The Importance of Tire Tread in Driving Safety
The earliest automobiles were essentially horse-drawn wagons or buggies that had been adapted to internal combustion engine power. They rode on wagon wheels where steel rims and wooden spokes were the order of the day. Braking, cornering and road conditions were issues to be addressed in future automotive design. Automotive history is on display at the Henry Ford Museum in Dearborn, MI where one can walk the twelve decades in an afternoon.
Steel and wooden wheels were adequate when the roads were dirt trails and engines offered limited power. Horse power increases brought increased speed and acceleration capability, with the accompanying demands on suspension and wheels. The carriage wheel evolved through the hard rubber tire into the balloon tire. Tire tread design began as a decoration and included such innovations as the manufacturers name molded into the road contact surface multiple times.
If ever there was an example of “racing improving the breed” it is the modern automotive tire. The implication for the average driver operating the family sedan is a safer and more durable tire. While racing “slicks”, tires without tread, are used on a dry racetrack, racing rain tires have deep treads and are most akin to the normal street tire. The tread’s purpose, both in racing and on the street, is to provide an escape route for water between the tire contact patch and the road surface. Contrary to a common misconception, it is not the tread that grips the road but the smooth rubber between the treads.
The term “hydro plane” refers to a condition where the tread can no longer drain the water from under the smooth sections. This is the mechanism that is at the root of control loss on wet roads. When the tread cannot expel the water the water forms a wedge between the tire and the road surface and traction is lost.
The solution to maintaining traction in wet conditions is two-fold and quite simple. There are warning strips molded into each tire within the tread. As the tire wears the warning strips become exposed at which point the tread depth is less than 3/32”. These indicators are pointing to the condition where the tread is no longer capable of properly draining the moisture from between the road surface and the smooth sections of the tire. Thus handicapped, a combination of speed and cornering load that would have previously been acceptable will now exceed the tire’s capability and a loss of traction will occur.
Most drivers understand that wet traction is lower than dry traction. Within the 18th edition of the Bosch Automotive Handbook is a list of friction coefficients of tire-road interface friction. It lists these coefficients for dry asphalt and concrete and for wet conditions with various depths of water. It also compares these coefficients for new and worn tires. Even when the tread is functioning well, there is still a film of moisture between the contact patch and road surface that reduces traction. Wet traction is reduced to approximately 60% of dry traction. Hydro planning is always a potential even for a tire in good condition if the water depth is such that the tread is overwhelmed. This is evident as the traction coefficient is shown by the Bosch Handbook to be reduced as the water depth increases.
As engineers continue to develop improved tires and tread design it remains in the hands of the driver/vehicle owner to understand the dynamics of a vehicle in motion. Traction control systems are incorporated into vehicle electronics to assist the driver. However, it remains within his purview to push the tires beyond their capability and thereby rearrange the roadside scenery with his vehicle.
