TTI’s technical editor, Joe Walter, recalls how a difference in specified tire pressure caused Firestone to recall millions of tires developed for the Ford Explorer
To understand vehicle rollover and what went awry with the Ford Explorer fitted exclusively with certain Firestone tires from 1991 until 1999, consider the origins of the sport utility vehicle in the USA, starting with the rollover-prone World War II US Army Jeep – the progenitor of today’s quintessential SUV.
More than 600,000 Jeeps were produced during the war effort. When hostilities ended in 1945, about 200,000 Jeeps remained unused but were never sold in a car-starved nation. Thus, the civilian Jeep (CJ) was born and produced in various embodiments by different manufacturers until 1986 – all with a checkered highway rollover history. But the vehicle was popular with returning military personnel and civilian off-roaders. More than 1.5 million Jeep CJs were eventually built with the same basic body style for more than four decades.
Ford designed its first SUV, the full-size Bronco, to compete with the trendy Jeep CJ in 1966; it then introduced the smaller Bronco II in 1984, jokingly known as the bucking Bronco. In 1991, a more passenger-friendly SUV, the first-generation Explorer, replaced Ford’s rollover-prone Bronco II – but it was based on the Ranger pickup truck with which it shared the same chassis, wheelbase and solid rear axle. The Explorer proved to be immensely popular with the motoring public, with sales exceeding 600,000 vehicles by 1999. Surging SUV sales were underway throughout the US. During the 1980s and 1990s, rollovers accounted for roughly 3% of all serious US highway crashes but were responsible for 30% of fatalities.
The National Highway Traffic Safety Administration (NHTSA) took notice, funding a multitude of research programs in part to develop theoretical vehicle models to predict and explain rollover. The simplest model, based on vehicle track width and height of the center of gravity, was shown to be highly correlated with both tripped and untripped rollover events. Unsurprisingly, wider track and lower center of gravity reduced the rollover propensity of all vehicle classes.
As was the auto industry norm, Ford designed the Explorer while specifying the tire performance specifications including inflation pressure – a relatively low 26psi (1.79 bar). Firestone then undertook development of a tire to meet its internal standards, government regulations and Ford requirements. At the confluence of these parameters, there was a basic problem with the tire design – a lack of adequate
load reserve at the low operating pressure for a heavy vehicle with large interior volume. Firestone recommended 30psi (2.1 bar) tire inflation to no avail.
Rollovers occurred on Explorers at troubling rates, with its exclusive Firestone fitments eventually leading to finger-pointing between long-time partners Ford and Firestone. By 2001, Firestone had recalled 14.4 million tires and severed its 100-year relationship with Ford in North America. The reputations of both companies were severely tarnished. Interestingly, the same tire specification was produced non-exclusively for the Ford Ranger pickup truck with 35psi (2.4 bar) tire pressure – and non-problematic service history.
Aside from the perfunctory resignations (by 2001) of Ford and Firestone CEOs Jacques Nasser and Masatoshi Ono, lessons learned and/or actions taken by affected parties as a result of this rollover debacle included:
- Ford’s redesigned Explorer (2002) prudently featured increased tire pressure, wider track and an independent rear suspension – and multiple tire suppliers. It probably no longer relies on exclusive contracts for safety-critical components for vehicles with large and expanding market size.
- NHTSA’s five-star rollover rating regulation surprisingly evolved as a consumer advisory in 2004, not a more rigorous Federal Motor Vehicle Safety Standard. The organization finally issued a major revision of federal standards governing tire safety in 2007. More importantly, it ushered in first-time requirements for tire pressure monitoring in 2008 and electronic stability control in 2008.
- Firestone upgraded its tire development and manufacturing standards, sharpened its quality control procedures and burnished its brand image with significant marketing events such as prestigious IndyCar racing.
While Firestone has seemingly regained the public’s trust, I have my fingers crossed – hoping for the best.