Originally Posted by Necromancer_tat
I'm also very interested in this topic as I'm doing research for my own build upgrades. I have seen elsewhere it suggested that the link separation at the axle side be a certain percentage of the intended tire diameter in order for them to control the axle effectively. I believe it was close to 25% if I remember right. I'll try and see if I can find the link and post it here. If it's correct a 40" tire should have a link separation of 10" at the axle, but perhaps using bigger joints and stronger mounts and control arms could reduce that amount? If that ratio is accurate then a 35" tire would require 8.75" of separation.
You're information is correct. The forces against the control arms are resultant forces that originate from the tire's contact patch.
Draw a straight line vertical from the tire's contact patch through/by the lower control arm pivot point and then through/by the upper arm pivot point. That's the lever that needs to be resolved to understand the forces. And that leverage changes with tire size AND control arm placement.
The force generated at the contact patch is magnified by the lever length between the ground and the lower control arm mount, which is the fulcrum. Then the tire's contact force is magnified in the opposite direction by the length of the lever that exists between the fulcrum and the upper arm's pivot point. The force at the fulcrum is the sum of the force at the contact patch and the force at the upper control arm.
This is a first class lever and follows "lever law" in its theory and calculation. Simple stuff once you define the lever.
If you play with the lever lengths that are affected by moving the control arm mounts and do the force calculations you'll see installing larger tires in combination with moving the control arm mounts further from the ground has a huge impact on the forces seen by the control arms and then transferred into the frame/body. That means a bigger effect on the same anti-squat percentage because that is more force being input, for example. The same anti-squat percentage will behave differently and probably need to be reduced in order to make the vehicle dynamics act in a similar fashion as with smaller tires and lower control arm mounts.