Rigging Physics Talk


A quick mini rigging physics talk.

So if I told you we were going to do a test on a 4:1CD MA system and a 6:1CD MA system, and we were going to raise a 100lb load and then do it with a 200lb load…..which would you choose as the system that would be the easiest for a person to haul up that load?

You should say the 6:1 if you are thinking what the Ideal/Theoretical MA is.

Let’s toss some friction in there and do a Practical MA test.


Rigging Physics Talk
Practical Test Setup

What we found was the 4:1CD outperformed the 6:1CD in both tests, meaning it took less effort to haul up the load. Now this was a very informal non-scientific test but you can see by the pictures that the sheave diameter in the 6:1 is almost a 1:1 ratio of rope diameter to the diameter of the sheave unlike the 4:1.

The 4:1 has high efficiency ball bearing sheaves and the 6:1 has plastic sheaves. Another issue was the friction of the lines rubbing on each other when hauling. Because the sheaves are so close to each other in the system any twist or movement caused the lines to rub. Friction is the enemy when hauling and our friend when lowering.

So something to think about…Ideal MA versus Practical MA.

Rigging Physics Talk
Twisting and rubbing of ropes adds friction to the system
Rigging Physics Talk
Sheave size in relation to rope diameter.