Solstice Rear Caster
The Pontiac Solstice enjoys the distinction of being the first GM vehicle to have adjustable rear caster (fig. 1). It will soon be joined by the Saturn SKY.
A procedure has been developed to measure rear wheel caster, using a new Digital Angle Gauge. But first, a little detail on rear caster.
Rear Caster
Rear caster adjustment is used on the Solstice and SKY to insure rear ride steer (bump steer) is within specification. Ride steer is toe change during vertical wheel travel.
On a front suspension, it’s common to check caster by doing a “caster sweep,” in which the wheels are steered through an angle, and the caster angle is indicated. Because the rear wheels don’t have steering gear, a caster sweep is not possible. So a direct measurement is necessary.
Digital Angle Gauge
A new Digital Angle Gauge CH-47960 (fig. 2) has been released by SPX Kent-Moore. It will be essential for all Pontiac and Saturn dealers.
Instructions for use are supplied with the gauge. Here are some highlights.
The gauge consists of three parts: a battery powered control unit, a sensor, and a connecting cable.
The control unit provides simple input buttons:
- to specify the side of the vehicle being checked (driver or passenger)
- to zero the instrument.
- to specify offset
There are two locating pins on the back of the sensor (fig. 3). They can be adjusted up and down by twisting to loosen and tighten.
The pins are inserted into two manufacturing gauge holes in the rear wheel knuckle (fig. 4).
Measuring Tips
Before using the gauge, it must be zeroed to the alignment rack (fig. 5). This is accomplished simply by placing the sensor on the rack and pressing the zero button. Zeroing accounts for variations in the levelness of individual alignment racks, so it’s never a concern. You don’t have to do any math or make allowances.
TIP: Be sure both the bottom of the sensor and the surface of the alignment rack are free of debris, to avoid an inaccurate zero.
Before installing the sensor to the knuckle, use a soft (nylon) bristle brush to clean debris from the gauge holes. Do not use power tools or abrasives.
Use the appropriate pushbutton on the control unit to indicate which side of the vehicle you’re working on, either driver or passenger.
A vertical line drawn through the gauge holes in the knuckle (B) is not quite perpendicular to the axis between the two balljoints (A). So you need to locate the specification of that angle and input it into the tool. This is called the offset (fig. 6), and in the case of the Solstice rear wheels, it’s +3.7°.
Finally, take the caster reading (the specification is -4° +/- 0.5°). By specifying the offset angle earlier, you set up the gauge so the readout indicates the actual caster angle (fig. 7). You don’t have to do any math.
TIP: If you study the front and rear suspension of the Solstice, you may notice that the driver front knuckle resembles the passenger rear knuckle, and the passenger front and driver rear knuckles resemble each other. This suggests that you could use the gauge to check front caster. This would require inputting “driver” into the Digital Angle Gauge when working on the passenger side, and vice versa. Don’t be tempted to do this, however, because the front offset angle is different from the rear, and this specification has not been published.
Alignment Tips
TIP: It’s important to complete the rear wheel alignment before aligning the front end.
IMPORTANT: If you do not have Digital Angle Gauge CH-47960, do not attempt to adjust rear caster. In this case, set only rear camber.
TIP: When you do a rear wheel alignment, you will observe adjustments for camber and toe from your alignment rack readout, and caster from your Digital Angle Tool.
There are two adjustment cams on the rear lower control arm (fig. 8). The front cam is used to adjust caster, and the rear cam is used to adjust camber. Toe is controlled by an adjustable rear toe link.
A Rear cam -- camber
B Front cam -- caster
It’s important to make the suspension checks and adjustments in the following order, to minimize having to make multiple passes.
1. Camber
2. Caster
3. Toe
4. Fine tune camber *
5. Fine tune caster *
6. Fine tune toe*
* if required
TIP: Both cams affect caster to some extent, but the rear cam affects camber about eight times as much as it affects caster. This is why you should set camber first, then caster.
