On another thread recently they were talking about caster angle and shackles. I did some more research and math after that. I found that less caster is needed with larger tires, since the tire/road contact point is further ahead of the knuckle swivel. The OEM spec for my YJ is 6.3° (+/-1), which turns out to be about 5° needed for a 35" tire (compared to a stock 29" tire).
Doing the caster math as it applies to shackles, changing the shackle length by 3/4" changes the caster angle by 1°. So, lengthening a shackle from the stock 4" means lowering the caster, which means for twitchier on-road handling and poorer on-road return-to-center (Jeep seems to have a mind of it's own). My present shackles are 5.75" long, and my caster is about 3.5° (had it measured by a shop), and the Jeep indeed has a mind of it's own on-road.
Actually, today's garage project is to fab up a new pair of front shackles that are maybe 4.5" long to get my caster much closer to the 5° I figure is good. I think it's worth a couple hours of shop time and 2' of steel to experiment.
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site