NTSB Reports Important Steer Axle Tire Failure Testing Results

February 11, 2010 by · Leave a Comment
Filed under: News, Testing, Tread Separation 

2009 02 11 - Human performance & Vehicle Group Chairmen's Report of Operational TestingBy Mark W. Arndt

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.

Problems with Tire Standard FMVSS 120

February 9, 2010 by · Leave a Comment
Filed under: Crash Reconstruction, News, Tread Separation 

By Mark W. Arndt

TSTI_01667  010The US Federal Motor Vehicle Safety Standard (FMVSS) 120 covers tires and rims for motor vehicle other than passenger cars.  The standard is applicable to multipurpose passenger vehicles, trucks, buses, trailers and motor cycles. The core of the standard attempts to assure that the tires and rims that are sold with a vehicle, and that the tires and rims that are specified on a tire label affixed to a vehicle, are capable of supporting the manufacturer’s specified gross axle load.   

A manufacture can sell vehicles weighing more than 10,000 lbs with tires and rims different from its tire label specified tires, rims and inflation pressures.  Such a vehicle must meet the requirements of FMVSS 120 with all sets of tires and rims listed on the label and mounted to the vehicle.  The tires and rims on a vehicle are evaluated with information from the sidewall of the tire, including the maximum load at the sidewall inflation pressure.  The tires and rims listed on a tire label are evaluated using a source like the Tire and Rim Association Yearbook to determine the load carrying capacity of each tire on the label at the recommended pressure.  The evaluation process requires that the sum of load carrying capacity for all tires on an axle equal or exceed the manufacture’s front and rear Gross Axle Weight Rating (GAWR).

There are several problems that are presented by this standard including:

  • There is no assurance that the tire inflation pressure listed on the tire label will be compatible with different tires in-use on a vehicle.  In other words, the inflation pressure listed on a tire label may be too low for the in-use tire given the load and service provided.
  • The requirement is that the combined tire carrying capacity of an axle must equal or exceed an axle’s weight rating, however in some vehicles large side to side weight differences exist rendering a tire or set of tires insufficient for the carrying load.
  • The standard does not assure compatibility for the variety of tire to rim and mixed tire combinations that might occur given differences between the tire label and the tires sold with a vehicle.  For example, the tires listed on a tire label may not properly fit a wider rim appropriate for larger tires sold with a vehicle.