Ford Aerostar 1986-1997 Repair Guide

Front End Alignment


If the tires are worn unevenly, if the vehicle is not stable on the highway, or if the handling seems uneven in spirited driving, wheel alignment should be checked. If an alignment problem is suspected, first check tire inflation and look for other possible causes such as worn suspension and steering components, accident damage or unmatched tires. Repairs may be necessary before the wheels can be properly aligned. Wheel alignment requires sophisticated equipment and can only be performed at a properly equipped shop.


See Figure 1

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Fig. Fig. 1: Caster angle (the forward or rearward tilt of the top of the front wheel spindle) affects straight line stability

Wheel alignment is defined by three different adjustments in three planes. Looking at the vehicle from the side, caster angle describes the steering axis rather than a wheel angle. The spindle (steering knuckle) is attached to the strut at the top and the control arm at the bottom. The wheel pivots around the line between these points to steer the vehicle. When the upper point is tilted back, this is described as positive caster. Having a positive caster tends to make the wheels self-centering, increasing directional stability. Excessive positive caster makes the wheels hard to steer, while an uneven caster will cause a pull to one side. Caster and camber are adjusted with shims and are closely interrelated; refer to the following camber procedure for caster adjustment.


See Figure 2

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Fig. Fig. 2: Camber (the amount that the centerline of the wheels tilt inward or outward from true vertical) influences tire contact with the road

Looking at the wheels from the front of the vehicle, camber adjustment is the tilt of the wheel. When the wheel is tilted in at the top, this is negative camber. In a turn, a slight amount of negative camber helps maximize contact of the outside tire with the road. Too much negative camber makes the vehicle unstable in a straight line.

Caster and camber adjustment is provided by shims on the upper control arm. The two different shims initially provided from the assembly plant include one 2mm thickness and one 6mm thickness, for a total shim stack thickness of 8mm at each leg of the upper control arm. These shims are added, removed, or switched from the front and rear legs of the upper control arms as required to adjust the front end alignment.

Camber adjustment is obtained by removing or adding an equal number of shims to the front and rear leg of the wire arm. Caster adjustment is obtained by removing shims from the front leg and installing them on the rear leg, and vice-versa. If the same amount is switched from one leg to the other, caster will be changed but camber will not be affected. If unequal amounts are removed and added to the front and rear legs, both caster and camber will be changed.


See Figure 3

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Fig. Fig. 3: Toe-in means the distance between the wheels is closer at the front than at the rear of the wheels

Looking down at the wheels from above the vehicle, toe alignment is the distance between the front of the wheels relative to the distance between the back of the wheels. If the wheels are closer at the front, they are said to be toed-in or to have a negative toe. A small amount of negative toe enhances directional stability and provides a smoother ride on the highway. On most front wheel drive vehicles, standard toe adjustment is either zero or slightly positive. When power is applied to the front wheels, they tend to toe-in naturally.

Toe-in should only be checked and adjusted after the caster and camber have been adjusted to specifications. Caster and camber adjustments change the position of the steering arms, thus affecting toe. Toe is defined as the difference between measurements taken between the front and rear of the tires. Positive toe or toe-in occurs when the front of the tires are pointed inboard of the rear of the tires. Negative toe or toe-out occurs when the front of the tires are pointed outboard of the rear of the tires. The toe specification is designed to provide optimum vehicle handling and tire life under a variety of driving and load carrying conditions.

All wheel alignment adjustments and readings must be performed on an alignment rack level to within1/16in. (1.5mm) side-to-side and front-to-rear. Refer all alignment checks and adjustments to a qualified repair shop.