The road force roller finds soft/stiff spots in the carcass and high/low spots on the wheel.
Photo credit: Hunter Engineering
The Hunter GSP9700 Road Force Touch
Photo credit: Hunter Engineering
Bad bushings can cause road vibrations too. This makes it easy to replace them.
Photo credit: SP Tools
Stud-centric wheels must be mounted on the balancer using an adjustable adapter.
Photo credit: Atlas Automotive Equipment
An electronic stethoscope lets users listen while the car is being driven.
Photo credit: Steelman
Proper torque of lugnuts and bushing fasteners will reduce the chance of vibration as miles accumulate.
Photo credit: AC Delco Durofix
Steel wheel weights are coated to protect against corrosion.
Photo credit: Hennessy Industries Inc
Tape weights are precise and easier on alloy wheels.
Photo credit: 3M Automotive Aftermarket
Snap-on Thebis is a six-channel noise and vibration analyzer/recorder.
Photo credit: Snap-on Tools
- 1998 Ford Explorer 4.0L, Vin E, Eng Cfg V6
- Average Reported Mileage: 80,305
There is a vibration that can be felt in the steering wheel and in the seat.
Vibrations can sometimes be narrowed down by when the vibration occurs. a. If the vibration can be duplicated in the stall simply by increasing the RPMs, it is likely engine related. Remove the serpentine belt; if the vibration is gone, check all driven components. b. If the vibration is only there with the vehicle moving, but goes away when put into neutral, it is likely a drive line vibration; begin checking driveshaft and rear differential. c. If the vibration is clearly speed related, but not RPM related, and it is unaffected by engine speed and transmission gear, suspect tires, wheels, or bearings.
When it comes to tires, balance and condition are important, but road force balance is becoming more important on today’s cars and trucks. If suspecting a tire/wheel problem, but the tires are balanced ok, have them road force balanced.
When traditional diagnosis will not work, there is an electronic vibration analyzer that may assist in diagnosis. This will tell you what frequency the vibration is happening at, low frequency like 10 or 13 hertz, will verify a tire/wheel/bearing issue.
Running or Rolling?
The techniques described in Step 1 will help you decide if the vibration is in the engine or chassis. Engine vibration can be found without any special tools, so we’ll focus on chassis vibrations.
If a vibration occurs only when the vehicle is moving, most techs check wheel balance first. But as many have discovered, a wheel/tire assembly can balance perfectly on the machine but still feel unbalanced on the road. Why?
Even a tire that looks good can have internal damage to the carcass. Tires develop a flat spot in the carcass when the vehicle sits for long periods. Usually the carcass recovers its shape as the tire rolls up to operating temperature, but sometimes the flat spot becomes permanent.
The carcass gives the tire shape and structure, and it supports the load being carried by that wheel. As it rolls down the road, the inflated tire acts like springs arranged like the spokes of a wheel. If the carcass is imperfect or damaged, stiffer at one spot or softer at another, it will act as though one of those spoke-like springs is stiffer or weaker than the others. As speed increases, the tire feels like it’s imbalanced, even if the balance is perfect. A high or low spot in the wheel will have a similar effect.
A wheel balancer with a road force roller will help you find the weak/stiff spots in the tire carcass and any high or low spots in the wheel.
Road Force Wheel Balance
A road force wheel balancer has a roller that puts up to 1,400 pounds of load against the tire as it spins on the balancer. It measures the force with which the tire pushes back. Ideally that push-back force should be equal all the way around the tire. Depending on the type of vehicle, if the push-back force at any one spot is high or low by more than about 15 pounds, you would probably feel it as the car rolls down the road. At 25 pounds difference, most of your customers would feel it too.
Car manufacturers have been using road force wheel balancers for decades because even new tires are not perfect. The imperfections are usually small enough to go unnoticed, unless they happen to be in the same place. By finding the imperfections in the tire and any high or low spots on the wheel, the tire can be mounted on the wheel with the imperfections 180 degrees away from each other, so they each cancel out. This is called match mounting, and it’s done at the factory on every wheel-and-tire assembly to create that impressive new-car-ride.
A road force wheel balancer is now available to the aftermarket too: the Hunter GSP9700. Most manufacturers “encourage” their dealers to own one of these machines, and virtually all manufacturers have issued Service Bulletins about using it to solve difficult wheel balance problems.
No matter what kind of wheel balancer you have, its calibration should be checked at least once a month. The job is easy because only three things are needed: a straight wheel, wheel weights and the owner’s manual.
The procedure varies depending on the manufacturer and the machine, but the basic concept is the same. A wheel of known dimensions is spun and a baseline measurement is taken. Next, a known amount of weight is added to create an imbalance. The wheel is spun again, and if the imbalance matches the added weight, calibration is correct. If it’s not correct, that’s when you need the owner’s manual.
It may be simple, but there are some important details. First of all, the wheel must be straight. Some balancers require a wheel of specific size, and if a cone is used to center the wheel on the balancer shaft, the wheel must be hub centric (many wheels are centered by the studs, not the hub). Some shops keep a new steel wheel as their ‘calibration’ wheel.
Some balancers are shipped with the calibration weights that were used when the machine was built. These must be used to check calibration in the shop too. Whether using those weights or your own, calibration weights are usually fairly hefty, at least 3 ounces (85 grams). This makes the procedure easier and more accurate.
Hub Centered vs Stud Centered
There are two different ways to center the wheel on an axle: hub-centric and stud-centric. Hub-centric wheels fit onto a slight extension of the hub or axle. This keeps the tire mounting bead concentric with the axle. Many aftermarket wheels are hub-centric but have an oversized hub hole, so centering rings are used to make them fit the vehicle. Trucks and SUVs typically use wheels that are centered by the studs. The hole in the center of the wheel may be centered, BUT NOT ALWAYS.
When mounting wheels on a wheel balancer, you need to know which type of wheel you’re working with. For stud-centric wheels, an adapter plate is needed. They are available with every wheel balancer, and there are several aftermarket adapters too. Most are adjustable for different bolt patterns. When mounting a wheel on an adapter, make sure to torque the nuts evenly.
Wheels and tires are only one possible source of road vibration, especially in our customer's older 3,700-pound full-frame 4WD truck. Vibration can come from other parts of the chassis, including axles or driveshafts, wheel bearings, worn suspension bushings and even faulty motor mounts.
Almost any vibration that can be felt produces noise too. Most techs have used a long screwdriver as a stethoscope to locate noise, but that’s not really possible when the car is being driven. However, there are tools designed for exactly that purpose.
Steelman's Chassis Ears are small radio transmitters that have a clamp with a built-in microphone. With the clamp attached to a suspected problem area, the transmitter is secured to the vehicle and the signal from this electronic stethoscope is sent to a 4-channel receiver. A tech can listen to each transmitter one at a time while someone else drives. It takes practice and experience with known-good vehicles, but this tool can help you quickly find the exact location of any noise or vibration from gears, bearings or suspension pieces while under load. It’s especially useful on trailers (transmitter range is 50 feet).
A Black Art
Like using scanners and scopes to diagnose engine problems, finding a chassis vibration is an art. A bad axle bearing or dragging brake caliper might produce more vibration than noise, and finding it requires imagination and resourcefulness, plus the ability to ‘think outside the box.’ So imagine how you would like to test your theory, then look in your toolbox, look around the shop and look in the pages of this magazine to find the tools that will help you get the job done.