Treading Close to Limits - Tire Review Magazine

Treading Close to Limits

At what tread depth should tires be taken out of service? For a long time, the standard answer has been 2/32-inches, but some in this industry are pushing for 4/32-inches. For good reason.
In Europe, the conversation has gotten pretty heated among tire manufacturers. Here, it is a barely audible conversation. So it comes down to what’s right for your customers.
Continental has come out strongly in favor of the 4/32-inch standard. The tiremaker states passenger tires should be “removed from service” when worn to 4/32-inch.
According laws in 42 states, tires are legally worn out when the tread depth is worn down to 2/32-inches (1.6 mm). There is no federal standard for con-­ sumer tires.
For commercial trucks, the federal limit is 4/32-inches for steer tires and 2/32-inches for all other positions.
The 2/32-inch standard has been adopted by most countries that have enacted treadwear limits. Tires sold in North America and Europe are required to have treadwear indicator bars (wear bars) in the main grooves that become level with the tread surface at approximately 2/32-inch of remaining tread. The wear bars are intended to give motorists a visual warning that their tires no longer meet minimum tread depth requirements.
How many of your customers even know about the wear bars on tires?
Over the last several years, in both Europe and North America, there have been efforts to move to 4/32-inch as the new removal standard. In North America, Consumer Reports magazine has perhaps been the most vocal and persistent champion of effectively doubling the depth at which car owners should begin shopping for new tires, especially if they drive on wet roads.
In making its recommendation, Consumer Reports cited its own and others’ research that show braking distances in wet weather increase mark­edly after tires reach 4/32-inch tread depth. Tire Rack, an online retailer/­wholesaler, has largely echoed those same arguments.
Among tire manufacturers, only Continental AG has come out strongly in favor of the 4/32-inch standard.
Continental takes the strongest position, flatly stating that passenger tires should be “removed from service” when worn to 4/32-inches, and has posted videos clearly showing the stopping-distance difference between a vehicle on tires worn to 2/32nds and those at 4/32nds.
Continental maintains that its studies and those of independent experts have shown that braking distances, particularly in wet conditions, are greatly increased after the tread depth falls below 4/32-inch. Continental’s rationale is that the 2/32-inch limit dates back to when cars had thinner tires and less power. With the trend towards larger, wider tires with lower profiles, the typical contact patch is larger … great for dry braking, but a problem in wet conditions. The larger contact patch increases the risk of hydroplaning and 2/32-inch of tread depth is insufficient to channel the water away from under the tire.
Switching Standards
So far, other tire manufacturers haven’t been moved by the arguments for the 4/32-inch standard, which contradicts the current 2/32-inch threshold. The consensus seems to be that the 2/32-inch minimum tread depth is prudent and provides a clear indication when to remove tires. Manufacturers also cite environmental and cost reasons: i.e., if the standard becomes 4/32-inch, manufacturers would have to build more tires and consumers would need to dispose of more tires.
And let’s not forget the legal argument that if a company supported one standard and then shifts to another, is it exposed to lawsuits from drivers injured in skid-related accidents, claiming “you are liable because you always said 2/32nds was okay.”
Nor do manufacturers other than Continental appear to be convinced that moving from 2/32-inch to 4/32-inch will have an impact on safety. They cite a lack of data to show that more accidents occur as a result of tires being operated at the current minimum tread depth.
Recent advances in technology mean that wet weather performance no longer necessarily declines in a linear way as the tread depth decreases. By incorporating dual-layer compounds where a second compound is exposed as a tire wears, wet traction can be maintained in the later stages of the tire’s life, some manufacturers claim.
Other technology causes “new” tread patterns to appear as the tire wears, allowing the tire to evacuate the same amount of water as when new.
Testing and Results
Consumer Reports and Tire Rack back their recommendations with very similar tests focused on wet braking where the depth of the tread of the tire is critical to stopping power. Remember that tires need to be in contact with the road in order to stop. The tire has to channel water away so the tire contacts the road surface and is not floating on a thin film of water – hydroplaning. Without sufficient tread depth, the tire can’t move the water out of the way and the tire hydroplanes.
