Over 1m views- what caused this crash?

[Jaybird180]

WARNING: The rider isn't wearing gear and he goes down in a strange way.

The bike is gyrating and wildly trying to throw him off. Best I can tell is that he tightened up on the bars, making it worse. But still no cause...

[Lnewqban]

Note that his left hand (and possibly the right one) was not on the handlebar at the very beginning of the vid.

That allowed the steering to initiate a small oscillation, perhaps induced by the air turbulence of the truck and/or some wheel's unbalance and/or worn steering head's bearings.

The tail luggage had the bigger lever to influence the steering and you can see how loose it becomes by the end of the fall.

Note that his knees were not hugging the tank; hence, the loose masses of rider and luggage created a tip-of-a-whip effect, extending the oscillation's amplitude.

The oscillations of the steering and bike increased because resonance:

https://en.wikipedia.org/wiki/Resonance

Skin and pavement don't mix  

[Jaybird180]

I found some writups that indicates the bike did this oscillation off-on for several miles and the rider decided to continue rather than check it out. In aviation we call that "getthereitis" (pronounced: get-there-itis).

Assuming it's not a mechanical failure, would being on the gas or off help solve the oscillation- even if temporarily to allow the rider to dismount and have a look?

[Lnewqban]

Many years ago, I had a motorcycle that loved doing that.

I tried all possible tricks to stop the oscillation before becoming uncontrollable, being the most effective leaning the upper-body forward and clutching in.

Braking was bad and accelerating was scary, although more effective: the problems are 1) the calipers/pistons are affected by the front shaking (to the point of reducing front brake power) and 2) to keep fine throttle input under those conditions.

The causes are many, as well as the differences among bikes, but one good thing is not to remove the hands off the handlebar, especially while coasting or decelerating, with cross-wind or bad road surfaces or while carrying a heavy/bulky tail luggage.

Bonus for your natural curiosity:

https://en.wikipedia.org/wiki/Speed_wobble

 

[Jaybird180]

Wow- I do not feel good about the wobble weave video.

[Jaybird180]

 So here's the question: do wobbles, weaves and tank slappers not follow the general rule of throttle = stability? 

Is this an engineering flaw of that era bikes?

 I recall an interview I read with Tadao Baba, the project lead for the CBR series from Honda who said that steering dampers should not be required if the bike is designed properly. 

[Lnewqban]

On 7/7/2017 at 2:14 PM, Jaybird180 said:

 So here's the question: do wobbles, weaves and tank slappers not follow the general rule of throttle = stability?

I would say do not.

The general rule of throttle induces stability in pitch (remember the three axis of rotation crossing at the CG?), but this problem is in yaw.

Comparing to aviation, this problem is very similar in nature to flutter of control surfaces: a rapidly oscillating mass fed by the energy of movement.

Engineers design and test machines to operate at regimes far from resonance, in "normal" circumstances.

As profile and softness of tires and rigidity of frames and telescopic forks have improved during the years, the problem is less common.

Those are only two of the many factors at play, it still can happen to any normally stable motorcycle when some of the remaining factors get aligned in the wrong direction.

Example: Worn bearings of swing-arm + tire hitting stone or road groove + specific speed of the bike.

Copied from Chapter 11:  "The less of a "whipping back and forth" mass you become, the quicker the bike will stabilize."

You can read more about steering shaking, dampers, and wobbles in Chapter 8.