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.

Will Electronic Stability Control Eliminate Tire Tread Separation Crashes?

October 1, 2009 by · Leave a Comment
Filed under: Tread Separation 


By Mark Arndt

Will Electronic Stability Control Eliminate Tire Tread Separation Crashes?

Probably not, but the technology holds great promise to substantially reduce these crashes.  Recently published testing results demonstrate that Electronic Stability Control (ESC) systems provide drivers a greater steering margin of safety when their vehicle experience a rear tire tread separation.

The study also found that not all ESC systems are alike in their potential benefit. Some systems provide a greater benefit to the driver in the event of a rear tire tread separation than others.

 A rear tire tread separation event can lead to loss of vehicle control as a result of an unexpected deviation to the vehicle’s intended path in combination with significant change that occurs to the vehicle’s steering characteristics. Vehicle designers have had difficulty providing substantial improvements in basic vehicle response after a tire tread separation, but ESC was shown to make a substantial improvement in rear tire failure events (video).

Fuel Fill Pipe Damage, Leakage and Fire by Tire Tread Separation

September 28, 2009 by · Leave a Comment
Filed under: Fill Pipe, Fuel Tank, Testing, Tread Separation 

TSTI Test 0013 146By Mark Arndt

Given knowledge of the dangers, the notion that tire failures occur on vehicles traveling at highway speeds is frightening. Yet, such failures occur and the general acceptance of tire failure is so deeply rooted in vehicle performance that spare tires are standard equipment. 

A specific rollover incident in which a rear tire tread separation caused tearing apart of a fuel tank fill pipe routed just behind the wheel well was recreated in a controlled test (watch video).  The incident resulted in fuel spillage, fire and burn injuries.  The October 2001 incident involved a 1995 Land Rover Discovery with a General AmeriStar tire failure.  The vehicle manufacture’s corporate representatives were shown the test in late 2002 during depositions. 

TSTI Test 0013 147High speed video captured contact between tire tread and reinforced rubber fuel hose as the flailing end of the tread separates from the tire.  Repeated contacts by the tread to the fill pipe fuel hose and vehicle underbody result in fill pipe failure and release of liquid from the fuel tank.  Tests of a peer vehicle did not produce a failure or leakage.

Regarding vehicle handling and tire tread separation, failures a relationship between vehicle design and loss of control is scientifically documented.  Only recently has an incremental improvement in vehicle handling following tire tread separation been demonstrated with Electronic Stability Control (ESC).  Regarding other aspects of vehicle performance in tire failures, specifically including tire tread separations, dangers exist that can enhance the chance of harm. 

It is a well know consequence of tire failures that the tire tread can damage the vehicle.  In tire tread separations substantial damage to the wheel well sheet metal is probable.  Tire tread failure induced damage has been documented to hydraulic brake lines, parking brake cables, tail lights, fuel fill pipes, wiring and bumpers.  Parts of a car, not to mention the tire tread, can be knocked free and onto the road surface.  Vibrations from a tire failure have tripped inertially activated fuel pump cut off switches resulting in unexpected engine cut-off.

Engineers can readily foresee similar scenarios for a variety of safety equipment that is taken for granted in motor vehicles.  For example:

  • A tire tread separation occurs at night, damages wiring that routes near the wheel well and renders driver’s suddenly blind to the road or hazards on the road,
  • A tread separation causes permanent damage to hydraulic brake lines or parking brake cables resulting in brake failure or compromised performance.
  • A tread separation causes damage to a light cover or reflector, often rear taillight breakage is observed.

That these are important events in a vehicle’s safety performance is simply supported by the fact that Federal Motor Vehicle Safety Standards (FMVSS) regulate the performance of vehicle systems that are directly dependent upon the key components described above.  

A tire failure event could be analogous to the Part 581 Bumper Standard. Low speed, often parking related, bumper contacts occur in normal driving. Comparatively, tire failures are also expected – driving manuals instruct how to react to a tire failure and most cars have spare tires. Anticipating low speed contacts, the Part 582 Bumper Standard covering all passenger motor vehicles sold in the United States prescribes protective criteria for: lamps, reflective devices and head light alignment; operation of doors; fueling and cooling systems; propulsion, suspension, steering and braking systems; impact energy absorbers; fasteners and joints; and, even separations of surface material, paints and coatings and permanent deviations of original contours.  Comparatively, following a tire failure a vehicle should be capable of performing at the minimum level of safety prescribed by applicable FMVSS.  Vehicle design interventions can effectively eliminate dangers from tire failures induced vehicle component damage.

