Physical Laws that Affect Car Crashes

Inertia - the law of motion. A property at rest wants to remain at rest.

Body in motion – keeps moving until something stops it. The object remains traveling at the same speed until acted upon by an outside force. When in a crash, the front of the car absorbs energy. This absorption slows down the rest of the car, but the occupants inside the car continue moving at the same speed until they strike the steering wheel and windshield or other objects in the vehicle. Inertia is one reason why safety belts are so important for they tie the occupants to the vehicle during a crash.

G – a standard unit of acceleration or deceleration. This force is experienced in car crashes, causing injury.

Extending the time of impact – is the basis for keeping people safe in a crash. It is the reason for airbags and crumple zones in vehicles and crash cushions and breakaway utility poles on our highways. It is also why some crashes cause severe injuries and some crashes cause minor if any injuries.

Car size and weight

When two vehicles of the same size and weight collide the occupants would experience the same deceleration, this would be the same as a single vehicle crashing into a rigid barrier.

In a collision of two cars with different weights and size (unequal mass), the more massive car would drive the less massive car backward during the crash, causing a greater speed change in the lighter car than the heavier car. The different speed changes occur during the same time so the occupants of the lighter car would experience much higher forces than the occupant of the heavier car. This is one reason why lighter, smaller cars offer less protection to the occupants than larger, heavier cars.

Larger size helps you in all kinds of crashes.
Weight is primarily an advantage in a crash with another vehicle.

Energy – the ability to do work (the stuff that makes things move)

Kinetic energy – the energy an object possesses due to its motion, the rapid transfer of kinetic energy is the cause of crash injuries. Managing kinetic energy is what keeping people safe in car crashes is all about.

Road Surface and Traction

Traction

The adhesion, friction or grip between the tires and the road surface. Without traction, a driver cannot steer, brake or accelerate. A loss of traction may cause a loss of control that can result in skidding.

Traction varies

Traction varies with the vehicle’s speed, tire condition and roadway surface. A driver controls the vehicle’s speed and tire condition, but they have no control over the road surface or its condition, so drivers must learn to recognize conditions that may indicate a change in traction, which will require a change in speed, position, or direction.

Conditions that can affect traction:

Surface materials – concrete, asphalt, brick or polished concrete, dirt, gravel, new pea stone, sand over hard surface. Ranked in order, from the greatest traction to the least.
Substances on the road – sewer covers, paint, vinyl strips, tar, wet leaves, sand, loose gravel or mud. Reduce the level of adhesion or traction available.
Surface conditions – rippled surfaces or potholes, wet, icy and snow-packed roadways, bridge surfaces, overpasses and shaded areas may freeze before other road surfaces. All of these contribute to a loss of traction.
Roadway design – banked or crowned roads, uneven shoulders.
- Banked roadway – usually found at a curve, is higher on one edge than the other.
- Crowned roadway – road that is higher in the middle than at the sides.
- Shoulders – frequently provide the only escape path in an emergency, shoulders of most highways provide less traction than the road surface. Some shoulders may be wide and paved but most are narrow and gravel. They can be rough or soft, covered with loose materials or littered with broken glass, debris or other waste and often are uneven with the road surface. Drivers must be prepared to “drive” on the surface that is available.

Natural Laws and Traction

Inertia

Traction is affected by inertia. Inertia means an object in motion continues to move straight ahead until acted upon by some outside force. This force creates problems when a driver changes a vehicle’s direction too suddenly.

As the car travels around the corner, inertia tends to keep the car moving forward in a straight line, rather than allowing the car to follow the curve or turn. Unless traction is great enough to overcome the force of inertia, the car will slide to the outside of the curve or turn.

Factors influencing the effects of inertia:

Sharpness of the turn or curve
Speed
Size, height, weight and load of the vehicle
Roadway slope or crown
Roadway surface condition

Gravity

The invisible force that pulls all things to earth. This force also affects vehicle traction and performance. Gravity’s downhill pull affects a driver’s ability to accelerate and maintain speed on hills.

Driving uphill – A driver must accelerate to keep a constant speed and overcome the force of gravity. On a slippery hill, acceleration may result in a loss of traction.
Driving downhill – The force of gravity tends to boost speed, increasing the stopping distance.

Vehicle Suspension, Balance and Traction

Single-vehicle crashes

More than 50 percent of occupant fatalities occur as a result of single-vehicle crashes. Basically, the driver loses control and the vehicle leaves the roadway and strikes a fixed object or overturns. These crashes normally involve improper steering or braking which can upset a vehicle’s balance.

Balance

Refers to the distribution of the weight of the vehicle on the chassis or the frame, wheels and body of the vehicle. A transfer of weight from one point of the vehicle to another occurs when the driver accelerates, brakes, turns, or performs a combination of these actions.

Weight distribution

When a car is in balance, the vehicle’s weight is distributed equally over the front and rear tires. However, the vehicle’s weight can be shifted from the center of the chassis to a position where the weight is concentrated over the front, rear, left or right tires. When the vehicle is stopped or traveling straight at a constant speed, its suspension and traction are stabilized.

When weight is transferred to or from a tire, the size of its contact patch will change. A tire’s contact patch is the area of tire rubber that is in contact with the pavement at any time.

The larger a tire’s contact patch, the more traction that tire has on the road surface. The average size of a tire’s contact patch is approximately equal to the surface area of an adult human palm.

Changing vehicle suspension load

Front to rear – when a driver accelerates, the vehicle weight or center of mass shifts to the rear. The rear suspension compresses, and the rear tires’ contact patches increase in size, while the front tires’ contact patches decrease in size. If acceleration is aggressive, there will be a noticeable rise of the vehicle’s front and a drop of the rear.
Rear to front – when a driver decelerates, the vehicle weight or center of mass is transferred to the front. If braking is hard, there is a noticeable drop of the vehicle’s front and a rise of the rear. Occupants will feel forward movement. Apply too little brake pressure, and the vehicle will not stop at the desired point or within the distance available. Apply too much pressure, and the wheels may lock up, resulting in loss of traction and directional control.
Side to side – When turning, the vehicle’s weight will shift in the direction opposite the turn. When turning right, the vehicle’s weight will shift to the left, causing the vehicle to lean to the left. When turning left the shift and lean will be to the right.

Maximizing traction

To maximize traction and minimize the chance of traction loss:
- Avoid sudden input when accelerating, braking or steering.
- Focus on executing one input at a time – brake, then steer, then accelerate.
Seating position

Drivers should sit in a comfortable, upright position directly behind the steering wheel to control vehicle balance and help maximize the driver’s view of the driving environment.