FUEL ECONOMY AND HEAVY TRUCK
REAR AXLE ALIGNMENT


GOALS:
To understand the importance of rear axle alignments on heavy trucks.

OBJECTIVES:
The students will:

1. Demonstrate how to test and adjust rear axle alignment on heavy trucks.
2. Understand how a misaligned rear axle causes tire wear and reduced fuel economy.

LESSON / INFORMATION:
A properly aligned front end has long been established as a necessity for long tire life, handling ease, and driving safety, as well as a factor in achieving optimum fuel economy. However, until recently, little attention has been focused on the alignment of the rear axle. A misaligned rear axle may result in: a) reduced tire life; b) difficulty in driving; c) an unsafe vehicle; and d) less than optimum fuel economy. According to a February, 1992, Fleet Equipment article, "Studies have proven that 80% of Class 8 tractors are operating misaligned, and over 90% of the trailers they pull have serious alignment problems." (Fleet, pg. 20) The majority look like those in Figure 2.
dsl-g45

A misaligned rear axle causes excessive tire-to-road drag and creates undesirable lateral forces. Figure 1 illustrates correct tandem axle alignment. Figure 2 illustrates some possible axle alignment conditions.

Even new trucks need rear axle alignment. Other studies cite that up to 70% of new vehicles need some form of rear axle alignment. In a study of a fleet of 100 trucks, those with proper rear axle alignment operated at .79 cents per mile for tires compared to the misaligned of 1.8 cents per mile. Other fleets reduced tire operating costs from 1.9 cents to 1.2 cents. Most studied fleets with correct rear axle alignment reported a gain of 3/10ths of a mile per gallon. For 100 trucks logging 125,000 miles per year, this translates into 1,100 gallons of fuel per year per truck. As an example, a tandem tractor operating 125,000 miles yearly, with the front axle 1/4" out-of-toe and the rear axles 1/4" out of parallel (see Figure 2) would "scrub" the road 230 miles sideways. According to recent studies conducted, this is typical of the tractors that operate on America's highways! Additional trailer drag and scrub contributes to even more fuel loss and decrease in tire life. "Sixty percent of steering axle tire wear is caused by drive axle misalignment." (Fleet, Dec 1991, pg. 9.)

All heavy trucks and trailer units should have their rear axle alignment checked and adjusted periodically or when any abnormal tire wear appears. Rear axle alignment should be checked after any collision repair. Minor shifts in the frame-to-suspension mounts may cause significant shifts in both front and rear axle alignments. To align the front axle will only fix half the problem.

Figure 3 illustrates various effects on fuel consumption. Drag and tire rolling resistance accounts for the single largest segment of fuel consumption - 42.2%. Any action taken to reduce rolling resistance, such as proper alignment, can and will reduce fuel consumption.
Figure 3
dsl-g46
Courtesy of Fleet Equipment. Used with permission.

It is possible for a given truck to have reciprocal rear axle misalignment conditions. If this were the case, it may not exhibit any poor driving characteristics. However, tires will still wear and fuel economy will be decreased.

Rear axles usually have little or no camber and toe requirements. Heavy truck rear axles use no caster alignment correction.

ACTIVITY I:
Use the activity sheet provided to:
1. Locate a tandem axle truck and a single axle truck.

2. In the appropriate shop manual(s), locate the pre-alignment inspection sequence(s). Note any special procedures for rear axle alignment for each truck.

3. From the shop manual(s), rear axle alignment specification section, determine the correct rear axle camber and toe settings.

4. From the shop manual(s), determine the correct adjustment procedures for camber and toe.

5. If the manufacturer permits bending the axle to correct minor out-of-tolerance camber and toe conditions, make a diagram of the suggested equipment configuration.

6. If four wheel alignment equipment is available, perform a four wheel alignment check on each truck and perform any necessary adjustment(s). If four wheel alignment equipment is not available, complete Activity II.

7. Summarize and compare the rear axle alignment check and adjustment differences between the tandem axle truck and the single axle truck.

