Singing In the Rain? Well, your tires may not be so happy after all. Summer heat is certain to bring showers and some unexpected wet road conditions. If you want to be certain your tires are safe for wet-weather driving, you need to make sure the treat has enough traction to avoid hydroplaning (when a tire “floats” over water on the road instead of gripping the pavement).
A small amount of extra rubber makes a big difference in stopping distances. At highway speeds (55+mph) an inch of tread depth remaining, resistance to hydroplaning is significantly reduced and stopping distances dramatically increase. If you let your tire tread go beyond the 2/32 of an inch (1.6mm) tread depth, then you could be sacrificing wet stopping ability.
An independent test compared the stopping distances of tires with 2/32 of an inch of remaining tread depth to tires with 4/32 of an inch of tread depth and to new tires. The tests were conducted at a 70-degree ambient temperature on a 600-foot long asphalt braking lane using a 2006 BMW 325i and a 2006 Ford F-150 SuperCab 4×2 pickup driving at 70 mph. Both vehicles were equipped with four-wheel disc brakes and anti-lock brake systems, and with tires that had been used as original equipment on equivalent models of the cars.
One set of test tires had full tread depth, while the other two sets were shaved to 4/32 and 2/32 of an inch of remaining tread and baked in an industrial oven at temperatures that replicate aging the rubber over several years of driving. All tire sets were broken in for approximately 100 highway miles. The asphalt test track was wetted by a watering system that maintained 0.05 to 0.06 an inch of water depth above the peaks of the asphalt’s aggregate. To bring another coin measurement into mix, this means you could lay a dime on the road and the water would flow around but not over it. The testers accelerated to just above 70 mph on dry road and then drove across about 75 feet of wet pavement before slamming on the brakes and recording a panic-stopping distance.
After repeated runs in the BMW with new tires, the average stopping distance was 195.2 feet in 3.7 seconds. On tires with 4/32 of an inch of remaining tread depth, the average stopping distance was 290.0 feet in 4.7 seconds, and the car was traveling at a little over 45 mph when it reached the distance it took the new tires to bring the BMW to a stop. Tires with the 2/32 of an inch of tread depth had an average stopping distance almost double from what it had been for the new tires, 378.8 feet, and it took 5.9 seconds to stop. Plus, the car was still traveling at around 55 mph when it reached the same distance it took the BMW to stop with the new tires.
For the same test in the Ford pickup, the average stopping distance when it was shod with new rubber was 255.9 feet in 4.8 seconds. Stopping the truck with tires worn down to 4/32 of an inch of tread depth took 377.8 feet in 6.0 seconds, and the truck was still traveling about 47 mph when it reached the distance it had taken the new tires to bring the pickup to a halt. On tires with 2/32 of an inch of remaining tread depth, it took the truck 499.5 feet to stop in 7.5 seconds — meaning that the road would have to be clear for almost 1/10 of a mile ahead to allow a complete panic stop without slamming into something. The testers also observed that the truck with the 2/32-inch tread-depth tires slowed only to about 58 mph when it reached the same distance at which the new tires brought the vehicle to a halt.
Although Brake-O-Rama isn’t necessarily recommending that car owners replace their tires when they reach 4/32 of an inch of tread depth, they do say that “By the time tires reach the 4/32 of an inch mark, drivers should be doing their research for their next set of tires and make that purchase soon thereafter”. While Brake-O-Rama has obvious financial incentives in recommending that car owners replace their tires sooner, the retailer acknowledges that there are trade-offs in doing so, such as an increase in driving cost per mile for consumers and more tires being discarded annually. But the company feels that that the monetary and incalculable costs from accidents due to driving on worn-out tires far exceed such concerns.
The company also wants to change long-standing rules on when a tire is out of warranty. “Some tire tread-wear warranties require that all four tires be worn down to approximately 2/32 of an inch of remaining tread depth before they can be submitted to the manufacturer for consideration” says Brake-O-Rama. “And if you live in an area with frequent snow and rain, we feel the consumer should be in a position to change the tires sooner to maintain better control of their vehicles rather than pushing it for the last couple of thousand miles to claim their tread-wear warranty.”
Brake-O-Rama also points out that the Penny Test standard dates back 40 years. As specified in the 1968 Federal Motor Vehicle Safety Standard 109, wear indicators with 2/32-inch raised bars were required to be molded across tire treads, under the premise that tires lose traction capabilities at about 2/32 of an inch of remaining tread depth. Of course, tires and vehicles have changed quite a bit in 40 years. According to Brake-O-Rama, a 1967 Chevrolet Impala was originally equipped with 14-inch tires with 4.5-inch wide treads, while a 2007 Impala uses 16-inch tires with 7.1-inch treads — over one and a half times wider.
Certainly new tires provide the greatest protection when it comes to the tire’s ability to avoid hydroplaning and brake in a short distance in wet conditions. “Our tests were intended to show that when tires wear down to 2/32 of an inch, it virtually doubles the stopping distance compared to a new tire. And by considering replacing tires at the 4/32 of an inch, it gives car owners more leeway in researching their tire purchase.”
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