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Front End Alignment vs Trunk Weight

Written by John Bellah

How much difference does vehicle loading really make? We put our test vehicle, a 1998 Ford Crown Victoria Police Interceptor, on the alignment rack to find out.

To obtain the before alignment readings, we started off with a full tank of fuel (which makes about a 100-pound difference on rear suspension loading) and removed everything from the trunk. The tire pressures were set, and the suspension components were checked for wear. The factory alignment specs for this vehicle are:

                                            LEFT                                                       RIGHT

               Minimum      Nominal     Maximum                  Minimum       Nominal        Maximum

Caster:   +4.75          +5.50       +6.25                       +4.75         +5.50         6.25 (degrees)

Camber:  -1.25            -0.50        +0.25                       -1.25         +0.50        +0.25 (degrees)

Toe:         -0.19            -0.07        +0.06                        -0.19        -0.07         +0.06 (degrees)

The alignment was then set as close as possible to the nominal, i.e., preferred specs. These “before” settings are:

                                                     LEFT                        RIGHT

Caster:                                    +5.4 degrees            +5.4 degrees

Camber:                                   -0.5 degrees             -0.5 degrees

Toe:                                         -0.06 degrees            -0.07 degrees

Then we began the test by adding 400 pounds in the trunk. This caused the rear to sag about 1 ½ inches, typical for many in-service CVPIs. We obtained the following results in this configuration:

                                                    LEFT                                  RIGHT

Caster:                                   +5.8 degrees                     +6.0 degrees

Camber:                                  -0.5 degrees                       -0.6 degrees

Toe:                                         -0.04 degrees                     -0.04 degrees

Then we put 215 pounds in the driver’s seat, leaving the right passenger seat empty. This was in addition to the 400-pound load in the trunk. We obtained these results:

                                                     LEFT                                     RIGHT

Caster:                                    +6.4 degrees                         +6.3 degrees

Camber:                                   -0.5 degrees                           -0.8 degrees

Toe:                                        +0.01 degrees                          -0.03 degrees

As you can see the added weight, and the weight distribution made a huge difference in the alignment. The caster went out of spec to the maximum (positive) side. The camber remained in-spec but moved to the minimum (negative) side. The toe remained in-spec but moved well to the maximum (positive, i.e., toe-in) side. Even worse, due to the driver’s weight on the left side, the left tire is now just slightly toe-in, while the right tire is slightly toe-out.

A police vehicle should be aligned with a full tank of fuel and all of the police equipment installed in the trunk. If the vehicle is to tow a trailer, the tongue weight of the trailer should be compensated for during alignment procedures.

The author would like to thank Dave Belski of Dave’s Brake and Alignment in La Habra, CA for conducting these alignment tests and measurements. Belski is a master mechanic/technician with more than 30 years of experience in front-end, brake and suspension maintenance.

John Bellah is the technical editor of Police Fleet Manager and a corporal with the California State University, Long Beach Police. He can be reached at jbellah@csulb.edu.


Published in Police Fleet Manager, Nov/Dec 2006

Rating : 8.0


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