July 20, 2007
The accident report read, "I was driving on the expressway with the flow of traffic at about 70 mph in moderate rain when all of a sudden I noticed traffic had come to a stop ahead. I hit the brakes hard, but nothing happened" While most drivers credit their vehicle's brakes when it comes to stopping quickly, many overlook the important role tires have in converting the brake system's capabilities into actual stopping distances.
After receiving feedback from drivers concerned about decreasing wet traction as their tires wore down, Tire Rack team decided this important safety issue deserved further investigation. We wanted to determine if the current replacement recommendations based on minimum tread depth are good or if they simply give drivers a false sense of security. By the time we were done we were surprised at the incredible reduction in wet road braking performance of tires with legal minimum tread depths that could only produce potentially lethal stopping distances!
Members of Tire Rack team conducted independent tests that compared the stopping distances of new tires to tires at the point of approaching being legally worn out in most states with 2/32nds-inch of remaining tread depth (easily measured using a penny), as well as tires with 4/32nds of remaining tread depth (easily measured using a quarter). Because we wanted to complete the testing before the ambient temperatures in South Bend, IN would represent typical driving conditions and needed to evaluate braking performance at typical 70 mph expressway speeds, we conducted the tests off-site at a tire and vehicle test center where 70-degree ambient temperatures were the norm and a 600-foot long asphalt braking lane was available.
We used one of Tire Rack's 2006 BMW 325i test cars to represent European sport sedans and arranged for the use of a 2006 Ford F-150 Super Cab 4x2 to represent full-size American pickups. Both vehicles were equipped with 4-wheel vented disc brakes and Antilock Braking Systems (ABS). We selected tires that have been used as Original Equipment on equivalent models: the Michelin Energy MXV4 Plus in the 205/55R16 91H size for the BMW sedan and the Continental Tire ContiTrac in the P255/65R17 108S size for the Ford pickup.
One set of test tires remained at full tread depth, while additional sets were shaved to 4/32nds and 2/32nds of remaining tread depth. The shaved tires were baked in an industrial oven for several weeks at temperatures that would help replicate the rubber aging experience by tires during several years of driving and wear. The final step of tire preparation was to break them in by driving all of them approximately 100 highway miles.
The tire and vehicle test center's 600-foot long asphalt braking lane was wetted by a watering system that maintained between 0.05-0.06" of water depth above the peaks of the asphalt's aggregate (this means that you could lay a Roosevelt dime on the road, and the water would flow around the coin, but not over it).
We accelerated to just above the 70 mph target speed on dry road and drove across about 75 feet of wet road before hitting the brakes and recording our panic stop from 70 mph. This procedure would be used for testing both vehicles and all of the tires to assure they began braking at equivalent speeds and experienced the same conditions. We recorded the tires' braking performances with our Vericom VC2000 and DriftBox performance analyzers. The Vericom would be used to record the limits of performance and the DriftBox would be used to confirm accurate test speeds and chart our results.
Even though the speed to begin braking the 3,400-pound BMW sedan was 20 mph higher than the 50-0 mph braking tests we conduct at Tire Rack, the braking forces developed by the new tires felt reminiscent of full tread depth tires we've experienced on our home track. The car pitched forward as the brakes were applied, quickly took a set and slowed the car with authority. After repeated runs we learned that the average stopping distance for the new tires from 70 mph was 195.2-feet in 3.7-seconds.
Testing tires with the 2/32-inch minimum legal tread depth taught us the probable origin of the driver's accident report statement, "I hit the brakes hard, but nothing happened." There was a perceptible delay while waiting for the braking forces to grow after initiating the stop. Looking at the graphs recorded by the DriftBox confirmed this discomforting situation when we realized we were still traveling at about 55 mph on the tires with 2/32-inch of remaining tread depth when we reached the same distance it took the new tires to bring the BMW to a complete stop! This time the repeated runs taught us that the average stopping distance for the still legal 2/32-inch deep treaded tires from 70 mph had almost doubled to 378.8-feet and took 5.9 agonizing seconds to accomplish. We had the same car and the same brakes, but the tires with minimum legal tread depths weren't able to generate enough traction on the wet road to bring us to a quicker stop.
Stopping with the partially worn tires with 4/32-inch of remaining tread depth increased our confidence, but we discovered the initial stopping forces felt more like the 2/32-inch deep tires than new tires, gradually gaining advantage over the 2/32-inch tires as the car's speed was reduced. Looking at the graphs recorded by the DriftBox confirmed a better situation, but we were traveling at a little over 45 mph on tires with 4/32-inch of remaining tread depth when we reached the distance it had taken the new tires to bring the BMW to a stop! Our runs indicated that the average stopping distance for the 4/32-inch deep worn tires was 290.0-feet in 4.7 seconds, basically splitting the difference between the new tires and tires that had legal minimum tread depths.
Then we turned our attention to the approximate 4,500-pound Ford F-150 Super Cab 4x2 pickup. While we expected its additional 1,100 pounds of curb weight to help push the tires down through the water to the road, we felt that the energy required to stop the extra weight would increase braking distances. As with the BMW, we accelerated to the 70 mph target speed on dry road and drove across about 75 feet of wet road before hitting the brakes to begin our panic stop.
