TechLinkOct02e

Table of Contents – October 2002
Wheel and Tire Balancing	Multi-port Flexible Injection System (MFI)	Sunshade Removal Tips	Key Release Difficulty
Valve Index Balancing	Pontiac Vibe EVAP System	Steering Clunk Noise	New Engine Coolant Fill Procedure
Class 2 Corner, Tip of the Month	Powertrain Quality Center Update	Blower Motor Inoperative	Headlamp Replacement Due To Moisture
Know-How Broadcasts for November	Poor Radio Reception	Hose Clamps Eliminated	Power Door Lock Programming
New Airbag Features in 2003 Full-Size Pickup Trucks	Visor Contacts Inside Mirror	Radio Programming	Bulletins

Note: Clicking on any picture or illustration will open a larger version of that art.

Wheel and Tire Balancing

Consider a Chevrolet TrailBlazer travelling at 60 miles per hour. The P245/70R16 tires have a diameter of 29.6 inches. At 60 miles per hour, they're running 703 rpm. If a tire is out of balance, at that speed it introduces 703 vibrations per minute into the vehicle. The tire is slapping the pavement 12 times per second (12 Hz). You think the customer is going to notice that?

Actually, tire related shake or vibration is one of the most common customer dissatisfiers, and it's most easily noticed when driving on a smooth highway. There are three causes: out-of-round, imbalance, and radial force variation (RFV). Each one of these conditions can result in shake or vibration, but the causes and cures are different for each. They are not necessarily related, but each of them must be addressed.

Here's where to find guidance in SI:

"build" the vehicle
General Information
Vibration Diagnosis and Correction
Diagnostic Information and Procedures
Vibration Analysis -- Tire and Wheel

The Vibration Analysis chart contains thorough test procedures. The exact path you will follow depends on what you find on the vehicle you're diagnosing, but in general, you will check in this order: runout, imbalance, and RVF.

Runout (Out-of-Round)
TIP: It's tempting to start with wheel balance, but it's important to measure on-vehicle runout first. If runout is the cause of the vibration, balancing won't fix it. But removing the wheel/tire assembly to check balance can disturb runout evidence. This is because a small amount of runout in the wheel may cancel or accentuate a small amount of runout in the mounting surface, depending on which position the wheel is mounted. This is referred to "stack-up" of runout.

Wheel/tire runout is measured two ways, on-vehicle and off-vehicle. The on-vehicle measurement includes possible runout of not only the wheel and tire but also the mating hub/axle flange and studs. Off-vehicle runout measures only the wheel and tire assembly. Further, it may be necessary to measure runout of just the wheel, with the tire dismounted. The diagnostic chart explains how to interpret and correct various runout conditions you detect.

One of the repair methods is called match-mounting (vectoring). This procedure permits you to move the tire on the wheel to determine if the low and high spots cancel each other out. In a similar fashion, it may be possible to match-mount the wheel to the mounting flange. See SI for details.

Imbalance
Wheel/tire balance is fairly easy to understand, and of all the conditions mentioned here, offers the most visible evidence. The weights are there for everyone to see. However, if runout or RFV is the cause of the vibration, checking balance is not going to fix the condition.

That having been said, there is an appropriate time to check and correct wheel/tire balance, according to the diagnostic chart.

TIP: Be sure to check for foreign materials, particularly inside the wheels, and particularly when the vehicle has been driven in snowy, icy or offroad conditions. Correcting an imbalance condition may be as simple as removing a chunk of mud.

The readily available off-vehicle, two-plane dynamic balancer is the equipment of choice for checking and correcting imbalance.

As with any precision tool, proper calibration, proper maintenance, and proper use are all important in obtaining satisfactory results.

Radial Force Variation
Radial force variation (RFV) refers to variations within the construction of the tire. All tires have splices in the various plies. These resulting stiffer spots do not cause problems unless they are excessive.

The causes of radial force variation are impossible to see, so RFV is the least understood cause of vibration.

TIP: RFV can masquerade as excess runout or imbalance. RFV can cause vibrations even though the wheel/tire assembly has acceptable runout and is properly balanced. If you've eliminated these two conditions, RFV is your likely culprit.

TIP: The Vibration Analysis chart in SI contains a link to an Isolation Test, which can help you identify which wheel/tire assembly is the source of the vibration.

As you can see in figure 1, it's helpful to think of the tire's sidewall as a series of coil springs. If one of the "springs" is considerably stiffer than the others, the tire will bump each time that portion contacts the pavement. This will be perceived as a vibration.

RFV is measured by specialized equipment in which the mounted tire is pressed against a load wheel. The load simulates the weight of the vehicle. As the tire is rotated, the equipment measures the variations in the tire's radial force.

Most vehicles will tolerate these RFV values:

P-metric tires on passenger cars	18 lbs (8.1 kg) or less
P-metric tires on light trucks	24 lbs (10.8 kg) or less
LT tires on light trucks	30 lbs (13.5 kg) or less

TIP: Technical Assistance has information on tire parameters that may deviate from these guidelines.

Correcting radial force variation is similar to correcting for runout. The stiffest part of the tire is matched up with the lowest part of the wheel.

Hunter GSP9700
The Hunter GSP9700 Vibration Control System (figure 2) is now available from GM Dealer Equipment (1.800.GMTOOLS).

A major benefit of the GSP9700 is that a single piece of equipment can:

measure off-vehicle runout
check and correct balance
check and correct radial force variation

The tire, spinning at 90 rpm, is contacted by a load roller (figure 3). The roller's radial position is affected by the varying stiffness of the tire as it rotates. While the tire rotates 4 revolutions, sensors take 128 readings per revolution.

Inner and outer Dataset® arms equipped with sensors measure the wheel's runout (figure 4).

TIP: There is a high correlation between inside and outside measurements on most wheels. In cases where external measurements cannot be made, the tire must be removed to measure the inside dimensions of the bead seat.

By combining the data from the roller and Dataset sensors, the GSP9700 can calculate vectoring and weight requirements. There is even a feature which determines where to place weights so they don't show on spoked wheels.

Important
When mounting a GM wheel to a balancing machine, always use the center pilot hole with a back cone. Use a flange plate to clamp the wheel to the cone and machine.

Any type of service equipment that removes tread rubber by grinding, buffing or truing is NOT recommended, and may void the tire warranty. Tires may have been ground by the tire company as part of the manufacturing process, and this is a legitimate procedure.

- Thanks to Dave Scribner at Hunter Equipment, and to Russ Dobson

figure 1

figure 2

figure 3

figure 4

Table of Contents – October 2002