ACTIVITY SHEET
Name:______________________________ Date Completed:__________________________

Tandem Axle
1. Year___________________________
Make__________________________
Model__________________________

Type of rear suspension:
_______________________________

2. Pre-alignment checks:
______________________________
______________________________
______________________________

3. Rear axle alignment specifications:
_______________________________
_______________________________
_______________________________

4. Adjustment procedures:
Camber ________________________
Toe ___________________________

5. Results of 4-wheel alignment check:
Rear camber ___________________
Rear toe ______________________

Single Axle
1. Year ___________________________
Make __________________________
Mode l__________________________

Type of rear suspension:
_______________________________

2. Pre-alignment checks:
______________________________
______________________________
______________________________,

3. Rear axle alignment specifications:
_______________________________
_______________________________
_______________________________

4. Adjustment procedures:
Camber ________________________
Toe ___________________________

5. Results of 4-wheel alignment check:
Rear camber ___________________
Rear toe______________________

Summary of findings:
Tandem Axle:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________

Single Axle:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________

ACTIVITY II:
Note:
This activity provides the technician with a rear axle alignment checking procedure only. It is accurate enough to determine if a rear axle is out-of-alignment, but is not accurate enough to accurately align an axle. This procedure can be used to initially align a rear axle after a major rear axle repair. Final alignment should be performed using appropriate alignment equipment.

Materials Needed:
1. A straight, eight foot long, 1-1/2" or 2" angle iron
2. Two large "C" clamps
3. Fifty feet of heavy cord
4. Four pieces of 2" x 4" lumber exactly four inches long
5. A carpenter's square
6. A twelve inch ruler
7. An adjustable trammel gauge

Note:
This checking procedure is written for NON-OFFSET axle trucks. Trucks with offset axles require compensations for axle offset.

Procedure:
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1. Drive truck onto a flat, level area.

2. With the front wheels pointed straight ahead, the engine off, the transmission in neutral, and the parking brake released, roll the truck back and forth a few feet to neutralize any residual suspension torsion as a result of steering and braking efforts. Set the parking brake and block the wheels to prevent further movement.

3. Using the carpenter's square and the "C" clamps, clamp the angle iron just ahead of the rear tires.

4. Attach a piece of cord to the frame just behind the rear tires; if a tandem axle is being checked, attach the cord to the rear of the front axle. Place a block at the rear of the outer tire sidewall and the front edge of the outer tire sidewall. Attach the cord to a safety stand that is placed ahead of the front of the truck. Bring your eye level to the cord and sight align the cord so that it is in a straight line.

5. Measure the distance between the outer side wall of the front axle tires and the cord.

6. Distance "X" and "Y" should be equal to within 1/8". If there is more than 1/8", the rear axle is definitely out-of-alignment.

7. If a tandem axle is being checked, use the trammel gauge to measure the distance from the intersection of the cord and the angle iron to the center of the hub of the forward axle. These left and right side distances should be equal.

8. If the forward axle checks "OK" with the trammel gauge, use the trammel to check the distance from the center hub of the forward axle to the center hub of the rearward axle. These distances should be equal.

9. Summarize your findings and make recommendations accordingly.

10. This procedure can be used to check the alignment of a tandem axle trailer by modifying the set up using the fifth wheel pin.
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INFORMATION CHECK
Directions: Indicate whether the statements below are true or false. If the statement is false, explain why it is false.

1._______ Only tandem axle trucks need rear axle alignments.


2._______ Even if the front wheels are aligned correctly, a truck may still pull or lead to one side.


3._______ A misaligned rear axle on a trailer causes reduced fuel economy.


4._______ Caster and toe are the two possible adjustments to be made on a rear axle.


5._______ Tire rolling resistance accounts for the greatest fuel consumption.


TEACHER'S NOTES
Recently the automotive service industry has focused on rear axle alignment. Some older technicians view this recent shift as a manufacturer's ploy to sell more equipment. Some consumers resist rear wheel adjustment due to additional costs. They see this as unnecessary since former trucks did not seem to need rear axle alignments. Part of this problem is a pricing structure that has evolved - charge the same price for "a check only" as to "check and adjust." For tandem axle and single axle trucks, proper rear axle alignment is necessary for optimum fuel economy and tire wear. Proper rear axle alignment makes the truck drive better. Significant out-of-specification (more than 2° camber and 1/4" toe) rear axle alignment could be dangerous, increases fuel consumption and decreases tire life. As a result, operational costs increase.

ANSWERS TO INFORMATION CHECK:
1. False. All axles require proper alignment for optimum fuel economy and tire life.
2. True
3. True
4. False. Camber and toe are the rear axle adjustments. There is no caster setting for rear axles.
5. True

RECOMMENDED READING:
Fleet Equipment, February, 1992. Pages 20-24.

Diesel Equipment II, Pages 122-136.

REFERENCES:
Schultz, Erich J. Diesel Equipment II. McGraw Hill, NY. 1982.

Fleet Equipment, February 1992. Volume 18, Number 2. Mis Alignment: The Tire Killer. Tom Gelinas, Editor.

Fleet Equipment, December, 1991. Volume 17, Number 12, Preventative Suspension Maintenance.


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