Pretty much as in the title, curious as to whether a change in the rear ride height approx plus 1.5” would make a significant difference to the caster, I.e. if the rear is lifted it is tilting the frame forward so the top ball joint in theory is moving forward relative to the contact patch of the front wheel?
Doing some basic calculation, if the wheelbase is 108” and the frame relative to the rear wheel has raised 1.7”, then the incline from the original ride height as a ratio is 1.7:108.
Using an online calculator that converts to 0.9 degrees forward tilt of the frame and correspondingly the front suspension, so presumably a 0.9 degree reduction in caster?
Raising the rear of the car will result in a loss of positive caster.
It's not "unmeasurable" either. Having checked the caster on my Duster with and without rear slip plates in place to match the height of the front turn plates I know on my car the ~2" change in height from the presence or lack of slip plates on the rear resulted in about a .5° difference in the caster measurements. Some of that obviously depends on the alignment settings, ride height and frame angles you're starting with and spring rates for the springs and bars on the car, but most of those would be a fairly minor effect. So for a change in ride height of ~1.5 to 2" at one end only I would say .5° of caster change is in the ballpark.
Doesn't sound like a lot but if you're running stock UCA's the positive caster is already pretty limited, so losing another half degree by raising the rear end of the car ~2" could be significant. If you're running stock UCA bushings you might only be getting +1.5 to 2° of positive caster to begin with so losing half a degree would be important. Even with stock UCA's and offset UCA bushings you might only be looking at ~+3.5° of caster, so a half a degree loss could still result in less than ideal steering characteristics.
So the next and separate (but related) question is when the front ride height is raised using the TB adjuster bolts does the change in the control arm geometry add more or less positive caster.
Here's a plot of suspension geometry changes over the range of suspension travel, which would be similar to the suspension geometry changes that occur when you raise or lower the front ride height. This is from Bill Reilly's article "Debated Usage" originally published in Mopar Muscle in March 2005. These are changes directly from suspension geometry changes from the control arms moving, not a change in the frame angle (which is held constant to plot these numbers).
Now, these exact numbers are not set in stone, the initial static alignment and ride height plays a role in how much they change too. This chart is plotted for a car that was about 1"lower than the factory ride height with the static alignment numbers listed. The car also had a ~1.5" rake with the rear being higher.. But in general the trends should be similar unless you really change the ride heights or alignment numbers dramatically.
So if you held the frame angle constant and raised the front suspension, that would be "negative dive" on the chart. Meaning, you would lose caster adjusting the control arms to raise the car. So, if you raise the back of the car you lose positive caster because of the change in frame angle, but if you raised the front to keep the frame angle the same you'd still lose caster because of the suspension geometry changes. And with little caster adjustment to begin with you won't be able to adjust all of that out.
This explains the method for getting the numbers, also, a direct comment from Bill Rielly that rear ride height directly affects caster.
Yep, that was a good start!
I believe you can still edit the thread title...
