The Truck Tire/Brake Relationship
Modern trucks have evolved significantly in recent years. Depending on the vantage point of the assessor, today’s over-the-road rigs have been described as innovative, streamlined, incredibly reliable, or as merely highly evolved, electronically assisted versions of some aging basic designs.
Most, however, would agree that today’s trucks move more of our North American freight more economically and reliably than ever before.
Unscheduled repairs – a euphemism for breakdowns ®“ are rare, and replacement of wearing/service parts is routine and planned. Tires are a big part of that scheduled maintenance.
Truck operators acknowledge tires as their second highest non-labor operating expense, with only fuel being higher. That’s not to say that tire technology hasn’t kept pace with the advancements in other componentry, as numerous vehicle subsystems require less routine maintenance than in prior years.
A prime example is the truck braking system. Anti-Lock Braking Systems (ABS) are now standard by Government-mandate on all new commercial trucks and trailers manufactured for the U.S. and Canadian markets. And further brake system changes are on the way.
How Brakes Impact Tires
As truck and truck trailer brake systems evolve, tire selection and performance can be significantly affected. More aggressive steer axle brakes, heavy duty air disc brakes, increased use of engine compression brakes and electronic brake-by-wire systems can impact tire wear rates, wear patterns, and influence tire type selection at both OE and replacement levels. Let’s look at some possible effects.
Prior to ABS, steer axle brakes were generally undersized, based mainly on the belief that it was better to maintain steering control in an emergency braking situation rather than risk the loss of lateral traction caused by a skidding front tire. Some operators went so far as to delete or disconnect steer axle brakes entirely.
With little or no braking torque transferred through the single mounted steer tires, irregular wear patterns – characteristic of free-rolling tires ®“ became common. Major tire manufacturers developed a variety of fixes, ranging from innovative technology to band aids addressing the wear issue, resulting in tires designed very specifically for steer axle use.
Trailer tires have become even more specialized and axle-specific. Even though they are subjected to braking forces, weight transfer during aggressive brake applications reduces their tractive ability, making flat-spotting a concern.
Additionally, trailer tires are not subjected to high side loads or cornering forces since they do not steer the truck and are mounted as duals. Current generation premium trailer tires feature shallow treads and relatively simple pattern designs.
Drive axle tires traditionally transferred most of the vehicle braking forces through their footprint or road contact patches in addition to driving the truck by accelerating or maintaining speed while overcoming aerodynamic and frictional forces. This lion’s share of torque transfer, both power and braking, caused drive tires to evolve as the deepest treaded tires, and the least susceptible to irregular wear tire type by axle on the typical highway rig.
As a result, high mileage drive tire designs have approximately twice the tread depth of premium trailer tire designs.
New System on the Horizon
As a general rule, the greater the magnitude and variety of forces transferred through the tire footprint, the less fragile the tire becomes in resisting unwanted wear pattern development. This translates into good news for truckers given the trend in truck brake system upgrades.
ABS on steer axles allows fitting larger, more powerful brakes, which will tend to retard irregular wear pattern development in addition to reducing overall truck stopping distances. Since ABS also tends to equalize braking torque at each axle end and limit brake effort just short of wheel lock, the probability of brake skids is significantly reduced – more good news for the tire watchers.
Brake-by-wire systems now under development will electronically signal the trail, drive and steer axles that braking action is needed. The brakes will be activated using the application timing, by axle, and degree of retarding force that best suits the vehicle and road conditions present.
This will reduce brake response time compared with today’s systems, which require compressed air to travel from the cab-mounted control valve to the various axles via long brake lines.
Keep in mind that trailer brakes normally apply first, followed by drive axle and finally steer axle brakes. This application sequence should happen quickly and more consistently with electronic controls.
Results are expected to include more consistent stopping distances and fewer instances of abusive, brake-induced tire wear.
Retarders Cause Excessive Wear
One other trend that can affect tire wear is the popularity of auxiliary engine compression brakes. International truck engineers estimate that 80% of today’s new Class 8 trucks are equipped with these devices – often referred to as "Jake brakes." They apply braking forces through the drive tires only, and are normally used to assist the regular braking system.
Aggressive use of these auxiliary brakes can increase overall drive tire wear, although the tendency is also to reduce any directional, torque-induced patterns such as heel/toe wear.
The bottom-line message for tire engineers, tire dealers and truck operators alike is that the changes in heavy duty truck brake systems can, and likely will, continue to impact tire wear rates and patterns. At this point, most of the changes appear to be for the better. Stay tuned.
