roll bar vs. roll cage

guys,
Im putting the interior back together after having the cage installed in the dart and have a question on the Simpson 5 point harness. Included with the harness is an anti submarine belt that attaches to the floor. Do i have to use it, or is it optional?

Please put the submarine belt in. Its the belt that goes between your legs. I had a friend not use it in his 8 second race car and crashed. He was seriously injured and lost one of his ears in the accident due to him slipping down into the footwell.

Ken I am glad you made it out alive, thats a horrific accident.