Tracked Vehicle Control Arm angle?

[Avadon77]

I'm building a tracked robot and trying to figure out what angle the axle arms/control arms should be set at? I measured the M1 Abrams tank and the road wheels are all set to 15dgs except for the leading wheel which is set at 40dgs.

But then I looked at other robotic tracked vehicles and they look to be around 40-60dgs. Is there some special angle? Of course this dictates how the suspension will work as well.

any clue what angle these axle supports should be placed at?

[Avadon77]

These road wheel arms on these two robots (Green is FIRST robotics team robot and the other is a Darpa Suspension robot) have a much higher angle.

So I'm wondering should the arms be much lower like 15dgs on the Abrams and other military tanks or should I be much higher like many other robotics chasis? and what is the difference?

Thanks,