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I was accelerating my bicycle around a corner and leaning hard to the left. I felt my rear wheel hop and looked back to see what I had just hit. What I saw was that the bicycle had rotated such that the wheel had flipped from leaning to the left to leaning horizontally to the right. The next moment I slammed into the sidewalk. From tire hop to crash happened in a second.

When I saw the rear wheel had flipped from left to right, my immediate reaction was "How did that possibly happen?" I then hit the sidewalk.

Since the accident, I've been trying to understand how the forces the bike and I were experiencing ended up rotating the bicycle such that it rotated some 120 degrees to the right. The 120 degree is a guess assuming I was initially leaning 30 degrees leftward and ended up horizontal with my head to the right.

I described the accident to a motorcycling friend the other day and he said "Sounds like a high side to me." Looking at some motorcycle videos I would agree except they appear to involve braking issues and I was accelerating when the accident happened. Both accidents entail the rear wheel losing contact.

So two questions:

  1. How do the force vectors end up rolling the bike up and over vertical when the rear wheel loses contact with the road? I would have expected the bike to slide such that my left side would hit the ground instead of the right side.

  2. Did dropping my head to peek under my left arm pit to see what happened affect the forces and help flip me to the right? i.e., would my chances of counteracting the flip have improved had I kept looking forward?

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    $\begingroup$ Would it possible to have a sketch added? $\endgroup$
    – Steeven
    Jan 17, 2020 at 18:12

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Here is how this happens. I'll begin with the example of a skidding car, which is analogous and easier to visualize.

When the rear wheels of a car lose traction in a (for example) sharp left turn, the rear end stops tracking the turn and instead wishes to continue on a path tangent to the turn curve at that point where the tires first broke loose. Since the rest of the car is still tracking left in the turn, the car rotates towards the left about its vertical axis and rear end swings out to the right. This swings the front end to the left, making the turn sharper, and as the front end tracks harder into the now-sharper turn, the rear end swings out more violently and the car "spins out".

On a bike, when the rear wheel lets go while the front wheel is still grabbing the road, the rear end begins to swing out like in the car example and the front end gets steered more steeply into a sharper turn. However, because the bike has only two wheels, something different happens:

The front wheel's tire contact point on the road is below the bike's center of mass, which wishes to continue in a straight line down the road. The friction force acting at the contact pad and the momentum vector of the center of mass thereby create a rolling moment which acts to tip the bike over to the right and the bike crashes over the high side.

Excellent high-speed videos of both high-side and low-side bike crashes can be seen on youtube, posted there by Rnickymouse.

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  • $\begingroup$ From your answer I surmise that tucking my head to see what happened didn’t materially affect the roll $\endgroup$
    – Michael
    Jan 18, 2020 at 20:10
  • $\begingroup$ probably not; the mechanism was in play the moment the rear wheel let go. -NN $\endgroup$ Jan 18, 2020 at 20:45

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