New Test Results: A Breakthrough in Understanding Front Tire Failure Crashes

September 25, 2009 by · Leave a Comment
Filed under: Crash Reconstruction, Random, Testing, Tread Separation 


By Mark Arndt

Not all tire tread separations are equal and new testing documents previously unknown differences between a front tire failure and a rear tire failure.  Almost universally, tread separation event testing is limited to rear tire failures. Most of the Ford Explorer/Firestone Tire crashes involved rear tires and the causes of these crashes are attributed to a variety of vehicle factors – the largest factor relates to adverse changes in vehicle controllability.  

So why do vehicles that have front tire tread separations get into crashes?

The answer, in part, is explained because despite decreased sensitivity to steering the failure event is startling, produces violent vibration and loud noise and pulling.  Pulling is turning of the vehicle without the driver turning the steering wheel.  Of course, the vehicle steering characteristics also changed suddenly and nonsymmetrically, complicating the driving task.  New testing of front tire tread separation demonstates for some vehicles a substantially increased pulling response comparable to equivalent rear tire failure.  New testing also documents a torque response transmitted through the steering wheel that may jerk the steering wheel from the driver’s grip.

As a rule of thumb, when a rear tire experiences a tread separation the resulting change in the vehicle’s understeer gradient, a key measure of the vehicle turning characteristics, is roughly three degrees per G (3 deg/G) . Where, G is equal to the acceleration of gravity. And, when a rear tire experiences a tread separation event all vehicles ever tested respond in dynamic maneuvers with oversteer – in other words, they spin-out.

It is perplexing that the same changes at the tire that makes a vehicle spin-out when there is a rear tire failure also makes a vehicle less likely to spin-out when there is a front tire failure – in other words, when there is a front tire failure the vehicle will understeer more and become less sensitive to steering.  The new testing results show that an external disturbance may play a greater role that previously understood.

Tread Separation: More Than a Gust of Wind

September 14, 2009 by · Leave a Comment
Filed under: Tread Separation 


By Mark Arndt

An expert made a statement about tire detreading.  He said, “The vehicle gets pulled left only about a foot, in an orientation of one to two degrees, like a gust of wind.”  His answer implied there was little danger and as evidence he referred to a paper I had written.   Attorney Mikal Watts of the Texas law firm, Watts Guerra Craft asked what I thought about the response given the extensive amount of research and testing our company has done on this topic.  

Stated simply, the statement is wrong in its content and it is wrong in it’s context.  The current state of knowledge on the consequences of tire tread separation events (tire detreads) is substantially greater than indicated.

First, when tires fail they fail in a variety of ways creating a broad distribution of response that depend upon the nature of the tread separation, the vehicle and the driving environment.  In other words, there is not just one type of tire failure and there isn’t just one response associated with tire failures. 

Having stated above, one question might be – which paper supposedly describes his response, “like a gust of wind?” The answer is none.  In fact, a tread separation event is always described as including vibration, noise and turning of the vehicle and the event has associated adverse changes in vehicle handling.  Not even a gust of wind has vibration and pulling.  And, the associated adverse change in vehicle handling makes any vehicle dangerous.  Driving at highway speeds with a detreaded tire and gusty wind substantially magnifies the danger and chances of a crash.

The numerous papers (19992000, 20012004, 2006, 2006, 2009) I have written on tread separation demonstrate by scientific methods that all tire tread separations cause violent vibration of the tire/wheel and perceptible yet unspecific vibrations of the vehicle.  Tire tread separations cause sudden and startling loud noise or noises from the separating tread hitting portions of the car and ground.  Tire tread separations cause an external pulling of the vehicle — in other words the vehicle turns without the driver turning the steering (hand) wheel.  Pulling could be minor or could force a vehicle off the road.  Finally, steering characteristics are degraded and in incidents involving rear tires a vehicle oversteer characteristic results.