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Calculate inertia from rope pull test

3 Posts
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Posts: 21
Topic starter


I hope someone could help me.

I'm trying to calculate the inertia of my dyno.

I have setup a rope pull test but am struggling to get any meaningful results.

My rope is wrapped round my roller which is 508mm diameter

My sensor uses 153 pulses per revolution 

I have a 2.5kg weight placed in  a bucket to which I have already added enough water to move against the friction of the system.

A have made multiple dyno runs with this setup falling from a pully on the ceiling which I have added here in a zipped folder with the raw log files. 

Is it possible to calculate inertia with any accuracy from these runs?

Any help would be much appreciated  


Posted : 27/04/2022 3:14 pm
Posts: 21
Topic starter


Thanks for the reply.

I have used that spreadsheet already.

The problem that I have is getting the accuracy, just a small variation in the timing can give a large difference in the calculated inertia. 

I have tried filming the drum rotation with an iPhone on 240fps but if you do 10 attempts you get 10 different answers.

There must be a way to get a more repeatable test or something that I am doing incorrectly?

Posted : 27/04/2022 11:07 pm
Posts: 21
Topic starter

I've spent all day trying to get a handle on this.

I've removed as many uncertainties as I can.

I have calculated the inertia from the dimensions of the drum to give me an idea what the results should be from the rope pull test.

Length of drop 1596mm

Weight 2575g

Diameter that rope is wrapped round 508mm

Time of drop 5.72 (from an average of 10 tests)

This results in a MOI 16.54Kg/m² using the spreadsheet but the calculated MOI is 8.2Kg/m²

There is something wrong here and it seems strange that the spreadsheet produces a figure that is pretty much double what I am expecting to see

Posted : 29/04/2022 8:40 pm
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