You change the camber by adding or subtracting the same amount of shims both front and rear of the arm. If you change the amount of shims on just one, it shifts the axis. So to make more caster you would take out shims in the front or add shims to the rear.

 

Think about it this way for visualisation. Lets say you have a stick suck into the ground and on the top of that stick you have a triangular bracket with one point to the stick and the flat side to a wall. If you pull out on one of the corners mounted to the wall, it will throw the stick forward or backward, depending on which corner you move. Does that help?

 

"Steering axis inclination" is the angle of the steering axis (imaginary line connecting the upper and lower ball joints) measured in the LATERAL plane - which you see when viewing the car from the front. It is fixed and non-adjustable other than the fact that it varies slightly with camber setting.

"Castor angle" is the steering axis angle measured in the longitudinal plane (as viewed from the side). Imagine the suspension "plan form" (view from above.) The lower ball joint is fixed, so the only way to change the caster angle is by moving the upper ball joint. If you remove a shim from the rear upper control arm mount and move it to the front mount you effectely move the upper ball joint rearward which will increase caster, but the camber will remain essentially the same.

Castor and steering axis inclination work together to create self aligning steering and steering feedback, and they also interact to create a "camber gain" curve, which is the change in camber with wheel jounce and rebound.

Duke

 

The angle you measured has nothing to do with castor. The upper control arm is angled relative to the horizonatal plane in order to provide brake anti-dive characteristics. Inertial force under braking is transmitted along the pivot axes of the front control arms and the vertical distance between these lines (at the point where they intersect the lateral plane of the CG) and the CG determines a vehicle's anti-dive characteristics.

There is a formula to compute this, and though I don't recall the formula, the C2/C3 upper control arm is angled to provide 50 percent anti-dive.

The nominal caster setting for C2/C3s is in the range of 1.5 to 2 degrees, not 7 degrees.

The Corvette's front suspension is an example of a "four bar linkage". There are three moving elements (knuckle, upper and lower control arms), and the ground plane (the frame in this case) is the fourth "bar". The study of moving linkages is known as "kinematics" and is a playground for mechanical engineers. Though somewhat simple in appearance, analysis of a four bar linkage can be very complex, and changing one parameter or dimension impacts just about everything else.

Back in the early sixties, Zora laid out the Corvette's front suspension based primarily on his intuition of suitable characterisitics for a sports car, and he did a very good job, but the geometry won't work very well with modern low profile tires. Back then all the analysis was done on hand drawn layout drawings. Nowadays computer programs crank out all the parameters such as track change, camber gain, caster gain, roll center, percent antidive, bump steer, etc. for any physical change to the design. Bear in mind that all these parameters change with wheel jounce and rebound, so you end up with a lot of numbers in the analysis.

The Chevrolet Power Manuals from the seventies and eighties have a good section on vehicle dynamics, and Fred Puhn's "How to Make Your Car Handle" (available from Classic Motorbooks) is a good primer for those who are interested in learning more about suspension design.

Duke





[Modified by SWCDuke, 8:54 AM 12/29/2001]

 

Excellent technical explanation (as usual) by Duke.

In layman's terms, the only adjustment available on Corvettes (and most other cars) is the position of the upper ball joint relative to the (fixed) lower ball joint, by varying the shim packs. To adjust camber only, the same shim thickness is added (to go more negative) or removed (to go more positive) from BOTH upper control arm shaft mounting points. To adjust caster only (to go more positive), shims are removed from the front or added to the rear, which tilts the steering axis rearward at the top in the side view; to go more negative, shims are added to the front or removed from the rear, which tilts the axis forward at the top in the side view. Realistically, however, these adjustments aren't independent of each other; due to the geometry, there is some interaction between camber and caster, and the technician must deal with both of them in order to change one without changing the other.

Most cars with upper and lower control arms are similar in concept, although some place the adjustment at the lower control arm pivot bushings by using cam-bolts instead of shims at the upper arm (Viper, for instance). We build Vipers with one quite thick "sport-shim" at each front upper control arm attachment; for track events, owners can loosen the nuts, pull out the "sport-shims" (which have little bent handles on them to facilitate removal), tighten the two nuts back up, and that adds one degree additional negative camber for improved racetrack handling (the Viper upper arm attachments are outboard of the frame, not inboard, so removing the shims makes camber go negative). When the track event is over, they loosen the two nuts, re-insert the "sport-shims", and camber returns to the normal street settting. With 13"-wide tires, camber setting on a Viper is critical to make sure the entire width of the tire contact patch is on the ground all the time - the special machine we had built to set alignment in the plant sets camber (at all four wheels) within 2/100ths of a degree.

Cars with McPherson struts (most front-wheel-drive cars) have the upper joint as the upper attachment of the (rigid) strut to the shock tower structure in the body - camber and caster are adjusted on these cars by loosening the upper strut attachment and literally sliding it fore-aft and cross-car, although some use cam-bolts on the lower control arms instead.

 

Point of order. Castor is an oil from the Castor oil plant. Caster is what we are discussing here.

 

Sorry - I'm fighting a cold.

Duke

 

Just having some fun with ya.

 

If castor oil comes from the castor oil plant, where exactly does baby oil come from? :lol:



[Modified by Mac, 12:00 PM 12/30/2001]

 

Baby oil plant.