In the Consumer Reports test, they flooded a test track with water 0.05-inch deep. A car and a full-sized pickup truck made full emergency stops from 70 mph. Stopping distance and time were measured for three different tire depths: new tires, tires with 2/32-inch tread and tires with 4/32-inch tread.
The stopping distance for the car with 2/32-inch tread depth tires was nearly double that of the car equip­ped with new tires. To put that into perspective, when the car with the 2/32-inch tires had braked for the distance required to stop the car with new tires, it was still going 55 mph.
Even with the 4/32-inch tread depth tires, the car was still going 45 mph at the point where new tires brought the car to a halt. The results with the truck were similar, with the stopping distances being even longer.
The tests conducted by Tire Rack again used new tires, tires at 4/32-inch depth and tires at 2/32-inch. Panic stops were again made from 70 mph with a sedan and pickup truck on asphalt covered by 0.05-inch of water.
With new tires, the car traveled 195 feet before coming to a stop; with 4/32-inch tires, it went 290 feet. The same vehicle with 2/32-inch tires traveled 379 feet, nearly doubling the stopping distance of the car with new tires. In fact, the sedan with 2/32-inch tread depth was still going 44 mph when it reached the final stopping point of the 4/32-inch vehicle.
As in the Consumer Reports test, the pickup truck test provided longer stopping distances but similar ratios (255 feet, 378 feet and 500 feet, respectively), and the pickup with 2/32-inch tread depth tires was still going 47 mph when it passed the stopping point of the truck with 4/32-inch. Tire Rack’s tests can be found at by searching for “panic stopping.”
It can certainly be argued the Consumer Reports and Tire Rack tests were very limited in scope and that all vehicles and all tires are not going to perform in the same manner. Some of the newer premium offerings with their dual compound treads and sophisticated tread patterns would have made for an interesting comparison. At the very least, though, these tests illustrated that tread depth remains key to safe wet weather performance.
While there are no separate legal tread depth removal standards for winter tires, at 2/32-inch any tire has almost no snow traction, even a specifically designed winter tire. More tread depth is needed in snow because tires need to compress the snow in their grooves and release it as they roll. Without sufficient tread depth, traction and mobility will be sacrificed.
Because tread depth is so important for snow traction, winter tires often start with deeper tread depths than all-season or summer tires and should be removed when tread depth reaches 5/32-inch if snow traction is a concern. At 5/32-inch and below, the tire is still usable as the equivalent of an all-season tire, but it will offer no real performance advantage in snow.
The Conclusion
So, what do you tell your custo­mer? Like many tire questions, much depends on how the customer uses the vehicle, what conditions they’re likely to encounter and their expectations. Asking about driving habits and anticipated weather and road conditions helps you make reasonable recommendations and helps the customer make good decisions.
While 2/32-inch may be acceptable for dryer climates, in areas of the country where rain is more likely, customers should plan to replace tires when the tread depth reaches 4/32-inch.
Similarly, winter tires should be considered for replacement if only 6/32-inch remain at the start of the season.
Undoubtedly, customers who replace tires at 4/32-inch may not get the full benefit of a tread warranty or know if their tires are wearing out too fast. They may be suspicious of the recommendations for “early” replacement because they seem self-serving.
Yet, the recommendation of an influential publication like Consumer Re­ports should give you some added credibility for those recommendations. Certainly there are a number of consumers who follow the recommendations of Consumer Reports for their tire purchases and they will likely be accepting of the magazine’s opinion.
Fortunately, the 2/32-inch vs. 4/32-inch debate is often not an issue because so many tires need to be replaced long before they reach 2/32-inch. More often than not, replace­ment is necessary due to damage or uneven wear rather than low tread depth alone. Most customers don’t come in with tread depth concerns, but rather with symptoms, such as lack of traction or hydroplaning. Frequently, the explanation is some type of uneven wear, even when there is tread left.
All tires have to be replaced eventually. The customer has to balance the cost of new tires versus the safety benefits they provide. However, at some point, new tires become inexpensive compared to the risk of property damage and injuries.

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