Re: Vibration Measurement

Joe Riel wrote: 
frkrygow@xxxxxxxxx writes:


Neither is, I'd think, instrumenting a bike with a rigid weight on the
saddle - but it's better.


I pondered this during my two hour ride this morning. Consider an
element, say a bike tube, attached at one end to an immovable object.
Any vibrations set up in the tube won't move the object, so an
accelerometer attached there would read 0. However, that doesn't
mean that nothing is happening to the object. A wave traveling
down the tube and reflecting back has to send twice the impulse
into the infinite mass, right? I'm an electrical engineer and used
to dealing with transmission lines, so assume that analogue occurs
in the mechanical case. 

To better understand this, consider a compression wave (presumably the
Zertz works on transverse waves rather than compression (longitudinal)
waves, but that's another issue) hitting the infinite mass as a series
of tennis balls bouncing off a brick wall.  The wall doesn't move.

technically, it does, but it's sufficiently massive that it's not apparent. an analogous an old trick in construction is to put a sledge hammer behind any piece of timber being nailed. the momentum difference it imparts allows the nails to be driven much further on each blow.

But it does "absorb" momentum.  The point being, measuring 0
acceleration at the seat doesn't indicate that the user won't feel
anything.

Joe


.

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