First Order Radial Force Variation
Radial Force Variation vs. Unloaded Runout
In the manufacturing community, tire uniformity is called radial force variation. The uniformity of most tyres manufactured today is measured with a machine in accordance to SAE practice J332. This practice is widely used in the tyre industry and describes tire testing equipment and procedures used to measure radial force variation of the tyre. This practice stresses the importance of measuring force variation while the tyre is under load and does not acknowledge unloaded runout measurement.
Many tyre assembly plants have large production lines to measure loaded tyre force variation. Tyres, which do not meet uniformity specifications, may be brought into specification through additional manufacturer's procedures called force grinding. Force grinding is done to improve radial force variation by removing small areas of rubber from the sides and footprint of the tread. Force grinding may not improve (and in some cases may increase) the unloaded runout measurement.
A tyre with large amounts of unloaded radial runout may be vibration free while a tyre with low unloaded radial runout may vibrate. In many cases, tyre manufacturers will forego unloaded runout measurement since this information is not as valuable as tire force variation when it comes to analyzing the causes of tyre ride disturbances.
In the past, when trying to resolve tyre/wheel vibration concerns, service facilities were unable to measure tyre force variation. The size and expense of the factory machines were cost prohibitive. In order to compensate for this lack of field service technology, many automotive and tire manufacturers have published service limits for unloaded runout in the tyre/wheel assembly.
A standard industry practice has been to measure unloaded runout in the center of the tire tread using a relatively inexpensive gauge. However, this measurement has little relationship to the actual amount of ride disturbance felt in the vehicle. For example, a set of springs may have an unloaded height measurement of equal length, yet when compressed may create different forces at the same compressed height.
Radial Force Vibration Placed in Perspective
In the past, most tyre/wheel assembly vibration was considered balance related. Because of this, tyre service professionals tend to relate tyre/wheel vibration in terms of balance weight. Road Force will be best understood when related to the amount of balance weight required to cause a similar vibration in a wheel that rolls round under a load. In other words, "How much Road Force creates a similar vibration caused by tire imbalance?
Most tire service professionals and factory service manuals agree that residual static imbalance should not exceed .30 oz. on average size wheels and .60 oz. on larger light duty truck wheels.
Radial Force is determined by measuring loaded radial runout. On an average passenger car tire/wheel assembly, one thousandth of an inch (0.001") of loaded radial runout is equivalent to approximately one pound of Road Force.
Tests on a Chevrolet Lumina were performed using a chassis dynamometer in a Detroit test lab. The purpose of the test was to determine how much balance weight would be required to produce the same magnitude of force as a measured amount of loaded radial runout.
The tests were performed with the vehicle running at different speeds. The first test was at 80 kilometers per hour and the second test at 120 kilometers per hour.
At 80 km/h:
A measured .030" (about 30 pounds) of loaded radial runout caused the same amount of vibration as 1.5 ounces (42 grams) of wheel imbalance at 80 km/h. This is 5 times greater than the .30 (1/4) ounce imbalance limit.
At 120 km/h:
A measured .030" (about 30 pounds) of loaded radial runout caused the same amount of vibration as .75 ounces (21 grams) of wheel imbalance at 120 km/h. This is 1 1/2 times greater than the .30 (1/4) ounce imbalance limit.