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Braking Adjustments for CMV
Air Brake EfficiencyWeight per AxleTire CompoundsDrum Brake vs. Disc BrakeHeat Build-up in BrakesMechanical LimitationsCenter of Gravity HeightWeight Transfer
General rules of thumb:
TT w/o front brakes: 25 – 40% (0.25 - 0.40)
TT w/ front brakes: 70 – 80% (0.70 – 0.80)
TTST w/o front brakes: 50 – 70% (0.50 – 0.70)
TTST w/ front brakes: 65 – 80% (0.65 – 0.80)
TTSTST w/ front brakes:75 – 90% (0.75 – 0.90)
Buses(Motor coach) 70 – 85% (0.70 – 0.85)
St. Truck 60 – 85% (0.60 – 0.85)
A 5-axle TTST slid to a stop in a distance of 200 feet on a dry, asphalt
surface. What was its beginning speed?
(There was no impact with anything else)
A straight truck (with a reefer) slid into another car after skidding for 150
feet on a dry, asphalt surface. The driver said he was only going 35 mph before he began to brake. Is he telling
the truth?
Your driver told you he was going 20 mph and had to skid to a stop. He did so in 30 feet on dry pavement. Does this conform to CFR-49 requirements
for braking?Does it meet or exceed the requirements?
Reconstruct and Analyze the Crash
If you have documented an adequate amount of crash data that was available
from the scene, the vehicles and the people involved, then this information
can be evaluated at a later time to more fully determine “how” and “why” the
traffic crash occurred.
Vehicle Speed (MPH)
Engine Speed (RPM)
Percent of Wide Open Throttle
Brake Switch Status (On or Off)
Pre-Crash Graph GM Air Bag Module