Road Force Measurement™
Road Force Measurement™ is new to the automotive service industry. This new measurement identifies tyre and wheel uniformity, which has been measured for years in assembly plants and manufacturing facilities. Road Force can be used to solve uniformity related tyre and wheel vibrations. Tyre/wheel assembly uniformity can only be measured under load.
The load roller on the GSP9700 performs a computer simulated “road test”. It measures the tyre/wheel assembly to determine how “round” the assembly is when rolling under a load. If a tyre were not exposed to the road surface, then balance would be more than sufficient. However, not all tyres roll round under a load. For example, an egg-shaped tyre/wheel assembly can be balanced about its axis, but an egg-shaped tyre-wheel loaded against a surface would not give a smooth ride.
To understand the effects of radial force variation on vibration, a model of a tyre can be used. The sidewall and footprint can be understood as a collection of springs between the rim and the tyre contact patch. If the “springs” are not of uniform stiffness, a varied force is exerted on the axle and causes it to move up and down as the tyre rotates and flexes. This movement creates a vibration in the vehicle unrelated to balance.
The GSP9700 load roller applies a force of up to 635kgs against the rotating tyre/wheel assembly as it performs the Road Force Measurement™. The GSP9700 measures loaded radial runout of the tyre/wheel assembly within 0.05 mm. It plots data points as the component is rotated and calculates the radial first harmonic of the tyre/wheel assembly and the first harmonic of wheel runout. The peak-to-peak value (Total Indicated Runout) and second, third and fourth harmonics of Road Force are also calculated and used for diagnostics. These measurements are all displayed on other screens.
The measurements of loaded radial runout are converted to Road Force Measurement in pounds, kilograms or Newtons using the following equation:
(Loaded Radial Runout) x (Tyre Spring Rate) = Road Force Measurement
Radial Force Variation
Radial force variation is an industrial measurement term describing the tyre uniformity under load, measuring the variation (up and down) of the load acting on the vehicle spindle (SAE practice J332).
All tyres have some non-uniformity in the sidewall and/or footprint due to variables in the manufacturing process. Tyre uniformity measurement values can be affected by rim width, rim condition and many diverse tyre mounting variables. Unlike balancing, there is often a small amount of RFV remaining in the tyre/wheel assembly after ForceMatching and this is generally acceptable.
First Order Radial Force Variation
Radial Force Variation vs. Unloaded Runout
In the manufacturing community, tyre 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 tyre 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 tyre 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 tyre 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 tyre 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 tyre imbalance?
Most tyre service professionals and factory service manuals agree that residual static imbalance should not exceed 8.5 grams. on average size wheels and 17 grams. on larger light duty truck wheels.
Radial Force is determined by measuring loaded radial runout. On an average passenger car tyre/wheel assembly, 0.056 mm of loaded radial runout is equivalent to approximately one kilogram 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 km per hour and the second test at 112 km per hour.
At 80 km/h:
A measured about 13 kgs of loaded radial runout caused the same amount of vibration as 42 grams of wheel imbalance at 80 km/h. This is 5 times greater than the 8.5 grams imbalance limit.
At 112 km/h:
A measured about 13 kgs of loaded radial runout caused the same amount of vibration as 21 grams of wheel imbalance at 112 km/h. This is 1.5 times greater than the 8.5 grams imbalance limit.