[nxracer]

Actually it's no shoulder harness without a cage.

The purpose of a sub belt is two fold. (B is the important one)
A) keeps one from submarining out of the belts.
B) keeps the lap belt properly positioned. Without a sub belt pulling down on the shoulder harness pulls the lap belt up into the stomach area and thats a very dangerous place.

I use a double sub (6 point harness) and it is OK to sit on your subs

 

[nxracer]

Actually mine are still on the front seat bolts BUT are tight enough to keep the lap belt down where it belongs