I have little experience with these things, either personally or studying.
From the video it looks to be too high frequency to be dependent on rider actions or reflexes.
And it also looks like, but you'd really have to look closely to see, that the bicycle has substantial deformation. So it looks to me like some kind of instability involving frame deformation, rolling contact, the mass of the bike and the passive mechanical impedences of the rider at the hands.
That is vague and general enough to be a pretty empty theory.
— Andy Ruina, Mechanical Engineering, Cornell
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Bike crash
A. J. R. Doyle,
bicycle.tudelft.nl/schwab/bicycle/doyle1987.skill.pdf
It is obviously impossible to make an informed comment with only that video as a guide, however, the following might be of interest.
As a young man I had an ‘interesting’ crash on a bike. I had never given any serious thought about how a bike worked and, as the young are inclined to do, just set off full of hope and excitement. I went into a turn that gradually tightened up. I was going too fast for my level of knowledge and skill and realized as I entered the start of the bend that I needed to turn tighter. As everyone knows, to increase the angle of lean one has to make a brief front wheel turn in the wrong direction to get the roll into the lean started. To my surprise and concern I felt the steering go suddenly very light and beyond that angle of lean the control movements I was used to were somehow working differently. Just as I was worrying about that I became aware that the radius of turn I had selected was no longer matching the radius of the turn of the road and I was going to end up hitting the far curb unless I tightened the turn. However in order to tighten the turn I needed more lean and that meant slacking off a turn that was already too slack. Yes! I hit the curb.
There are a number of relevant observations:
1. The maximum rate of turn for a bike is limited by the angle of lean. When the tyres slip you have exceeded the limit.
2. Any turn requires a force at right angles to the direction of travel towards the centre of the turn. The tighter the turn and the faster the speed the greater this force must be. The action of the tyres on the road produce this force. Both front and back tyres contribute. Any slackening of this force leads immediately to an increase in the radius of turn.
3. The force towards the centre of the turn forms a couple about the centre of gravity in the vertical plane tending to rotate the rider back to the upright position. This couple must be balanced exactly by the angle of lean. At extreme angles of lean changing the angle becomes awkward. To increase the angle of lean, the radius of turn must be briefly increased, which may be enough to put the rider into the curb. To decrease the turn the handle bar angle must be momentarily increased to provide the extra force needed to start the recovery from the lean. There are two problems here. The first is that in a really tight situation this may exceed the adhesive limit for the front tyre. The second is that at high angles of lean any added steering angle starts to move the point of contact of the front wheel round to the front of the wheel and the action becomes as much one of lifting as steering.
4. Finally the lightning of the steering at high angles of lean. The front wheel geometry puts the contact patch of the front tyre behind the point where the steering axis meets the road. This ‘trail’ provides a degree of castoring tending to iron out random disturbances. As the angle of lean increases so the trail distance reduces. At some angle of lean, exactly where depends on the design detail, the trail distance reverses and the castoring effect now exaggerates disturbances instead of damping them. Up to that point the rider has been controlling the bike with pushes on the bars which balance out the castoring forces. To command a turn to the right he applies a torque to the steering by applying an angle independent force to the right handle bar. This overcomes the castoring torque and forces the bike to lean to the right. The more the bike leans to the right the stronger the castoring torque trying to restore upright running. The rider keeps on overpowering this torque until it is judged that the angle of lean is sufficient, at which point the push is reduced so that the two torques balance out and the angle of lean remains constant. Doing this makes the bike run in a curve but control is exercised round the angle of lean not the angle of progress. At high angles of lean the castoring couple gets less and less until at some critical point it starts working in the opposite direction. At the point where it is zero there is no longer any torque from the front wheel geometry and controlling the bike requires a similar skill to that used when riding very, very slowly, where the castoring couple is virtually nil. Beyond that zero point controlling the bike gets more like trying to ride the kind of trick bike where the front wheel has been changed to work in reverse. If all this happens just as you suddenly realize that you desperately need to turn tighter, loss of control is more or less inevitable.
Cheers,
Tony Doyle
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