He didn’t defy gravity — I’m sure it was there the whole time.
If stuntman Steve Truglia had been too timid in his acceleration, his yellow Toyota would have reached the top of the track and dropped like a stone.
Not quite. If his speed was insufficient, he would not have reached the top. But the car would have dropped like a stone.
The Toyota had to be travelling fast enough that the centripetal force generated by its circular motion ‘offset’ the downward pull of gravity. This required the stuntman to enter the loop at exactly 37mph, immediately change out of gear and slow to 16mph as the vehicle swung round the top.
Well, no. The centripetal force is the gravitational force in the limit of the slowest speed that allows you to complete the loop, and the speed will naturally decrease as kinetic energy is converted to potential energy. Since the loop is 40 ft tall, we can actually calculate this. An object entering the loop and rising 40 feet to be traveling at 16 mph must be going 38 mph as it enters. The article says 37, but car is a little off the ground, so the actual change in potential energy is smaller. (The actual change in height is 37.4 feet using those numbers, putting the CoM a little over a foot off the ground. Close enough)
The downshifting isn’t there to slow the car down — the only thing the engine needs to do is compensate for losses. The downshifting is because the car will slow down, and you don’t want it to stall as the result of being in the wrong gear. An ideal car (of which a Toyota does not qualify) could simply coast after entering the loop. It’s entirely possible to enter the loop at a slower speed, but have the engine make up the additional energy needed while in the loop, but that would not have been the safe move from the he-doesn’t-so-much-loop-as-plummet angle .
And, from a physics point of view, he could have gone faster. 16 mph gets you about 1g of downward acceleration, i.e. you are basically in freefall under that scenario. The numbers don’t quite jibe — even when I use the smaller radius from above, the acceleration is a little lower than 1g. So undoubtedly some rounding went into the story already. Going faster would just mean that the track was exerting some force on him while at the top.
As far as the danger of blacking out, that’s why he wanted to be going near the minimum speed at the top, because near the bottom is where he would pull the most g’s — about 5 of them, at that speed, assuming the track is circular and not flattened to reduce the force.