Some of that I kinda get / understand , But not the change in pinion and drive shaft angle. why would those two change? because to me those should not be moving I would'nd think??
I didn't consider it either. But if you are going with a tire that is, say, four inches shorter then that means you have to raise up the back of the car TWO inches with respect to the axle centerline to maintain the same ride height. Doing that lowers the back end of the driveshaft relative to the surrounding chassis tubes and tunnel so you have to be sure there is still adequate clearance to everything. Depending on where the theoretical pivot point of the rear axle is - the instant center - the pinion angle may also change. Fortunately in my case there was still adequate clearance for the new driveshaft location when it was cycled through its new arc of travel and the pinion angle looks to still be close to optimal. The instant center will change because the back two 4-bar pivot points are now 2" closer to the earth while the front two are unchanged, effectively raising the instant center of the rear suspension. I think I may actually want the I.C. higher as it will cause a harder initial hit to plant the smaller tire. The shock rate needs fine tuning too since I put on longer shocks with more travel and a little softer spring rate.
It all becomes a big fascinating juggling act. Stay tuned...
