Corvette V8-6.2L (2008)
of vibration is normally felt more in the "seat of the pants" than the steering wheel.
5. Next, record the Hertz (Hz) reading as displayed by the EVA onto the tire data worksheet found at the end of this bulletin. This should be done
after a tire break-in period of at least 16 km (10 mi) at 72 km/h (45 mph) or greater, in order to eliminate any possible tire flat-spotting. This
reading confirms what the vehicle vibration frequency is prior to vehicle service and documents the amount of improvement occurring as the result
of the various steps taken to repair. Completing the Steering Wheel Shake Worksheet below is required. A copy of the completed worksheet must
be saved with the R.O. and a copy included with any parts returned to the Warranty Parts Center for analysis. A reading of 35 to 50 Hz typically
indicates a first order propshaft vibration. If this is the situation, refer to Corporate Bulletin Number 08-07-30-044D. Generally, a reading between
10 and 20 Hz indicates a tire/wheel vibration and if this is the reading obtained, continue using this bulletin.
If the tire 1st order vibration goes away and stays away during this evaluation, the cause is likely tire flat-spotting. Tire flat-spotting vibration may
come and go at any speed over 72 km/h (45 mph) during the first 10 minutes of operation, if vibration continues after 10 minutes of driving at
speeds greater than 72 km/h (45 mph), tire flat-spotting can be ruled out as the cause for vibration.
6. If flat-spotting is the cause, provide the explanation that this has occurred due to the vehicle being parked for long periods of time and that the
nature of the tire is to take a set. Refer to Corporate Bulletin Number 03-03-10-007E: Information on Tire/Wheel Characteristics (Vibration,
Balance, Shake, Flat Spotting) of GM Original Equipment Tires.
7. If the road test indicates a shake/vibration exists, check the imbalance of each tire/wheel assembly on a known, calibrated, off-car dynamic
balancer.Make sure the mounting surface of the wheel and the surface of the balancer are absolutely clean and free of debris. Be sure to
chose the proper cone/collet for the wheel, and always use the pilot bore for centering. Never center the wheel using the hub-cap bore
since it is not a precision machined surface. If any assembly calls for more than 1/4 ounce on either rim flange, remove all balance weights and
rebalance to as close to zero as possible. If you can see the vibration (along with feeling it) in the steering wheel (driving straight without your
hands on the wheel), it is very likely to be a tire/wheel first order (one pulse per revolution) disturbance. First order disturbances can be caused by
imbalance as well as non-uniformities in tires, wheels or hubs. This first order frequency is too low for a human to hear, but if the amplitude is
high enough, it can be seen.
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 following assembly radial force variation numbers should be used as a guide:
When measuring RFV and match mounting tires perform the following steps.
Measuring Wheel Runout and Assembly Radial Force Variation
Important
The completed worksheet at the end of this bulletin must be attached to the hard copy of the repair order.
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Measure radial force variation and radial runout.
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If a road force/balancing machine is used, record the radial force variation (RFV) on the worksheet at the end of this bulletin. It may be of benefit
to have the lowest RFV assembly to the front left corner. If the machine is not available and the EVA data suggests there is an issue, swap the tire
and wheel assemblies from the front to the back. Re-check on the EVA and if the problem still exists, test another vehicle to find tires that do not
exhibit the same frequency and swap those tires onto the subject vehicle.
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If a runout/balancing machine is used, record the radial runout of the tire/wheel assemblies on the worksheet at the end of this bulletin. If one or
more of the tire/wheel assemblies are more than.040 in (1.02 mm), match mount the tire to the wheel to get below.040 in (1.02 mm). For sensitive
customers, readings of 0.030 inch (0.76 mm) or less are preferable, it may also be of benefit to have the lowest runout assembly to the front left
corner. If the machine is not available and the EVA data suggests there is an issue, swap the tire and wheel assemblies from the front to the back.
Re-check on the EVA and if the problem still exists, test another vehicle to find tires that do not exhibit the same frequency and swap those tires
