Richie
It looks like you are building a motorcycle streamliner
If you have a rule book , you should look at rule 7.H.4
Roll Cage:
Shall completely surround the rider and shall be fitted in the rider’s compartment. Minimum diameter is 1-1/4 in. with .090 in. nominal wall
thickness, mechanical steel tubing. No galvanized pipe, black water pipe or threaded fittings are permitted. The design of the roll cage shall
incorporate the following features as a minimum: Two (2) roll bars, (one forward and one after the rider’s head), which shall be tied together and
capped with a steel plate .090 in. thick. The cap shall cover the upper 140 deg. of the rider’s head. The roll bar shall be braced with a tube of the
same dimensions on each side. Rider head movement shall be limited to no more than 2 in. to each side, top, or rear, with rider’s head in the
normal position. Roll cage padding meeting SFI specification 45.1 for round tube roll cage padding and SFI specification 45.3 for flat roll cage
padding is required in the vicinity of the driver’s helmet.
Forward movement: All motorcycle streamliners presented for inspection shall have an engineered and tested SFI
spec 38.1 type head and neck restraint system. All motorcycle streamliners shall have an engineered and tested SFI spec 38.1
type head and neck restraint system.
Lateral movement: Shall be constructed such that the helmet can not exit the outer plane of the roll cage,
You will note is states "steel tubing" so aluminum is out
The other thing you should consider is buying the correct roll cage padding and HANS type neck restraint, then working out if you fit in the cage
Also, I'm not keen the way the cross bar is made in sections. It think would be better to make the round section and weld the front to back bars to it.
G