Unfortunately, I'm going to use formulas in this (but only the really basic ones).

As a bonus, we'll explore the natural frequencies of an esk8.

Second post in this project, catch up the first here.


Background story

Your board is doing the wobbling - kind of swaying back and forth around the length axis. This wobbling may be unpleasant.

Now, you can tighten up the bushings (or do other stuff) - stiffen the structure in this way. The wobbling will go away or at least be somewhat tolerable.

This also makes the board feel every bump along the way with the stiff bushings.


What if you could add something to wobble instead of the board?

The theory says you can.


The basic theory

Adding the damper to an existing board means something like this:

Annotation 2019-12-03 225628
Annotation 2019-12-03 225629
Extremely schematic board
Extremely schematic board

The goal is to reduce the vibrations (x) as much as possible. This means adjusting the mass and the stiffness of the damper so it will match the frequency of the wobbles:

Annotation 2019-12-04 213717

The big question

What is the frequency of the wobbles? You usually don't know.


But we can estimate:

    1. These are low frequency vibrations - we feel those the most.
    2. It's probably below the natural frequency of the board - otherwise it'll be too violent.


So, what's the natural frequency of the board?

A modal test

This is one of my all time favorite tests, now I've got to do it on my board.

You attach an accelerometer to the board, hit it with a hammer and check the forces and responses. The natural frequencies and natural modes can be calculated from the data.

My test setup
My test setup

The results from the test can be shown in a funny looking graph with peaks at the natural frequencies.

Annotation 2019-12-051

The lowest "wobbling mode" was recorded at 11Hz, as a bonus I've got the first bending of the deck at 38.1Hz

Wobble mode
Wobble mode

So, what now?

First of all, how cool is that we know the dynamics of the board?

Second, you'll have to adjust the damper to your liking, and probably stay below 11Hz.


The cool thing is you don't have to be precise to see an improvement, being around the wobble frequency is enough.

Stay tuned for the next post, when we take the numbers to the real world and make the damper.





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