AKRON—Has the widely accepted minimum tread depth of 2/32nds inch—a relic of the bias-ply tire era—outlived its usefulness and should it be increased to improve safety on the nation's highways?
That's the contention of a vehicle dynamics engineer from Palo Alto, Calif., who has studied the situation for years, and the subject of two studies published recently in Europe.
William Blythe, a retired mechanical engineer who taught applied mechanics at San Jose State University for 42 years, has published a paper showing wet traction falls off “significantly” as tread depth falls below 4/32nds inch—or twice as deep as most states allow.
In his study, presented Sept. 12 at the International Tire Exhibition and Conference (ITEC) in Akron, Mr. Blythe notes there is no national standard for minimum tread depths. Instead, each state sets its own and is responsible for policing it.
Federal law governing tires does, however, require tires sold in the U.S. to be built with wear indicators to show when a tire has worn to 2/32nds inch.
In the U.S., 42 states consider 2/32nds the minimum tread depth, California and Idaho consider 1/32nd the minimum, and Arkansas, Montana, New Mexico, North Dakota, South Carolina and West Virginia have no standards, according to Mr. Blythe's research.
Federal law does mandate 4/32nds inch tread depth on the steer axle tires of medium and heavy trucks and 2/32nds inch depth on the drive and trailer axle tires. In addition, several U.S. states in the snow belt or mountain states proscribe 6/32nds as the minimum tread depth for winter tires to be used without chains in critical areas.
In Canada, the minimum tread depth of 2/32nds for cars/light trucks is a federal standard, enforced by local police under the authority of the Motor Vehicle Safety Act, according to information from the Rubber Association of Canada.
Regarding the commonly accepted 2/32nds, Mr. Blythe said, “No rational basis relating to roadway safety has been found for the minimum tread depth requirement of 2/32nds (1.6 millimeters) of an inch.”
Comments from the ITEC audience listening to Mr. Blythe's presentation supported his supposition that the 2/32nds limit stems from warranty claims.
In addition, engineers and industry experts familiar with the situation say 2/32nds worked relatively well when tires were still primarily 80- and 90-series and relatively narrow, and when speed limits were lower. Such tires have a longer, narrower footprint, which cuts through standing water more effectively than the lower-profile wider tires common today.
Harold Herzlich, an independent engineer/consultant who worked with the former Pirelli-Armstrong Tire Co. and Amstrong Tire Co., petitioned the National Traffic Safety Administration (NHTSA) in 1995 to increase the minimum tread depth, but that petition was denied.
Mr. Herzlich, who also is the technical editor for Rubber & Plastics News, a sister publication of Tire Business, said NHTSA denied the petition because it considered it anti-consumer—that increasing the minimum tread depth would reduce the value of a tire to consumers because it would shorten the tire life and increase the number of scrap tires.
Mr. Blythe's data are based on laboratory tests designed to show at what speed aquaplaning starts to occur at various depths of water.
The shallowest depth tested, one-half inch, still is considered quite deep by industry experts who participated in a “peer review” at the ITEC event of Mr. Blythe's paper.
As such, many of those who attended the peer review session would prefer to see more comprehensive testing in real-world conditions to determine to what extent braking or handling is affected in a 4/32nds vs. 2/32nds comparison.
While many in the industry consider Mr. Blythe's proposal a move in the right direction from a safety standpoint, some are concerned any move to raise the minimum tread depth would be perceived as a self-serving initiative that would shorten tire life and allow the industry to sell more tires.
Making the situation worse, though, is the fact that many consumers don't change tires even after they've reached the 2/32nds minimum. Research in 2001 by NHTSA showed that 9 percent of cars the agency surveyed had at least one tire at or below the minimum.
In Europe, the Tyre Industry Council (TIC) in the United Kingdom, an initiative funded by tire manufacturers and the majority of U.K. tire retailers, has supported research into the the tread depth question. It has begun lobbying actively for a change in the minimum tread depth—to 3mm (approximately 4/32nds inch) from 1.6mm—based on the results of those tests.
Separate but similar testing conducted by RoadSafe1—an independent partnership of leading companies in the motor and transport industries in Great Britain, the U.K. government and road safety professionals—concluded that cars running on tires with 3mm tread depth stopped 25 percent faster from 50 mph than did the same cars running on tires with 1.6mm. That translated into about two car lengths extra stopping distance.
German tire maker Continental A.G. followed up earlier this summer in cooperation with the U.K. consumer magazine Auto Express to conduct a series of similar tests using four different vehicles. That exercise again confirmed the results of the TIC and RoadSafe1 tests: Cars running on tires at 2/32nds depth required 130 extra feet—nearly nine car lengths—to stop from 74 mph than the same cars on tires at 4/32nds.
In fact, the U.K. government mandates that all government vehicles change tires when they've reached 2.5mm (approximately 3/32nds inch), according to the TIC's Chris Wakley.
Conti's Semperit Ireland subsidiary has taken the exercise to the next level, officially lobbying the Irish government to consider raising the minimum tread depth there to 3mm.
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