WHEEL BALANCE AND MPG

GOAL:

To understand how properly balanced tires can improve vehicle mpg.

OBJECTIVE:

The student will:

1. Learn how to identify an improperly balanced tire.

2. Understand the relationship between tire imbalance and reduced mpg.

LESSON/INFORMATION:

Rarely do technicians associate poor tire-wheel balance with reduced mpg. Most technicians associate properly balanced tires with increased tire life and a smooth, controllable ride. However, when tires are not properly balanced, tire to road traction is reduced. Reduced traction results in tire slippage, a source of energy loss.

Figure 1. Condition of balanced and unbalanced tires and road contact.

Loss of fuel economy may range from minimal to considerable, depending on the degree of tire imbalance and the vehicle's operational speed. The greater the imbalance and the greater the speed, the greater the loss of traction and a corresponding loss of fuel economy. The loss of fuel economy due to out-of-balance wheels applies only to the driving wheels. Improperly balanced non-driving wheels affects only ride and tire life. Unbalanced tires exhibit a spotty, erratic, cupped wear pattern. Force dynamics create this wear pattern. Energy is lost scuffing away the rubber. A tire 1 ounce out of balance and traveling at 60 mph exerts 7.73 pounds of excessive force each revolution! Energy is lost! See Figure 2 for tire wear pattern.

Figure 2 Illustration courtesy of Moog Automotive.

Tire life impacts energy reserves since crude oil is a raw material used to produce tires. Tires that wear out prematurely waste oil stock since replacements are required. Disposal of used tires is an environmental problem. Presently, efforts are underway to use spent tires as a fuel source and as an extender for road construction base material.

ACTIVITY:

1. Inspect several vehicles. Notice tires that show signs of imbalance, especially driving wheels. See Figure 2 for a typical imbalanced tire wear.

2. From the owner/driver, obtain the following information:

• a. Does the vehicle vibrate? ____Yes ____No
  If yes, speed at which vibration is most prominent: ____________ mph.
• b. Average fuel mileage: ________ city _______ highway _______ combined
• c. Noticed any change in mpg? _____Yes _____No
  If yes, how much: ____________________________
  

  Did this occur near the time vibration started? _____Yes _____No

3. With owner/driver's permission, road test the vehicle and verify tire imbalance and speed(s) where imbalance is most perceptive. NOTE: BE SURE TO WEAR SEAT BELT. OBSERVE TRAFFIC LAWS AND SPEED LIMITS.

4. Perform a normal tire/wheel inspection and wheel balance.

5. Road test again for accurate repair.

6. Request owner/driver report on this vehicle's mpg for the next three tanks of fuel.

INFORMATION CHECK

Directions. Indicate whether the statements below are true or false. If false, please correct the statement to make it true.

1. _______ Only imbalanced driving tires cause loss of mpg.

2. _______ Tires that wear out prematurely deplete crude oil stock.

3. _______ Any imbalanced tire results in loss of mpg.

TEACHER'S NOTES

Rarely do student technicians correlate improper wheel balance with mpg. Loss of traction on driving wheels causes loss of mpg. This exercise will help students understand this principle.

ANSWERS TO INFORMATION CHECK:

1. True

2. True

3. False - Non-driving tires will still wear prematurely and cause a poor ride and possibly reduce steering control (the front tires of a rear wheel drive vehicle) but they will not affect mpg.

RECOMMENDED READING:

Steering Suspension and Brakes. Pages 50-51

Principles of Wheel Alignment and Service, 3rd Edition. Pages 140-142

REFERENCES:

Ellinger, Herbert E. and Richard Hallaway. Automotive Suspension, Steering, and Brakes. Prentice Hall, Englewood Cliffs, NJ. 1980.

Bacon, E. Miles. Principles of Wheel Alignment Service, 3rd Edition. McGraw Hill, NY. 1985.

Birch, Thomas W. Automotive Suspension and Steering System. Harcourt Brace Jovanovich, Orlando, FL. 1987.

Coghlan, David A. Vehicle Chassis Systems. Duxburry Press, North Scituate, MA. 1978.

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