident or after the first stage of an accident.
External conditions that affect handling
Weather
Weather affects handling by making the road slippery. Different tires do best in different weather. Deep water is an exception to the rule that wider tires improve road holding. (See aquaplaning under tires, 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.
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 a small amount of understeer, so that it responds predictably to a turn of the steering wheel and the rear wheels have a smaller slip angle than 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 center of gravity and have similar tire loading, camber angle and roll stiffness in front and back to minimise the variation in handling characteristics. A driver can learn to deal with excessive oversteer or understeer, but not if it varies greatly in a short period of time.
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 tires 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 effect of irregularity of a road surface on the angle or motion of a car. It may be the result of the kinematic motion of the suspension rising or falling, causing toe-in or toe-out at the loaded wheel, ultimately affecting 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, tires and tire pressures. If suspension travel is exhausted the wheel either bottoms or loses contact with 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 tires 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 center 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 center. This is partly caused by body roll. Other causes include tires with high slip angle, and yaw and roll angular inertia. Roll angular inertia aggravates body roll by delaying it. Soft tires aggravate yaw angular inertia by waiting for the car to reach their slip angle before turning the car.
Compromises
Ride quality and handling have always been a compromise - technology has over time allowed automakers to combine more of both features in the same vehicle.
High levels of comfort are difficult to reconcile with a low center of gravity, body roll resistance, low angular inertia, support for the driver, steering feel and other characteristics that make a car handle well.
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.
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 tires, 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 tires, than MacPherson strut, but it takes more space.
The older Live axle rear suspension technology, familiar from the Ford Model T, is still widely used in most sport utility vehicles and trucks. The live axle suspension is still used in some sports cars, like the Ford Mustang, and is better for drag racing, but generally has problems with grip on bumpy corners, and stability at high speeds on bumpy straights. Having said that a good live axle can be superior to a poor independent rear suspension system, in most circumstances.
Aftermarket modifications and adjustments
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 aluminum or magnesium) wheels improve handling as well as ride comfort.
Moment of inertia can be reduced by using lighter bumpers and wings (fenders), or none at all.
Cars with unusual handling problems
Certain vehicles can be involved in a disproportionate share of single-vehicle accidents - their handling characteristics may play a role:
- early Porsche 911s — suffered from treacherous lift off oversteer (where the car unpredictably leaves the road tail first); also the inside front wheel leaves the road during hard cornering on dry pavement, causing increasing understeer. 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. A skilled driver can use the 911's other features to his/her advantage, making the 911 an extremely capable sports car in expert hands. Later 911s have had increasingly sophisticated rear suspensions and larger rear tires, eliminating these problems.
- Triumph TR2, and TR3 — began to oversteer more suddenly when their inside rear wheel lifted.
- Volkswagen Beetle — (original Beetle) senstitivity to crosswinds, due to the lightness of the front of the rear engine car; and poor roll stability due to the swing axle suspension. People who drove them hard fitted reversed wheels and bigger rear tires and rims to ameliorate.
- Chevrolet Corvair - cited for dangerous handling in Unsafe at Any Speed caused by poor roll stability due to the swing axle rear suspension similar to that used in the Volkswagen Beetle. These problems were corrected with the redesign of the Corvair for 1965, however, it died from its negative publicity.
- The large, rear-engine Tatra (known as the 'Czech secret weapon') killed so many Nazi officers during World War II that the German Army eventually forbade its officers from driving the Tatra.
- Some 1950s American "full size" cars responded very slowly to steering changes because of their very large angular inertia, softly tuned suspension which made ride quality a priority over cornering, and comfort oriented cross bias tires. Auto Motor und Sport reported on one of these that they lacked the courage to test it for top speed, probably due to their familiarity with smaller European cars and their unfamiliarity with large American cars.
- 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 center 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, as theoretically predictable by the Samurai's being exceptionally short and narrow. See http://www.safercar.gov/Rollover.
- Mercedes-Benz A-Class — a tall car with a high center of gravity; early models showed excessive body roll during sharp swerving manoeuvres and rolled over, most particularly during the Swedish moose test. This was later corrected using Electronic Stability Control and retrofitted at great expense to earlier cars.
- Ford Explorer — a dangerous tendency to blow a rear tire and flip over. Ford had constructed a vehicle with a high center of gravity - the tendency to roll over on sharp changes in direction is built in to the vehicle. Ford attempted to counteract the forces of nature by specifying lower than optimum pressures, in the tires in order to induce them to lose traction and slide under sideways forces rather than to grip and force the vehicle to roll over. For reasons that were never entirely clear, these vehicles then suffered from sudden tire blow outs, which led to a spate of well publicized single-vehicle accidents.
- Ford and Firestone, the makers of the tires, pointed fingers at each other, with the final blame being assigned to quality control practices at a Firestone plant which was undergoing a strike. Tires from a different Firestone plant were not associated with this problem. An internal document dated 1989 states
Engineering has recommended use of tire pressures below maximum allowable inflation levels for all UN46 tires. As described previously, the reduced tire 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 tire 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 tire deterioration under sustained high speed use, and eventual failure of the most highly stressed tire. Of course, the possibility that slightly substandard tire construction and slightly higher than average tire stress, neither of which would be problematic in themselves, would in combination result in tire 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 tire pressures, as well as sizes, to be optimised for light load.)
- The Jensen GT (hatchback coupe) — 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 Ace, 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.
See also
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