February 11, 2010 by markarndt
After conducting testing of natural tire delamination and simulated tire blowout failures on the steer axle of a motorcoach, the US National Transportation Safety Board (NTSB) reported important results that questioned pervasive thought and conventional guidance to drivers regarding such events. Conclusions derived from the report include:
Tire delaminations produced rotation and torque at the handwheel,
Some tire failures trials produced lateral force impulse that presented considerable difficulty for maintaining control, while others were “no problem” and presented little challenge,
Tire delaminations produced sudden, though different from event to event, turning of the vehicle,
Detection of underinflated tires by visual inspection or “thumping” was not accurate and not advisable. A tire gauge must be used to evaluate tire pressure,
Progressive loss of air pressure – even to less than 50% of manufactures recommended pressure – were imperceptible while driving and
Braking, contrary to conventional wisdom and guidance, was shown to not degrade, but improved vehicle control.
The testing was commissioned in an effort to explain factors that may have contributed to a driver’s loss of control of a motorcoach. To this end the NTSB conducted a literature review and ultimately testing regarding effects of steering axle tire failures on handling characteristics and dynamics of a bus or motorcoach. The Board cited numerous studies it found instructive, but no published studies examining the dynamics of buses or motorcoaches in response to tire failure. For this reason, the NTSB undertook its tests to evaluate the effects of steering axle tire delamination or blowout failures on a driver’s ability to maintain control of a motorcoach. Although it was not part of the original plan, the effect of braking on vehicle control was also specifically evaluated.
The NTSB tests were supported by the parties to its investigation and with the active participation, advice and assistance of individuals associated with: Continental Tire, Federal Motor Carrier Safety Administration, Motor Coach Industries, Inc., Greyhound Lines, Inc., The Goodyear Tire & Rubber Company, SmarTire Systems, Inc., Continental AG, Detroit Diesel/Allison Transmission Distributor and TRW, Inc.
Test conditions and response were recorded via the Last Stop Record which provided a snapshot of vehicle data for the 104-second interval preceding engine shutdown, including the vehicle speed (mph), engine speed (rpm), engine load percentage, throttle percentage, and whether the brakes were applied at the time of the tire failure and/or in the intervening period until the vehicle was brought to a stop. Sensors to monitor tire pressure and temperature and steering wheel torque were used. Global Positioning System (GPS) data were recorded to track the location of the test vehicle on the track.
