Hauling systems, free body diagrams, forces--help!

Lock this thread. Once the t test come out it’s all downhill…

Mark Pilate wrote:

It’s empirical data.  Agree that Theoretically, they should be the same. But every time I rig it on my own “drop tower” or basement “lab” using same microtrax, same pulleys, same rope, same force gauges, etc….all hanging from same master point. the C on top of the Z always uses a few less pounds of force to haul up the same weight vs stacking a Z onto a C. Try it.

 I verified it by repeating it just now.   A 100 lb load needed an avg of 26 lbs of pull vs an avg of 28.5 lbs of pull (5x pulls each).  About 9% difference 

I haven’t really tried to micro analyze where the Delta is coming from. 

Hey Mark

Some possibilites might be that the differences are due to 1) different ropes lenghts/stretch 2) pulleys moving at different speeds/friction.

Bruno Schull wrote:

Hey Mark

Some possibilites might be that the differences are due to 1) different ropes lenghts/stretch 2) pulleys moving at different speeds/friction.

Yep, it’s almost certainly the pulley friction.  In the drop C, there is more weight on that moving pulley/biner increasing friction, vs the lighter loads on the moving pulleys/biners with the C up on top.  

I’ve also found that the delta between the two increases as the load increases.  Thus I predict Kyle tested with 50lbs or less to get 3%.   If you’re hauling over 200lbs of gear and victim, the difference may be decently large   (Another factor can also be due to instrumentation accuracy/scale coming more into “focus” or sweet spot as the weights vary)

But, all this is assuming straight up against gravity only.   Now throw edge friction in, and it’s a wash or maybe goes the other way per Coppilillo.

I agree that in the end, and with all the variables at play, they are “about the same” when all is said and done.  Parsing the fine points are merely for armchair geekery and amusement.  

Thanks for that Mark--your words have a way of most always making sense :)

Mark Pilate wrote:

Yep, it’s almost certainly the pulley friction.  In the drop C, there is more weight on that moving pulley/biner increasing friction, vs the lighter loads on the moving pulleys/biners with the C up on top.  

I didn't use any pulleys and used all the same carabiner (Camp Photon) for the sake of keeping the test simple and repeatable.  I agree that the results will vary slightly depending on how many pulleys you use and where you use them, perhaps that explains the difference.

Thus I predict Kyle tested with 50lbs or less to get 3%.

Incorrect.

Parsing the fine points are merely for armchair geekery and amusement.  

Agreed.  Other folks who don't find this interesting could always just not open the thread, right?

Ok, this is just for fun and curiosity, but what are better reaons?

I made two models, one for a C drop with a Z drag added (I'll call this a C + Z), and one for a Z drag with a C loop added (I'll call this a Z + C).

• If you do not consider edge friction, the MA and the force on the anchor are the same for each.  
• If you do consider edge friction, the MA and the force on the anchor are higher for the Z + C, as discussed upthread.
• If you change the efficiency of the individual parts, the two systems show varying behavior depending on which pulley you change. For example, if you change the pulley closest to the haul strand from 1 to 0.5 % efficiency, the MA changes from 6 to 4 for the C + Z, and from 6 to 4.5 for the Z + C.  The same difference is seen when you change the efficiency of the pulley farthest from the haul strand, but the effects are reversed (On this case the C + Z changes less than the Z + C).
• Changing the efficiency of the progress capture device changes the MA and the force on the anchor, but these changes are consistent between the two systems.  
• The systems require different amounts of rope, the individual strands of varying length will stretch by different amounts, the ropes will be entering the pulleys from different angles, the pulleys will be moving at different speeds, both rotating and relative to each other, and so on.  These kinds of differences likely contribute to differences in overall efficiency, but you'd have to test both to tell (as Mike did). 

Take away lessons from all this--my goto crevasse rescue system remains the C + Z with the Z + C as an option when less rope is available.  Stopper knots on the main line.  Looking at some numbers makes me even more convinced than before that carrying real pulleys and a microtraxion on glaciers is a good idea, and I'm going to keep exploring weird options, like having one main rope with knots between climbers, and each climber prepared with a length of thin, light, low-friction, low-stretch static line.  

Thanks for the ideas folks.