There are some new terms bouncing around the tire industry these days — force variation, road force and dynamic force — that are used primarily in discussions about tire quality, ride disturbance and vehicle vibration.
Force variation refers to one aspect of tire uniformity. The tire is mounted on a specially machined wheel with a preset load, and the tire/wheel assembly is rotated. Small variations from the preset load are then measured, representing the tendency of the tire to bounce, wobble or surge. But when a tire is mounted on a “real” wheel, the wheel itself can influence force variation, as can components such as hubs and axle flanges.
Road force and dynamic force are generally used to describe the total force on the suspension system when axle end components are turning. In the real world, these may or may not correlate accurately with the force variation of the tire alone. First, other components can amplify or cancel the forces delivered to the vehicle. The footprint on the road surface can produce results different from those measured in the lab, and forces transmitted to suspensions vary with speed.
For years, the industry has recommended balancing tire/wheel assemblies as the first line of defense against vibration, an approach that has done a reasonably good job of providing a vibration-free ride. However, the terms “static” and “dynamic” balancing are somewhat misleading, since they refer to single-plane and dual-plane balancing that's done to correct vertical and lateral components of mass distribution. Even more important, tire/wheel balancing is performed on unloaded assemblies, so there's no contact patch or footprint deflection.
When addressing ride complaints, maintenance people have traditionally used a diagnostic tree analysis to determine whether vehicle vibration is related to the driveline or axle-end. If the axle-end is suspect, and the wheel is fitted properly to its mounting surface, tire assembly balance and runout are checked.
If there are no obvious out-of-balance or high runout conditions, the next step is often to replace parts until things improve. Sometimes, an excessively worn or out-of-spec part is identified. But often ride is merely improved to a point where it's acceptable, with no clear understanding of the problem.
Many new vehicle designs weigh less overall than earlier versions used for the same application. Modern suspension and brake designs have also resulted in reduced vehicle unsprung weight, since the percentage of total weight made up of safety enhancements and passenger convenience items has increased.
These trends, as well as increased emphasis on driver comfort and fatigue avoidance, favor axle end components that are more uniform, resulting in less force transmitted to the vehicle suspension.
Some engineers believe the answer to achieving significant ride improvements will require approaches and materials beyond those commonly used today. Premium gas-charged shock absorbers, for example, provide improved damping and isolation from certain ride disturbances compared to older-generation shocks.
Several explanations of how tires react to road forces and transmit them to other components are being explored. In theory, if forces created by the tire/road interaction could be dampened before they're transmitted outside the tire, we could improve ride beyond that possible with traditional tire balancing and assembly runout control.
With so many fertile minds at work, we'll eventually find a solution. But for now, the forces are still with us.