Automobile handling
Automobile handling and vehicle handling are descriptions of the way a wheeled vehicle responds and reacts to the inputs of a driver, as well as how it moves along a track or road. It is commonly judged by how a vehicle performs particularly during cornering, acceleration, and braking as well as on the vehicle's directional stability when moving in steady state condition.
In the automotive industry, handling and braking are the major components of a vehicle's "active" safety. They also affect its ability to perform in auto racing. The maximum lateral acceleration is, along with braking, regarded as a vehicle’s road holding ability. Automobiles driven on public roads whose engineering requirements emphasize handling over comfort and passenger space are called sports cars.
Design factors that affect automobile handling
Weight distribution
Centre of mass height
The centre of mass height, also known as the centre of gravity height, or CGZ, relative to the track, determines load transfer from side to side and causes body lean. When tires of a vehicle provide a centripetal force to pull it around a turn, the momentum of the vehicle actuates load transfer in a direction going from the vehicle's current position to a point on a path tangent to the vehicle's path. This load transfer presents itself in the form of body lean. In extreme circumstances, the vehicle may roll over.Height of the centre of mass relative to the wheelbase determines load transfer between front and rear. The car's momentum acts at its centre of mass 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 centre of mass, the effect on over/under steer is opposite to that of an actual change in the centre of mass. 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.
A lower centre of mass is a principal performance advantage of sports cars, compared to sedans and SUVs. Some cars have body panels made of lightweight materials partly for this reason.
Body lean can also be controlled by the springs, anti-roll bars or the roll center heights.
| Model | Model year | CoG height |
| Dodge Ram B-150 | 1987 | |
| Chevrolet Tahoe | 1998 | |
| Lotus Elise | 2000 | |
| Tesla Model S | 2014 | |
| Chevrolet Corvette Z51 | 2014 | |
| Alfa Romeo 4C | 2013 | |
| Formula 1 Car | 2017 | 25 centimetres |
Centre of mass
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 seeks to achieve the ideal center of mass, though front-engine design has the advantage of permitting a more practical engine-passenger-baggage layout. All other parameters being equal, at the hands of an expert driver a neutrally balanced mid-engine car can corner faster, but a FR layout car is easier to drive at the limit.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 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 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, thus reducing corner-entry understeer.
Using wheels and tires of different sizes 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 height and width, and 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 also increase with scale. 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. If most of the weight is in the middle of the car then the vehicle will be easier to spin, and therefore will react quicker to a turn.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.
Spring rate
The flexing of the frame interacts with the suspension. The following types of springs are commonly used for automobile suspension, variable rate springs and linear rate springs. When a load is applied to a linear rate spring the spring compresses an amount directly proportional to the load applied. This type of spring is commonly used in road racing applications when ride quality is not a concern. A linear spring will behave the same at all times. This provides predictable handling characteristics during high speed cornering, acceleration and braking. Variable springs have low initial springs rates. The spring rate gradually increases as it is compressed. In simple terms the spring becomes stiffer as it is compressed. The ends of the spring are wound tighter to produce a lower spring rate. When driving this cushions small road imperfections improving ride quality. However once the spring is compressed to a certain point the spring is not wound as tight providing a higher spring rate. This prevents excessive suspension compression and prevents dangerous body roll, which could lead to a roll over. Variable rate springs are used in cars designed for comfort as well as off-road racing vehicles. In off-road racing they allow a vehicle to absorb the violent shock from a jump effectively as well as absorb small bumps along the off-road terrain effectively.Suspension travel
The severe handling vice of the TR3B and related cars was caused by running out of suspension travel. 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, 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 depth of tread remaining greatly affects aquaplaning. Increasing tire pressures reduces their slip angle, but lessening the contact area is detrimental in usual surface conditions and should be used with caution.The amount a tire meets the road is an equation between the weight of the car and the type of its tire. A 1000 kg car can depress a 185/65/15 tire more than a 215/45/15 tire longitudinally thus having better linear grip and better braking distance not to mention better aquaplaning performance, while the wider tires have better cornering resistance.
The contemporary chemical make-up of tires is dependent of the ambient and road temperatures. Ideally a tire should be soft enough to conform to the road surface, but be hard enough to last for enough duration to be economically feasible. It is usually a good idea having different set of summer and winter tires for climates having these temperatures.
Track and wheelbase
The axle track provides the resistance to lateral weight transfer and body lean. The wheelbase provides resistance to longitudinal 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 to the vehicle's ability to swerve quickly.The wheelbase contributes to the vehicle's turning radius, which is also a handling characteristic.
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. Unsprung weight is more properly regarded as a mass which has its own inherent inertia separate from the rest of the vehicle. When a wheel is pushed upwards by a bump in the road, the inertia of the wheel will cause it to be carried further upward above the height of the bump. If the force of the push is sufficiently large, the inertia of the wheel will cause the tire to completely lift off the road surface resulting in a loss of traction and control. Similarly when crossing into a sudden ground depression, the inertia of the wheel slows the rate at which it descends. If the wheel inertia is large enough, the wheel may be temporarily separated from the road surface before it has descended back into contact with the road surface.This unsprung weight is cushioned from uneven road surfaces only by the compressive resilience of the tire, which aids the wheel in remaining in contact with the road surface when the wheel inertia prevents close-following of the ground surface. However, the compressive resilience of the tire results in rolling resistance which requires additional kinetic energy to overcome, and the rolling resistance is expended in the tire as heat due to the flexing of the rubber and steel bands in the sidewalls of the tires. To reduce rolling resistance for improved fuel economy and to avoid overheating and failure of tires at high speed, tires are designed to have limited internal damping.
So the "wheel bounce" due to wheel inertia, 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.
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 suspension is completely unsprung. The main factors that improve unsprung weight are a sprung differential and inboard brakes. Wheel materials and sizes will also have an effect. Aluminium alloy wheels are common due to their weight characteristics which help to reduce unsprung mass. 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.