I was looking at the Cater... front ARB, trying to copy what it looks as a very simple design. I tried a template bar in my car but I see that the arc that the "arms" describe is ver different to the one described by the upper suspension arm. I understand they use a rubber bush to attach the ARB to the suspension arm but it doesn´t look enought to me.

The different arc is because the ARB is rotating but the suspension arm is only going up and down in the same position. The ARB end is not only going up and down but is going away from...

Any suggestions?

Cheers!

The first Fiat Unos and the FWD Escort had designs similar to the original Lotus Seven, where the ARB is the trailing ling that positions the front arm longitudinally.. The ARB does describe an arc longitudinally, while the upper arm describes an arc transverse to the car axis.
When you put those togetther, you have a slight change in caster as the suspension goes up and down, but that doesn't affect suspension performance that much, and the Seven is a proof of that. All of the cars I mentioned have rubber bushings connecting ARB to the suspension arm, and rubber bushings in the upper arms, to allow the arm to twist as it rotates during suspension travel.
In the Caterham design the tip of the ARB moves inside a rubber bushing, which absorbs the unequal movements of the a-arm and ARB. Teh ARB tip will yaw and slide in the bushing.
Was that very confusing?
Cheers