tt9714

I recently rebuilt my front suspension, and I'm in the process of adjusting the alignment settings enough to get to a shop a few miles away.

I understand toe and how to adjust. Same for camber.

However, I'm having a hard time visualizing the effect of caster changes. Most textbooks seem to always show a side view of the car. On the 944, the caster adjustment is through the rear A-arm mounting bolt (eccentric).

It seems like the eccentric has about 180 degrees of movement. Over this range of movement the rear of the A-arm moves moves in an arc from left to right. Which orientation will give maximum (+) caster and which will give minimum (-) caster? Approximately where is 2-3 degrees relative to the adjusting "tab" that is on the eccentric bolt?

Riff

Caster is the angle of tilt of front spindle from verticle (think of it as an imaginary line from the top of the strut mount to the ball joint). Pos. castor is more tilt rearwards. Since movement of the bolt swings the arm right or left, it moves the ball joint forward or back relative to the upper strut mount. This will increase or decrease the angle of tilt.

As how to set it to get to the ball park figure, I can tell you that currently my car the tab on the driver's side is set at ~7 o'clock when viewed from the rear of the car. Passenger side is mirror image. Should be enough to get you to the alignment shop.

HTH,

tt9714

OK, I think that I am starting to understand how the caster eccentric works. The most obvious thought is that by turning the eccentric, the control arm moves in a direction ~perpendicular~ to the axis of the eccentric bolt (i.e. from left to right across the car).

When the eccentric is turned all the way to the right (driver's side, viewed from rear), the back of the control arm is closest to the centerline of the car. Conversely, when the eccentric is turned all the way to the left (driver's side, viewed from rear), the back of the control arm is farthest from the centerline of the car.

In addition, an analysis of the geometry will show that the control arm also moves in a plane parallel to the axis of the eccentric (i.e. from front to back of the car). This is why all of the "Wheel Alignment 101" website show a ~side~ view of the car. Adjustment in this direction is consistent with the concept of caster as described in most texts.

Here's the key question, when viewed from the side, there are two positions of the eccentric that will yield essentially the same result. Consider that if the eccentric tab is pointing straight down, this will push the rear of the control arm the greatest distance forward giving the highest possible positive caster. Now if the eccentric is turned left or right the same amount, it should yield approximately the same decrease in caster. However, the position affects the orientation of the control arm laterally (left to right side of car). So which is the "correct" orientation?

Riff

Not really sure how to answer your question, but I do not think that there are 2 positions on the eccentric that will provide the same results. When viewed from above, the arm moving closer or further from the centerline of the car (pivoting about the front mount to the cross member) will locate the ball joint forward or rearward affecting caster. Please see attached image for clarification.



HTH

special tool

In case anyone is interested in the purpose of the tabs - the more the caster (top suspension pivot moving to rear) the higher the steering input effort; the less the caster (top suspension pivot moving to front) the less the self-centering action after turn.