How it Works> Measuring Drag Factor
How the Vericom measures drag factor
Acceleration – Deceleration Graph
In the braking mode the Vericom measures speed, time, distance and G-force from the point when the tire braking force begins (B) to the point when the vehicles come to a complete stop (D).
Or we can say in the braking mode the Vericom starts when the tire braking force starts (B) and stops when the vehicle stops (D).
A is the point at which movement of the brake pedal begins. This is normally when the foot touches the brake pedal.
B is the point at which tire slip friction begins. This occurs when the pads touch the rotor (disc) or the brake shoes touch the brake drum. Wheel slip occurs on the deceleration curve between -.10 G and -.20 G. Good repeatability occurs when using -.20 G because premature activation is greatly reduced. Therefore -.20 G is the selected default activation threshold.
C is when the first peak G occurs. Typically vehicles with ABS will have several peak Gs or hold the peak G throughout the entire skid. Vehicles with standard brakes will typically have only one peak G.
D is the point at which longitudinal motion of the vehicle has ceased.
E is the point at the end of the skid when accelerometer reads zero Gs. This occurs approximately .080 to .220 seconds after the vehicles stops but the vehicle is still rocking on the suspension system.
Supporting documents:
Brake Design and Safety 2nd edition, by Rudolf Limpert ©1999.
page 13: After the brake system application time has passed, the brake shoes
contact the drums and vehicle deceleration begins.
page 428: Tire traction forces such as longitudinal or braking forces as well as side forces can
only be produced when a difference exist between the speed of the tire circumference and the
speed of the vehicle relative to the road surface. It is common to relate tire braking force data to
tire braking slip.
Fundamentals of Vehicle Dynamics by Thomas D. Gillespie ©1992.
page 55: Both Adhesive and hysteretic friction depend on some small amount of slip occurring at
the tire-road interface……. Brake force and slip are coexistent.
