That drawing is incomplete. There are two sets of arms that lie in the same plane - you need to specify front or rear control arm. This thread encompasses discussions about all three arms: front transverse arm, rear transverse arm, and the longitudinal arm. The one that does the most work is the front transverse arm. This arm is the U-crosssection piece that provides caber control. An adjustable arm there would provide camber adjustability in the rear, as well as less flex in the rear suspension. The difficulties with this arm are because it bears so much load, and needs to have the swaybar link attatched to it. No one wants to make such a safety critical part. The toe control links, that is, the rear transverse links, run from the wheel to the subframe as well, and are much thinner. The adjustability is lousy, but good enough, I think. Tubular members would compliment tubular arms in the front position to provide a more laterally rigid suspension. The longitudinal arms are plenty stiff enough, but have soft bushings like the other arms. They are also very heavy, and while a tubular fab could be difficult, increasing the control of toe under bump; reducing bind; and losing weight might make it worth adding that member to a complete tubular rear.
But no one is actually offering anything for sale.
