Here are some pics of the wheelie bars I made, not saying saying this is how to do it, just that it's how I did it. 2 upper bars, one lower, all 1'' .120 MS DOM, 1/2" bolts at spherical rod end attachment points, left-hand / right-hand threads on the uppers to allow lateral adjustment, double shear everywhere. Two machined side plates at the wheel end, spaced apart by the caster, an aluminium spacer, the upper tube rod ends, (and spacers) and lower tube attachment / spacer blocks flat on the sides and contoured to fit the diameter of the lower bar, which is sandwiched between them, with two 3/8" bolts thru all. Not clearly visible in pics, but the rear side plates have two attachment point options for the upper tubes. Rod ends at both ends of the upper tubes loads them purely in compression, unless a crash situation whacks them sideways or elsewhere. The rear tube upper attachment as shown has the upper tube centerlines passing somewhat above the wheel centerline, the effect is to bow the single lower bar upward when the wheel hits the track. The lower mounting hole for the rear of the upper bars makes the upper tube centerlines pass thru the wheel centerline only slightly above it, so the bow effect is less if the wheel is loaded upward, but it's still somewhat springy.
I asked a friend of mine who has some involvement with fuel cars and such, what he thought about the design thinking, he said I was overthinking the thing, (I heard that somewhere recently). The whole setup is certainly more clunky and heavy than what fast cars are and have been running, but it was fun to build. And it certainly resists wheelies in a smooth way.
On the safety side, I completely agree that two upper bars have to be better than one, in the event of backing the thing directly into a wall or something. No one can predict every possible angle of impact, etc., but I think the top-heavy triangle mounting gives me the best chance of not having tubes through my torso or up my... well, ya know...
