VEHICLE DYNAMICS
Vehicle dynamics is a complicated analytical and
experimental technology that is used to study and understand the responses of a
vehicle in various in-motion situations.
In the driver education field, it is not necessary to deal with the
specifics of this technology but rather with some of the basic physical
principles involved in it. The
following principles will be discussed in this unit.
I.
Kinetic Energy
II.
Centrifugal Force
III.
Inertia
IV.
Friction
V.
Traction
There
is no intent to give a complete technical definition of each principle but to
present them in a manner that will be useful in understanding why a vehicle
acts as it does.
Kinetic energy is a term that describes the energy a
vehicle has due to its mass and speed.
Its formula is simple, yet tells a great deal.
Kinetic energy = ½ (mass) x
(velocity)2
This shows that the kinetic energy of the vehicle
increases as the square of the velocity.
This means that if speed is doubled, the energy increases four
times. This energy increase causes no
problem unless it must be dissipated or redirected quickly.
One
way kinetic energy can be dissipated very quickly is when the vehicle strikes a
solid object. In this case, when the
speed is doubled, four times the energy is available to damage the vehicle and
injure its passengers. The kinetic
energy of a 4,000 pound vehicle traveling at 100 mph is equal to 1.36 million
foot pounds – enough to lift a 175 pound man 1.5 miles. To stop this vehicle tremendous energy must
be dissipated. This can be accomplished
by impact or by the brakes. Stopping
distance is related to the square of velocity; therefore, 30 mph requires four
times the distance to stop than 15 mph.
Many drivers never think of the consequences of increased speed but they
should be aware of the risks involved.
When
a vehicle turns, centrifugal force acts on the vehicle and tries to push it to
the outside of the curve. The formula
is:
Centrifugal Force = (mass) X
(velocity)2/ radius of turn
This
shows that centrifugal force increases as the square of velocity. Also, at a given speed, small (tight) radius
turns produce more force than large radius turns. Large amounts of centrifugal force require equally large amounts
of counteracting force from the tires if the vehicle is to remain on the
road. The tires can be thought of as
strings from each end of the vehicle to the center of the turn. If the centrifugal force is higher than the
tires can counteract, one or both of the strings break. The vehicle will then leave the turn.
Inertia
is the resistance to change the direction or velocity of a body, either at rest
or in motion. In this case, it is
related to changing the heading, or direction, of a vehicle; that is, changing
from straight ahead driving to a turn.
The
importance of inertia and weight distribution as they relate to driving is that
they affect the amount of time required to make a transition from straight to
turning or vice versa. Although these
changes with the usual loading of a vehicle are not large, a driver should
recognize the unusual loading of a vehicle, such as the placing of a large load
on the tailgate of a station wagon (or the addition of a heavy load on the
vehicle roof) will cause changes in the way the vehicle drives and adjustments
should be made in driving accordingly.
Since
inertia dictates that a body in motion will continue to move in a straight
line, a force must be applied to cause a vehicle to turn. This force is called Centripetal force,
and is a result of tires stretching to pull the car from a straight path. Centripetal force must exceed centrifugal
force for the vehicle to turn.
MOMENTS OF INERTIA:
A.
Pitch – the force felt in acceleration or braking movement around
(Horizontal axis) of vehicle
B.
Roll – the force felt in cornering, side to side movement (Lateral
axis) of the vehicle
C.
Yaw – the force felt in a spin movement around (Vertical axis) of the
vehicle
POLAR MOMENT OF INERTIA
A very important handling concept, which dictates
the willingness of a car to change directional position if called Polar Moment
of Inertia. “Poles of inertia” are just
another way of saying “center of weight concentration”. The “moment” in this concept is determined
by the front-to-rear location of the center of gravity. The car turns (changes direction) about its
center of gravity in a corner so the further away the centers of weight
concentration are located from the center of gravity (which is their common
center), the bigger the “moment”.
A high polar moment of inertia is present when the
weight concentrations are heavy and are far apart. The low polar moment of inertia is found when weight
concentrations are light and are close together. In other words, it is easier to steer a vehicle with a low polar
moment of inertia.
A vehicle with a low polar moment of inertia gives a
quick response to steering commands. A
vehicle with a high polar moment has high directional stability (meaning it
resists changing its direction).
Friction
is defined as the resistance to motion between two surfaces. There are four basic types of friction.
A.
Static – the holding force between two surfaces at rest
B.
Sliding – the resistance to motion between two surfaces which are
moving across each other
C.
Rolling – the resistance to motion of a rolling object like a ball,
cylinder or wheel
D.
Internal – the resistance to motion within elastic objects (tires get
warm from internal friction as they flex)
The
amount of friction between two surfaces depends upon:
1)
substance of material
2)
roughness of the surfaces
3)
amount of force pushing the surfaces together
4)
presence of lubricants
The
amount of friction between two surfaces is called coefficient of friction.
COEFFICIENT OF FRICTION
The term coefficient of friction is defined as
the maximum force that can be created by a tire on a given road surface
condition, divided by the weight on the tire.
Its formula is:
Maximum Force Available
Coefficient of Friction = Weight on the Tire
OR
Maximum Force Available = Coefficient of Friction
X the Wheel Load Weight
Thus, the maneuvering ability of a vehicle on a dry
road depends primarily upon road surface and vehicle weight. On a wet road, other factors such as tire
condition must also be considered.
As the vehicle accelerates or slows down more
rapidly, or as the vehicle corners at faster speeds, it demands greater
traction forces from the tire-road combination. The tire and road combination will produce these forces up to the
friction limit.
Traction
is defined as the adhesive friction of the tire to the road surface. There are three traction forces:
1)
Driving Traction – To accelerate the vehicle
2)
Braking Traction – To slow or stop the vehicle
3)
Cornering Traction – To turn the vehicle
At
any time traction force becomes greater than the coefficient of friction the
vehicle will go out of control.
A
driver has the potential for exerting three forces. For any given situation, there is a level of friction
(coefficient) available for exerting these forces, and therefore, maneuvering
the vehicle. When a driver exerts
either a braking or acceleration force while at the same time exerting a
cornering force, you must add the forces when considering the available
friction. In other words, the sum of
driving or braking traction and cornering traction must not exceed the
friction limit, or the vehicle will go out of control. Whenever possible, avoid braking or
accelerating while cornering. This
allows all available friction to be used in cornering.
A
spinning tire cannot provide full driving traction when accelerating. If a driver causes drive wheel spinning when
cornering, the vehicle may go out of control.
A
locked tire provides no cornering traction and reduced braking
traction. When a driver locks the
wheels in a corner, there will be no response to the steering input. During braking, the maximum coefficient of
friction; therefore, maximum braking ability, is when the driver applies the
brakes at a level of 15% slippage.
Comments: Webmaster - EOE - Privacy Policy - March 24, 2009