Rudimentary pneumatic tires at the end of the 19th century all featured smooth treads similar to their existing solid rubber counterparts. Tire tread patterns first appeared on cars in 1904, with Dunlop and Continental producing designs containing protuberances or indentations of raised rubber buttons or depressed transverse slots as ‘anti-skid’ measures. Firestone tires in 1908 featured treads with reverse molding; these cleverly laid down the words ‘NON SKID’ on snowy roads or dirt surfaces.
If it didn’t rain and roads remained dry, there would be no need for tread patterns in all their detailed variety. Treadless racing slicks provide unexcelled traction on dry roads, but roads are often wet. Thus, highway treads generally feature a geometric array of circumferential grooves, transverse slots, and innumerable tread elements that promote grip by providing passages for removing and mopping-up water in the footprint.
The cycle industry first introduced a non-smooth tread during the 1890s with a molded continuous circumferential rib to help prevent lateral skidding of bicycles. Sometime thereafter, skidding countermeasures for smooth-treaded automobile tires were promoted by tire repair shops and remolders with traction innovations based on, for example, protective leather straps fit with steel buttons, which would be strapped to the tread surface to improve traction. These products and processes ultimately evolved into today’s retreading industry, which was in full operation by 1920. Companies large and small were now producing their own tread patterns and associated designs for new and retreaded tires to meet the demands of a rapidly growing car population.
Today, the overall layout of PCR tread patterns can be classified as symmetric, asymmetric, or directional, and they are available in summer, winter or all-season applications. Symmetric patterns can be rotated to any of the four wheel positions of an automobile, while directional and asymmetric patterns are restricted to specific mounting arrangements as indicated by the tire sidewall lettering. Patterns can also be characterized as rib, lug, all-terrain, etc, depending on the vehicle application. In contrast with the pattern, the tread design provides the nuanced geometric details (such as the percent void, noise treatment, and sipe placement, among other considerations) appropriate to expected usage. Element lengths are scrambled to prevent tonal noise generation, while being scaled relative to tread depth to insure adequate fore-aft and transverse stiffness to inhibit rapid wear.
Tire tread patterns for all vehicle classes have to meet the performance requirements for the service conditions experienced by the intended class of transportation. For example, agricultural tires traditionally feature chevron-like patterns that are designed with deep and widely spaced traction bars that maximize drawbar pull in soft soil. High load concentration on the bars promotes soil penetration to the detriment of a smooth highway ride. Contrarily, tractor tires fit to non-driven wheels and aircraft tires work best with circumferential ribs devoid of elements. The FIA at one time mandated grooved treads in Formula 1 to limit open-wheel race car cornering and acceleration speeds – not surprisingly resulting in driver complaints.
Asymmetric tread patterns are an emerging market trend. The Michelin XAS was the first (1965) to feature tread pattern asymmetry and the first production tire designed to operate at 210km/h. It remained popular through the 1970s in European racing circuits, rallies and hillclimbs with large, closely spaced, interlocking elements on the outboard shoulder (for handling) with a more wide open design on the inboard shoulder (for traction).The trend has now spread to street-legal race tires with shallow tread depths (6/32in). Some tires also contain internal construction asymmetries and dual-zone tread compounds that complement the asymmetric design. Today, these niche products are widely available in the OEM- and aftermarkets, providing over-engineered tire performance properties that most drivers are unaware of.
The Tire Tread Design Guide, published annually in the USA since 1966, depicts more than 200,000 photos of currently available passenger, light truck, medium truck, off-the road, agricultural, motorcycle and retread patterns. For the past decade, tire company management often complained about the number of SKUs required to service the ballooning population of plus-sized tires. Now, they only have themselves to blame for the additional SKUs needed to handle the onslaught of new designs.