The Forces Are With Us
There are some new terms being used in the tire and vehicle servicing industries today and you may want to learn more about them as they’re not likely to disappear.
Most of these terms include the word "force". Such as "road force", "force variation", "dynamic force", and half a dozen other variations on the theme.
These words are often used in discussions about ride disturbance, vehicle vibration, and tire quality. Force variation has been used since the early 1960s, and was originally coined by tire and vehicle ride engineers to describe one aspect of tire uniformity.
It is typically measured by mounting the tire on a specially machined true wheel, then forcing this assembly against an instrumented test fixture to a preset load. Once loaded, the distance between the true wheel axle center and the test fixture is locked in and the tire/wheel assembly is rotated.
Small variations from the preset load are then measured, typically in pounds. These differences may be thought of as variations in the spring rates, or deflection characteristics of the tire. The measurements can be taken in three directions or planes – radial (up/down), lateral (side-to-side), and tangential (fore/aft). In simple terms, these represent the tendency of the tire to bounce, wobble or surge, respectively.
When a tire is mounted on any wheel other than a perfectly machined test wheel, the resulting force variation of the assembly can be significantly influenced by both the tire and wheel. Wheels do not have force variation signatures in the same sense as tires, as the spring rate of a wheel doesn’t vary. They may, however, have radial and lateral runout, which will contribute to overall assembly force variation.
Once fitted to a car or light truck, other additional components, such as hubs, axle flanges, wheel attaching hardware, rotors and drums, even wheel bearings can contribute to the total force variation measured on that axle-end assembly.
The terms "road force" and "dynamic force" are less specifically defined and are generally used to describe the total forces input to the vehicle suspension system by rotating axle-end components. In real world use, these may or may not correlate well with force variation of the tire alone. Many additional components can contribute, as mentioned above, tending to amplify or cancel out the total forces delivered to the vehicle. Also, the tire footprint on a real world road surface can produce results different from those measured on a laboratory test fixture, most of which are curved steel surfaces less than a yard in diameter.
The actual forces transmitted to vehicle suspension components also vary with speed, in contrast to tire force variation, which is defined as a measurement at one specific set of load and speed parameters independent of any other vehicle components.
No Simple Solutions
The tire and vehicle servicing industries have, for many years, recommended balancing tire/wheel assemblies as the first line of defense against vehicle vibration. Balancing is simply an effort to equalize the distribution of mass around the center of a tire/wheel. This approach has done a reasonably good job of providing drivers a vibration-free ride.
However, the terms "static" and "dynamic" balancing are somewhat of a misnomer technically, since these processes are single-plane and dual-plane balancing, respectively, and deal only with vertical and lateral components of mass distribution. Perhaps more importantly, tire/wheel balancing is performed on assemblies that are unloaded and, therefore, have no contact patch or footprint deflection.
While rebalancing tire/wheel assemblies can resolve many ride quality complaints, today’s tight tire tolerances and lighter weight vehicle suspension, steering and brake systems make investigating and correcting many ride complaints difficult.
Finding The Source
Traditionally, the approach used by service personnel when addressing ride complaints has been to use a diagnostic tree analysis to determine first whether the vehicle vibration is driveline related (engine, clutch, transmission, driveshaft, motor/transmission mount), or axle-end related (tire, wheel, brake drum/rotor, spindle/axle flange).
If the latter is suspect, and proper fit of the wheel to its mounting surface is confirmed, tire assembly balance and runout are checked. These procedures are detailed in service manuals for vehicle types ranging from autos to large Class 8 trucks.
In situations where there are no obvious out-of-balance or high-runout conditions, the next step is often to replace parts until the condition improves. Occasionally, an excessively worn or out-of-spec part can be identified, but in many instances, vehicle ride is merely improved to a point where it is acceptable without clear understanding or identification of a specific single cause.
New Materials, Theories?
Many new vehicle designs weigh less overall than earlier versions used for the same service. Modern suspension and brake designs have also resulted in reduced vehicle unsprung weight, as the percentage of total weight made up of safety enhancements and passenger convenience items has increased. These trends, together with increasing emphasis on driver comfort and fatigue avoidance, favor axle-end components that are more uniform and result in lower force transmission to the vehicle suspension.
With this in mind, some industry engineers believe the answer to achieving significant ride improvements will require approaches and materials beyond those commonly used today. For example, premium gas-charged shock absorbers provide improved damping and isolation from ride disturbances compared to older generation shocks.
Several interesting explanations of how tires react to road forces and transmit them to other vehicle components are being explored today. In theory, if forces created by the tire/road interaction could be dampened before they are transmitted outside of the tire structure, vehicle ride would be improved beyond that possible with traditional tire balancing and assembly runout control.
A variety of people from tire/vehicle design and servicing backgrounds are studying such possible improvements – therefore, the variety of terminology. With such fertile minds at work, surely progress will be made, but for the present the forces are still with us.