The lower V-brace is a way to make the car better for seating for tall drivers; it allows the seats to be moved back more than the X-brace. If you do the V-brace, you need to add a horzontal bar across the hoop at the top of the v. Also, the top of the V should not be too low; otherwise, it is a worthless brace. I would not place the tops of the V-bars any lower than 60% of the height of the floor to the top of the main hoop. Also, make the V out of one piece of tubing, tied to a decently large floor plate.
With the V brace, it is important to use use an X-brace in the backstays, and add the diagonals between the main hoop and the tubes over the doors (beside your head), to brace the upper part of the main hoop. Frontal impacts that get transmitted through the door bars to the lower portion of the main hoop can cause the top of the main hoop to bend forward (actually, less backwards). These braces help prevent this bending of the main hoop.
Note that the FIA rules have moved away from allowing the V-brace, which was pretty common uin Euro rally cars some years back.
The alternative for room for tall drivers is to move the main hoop and X-brace further back in the car, behind the B-pillar. This is OK unless you get carried away; if too far back, then the length of the tubes over the door openings gets long and more susceptible to bending, and the backstays get more and more vertical and begin to lose their effectiveness in bracing the whole cage in a fore-aft direction. So don't move the main hoop too far back. Also, when you move the main hoop behind the B-pillar, you have to start becoming concerned about the B-pillar being crunched in upon your upper body; hence the diagonals from the main hoop to the tubes over the door opeings are again important.
The new homologated cages are solving this seat room in a different way, but it is not as simple to fabricate, and thus may not find its way into the rules intended for local shop fabrication of cages.
KEY POINT: All of this says that at some point we all need to stop trying to design the perfect cage on paper, and instead get racing seats early on the process and find where the seats will be located for you in your car. Once you have seat locations established, then you know what you need to do to provide room for the seat backs and THEN can do a final selection on the main hoop location and bracing style.
Regards,
Mark B.
