Tires - Radial Force Variation (RFV) Information
Information IB08-08-S002Issue Date: JANUARY 2008
Information On Tire Radial Force Variation (RFV)
(This Bulletin Supercedes IB05-08-S001)
Affected Vehicles:
2007-2008 Isuzu Ascender
2007-2008 Isuzu i-290/i-370 (TI)
2006 Isuzu i-280/i-350 (TI)
Service Information
This bulletin is being revised to update model years and to adjust the allowable force variation chart.
IMPORTANT:
Before measuring tires on equipment such as the Hunter GPS9700, the vehicle MUST be driven a minimum of 16 km (10 miles) to ensure removal of any flat spotting. Refer to Bulletin IB05-08-S002 "Cold Tire Characteristics of The Manufacture GM Original Equipment Tires". Equipment such as the Hunter GSP9700 MUST be calibrated prior to measuring tire/wheel assemblies for each vehicle.
The purpose of this bulletin is to provide guidance to Isuzu dealers when using tire force variation measurement equipment such as the Hunter GSP9700. This type of equipment can be a valuable tool in diagnosing vehicle ride concerns. The most common ride concern involving tire radial force variation is highway speed shake on smooth roads.
Tire related smooth road highway speed shake can be caused by three things: imbalance out of round and tire force variation. These three conditions are not necessarily related. All three conditions must be addressed.
Imbalance is normally addressed first because it is the simplest of the three to correct. Off-vehicle two plane dynamic wheel balancers are readily available and can accurately correct any imbalance. Balancer calibration and maintenance proper attachment of the wheel to the balancer and proper balancing weights are all factors required for a quality balance. However a perfectly balanced tire/wheel assembly can still be "oval shaped" and cause a vibration.
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. Equipment such as the Hunter GSP9700 can address both (it is also a wheel balancer).
Tire radial force vibration (RFV) can be defined as the amount of stiffness variation the tire will produce in one revolution under a constant load. Radial force variation is what the vehicle feels because the load (weight) of the vehicle is always on the tires. Although free runout of tires (not under load) is not always a good indicator of a smooth ride it is critical that total tire/wheel assembly runout be within specification.
Equipment such as the Hunter GSP9700 loads the tire similar to on the vehicle and measures radial force variation of the tire/wheel assembly. Note that the wheel is affecting the tire's RFV measurement at this point. To isolate the wheel its runout must be measured. This can be easily done on the Hunter without the need to set up dial indicators. If the wheel meets the runout specification the tire's RFV can then be addressed.
After measuring the tire/wheel assembly under load and the wheel alone the machine then calculates (predicts) the radial force variation of the tire. However because this is a prediction that can include mounting inaccuracies and the load wheel is much smaller in diameter than used in tire production this type of service equipment should NOT be used to audit new tires. Rather it should be used as a service diagnostic tool to minimize radial force variation of the tire/wheel assembly.
Equipment such as the Hunter GSP9700 does an excellent job of measuring wheel runout and of finding the low point of the wheel (for runout) and the high point of the tire (for radial force variation). This allows the tire to be matched mounted to the wheel for lowest tire/wheel assembly force variation.
The machine will simplify this process into easy steps. The assembly radial force variation numbers shown should be used as a guide.
If match mounting tires to in-spec wheels produces assembly values higher than these tire replacement may be necessary. Replacing tires at lower values will probably mean good tires are being condemned. Because tires can sometimes become temporarily flat-spotted which will affect force variation it is important that the vehicle be driven at least 16km (10 miles) prior to measuring. Tire pressure must also be adjusted to the usage pressure on the vehicle's tire placard prior to measuring.
Most GM manufactured Isuzu vehicles will tolerate radial force variation up to these levels. However, some vehicles are more sensitive and may require lower levels. Also there are other tire parameters that equipment such as the Hunter GSP9700 cannot measure that may be a factor. In such cases Isuzu Technical Assistance Line should be contacted for further instructions.
IMPORTANT:
When mounting a GM manufactured Isuzu vehicle wheel to wheel balance/force variation machine, always use the wheel's center pilot hole. This is the primary centering mechanism on all GM manufactured Isuzu vehicle wheels; the bolt holes are secondary. Usually a back cone method to the machine should be used. For added accuracy and repeatability, a flange plate should be used to clamp the wheel onto the cone and machine. This system is offered by all balancer manufactures in Isuzu's dealer program. Any type of service equipment that removes tread rubber by grinding, buffing, or truing is NOT recommended, and may void the tire warranty. However, tires may have been grounded by the tire company as part of their tire manufacturing process. This is a legitimate procedure.
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