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Had a strange discussion in the coffeeroom today.

The background was a story about a guy that took his motorcycle to transport himself to the airstrip to do a skydive.

Since there is no place to put a parachute on the motorbike, he put the parachute on his back (since it is like a backpack).

And apparently the parachute almost opened during the drive, but he was lucky and nothing happened.

Now we come to the actual question, what would happen if you ride a motorbike and the parachute opens? Is it really that different from a skydive?

Let's compare the two cases.

The first case, the skydive, I guess free-fall speed is around 150-200km/h, and then you open the parachute and the speed drops and eventually speed is so low that you can survive the touchdown. And all the motion is vertical.

In the second case, the motorbike, we can have the driver going at the same speed 150-200km/h. And we assume that he travels on a big open space, with flat tarmac, so there is nothing that can come in his way (like trees, or other cars). Also he is wearing full protective clothing, like a proper race driver. In this case all the motion is horizontal.

What happens when the parachute opens?

Will the biker die? or will he glide on the ground until friction and the parachute stops him?

Is there really any difference if you are going vertically or horizontally? Except for the proximity to the ground?

Figure with biker and skydiver

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The Parks College Parachute Research Group did some really interesting testing in the late 1990s about the opening profiles of various parachutes. One interesting fact (as an aside) is that smaller chutes actually decelerate you with up to 40% more force. They also published a series of force-load profiles during opening, examples of which can be seen below.

Parks College Testing

Tracing out the curves for the medium chute, which has the best release profile for our rider, we see something that can be simplified to:

Simplified force curves

Some assumptions:

  • The rider can hold onto the bike until there is a 1g rearward force. A really quick sportbike can accelerate at up to 1.1g, so it is reasonable to assume that a practiced rider can hang on at that kind of rearward force
  • There is no upward component of force from the parachute; it is only straight back. This is a weaker assumption, as I’d assume that given the the airflow profile off the back of the bike, the chute would lift up somewhat, but it’s easier to assume it doesn’t
  • The rider takes 0.45s to fall from 1m in height, i.e. his fall is not slowed by an air resistance. Again, not the best, but it will give us the worst case scenario.
  • The rider is going 120mph (he’s on the autobahn)

In any case, by integrating the force curve over time, we get the delta-p of the rider; we can then divide by (for the first .54s) the rider+bike mass and for the last .45s the rider mass to get his delta-v.

Drum roll please….

If he pops the chute at 120mph, he hits the ground going 82.35mph. So it helps! But probably not enough.

The saving grace / caveat is that if there is any upward component, it seems like the chute will work.

On the downside, if we assume that the parachute force is, like most fluid drag forces, a function of v^2, then at highway speeds of 60mph, the parachute will provide ~1/4 the force as at 120mph.

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Air friction is the only thing really acting on the chute. Sure there is some gravity, but it should act basically the same vertically / horizontally. Think dragster at the end of the strip. The chute will put the brakes on the rider, not the bike. So chute opens, biker ripped from machine, biker stops with a few bruises, bike keeps going...

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  • $\begingroup$ I was thinking along the same line. $\endgroup$
    – Johan
    Commented Mar 20, 2014 at 10:23
  • $\begingroup$ Depending on how fast the chute opens (it's not instantaneous), and how much force it takes to yank the biker off, he may well get more than a few bruises. $\endgroup$ Commented Sep 28, 2015 at 17:35
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If you assume that the biker is 1m above the ground on his motorbike, and he opens the parachute, then regardless of his speed he will fall to the ground in the same amount of time that it takes to drop an object from 1m. Simple mathematics shows that it takes about 0.45s to hit the ground, a time not enough to reduce the biker's speed to a safe level.

Most likely the guy will die.

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  • $\begingroup$ Please clarify what force will kill him? It is not lethal to free-fall 1m, and it is not lethal to glide on the tarmac at 200km/h. $\endgroup$
    – Johan
    Commented Mar 20, 2014 at 16:21
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    $\begingroup$ @Johan "it is not lethal to glide on the tarmac at 200km/h" - citation very much needed. $\endgroup$
    – user10851
    Commented Sep 4, 2014 at 12:11
  • $\begingroup$ All energy is still there in form of sideways movement, and will leave in form of friction (heat) towards the surface (and air with the parachute) when the biker glides. Look at this video, as long as they glide, nothing happens, it is when they finally hit the side of the road that the concrete barrier that sideways force has to become something else (and may hurt the biker) youtube.com/watch?v=4WmjeeZ44B8. But in my example there is no barrier to hit, so "my biker" would continue to glide until the parachute has absorbed all energy, and that should not be harmful. $\endgroup$
    – Johan
    Commented Oct 6, 2015 at 9:56
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If the rider can manage to hold onto the bike and crouch as low as possible, he might be able to avoid getting ripped off the bike backwards, or have the bike flip over backwards. If he can stay on the bike and the bike stay upright, then there should be no harm at all. He and the bike will slow down due to the backwards force exerted by the parachute.

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