Cars - Car handling

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	Cars - Car handling

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.

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.

Center of gravity height is the principle difference between a sports car and a sedan or SUV.

Body lean can be controlled by the springs, anti-roll bars or the roll centre 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. 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 tires or jacking the car up higher or lower at the suspension springs. Jacking is frequently done with screws or shims at the springs.

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