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Cars, for use on public roads, whose engineering requirements emphasize handling above passenger space and comfort, are called Sports Car s. 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 months 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 tires, tire 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 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. Weight distribution See Also: 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. The center 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 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 principle performance advantage of Sports Car s, compared to sedans and (especially) SUV s. 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 Bar s or the Roll Center heights. 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. 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 "45/55" and "40/60." 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. 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. 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. 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 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 , 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.) 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 Wheel s 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 aggravate 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 Absorber s 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 Brake s, 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 Brake s. (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 Alloy Wheel s 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 Center 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 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. 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. 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 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 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. Alignment of the wheels Of course things should be the same, left and right. Camber affects steering because a tire generates a force towards the side that the top is leaning towards. This is called camber thrust. 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 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;
COMPROMISES Ride & 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 Brake s 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 TO AFFECT HANDLING See Also: Racing setup 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 Accident s - their handling characteristics may play a role:
: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 Car s is that their very lack of carrying capacity allows their standard tire pressures, as well as sizes, to be optimised for light load.)
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