Quote:
Originally Posted by phunt
Proper suspension geometry is based on the difference between the measurements of a and b. The 108mm that you call measurement c. Meaning it sets the angle of the lower control arm. From its pivot point to the ball joint. To maintain that angle (which will affect bump steer) when you raise the spindle 19mm you need to set that c measurement at 127mm. The 108 plus the 19 of raising the spindle. Otherwise you would reduce the angle of the control arm from the pivot point to ball joint. Bump steer is worsened by not having the steering control arm not parallel with the lower control arm taken from the center of the ball joint to the center of the pivot point. Those two arcs need to be as close as possible so no change to the dimensional measurements. Causing a pull or push on the steering knuckle. Additionally the distance from where the strut mounts on the ball joint to where it sets at the top on the chassis will not change. Maintaining original travel of the shock. That’s stock geometry. That’s why you lower a car with raised spindles vs just changing springs or spring orientation. By maintaining these angles you may not need to raise or lower the steering rack. It would depend on where the knuckle is on the strut. Weather it was raised or not.  |
You can measure either way, it's just that the original a - b hides the actual height w/i the formula, These days there are so many possible front tires used that , JIMHO, b is the metric I would want to know when setting the car up, Most of us aren't changing tires when we do this chore, so we really don't care. It's something that is used to initially determine what tire to use but it's not relevant after that as it's a fixed quantity.
and, yes, roll steer is the result of the different arcs defined by the A-arms and the steering arms. The #s I quoted above are just a way to keep track of the changes from stock and how to restore stock geometry in all relevant respects.
I initially began thinking about this when trying to compare my 993 w/ my 911 despite the different suspensions, It's amazing to me how similar they are in the heights of the fender tops.
here's what a stock RoW 3.2 Carrera toe and camber curves look like at stock RoW ride height w/ 205/55 x16 tires
In both cases a vertical trace is desired, the more horizontal the more change per mm of compression, the red line shows a compression of 40mm w/ corresponding toe change of -34' That's as good as it gets w/ a stock 911 strut front. It's also what gives these cars the feeling that they are alive because it causes the wheel to move in you hands.
