The generally accepted minimum tread depth of 2/32 inch is passe and should be increased to improve highway safety, according a speaker at the ITEC 2006 conference.
William Blythe, a retired mechanical engineer from Palo Alto, Calif., who taught applied mechanics at San Jose State University for 42 years, presented his paper that shows wet traction falls off significantly as tread depth falls below 4/32 inch-or twice as deep as most states allow.
In his study, presented Sept. 12, 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 2/32 inch.
In the U.S., 42 states consider 2/32 the minimum tread depth, California and Idaho consider 1/32 the minimum, and Arkansas, Montana, New Mexico, North Dakota, South Carolina and West Virginia have no standards, according to Blythe's research.
Federal law mandates 4/32 inch tread depth on the steer axle tires of medium and heavy trucks and 2/32 inch depth on the drive and trailer axle tires. In addition, several U.S. states in the snow belt or mountain states prescribe 6/32 as the minimum tread depth for winter tires to be used without chains in critical areas.
In Canada, the minimum tread depth of 2/32 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.
``No rational basis relating to roadway safety has been found for the minimum tread depth requirement of 2/32 (1.6 millimeters) of an inch,'' Blythe said.
Comments from the ITEC audience listening to Blythe's presentation supported his supposition that the 2/32 limit stems from warranty claims.
In addition, engineers and industry experts familiar with the situation say 2/32 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 Armstrong Tire Co., petitioned the National Traffic Safety Administration in 1995 to increase the minimum tread depth, but that petition was denied.
Herzlich, who also is the technical editor of Rubber & Plastics News, 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.
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, 1/2 inch, still is considered quite deep by industry experts who participated in a peer review of Blythe's paper at ITEC. Many said they prefer to see more comprehensive testing in real-world conditions to determine to what extent braking or handling is affected in a 4/32 vs. 2/32 comparison.
Many said they consider 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 2/32 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 in the United Kingdom, an initiative funded by tire manufacturers and the majority of United Kingdom tire retailers, has supported research into the tread depth question. It has begun lobbying actively for a change in the minimum tread depth-to 3mm (approximately 4/32 inch) from 1.6mm-based on the results of those tests.
Similar testing was conducted by RoadSafe1, a partnership of companies in the motor and transport industries in Great Britain, the U.K. government and road safety professionals. It concluded 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. It said cars running on tires at 2/32 depth required 130 extra feet-nearly nine car lengths-to stop from 74 mph than the same cars on tires at 4/32.
The U.K. government requires all government vehicles change tires when they've reached 2.5mm (approximately 3/32 inch), said TIC's Chris Wakley.