Before they're fitted onto a vehicle, tires are completely circular. They're designed to possess a springy quality, able to adjust to the road beneath without sacrificing structural integrity. To ensure that this flexibility does not compromise the safety of a tire, tires are put through rigorous tests by their manufacturer.
There are various forces put on the tire as it rolls forward. There are additional forces that act on the tire during various driving maneuvers, such as turning. Forces produced from the center of a tire outwards are acted out as a tire rolls and maneuvers from front-to-back and are known as Radial Force Variation (RFV). The forces that work from side-to-side are known as Lateral Force Variation (LFV).
What are the Differences in Force Variation?
Force variation plays directly into the process of achieving tire uniformity. This is the standard by which all tires are tested and measured against in order to pass a manufacturer's inspection. Different tires will possess different standards for specified use, yet all tires must meet basic specifications of radial and lateral force variation determined by the design engineers.
All tires will experience force variation. The radial force variation is a measure of the vertically applied force on the tire, while the lateral force variation measures the horizontal force applied to the tire. Radial force is the upward force that supports a vehicle. The radial force variation is a simple measure of the differences of force, or the fluctuation between the set measurements of force that occur during the driving process. Even on ideal road surfaces, there will be an occurrence of force variation. Lateral force variation will also fluctuate in force, even if driven in the most consistent manner on a flat, paved road surface.
Lateral force variation will play more of a factor in a vehicle's steering capability, or any inclination to pull in one direction or the other. Tire engineers aim to reduce lateral force variation. There are other variables that play into this equation during driving, but in a consistent testing process, a high quality tire will experience minimal lateral force variation.
Putting Force Variation to the Test
Tires are designed with radial, lateral, and tangential force variation in mind. While tangential force variation is important in developing low rolling resistance tires, and efficient high performance tires, this force variation won't affect the vehicle ride and handling capabilities as much as radial and lateral force variation will.
Radial and lateral force variation are measured on a machine known as a force variation machine. This test occurs after the tires have been manufactured. A tire must pass this testing process, or the tires will not be sold. The force variation specifications will vary from tire to tire, based on their intended use.
When tires are put to the test with a force variation machine, many variables are considered, including the tire's load index and its speed rating. In order to enjoy tire performance and safety, remember to always heed the limitations of a tire, which have been determined in regard to the force variations of the tire.