CRITICAL SPEED

Question 1

According to Sir Isaac Newton, inertia First Law of Motion

Answer 1

Everybody continues in its state of uniform motion in a straight line, unless acted upon by external forces so applied to the body that the direction of motion maybe
changed.

Question 2

Centripetal Force,

Answer 2

also known as the adhesion of the tire to the roadway

surface. This is the force that is trying to keep the vehicle on its curved path.

Question 3

Centrifugal Force,

Answer 3

the force that is attempting to make the vehicle leave its path of curvature. aka “inertia”,

Question 4

Newton’s Second Law says:

Answer 4

the greater the mass of the object being accelerated, the greater the amount of force needed to accelerate the object.

Question 5

Mass

Answer 5

is the amount of matter an object contains

Question 6

acceleration

Answer 6

“is the rate change of velocity with respect to time”

Question 7

Newton’s Third Law says:

Answer 7

For every action there is an equal and opposite reaction.

Question 8

A vector quantity

Answer 8

is any quantity that must be described by both

the magnitude and a direction.

Question 9

A scalar quantity

Answer 9

maybe described by magnitude alone.

Question 10

yaw mark

Answer 10

is the sideways movement of a vehicle when rounding a corner or curve, where the rear of the vehicle side slips and moves out from the curved path in which it was moving.

Question 11

Center

Answer 11

the point within the circle that is an equal distance from all other points on the circle.

Question 12

Chord“

Answer 12

A line segment whose endpoints lie on the circle. This measurement should be at least 20 feet in length.

Question 13

Circumference

Answer 13

The length of one circuit along the circle.

Question 14

Diameter

Answer 14

The longest chord; a line segment whose endpoints lie on the circle and which passes through the center; or the largest distance between any two points on the circle.

Question 15

Radius

Answer 15

A line segment joining the center of the circle to any point on the circle itself; or half the diameter.

Question 16

Tangent

Answer 16

a straight line that touches the circle at a single point.

Question 17

critical speed yaw mark

Answer 17

is created when a vehicle is traveling in a curved path at a speed greater than the surface will support, and the centrifugal force begins to overcome the centripetal force.

Question 18

The front tire

Answer 18

is the tire which will leave the most visible and distinguishing yaw mark and it is this tire which is usually used to determine the radius of a yaw mark.

Question 19

Critical speed yaw marks are considered definite

Answer 19

at the “point of cross-over” or where the rear tire begins to track outside of the front tire. This is also the point which you use to begin your measurement to determine the radius of the mark.

Question 20

All critical speed marks are yaw marks,

Answer 20

but not all yaw marks are critical speed yaw marks.

Question 21

Calculating Critical Speed

Answer 21

1. The marks are curved. (ALWAYS)
2. The rear wheels are tracking outside of the front wheels. This is confirmed by the “cross-over” of the marks.
3. There are striations present in the marks that are parallel to the direction of the slippage.
4. The tire is still rotating as it’s side-slipping.

Question 22

A vehicles track width

Answer 22

is measured from the outside of the left front tire to the outside of the right front tire.

Question 23

To calculate the critical speed of a vehicle, you will need 4 items.

Answer 23

1. The radius of the yaw mark or center of mass path.
2. The track width of the vehicle.
3. The coefficient of friction (drag factor).
4. The roadway super elevation (cross grade).

Question 24

To calculate the critical speed of the curve, you will need 4 items.

Answer 24

1. The radius of the curve.
2. The width of the lane of travel.
3. The coefficient of friction (drag factor).
4. The roadway super elevation.

Question 25

To verify that a vehicle is actually in a critical speed yaw and not a spin, you will

Answer 25

need to identify the location where the rear tire begins to track outside the front tire. (cross-over)

Question 26

The hypotenuse is defined

Answer 26

as the side opposite the 90 degree (right) angle of the triangle. The hypotenuse will be designated as side c.

Question 27

The Law of Pythagoras (or the Pythagorean Theorem)

Answer 27

the hypotenuse of a right triangle equals the sum of the squares of the legs.

Question 28

slip angle

Answer 28

the difference between where the car is pointing (heading) and where it’s actually going.

Question 29

For vehicles in critical speed yaws

Answer 29

this angle should be approximately 20° or less. The slip angle should be measured at point B, or the end of the chord.

Question 30

Acceleration equals

Answer 30

the change in velocity with the respect to time