falloutboy's Fundgrube und Texte

Vibration Analysis, Part 1: Tires

[falloutboy]

Vibration Analysis, Part 1: Tires
by GM

There have been several vibration conditions on 2014 Silverado 1500 and Sierra 1500 and 2015 Silverado, Tahoe, Suburban, Sierra, and Yukon models that have proved to be difficult to diagnose. These vibrations often occur at speeds of 35–45 mph (56–72 km/h) or 60–70 mph (96–120 km/h) and are felt in either the seat or steering wheel.

For example, one case from the Technical Assistance Center (TAC) was a concern about a vibration experienced at 70 mph and higher on a 2014 Silverado. At the dealership, after Road Force Balancing all tires, the Road Force Variation (RFV) measurements were: LF – 5 lbs., RF – 10 lbs., LR – 16 lbs., and RR – 20 lbs. In addition, the rear shocks were replaced.

A road test by a field service engineer using the CH-51450 Oscilloscope Diagnostic Kit with NVH showed a tire vibration amplitude of 22 mg at 70 mph. (Fig. 1) The tool’s sensor was placed on the seat track vertically.
fig01

Based on this information, the tires were moved from front to back on the same side. This put the highest RFV numbers on the front and the lowest on the rear of the vehicle. A second road test showed a greatly reduced tire vibration amplitude of 0.804 mg at 69 mph. (Fig. 2)
fig02

Some of the vibration cases may be difficult – but with the right approach and the right tools, a successful diagnosis can be achieved quickly.

Diagnostic Information
A variety of helpful information is available in the Service Information.
Information on Vibration Analysis and Diagnostics – #PI1354B provides detailed information on vibration analysis and diagnosis for several different conditions. It outlines the recommendations and procedures for diagnosing and repairing vibrations caused by wheels and tires, axle components and propeller shafts. It also includes a vibration diagnostic worksheet to record vibration measurements.

Vibration Analysis Worksheet – Bulletin 03-00-91-001G is a vibration analysis worksheet that is to be used when road testing vehicles exhibiting vibration conditions. The worksheet lists the necessary data needed in conjunction with the appropropiate testing procedures in the Service Information.

Information Needed when Calling TAC – When calling TAC for diagnostic help on vibration conditions, there is some basic diagnostic information needed in order to provide proper direction in repairing a vehicle. Before calling, technicians should use #PIT5345 to understand what is needed on a vibration condition. The measurements listed in the PI should be gathered using the appropriate tools.

First Steps

The first step in determining the cause of a vibration is a test drive with the appropriate diagnostic equipment installed on the vehicle. If the correct tools are not used or the proper procedures are not followed, an incorrect diagnosis will result.

• Inspect the truck for any aftermarket equipment installations, such as non-factory tires, lift kits or running boards.
• Mark each tire valve stem location on the tire to check for tire slippage on the rim. After the road test, verify that the tires have not slipped on the rim.
• Use the CH-51450 Oscilloscope Diagnostic Kit with NVH for vibration diagnosis. The oscilloscope kit provides an accurate analysis of vehicle noise, vibration and harshness conditions and uses the display of your laptop computer to present clear, easy-to-read results and actions for repair.
• For seat vibrations, mount the oscilloscope kit sensor to the rear seat bracket. (Fig. 3, #1)
• For steering wheel vibrations, mount the oscilloscope kit sensor to the steering wheel bracket under the instrument panel. (Fig 3, #2)
• Moving the oscilloscope kit sensor from a vertical position to a horizontal position may indicate higher amplitude, which may be beneficial to help in diagnosis.
• Measure the vibration by driving the truck with the transmission in M5 to keep the vehicle from switching in and out of Active Fuel Management (AFM).
• Once the vibration readings have been recorded on a road test, verify the vibration data in the service bay. If using a hoist, the suspension must be at the same trim height as when the vehicle normally sits on the road.

fig03

TIP: If the vibration can be duplicated on the rack, the test should be performed a second time with the wheel and tire assemblies removed from the vehicle and the wheel nuts installed to retain the brake discs and/or brake drums. If the vibration is eliminated, focus on the wheel and tire assemblies as the source of the vibration. If the vibration is still present, focus on the vehicle driveline as the source of the vibration.

Another case example from TAC shows the importance of understanding how to use the right tools. The case was a vibration condition on a 2015 Tahoe felt at 40-50 mph and at 60-70 mph.

Four different tires, a rear driveshaft and a rear differential assembly had been installed to address the vibration condition.

After a road test, the field service engineer determined the primary vibration was a 2nd order tire disturbance.

Tire vibrations were measured with the CH-51450 Oscilloscope Diagnostic Kit. The initial RFV measurements for three tires were 25 lbs., 16 lbs., and 12 lbs. The fourth tire had a measurement of 8 lbs. (1st order disturbance), but also a 2nd order disturbance of 21 lbs. (Fig. 4)
fig04

Reviewing the Hunter GSP9700 Road Force Balancer results for the tire with the 2nd order disturbance showed the 1st order harmonic was below specification, but the 2nd order specification was 21 lbs. (Fig. 5) It’s important to look at all harmonic measurements when reviewing the road force measurements and not to dismiss a particular tire based on only one measurement. If present, the CH-51450 Oscilloscope Diagnostic Kit tool will display the primary vibration as a 2nd order disturbance. Be sure to use this information and look at the other harmonic measurements on the Hunter GSP9700 Road Force Balancer.
fig05

The vibration was corrected by replacing and match-mounting (or vectoring) all four tires. The RFV measurements were 1 lb., 4 lbs., 4 lbs., and 7 lbs. (Fig. 6)

#PI1354B lists a RFV specification of 15 lbs. for light truck tires. This specification is lower than the current specification listed in the Service Information. It should only be used if there is a speed-related tire disturbance. If the RFV of the tire is over the specification, match-mount (vector) the tire on the wheel. If that doesn’t bring down the measurement to within the specification, the tire should be replaced.
fig06

TIP: When replacing tires, the road force should be checked before a test drive and after a test drive (minimum of 10–15 miles or 16–24 km/h). Road force on new tires will change dramatically after the tires are warmed up (as much as a 20-lb. reduction). After the test drive, the tire’s road force should be checked. If acceptable RFV cannot be achieved, first try vectoring the tire on the rim before an alternate tire is used. Refer to Bulletin #13-03-10-002: Diagnostic Tips for Difficult to Resolve Tire/Wheel Vibration Concerns.