I am trying to sort out my wishbones, what is the reason the top wishbone is shorter than the bottom one. I can find camber,caster, tow in and out anti roll bar and Tyre scrub but nothing on wishbones on internet. Help please

To avoid or at least keep low the variation of camber angle in external wheel during rolling of chassis in a curve

It helps to think of an extreme example. Imagine two parallel wishbones of the same length, directly above each other. As you move the wheel upwards, the angle stats the same.

Now imagine the top wishbone is only 50mm long. If the wheel moves up by 50mm, the bottom wishbone moves up and slightly in (following the arc of the circle). The upper wishbone moves up by 50mm, but because that's the length (or radius of a circle) it also moves in by 50mm. This relative movement of the upper in by 50mm, whilst the lower moves in only a little bit, adds some positive camber.

This is known as camber gain(despite it being negative), and is desirable because as the car rolls the outside tyre tends to lose camber (goes positive) and therefore reduce grip. Camber gain counteracts that.

It is also a bad thing. Think about a braking phase. You don't want any camber change because there's no roll in the car, but the front axle dives down, causing the motion which generates extra camber, reducing the contact patch between the tyre and the road.

It's called an SLA (short-long arm) geometry. Hope that helps a bit!

Have a play with this & you'll what's being explained above...

http://www.racingaspirations.com/apps/suspension-geometry-calculator/