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Office of Highway Safety
Commercial Vehicle Tires and TestingCommercial Vehicle Tires and TestingJennifer Russert
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2002 Chevrolet Suburban2002 Chevrolet Suburban
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1996 Dina Viaggio Motorcoach1996 Dina Viaggio Motorcoach
SteerDriveTag
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Minimum Tire Tread Depths (inches)Minimum Tire Tread Depths (inches)
Left Right
Steer axle 14/32 15/32
Drive axle3/322/32
6/325/32
Tag axle 8/32 5/32
FMCSA
4/32
2/32
2/32
CVSA
2/32
1/32
1/32
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Motorcoach Tires – Tread DepthMotorcoach Tires – Tread Depth
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Motorcoach Tires – Tread DepthMotorcoach Tires – Tread Depth
• Low Tread Depth– Decreases ability for tires to channel water– Limits the ability of tire to maintain contact
with the roadway
• Effect on friction and handling• Tire testing
– All drive axle tires (2/32, 3/32, 5/32, 6/32)– Right steer axle tire (15/32)
• Testing and simulation
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Tire FrictionTire Friction
• Testing at 60 mph, all tested water depths– Steer axle tire friction = 0.28 to 0.30– Right drive axle tire friction = 0.12 to 0.16– Left drive axle tire friction = 0.10 to 0.15
• Speed, water depth, and tread depth affect friction
• Drive axle tire friction equivalent to ice (0.12 to 0.25)
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Effect on Motorcoach HandlingEffect on Motorcoach Handling
• Lower friction available for rear tires• Passenger car testing
– Friction reduced when tires worn– Handling changes when worn tires are in
rear
• Differing friction on front and rear tires creates instability
• Results in rotation and loss of control• Differing friction on left and right tires
increases instability
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Tire FrictionTire Friction
• Drive axle brakes locked sooner than other axles due to friction available
• Earlier lock up of drive axle contributes to loss of control
• If drive axle had same tread depth as steer axle, it would not have locked up
• Driver would likely have been able to maintain control
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