00-03-10-006I Information on Tire Radial Force Variation (RFV) - (Jan 28 2013) Tires - part 3
If a vibration or shake still exists after balancing, any out of round conditions,of the wheel, and force variation conditions of the tire, must be addressed. GM approvedtire force variation measurement equipment can address both (it is also a wheel balancer).
Tire radial force vibration (RFV) can be defined as the amount of stiffness variationthe tire will produce in one revolution under a constant load. Radial force variationis what the vehicle feels because the load (weight) of the vehicle is always on thetires. Although free runout of tires (not under load) is not always a good indicatorof a smooth ride, it is critical that total tire/wheel assembly runout be within specification.
Tire force variation measurement equipment loads the tire, similar to on the vehicle,and measures radial force variation of the tire/wheel assembly. Note that the wheelis affecting the tire's RFV measurement at this point. To isolate the wheel, its runoutmust be measured. This can be easily done on GM approved tire force variation measurementequipment, without the need to set up dial indicators. If the wheel meets the runoutspecification,the tire's RFV can then be addressed.
After measuring the tire/wheel assembly under load, and the wheel alone, the machinethen calculates (predicts) the radial force variation of the tire. However, becausethis is a prediction that can include mounting inaccuracies, and the load wheel ismuch smaller in diameter than used in tire production, this type of service equipmentshould NOT be used to audit new tires. Rather, it should be used as a service diagnostictool to minimizeradial force variation of the tire/wheel assembly.
GM approved tire force variation measurement equipment does an excellent job of measuringwheel runout, and of finding the low point of the wheel (for runout) and the highpoint of the tire (for radial force variation). This allows the tire to be matchedmounted to the wheel for lowest tire/wheel assembly force variation.
The machine will simplify this process into easy steps. The following assembly radialforce variation numbers should be used as a guide:
P-Metric tires of passenger cars
18-lbs (8-kg) or less
P-Metric tires on light trucks
24-lbs (11-kg) or less
LT-tires on light trucks
35-lbs (16-kg) or less