I'm thinkin' shackle angle is closely associated with ride quality. The more vertical your shackles are, the more energy is transfered into the chassis ( = rough ride). On the other hand, you don't want to bottom your shackles out on the frame when the tire's at full compression, limiting your flex. A long vertical shackle will also 'move' the axle backwards while it pivots under compression(?). With a flatter angle, there's less induced steer with your suspension flex.
Check out 4crawler.com, Roger Brown's page on this one. He's got a lot of info on shackles.
http://www.4crawler.com/4x4/ForSale/Shackles.shtml#IntroductionI'm still a noob on this stuff, so if a buch of people throw angry posts after this... well, you know...
