## What If? Rejects #10.2: Stunt Bike

So it turns out some people are actually crazy enough to try this.Previous post in this series: Knife Wounds

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Q: If I were on a motorbike and do a jump off a quarter pipe ramp, how fast would I need to be moving to safely deploy and land using the parachute?

Randall’s response: No response.

My response: If you mean just yourself and not the bike, possibly as low as 55 miles per hour, but only if you have the right parachute.

WARNING: Do not try this at home!

The real question here is, how high do you need to get to successfully deploy a parachute? An ordinary skydiver’s parachute is rated to deploy at 2000 feet. The reserve chute is rated for 700 feet. However getting to both of these altitudes would require going faster than terminal velocity—with a rocket-powered motorbike, perhaps. Terminal velocity is about 120 miles per hour, and that’s before air resistance, so you’re probably well outside the operating capacity of most motorbikes.

However, military parachutes for paratroopers can be rated for as low as 250 feet. To reach this altitude, you don’t need to get up to terminal velocity. In fact, you can calculate your takeoff speed using the simple equation:

V = SQRT(2gh)

Here, V is your takeoff speed, h is your maximum height, and g is the acceleration due to gravity of 32 feet per second squared. (I’m using feet because most of my readers will probably understand motorbike speeds best in miles per hour. As an astrophysicist, the correct figure is obviously 980 cm/s2.) Plug 250 feet into this equation, and you get a takeoff speed of 86 miles per hour. Granted, that’s without air resistance, but you’re also not going to be leaving the quarter pipe ramp from zero altitude, especially if it’s big enough to drive a motorbike on, so let’s assume it balances out.

But even this is not the limit. BASE jumpers routinely make jumps from less than 250 feet. In fact, some have jumped from as low as 100 feet. From that height, you don’t even need a parachute. You only need a swimming pool, since the official high-diving record is 172 feet (although dives from that height invariably result in broken bones).

To reach an altitude of 100 feet, you only need a takeoff speed of 55 miles per hour. You’ll probably want to make it 60 for safety (or more…a lot more), but in theory, if you take off with a speed of 55 miles per hour, and if you dismount from the bike, and if you have perfect timing, you could deploy a BASE jumping parachute at the top of your arc and land safely.

But note the video above: no one has actually jumped high enough to do this. Also, try not to land on the crashed-and-burned motorbike.