Bad Vibrations

How to find shakes and rattles while the car rolls


Vehicle Application:

  •  1998 Ford Explorer 4.0L, Vin E, Eng Cfg V6
  •  Average Reported Mileage: 80,305

Customer Concern:

There is a vibration that can be felt in the steering wheel and in the seat.

Tests/Procedures:

STEP 1:

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.

STEP 2:

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.

STEP 3:

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.

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