Road type can be a major variable in tire performance



Most of us have participated in or listened to fairly animated discussions about the comparative performance of competing brands or types of tires. We've also been exposed to the wide variety of conclusions various “experts” have derived from their experience. As a friend of mine used to mutter while one of his peers preached about their findings, “that's the thing about his experience — it's his.”

Nevertheless, it's sometimes difficult to understand how there can be such vastly differing opinions of tire performance. A number of factors that might explain these differences have been investigated, including vehicle configuration, drivers, load status and season of the year during which new tires are applied.

However, road surface is a major variable that's often overlooked. Variations in road type can significantly affect tire wear, as well as influence other performance concerns.

Fuel economy, for example, is generally improved when a vehicle runs on a harder surface such as concrete, rather than a softer one like multi-level asphalt. The theory behind this is that pressure from the tire footprint causes softer surfaces to deform, which means additional energy is lost in rolling resistance.

Other research has demonstrated differences in treadwear and traction on new vs. worn road surfaces of the same pavement type. This is likely caused by the “polishing” effect of constant use that tends to round the sharp edges of aggregate material in the top layers of roadmix.

Many tire engineers agree that the roadmix itself can be a major factor in determining wear rates and tread life expectancy. Despite successful efforts to standardize pavement-mix specifications, most still include locally quarried aggregate, which varies significantly from one part of the country to another.

Certain parts of Florida, for example, have tire wear rates up to 40-45% faster than the national average, most likely because local aggregates contain coral and other shell-type marine materials. Other areas have higher-than-average tire wear due to the hard, sharp edges created when local stone is fractured to size for use in roadmix. This characteristic is due to the geologic origin of the local rock.

The percentage and severity of changes in elevation can also play a role, especially in combination with stop-start driving cycles. Tread life is also dramatically reduced when tires are driven on curved roads, especially in combination with higher speeds. The primary mechanism at work in these examples is abrasion at the tire/road interface.

Whether tires run on primary or secondary roads can also affect wear rates, even if both have similar pavements and finishes. Most secondary roads are crowned (high in the middle and tapering down on both sides), while many modern primary roads are simply sloped slightly from one side to the other, sufficient for water drainage. Vehicles finely tuned for “neutral alignment” on level surfaces may pull slightly on sloped roads, requiring a small but consistent steering input, with resulting steer tire wear.

Today, most large trucks with modern radial ply tires employ small side-to-side variations in camber, caster or axle setback to improve straight-ahead road feel on mildly sloped Interstates. This contrasts with generally higher camber and toe-in settings, which were more appropriate when crowned secondary roads.

Maybe there is some truth to my friend's observation about individual experiences — depending on where you're from and what type of rocks you drive over.