Agricultural ROPS (tractor roll bars or roll cages) often utilize a
hinge to allow orchard work, as getting under tree limbs is important.
These units use a hinge structure that is extremely strong. In fact,
the entire ROPS standard exceeds all the racing standards I know of by a
huge margin. So duplicating their hinge structure will certainly be
more then strong enough for a mere Spitfire.
A sliding tube roll bar could be made quite strongly utilizing ball
release pins. This is much like how typical sliding tube jack stands
position. You could also utilize aircraft grade press locks for an even
cleaner setup.
Do be aware that no racing sanctioning body that I am aware of will
sanction such an adjustable roll bar. SCCA and NHRA certainly will not.
You may even have problems at an SCCA autocross as a result of having
such a roll bar, depending on how sensitive the tech inspectors are.
When you build the cage or bar, don't neglect the distance you will
rise out of the seat when inverted. A bar that is only an inch above
your head will be inadequate, allowing the top several inches of your
brain to be scraped off in an inverted slide. Loss of several inches of
brain negates the perceived advantages of having a roll bar. Having
witnessed many flips rolls and slides and studied many more footages of
same, I can confidently state that under no conditions are you ever
going to be able to hunker down and hold on. You will flail around to
the maximum extent allowed by the harness, so design accordingly.
The GRMS roto-Spit has a modified convertible top designed to fit over
the higher rear hoop of a racing roll cage. The fabric is extended
several inches on the side. You can do a similar modification to your
Spitfire, eliminating the attempts at a raising roll bar. The
appearance is of a 70's era custom dune buggy top.
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