While not experiencing as much absolute braking force, the pickup pitched forward a bit more than the car when the brakes were applied, took a set and slowed a bit more leisurely. We learned that the average stopping distance for the pickup on new tires from 70 mph was 255.9-feet in 4.8-seconds.
Testing tires with 2/32-inch of remaining tread depth on the pickup further emphasized the driver's accident report exclaiming, "nothing happened" when they hit the brakes. There was a noticeable hesitation waiting for the braking forces to grow after initiating the stop. Looking at the graphs recorded by the DriftBox showed we initially only slowed 12 mph on the tires with 2/32-inch of remaining tread depth when we reached the same distance at which the new tires brought the F-150 to a complete stop! This time the repeated runs taught us that the average stopping distance for the legal 2/32-inch deep tires from 70 mph was a staggering 499.5-feet in 7.5 excruciating seconds. In addition to the hydroplaning risks associated with shallow treaded tires, this means that the highway would have to be clear for almost 1/10 of a mile ahead of the pickup to allow this combination to complete a panic stop without hitting something.
Stopping with the partially worn tires with 4/32-inch of remaining tread depth again confirmed that the initial stopping forces again felt more like the 2/32-inch deep tires than new tires. Looking at the graphs recorded by the DriftBox showed an improved, but disquieting situation since we were still traveling about 47 mph on tires with 4/32-inch of remaining tread depth when we reached the distance it had taken the new tires to bring the pickup to a complete stop! The runs indicated that the average stopping distance for the 4/32-inch deep worn tires was 377.8-feet in 6.0 seconds, again splitting the difference between the new tires and those that had legal minimum tread depths.
For years the rule of thumb has been that tires should be replaced when their treads wear down to 2/32-inch (1.6mm) of remaining tread depth (the distance from the circumference of a U.S. penny to the top of Lincoln's head). Beginning August 1, 1968, wear indicators with 2/32-inch raised bars were required to be molded across the tread as specified in U.S. Federal Motor Vehicle Safety Standard 109 (FMVSS). This standard was partially based on a 1967 report (Skidding Accidents on Runways and Highways Can be Reduced) and the long-standing premise in the tire industry that tire treads lose their traction capabilities at about 2/32-inch of remaining tread depth and should be replaced. Today's FMVSS 139 continues the same standard and assumptions.
However, it's important to realize that the tire's ability to perform in wet conditions will be reduced as it wears. With only 2/32" of remaining tread depth, resistance to hydroplaning while driving in the rain at highway speeds will be significantly reduced and stopping distances increased. Tires easily push air around and through their tread designs as they roll; so tread depth isn't an issue in dry conditions. But when water builds up on the road during rainstorms, the water depth, vehicle speed and vehicle weight, as well as the tires' tread design, width and depth collectively determine when and if the tires will be forced to hydroplane on top of a film of water and how quickly they can stop a vehicle.
It is also very important to recognize how much tire sizing and vehicle applications have changed in the last 40 years. The 1967 Chevrolet Impala weighed 3,897 pounds and was originally equipped with 7.75-14 sized tires that featured about 4.5-inch wide treads. The 2007 Chevrolet Impala weighs 3,674 pounds and uses P225/60R16 97S sized tires that have 7.1-inch wide treads. In contrast, today's Impala weighs 233 pounds less and uses tires that feature a tread width that is over one and a half times wider than tires used in 1967. While this undoubtedly provides more traction and shorter braking distances on dry roads throughout the tires' life, how do the tread depths of today's wider tires influence stopping performance on wet roads when the tires are new, as they wear, and ultimately as they reach the legal minimum tread depth?
Legal tread depth requirements also have an indirect influence on tire warranties. Tires remain legal (in most states) and their manufacturer's materials and workmanship warranties remain in force until they have worn to 2/32" of remaining tread depth. However, tires also backed by a tire manufacturer's or retailer's tread life mileage warranty must be worn to 2/32" according to the written details of their coverage before they can be submitted for consideration of replacement. We question if this practice encourages consumers to drive longer on tires that don't provide enough poor-weather traction.
If rain and wet roads are a concern, consider replacing your tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, you need enough tread depth to allow rain to escape through the tire's grooves. If the water can't escape fast enough, your vehicle's tires will be forced to hydroplane (float) on top of the water, losing traction and increasing stopping distances. Fortunately, measuring 4/32" of remaining tread depth with a U.S. coin would simply require replacing the outdated Penny Test with a up-to-date Quarter Test (measuring with the distance from the coin's circumference to the top of Washington's head).
While there are obvious trade-offs when replacing tires before they are fully worn out (such as an increase in driving cost per mile and the reality that more tires would be discarded annually), we don't believe any of these reasons exceed the emotional and monetary costs of drivers and passengers recuperating from injuries or repairing a vehicle after an accident.
Some tire manufacturers will provide a consumer concession on a one-by-one basis that allows tire removal before reaching 2/32" for drivers facing seasons of the year that are accompanied by significant rain and snow. We believe the purpose of these consumer concessions needs to be formalized and be written into all tread life mileage warranties. We know these changes won't happen overnight so we're encouraging all drivers to pay more attention to their tires now.
Do you really want to put yourself at this much risk by running your tires until they are legally worn out?
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