What’s Causing That Vibration?
| 02 September 2002
We often say that a wheel/tire assembly is out of balance when we sense rolling vibration in our cars. But so-called wheel balance is a complex topic. Researching the subject, with Hunter Engineering Company as the primary source, showed just how complex it can be. Weight distribution or redistribution (balancing) is only part of the story. Certainly the vehicle itself can cause apparent wheel/tire vibration if there are issues with items such as an out-of-balance driveshaft, worn suspension or steering parts, bent hubs, improper (off-center) wheel mounting, radial runout of the mounting lugs, or out-of-balance brake drums/rotors. Assuming that all of those characteristics are within specification, the wheels and tires can contribute their own set of variables. This pony trick will concentrate on wheels, tires, and the wheel/tire assembly.
Besides weight imbalance, variables relating directly to wheels and tires include bent or damaged wheels, runout (out-of-round) of the rim, runout of the tire, and/or runout of the rim/tire assembly. In addition, there can be excessive radial force variation in a tire or improper mounting of a wheel/tire assembly on the tire-balancing machine at the tire store. Bent or damaged wheels can usually be detected by close visual inspection. Sometimes you need to mount the wheel and rotate it by hand to see the damage. Runout can occasionally be detected visually but it’s usually more subtle and requires a measuring device to detect its presence and severity. Radial force variation is the localized change-in-resistance-to-deformation of the tire sidewall. Picture a tire rolling and then visualize at any given moment a different section of the sidewall directly above the part of the tire (the footprint) that’s in contact with the road. If the different sidewall sections are softer or harder than their neighbors, each one will flex more or less depending on the hardnesss difference. The net effect is that the tire acts like it’s out-of-round because parts of it are exerting more or less pressure (force) between the wheel rim and the tire tread; i.e., radial force variation. The momentary out-of-round locations caused by differences in radial force can cause vibration. Specialized equipment is required to measure this characteristic.
Finally let’s discuss the mounting of rim/tire assemblies on the tire-balancing machine. An important issue to consider is the type of wheel being mounted. There are lug-centric wheels and there are pilot-hole-centric wheels. The important consideration when balancing one type or the other is the need to mount the wheel on the balancing machine in the same way that it mounts on the car. The lug-centric wheel is centered on the wheel hub by the lugs and lug nuts that fasten it in place; i.e., the lugs perform the dual function of centering and attaching. The pilot-hole-centric wheel is centered by the center hole or pilot hole in the spider (center part of the wheel) because it is designed to fit with minimal clearance over the hub. With this type of wheel, the pilot hole does the centering and the lugs do the fastening. By way of comparison, the lug-centric wheel also has a pilot hole but it is so much larger than the wheel hub that it contributes nothing to centering.
Typically when we ask for dynamic or spin balancing of a wheel/tire assembly, the tire shop will attach the assembly to the balancing machine in a pilot-hole-centric fashion using a cone adapter and a jam-nut. This can easily result in off center positioning if the wheel has a lug-centric design. The distinction is very important to first generation Mustang owners who have the popular styled steel wheels because styled steel wheels have a lug-centric design. If you have a balance problem with styled steel wheels, it could be that your tire shop didn’t use the correct method of centering the wheels when they were mounted on the balancing equipment. In fact, personal experience has shown that it’s quite difficult to find a shop where the balancing machine has lug-centric capability.
Speaking of capability, what about the capability to measure runout and radial force variation? Hunter’s solution is a machine that they call the GSP9700. Their information indicates that the GSP9700 can correctly mount both types of wheels and then perform a dynamic balance. In addition, it can measure runout and force variation and suggest different positioning of the tire on the rim to “make” the assembly “look” round under operating conditions. It can also detect when a wheel or tire needs to be replaced because it’s too far out of specification for use.
The writer has no personal experience with the Hunter GSP9700 on Mustang styled steel wheels but the equipment resolved an ornery vibration problem with a pickup truck tire when it was determined that radial force variation was the issue. So if you have an aggravating vibration that doesn’t respond to spin balancing, you might want to locate a shop where the equipment can measure some of the other characteristics that contribute to vibration.