I think the roll cage looks great and I'm actually silly enough that it's a possibility I might try one LOL!
Gotta remember that unlike the Miata race car you linked, the Micra already has a roof and pillars. On top of that, consider that the car is light, it's a one make series, low power and it's run on proper race tracks...I believe the bolt in cage will be plenty. For street use, it would be better if it were all welded construction for obvious reasons. Might not stop a nutter like me though!
Ten Grand Tin Can
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View my fuel log 2015 Micra S manual: 5.3 L/100 km ... 53.2 mpg (Imp) ... 18.8 km/L ... 44.3 mpg (US) ...