B. TRACTION (FRICTION, HYDROPLANING, STOPPING DISTANCES, CENTRIFUGAL FORCE)

1. Mother Nature and driving - Mother Nature has laws that affect us while driving. When we violate them, a break in smooth driving always results, even if no collision occurs. Mother Nature punishes you right then. There is no second chance. The laws that you should understand include the laws of gravity, friction, kinetic energy, inertia, and centrifugal effect. Driving too fast on a sharp curve may cause a vehicle to swerve out of its lane. If unbelted, you may crash into the steering wheel and/or instrument panel if your car collides with a stationary object, even at low speeds.

2. Gravity - The force that holds objects on or near the earth is called gravity. As a result, a car going down a hill requires a longer stopping distance than a car going up a hill.

3. Center of gravity or center of mass - In every object is a center of mass, a place around which its weight is balanced evenly. A car with a low center of mass is easier to handle, especially in turns and curves, and this also reduces the possibility of a roll-over. Let’s say that you go by and pick up Aunt Bea, Uncle Harry and Aunt Em to take them out for Sunday dinner. They each weigh 300 lbs. They all get in the back seat. Will your car handle differently? Of course. If you pull a boat or trailer, will your car handle differently? Every time you change the center of mass, don’t overlook the fact that your car will handle differently.

4. Friction - This is the resistance to the relative motion of a body or portion of matter in contact with another body or portion of matter. Friction may occur when any form of matter - solid, gas or liquid - moves against another. Without friction, tires would spin and not grip the road. When a tire does grip the road, it is called traction. Friction is used for braking and accelerating. The best road friction and traction is produced by properly inflated tires with good, deep tread on clear roads.

Many circumstances can reduce road friction and traction. Among the more common ones on the roadway itself are water, snow, ice, bumps, railroad tracks, leaves, oil and dirt. Worn tire tread as well as over or under inflated tires also can reduce road friction and traction.

5. Hydroplaning and skidding - This occurs when friction is reduced because of wet road surfaces. You will experience a loss of steering control. Hydroplaning can be caused by driving too fast on a wet surface with as little as 1/10” of water on it. Worn tires and water covered tread, riding on mirror-smooth pavement, create a stage for “water-skiing on four wheels.” If you start to hydroplane, take your foot off of the accelerator, do not touch the brake, and then steer in the direction you want to go.

6. Stopping - A car’s total stopping distance at a given speed depends on three things:

Perception - This affects the time required to see and identify a potential danger and the distance traveled during that time. The average person takes about three-fourths of a second to recognize that danger in ideal conditions.

Reaction - This affects the time it takes to depress the brake pedal after the danger is perceived and the distance traveled during that time. The average person needs about three-fourths of a second to react in ideal conditions.

Braking - This affects the time it takes to stop the car and the distance traveled after the brakes are first applied. Car manufacturers use this distance when advertising stopping distances.

Calculating stopping distances:

Your stopping distance is the sum of your perception, reaction and braking distances. To calculate this distance, you can use the formula (v²/2d) + vt, where d is the deceleration rate, v is the speed in feet per second, and t is the perception and reaction time (1.5 seconds for the average person). We need to convert miles per hour to feet per second to get the value for v, so to get this we multiply the speed by 1.467. For d, we will use 24 ft/s², which is the average deceleration rate for passenger vehicles on a dry surface. This gives us the simplified formula:

(v²/48) + (v x 1.5)

So how does this apply to you? We will use 30 mph and 60 mph to get the approximate stopping distances. But first we need to convert them to feet per second.

30 mph = (30 x 1.467) = 44 ft/s²

60 mph = (60 x 1.467) = 88 ft/s²

Now we can figure out the total stopping distances on a dry surface.

For 30 mph, (44²/48) + (44 x 1.5) = 106 feet.

For 60 mph, (88²/48) + (88 x 1.5) = 293 feet.

To compare, the distance between bases in a baseball field is 90 feet, and the length of a football field is 300 feet, not counting the end zones. These examples should help you understand why it is vital not to drive over the posted speed limits, especially in school zones.

Some facts to remember:

• Traction is reduced on wet surfaces.

• Stopping distance required on wet surfaces will be two to three times the norm.

• The centrifugal force will pull the vehicle left on a right hand curve, and right on a left hand curve.
  
• In hydroplaning, never hit the brakes hard.
  

Factors affecting braking distances:

• The greater the speed, the longer the braking distance.

• Worn tires, shock absorbers and brakes can reduce traction, increasing the braking distance.

• If the roadway is rough or unpaved, the friction decreases and the braking distance increases.

• Cars traveling downhill tend to go faster and braking distance increases.