The Story Skid Marks Tell

1
The Story Skid Marks Tell
http//www.ketv.com/video/4638347/detail.html?taf
oma
2
Tire marks are produced when a tire slides across
a paved surface, leaving dark rubber particles on
the pavement.
Similar to the way an eraser leaves particles on
a piece of paper
3
Types of Tire Marks
4
Skid Marks

• When you brake suddenly, the wheels of your car
  lock.
• You no longer have the ability to steer.
• The car will slide straight ahead.
• Skid marks are usually straight.
• The skid mark provides evidence of the distance
  over which braking took place.

5
Skid Marks
This picture shows skid marks that begin just
past a seam in the pavement. Does this show where
the car began braking?
6
ABS and Skid Marks

• Anti-lock brake systems (ABS) prevent locking and
  skidding by turning the brakes on and off several
  times a second.
• The driver can maintain steering control.
• Vehicles equipped with ABS may leave faint,
  intermittent skid marks.
• These skid marks may be in a curved path.

7
Yaw Marks

• Yaw marks occur when a vehicle travels in a
  curved path faster than the tires can handle.
• The adhesion limit of the tires is reached, so
  they begin to slip.
• Tires squeal and leave marks on the pavement.

8

• Yaw marks are always curved.
• There are often striations at an angle to the
  length of the mark.

9
Tire Scrubs

• Tire scrubs are caused by a damaged or overloaded
  tire.
• They happen during or immediately after a
  collision.
• Tire scrubs are usually curved and irregular in
  width.
• They may have striations at an angle.

10

• This picture shows a straight skid mark that
  changes to an irregular, curved tire scrub at
  impact.
• The dark mark is a tire scrub from a second car.

11
Measuring Skid Marks

• If all four wheels lock at the same time, the car
  will skid in a straight line.

• The marks from the rear wheels will
• overlap the marks from the front wheels.

12

• Skid marks from the front wheels have two thin
  lines on the outer edges.

• Rear wheel skid marks have a dark streak in the
  center.

13
Measuring Skid Marks

• If 4 marks are found, they should be measured
  individually.
• To get the average skid distance, add the four
  measurements together and divide by four.

14

• If three skid marks are found, add the three
  together and divide by three.
• The same applies to two marks.
• If only one mark is found, measure the entire
  length and use this as the skid distance.

15
Drag Factor

• Tires slide on some road surfaces easier than
  others.
• Can you think of road conditions under which
  tires would slide easily?

• What type of road surface would be the
• hardest for tires to slide on?

16
Examples of Drag Factor

• Asphalt 0.50 to 0.90
• Gravel 0.40 to 0.80
• Ice 0.10 to 0.25
• Snow 0.10 to 0.55

17
Braking Efficiency

• Each wheel on a car provides a certain amount of
  the total brake force available.

• If all four wheels are braking evenly, then
  braking efficiency is 100 or 1.00.

18

• For rear wheel drive cars, the brake force is 30
  for each front wheel and 20 for each rear wheel.
• What is the braking efficiency of a car if the
  rear brakes are not working at all?

19
You Have Arrived!!

• You now have the three variables required to
  complete the minimum skid speed formula.

20

• In this formula
• S speed in miles per hour
• 30 a constant value
• D skid distance in feet
• f drag factor for the road surface
• n braking efficiency in decimal form

21
Example

• A car skids to a stop, leaving four skid marks
  with an average length of 60 feet.
• The road is asphalt. Skid tests reveal a drag
  factor of 0.75.
• Since all four wheels are braking, the braking
  efficiency is 100 or 1.00.
• How fast was the car traveling?

22
36.7 miles per hour

• It is important to understand this is a MINIMUM
  speed for the car at the beginning of the skid.
• Since the driver is likely to have applied the
  brakes before they locked and skidded, his speed
  was certainly greater than 36.7 mph before he
  started braking.

23
Summary

• One of the most common tasks in accident
  reconstruction is the estimation of vehicle speed
  from measured braking skid marks.
• This involves careful measurement and proper use
  of formulas to arrive at valid conclusions.