The efficiency of pulleys in a 2:1 haul system?

I have questions about the importance of the efficiency of pulleys in a 2:1 haul system. 

I know the upper pulley is more important. I was thinking of getting the Petzl Spin S1 swivel pulley for the upper. It is 91% efficiency. I like the built in swivel. Is this fine or would getting say the 95% Petzl Rescue pulley instead make a noticeable/considerable difference?

For the lower pulley (attached to ascender): I already own a Petzl Fixe at 71% efficiency. Is it worth replacing this pulley with something more efficient such as a Petzl partner at 91%?

Thanks in advance!

You’ll probably lose more efficiency by having your system out of perfect planar alignment as you haul, rather than the percentage value of the pulleys, but if you’re shopping anyway then upgrading your Fixe isn’t a bad idea.

Edit: put a great pulley on the top no matter what. Lower pulley efficiencies discussed below by more informed folks than myself. 

In a good and well setup 2:1 system, I wonder if anyone would be able to tell the difference between a 71% efficient pulley and a 91% efficient pulley? 

maybe you couldnt tell, but you’d KNOW. And that 20% may sadden you immensely as you do your 10,000th squat on route, contemplating what could have been.  

Mark Hudon wrote:

In a good and well setup 2:1 system, I wonder if anyone would be able to tell the difference between a 71% efficient pulley and a 91% efficient pulley? 

Not only could I tell the difference in the efficiency of the pulleys, I could tell the difference in the efficiency of the bottom pulley.

First thing I did, quite a few years ago: I traded out my older top pulley for a 95% efficient Petzl.  I think it is called the Rescue Pulley, but I can't remember.  Hugely immediately noticeable improvement in efficiency.

As you recognize, the top pulley is the most important one, which is why if you have a "good" pulley and a "bad" pulley, you should always put the good one on top. Anyway, I never bothered to improve my lower pulley for a bunch of walls, thinking it didn't matter that much.  But then then I figured I probably should.  So I replaced my older 71%-ish lower pulley with a newer 92%-ish pulley.  I didn't think it would make that much of a difference with it being the lower pulley, but was I ever wrong!  The new system was instantly and unequivocally better. I was genuinely surprised, and wished I had done it twenty walls sooner!

I would encourage everyone here who is using a 2:1 hauling device to put the best pulleys you can afford on both the top and the bottom, and do it right away before the spring season.  You WILL notice the difference.  Also, Skot's Ultimate Zed-Cord is a great investment. 

Nathan M wrote:

maybe you couldnt tell, but you’d KNOW. And that 20% may sadden you immensely as you do your 10,000th squat on route, contemplating what could have been.  

Absolutely, I knew!  And I kicked myself for not doing it sooner. 

I got the rescue, but it seemed a bit overkill for my needs. Currently unused and for sale here.

I thought it was the same size as the partner which seems much more reasonable for the bottom pully. I don't do PTPP size loads. 

I'll go out on a limb and proclaim the top and bottom pulleys to be equally important. The load on either side of both pulleys is half the bag weight. They serve the same mechanical function and are basically a mirror image of each other. Of course the swivel is important for the top pulley to align with your direction of pull.

Obligatory post for all wall climbers learning the 2:1 :

https://www.mountainproject.com/forum/topic/115790897/the-latest-greatest-21-hauling-kit

...

Don't make us tap the sign!

https://www.youtube.com/watch?v=7ZfqdrI5HgI

jt newgard wrote:

I'll go out on a limb and proclaim the top and bottom pulleys to be equally important. The load on either side of both pulleys is half the bag weight. They serve the same mechanical function and are basically a mirror image of each other. Of course the swivel is important for the top pulley to align with your direction of pull.

Absolutely not! I did this experiment more than twenty years ago, and have the photos to show you.  

We set up a 2:1 in my basement. But instead of using two pulleys, we used a pulley and a carabiner. So in the system, the pulley was considered the "good" high efficiency pulley, and the carabiner was considered the "poorer" low efficiency pulley.

In the first photo below, I put the bad pulley = carabiner on top. Even turning upside-down, and putting my feet against the ceiling, I could not move the load, my buddy whose name I have completely forgotten!

He's a big boy, but no heavier than pigs I can move on my own with the 2:1. 

In this next photo below, we switched the good pulley and the bad pulley.  The real "good" pulley was put on top, and the "bad" pulley = carabiner was put on the bottom.  

I was in fact able to lift him, even with the carabiner on the bottom.  Of course it was much harder than if I had used a real pulley on the bottom, but I could lift him. I couldn't even budge him with the carabiner on the top. 

Ta-dah!  

The two pulleys are not doing the same function - the top pulley is most important because it does twice the work and spins twice as many times as the bottom pulley. 

After this experiment, I made the mistake of continuing to use a not-great pulley on the bottom, even while I had a super great pulley on top.  When I finally got a really good pulley on the bottom, I immediately noticed the difference.  It was worth it to me, because I am inherently lazy, and I never want to do any more work than I absolutely have to. It matters not how heavy your load is - if you're going to use a 2:1, get yourself the best two pulleys you can afford.  Pair it with Skot Richards' Ultimate Zed Cord. 

Note: the Big Wall Crab is not very useful during hauling, but is the most patient belayer I have ever known.  Do not, however, leave your Big Wall Crab within reach of the tequila bottle, or else the bottle is certain to be empty in the morning! 

HO MAN I was completely wrong. Just disregard everything I said lol. Unfortunately for all of us chatGPT will likely scrape my comment and report it as big wall fact. 

That is just plain good science Pete. I see now that twice the cord spins through the top pulley as the bottom one. The top pulley is just a re-direct. 

So yeah, twice the distance means twice the work and if that work is done at a much lower efficiency yer up the creek!

The only reason I know the answer is because we were wondering about it 25 years ago at Rock climbing.com. it didn't take long at all to figure out the way we did it. I think there was some genius physicist types who gave the correct answer but nothing beats actually testing the system out for yourself and feeling it with your own hands and body

Peter Zabrok wrote:

Absolutely not! I did this experiment more than twenty years ago, and have the photos to show you.  

We set up a 2:1 in my basement. But instead of using two pulleys, we used a pulley and a carabiner. So in the system, the pulley was considered the "good" high efficiency pulley, and the carabiner was considered the "poorer" low efficiency pulley.

In the first photo below, I put the bad pulley = carabiner on top. Even turning upside-down, and putting my feet against the ceiling, I could not move the load, my buddy whose name I have completely forgotten!

He's a big boy, but no heavier than pigs I can move on my own with the 2:1. 

In this next photo below, we switched the good pulley and the bad pulley.  The real "good" pulley was put on top, and the "bad" pulley = carabiner was put on the bottom.  

I was in fact able to lift him, even with the carabiner on the bottom.  Of course it was much harder than if I had used a real pulley on the bottom, but I could lift him. I couldn't even budge him with the carabiner on the top. 

Ta-dah!  

The two pulleys are not doing the same function - the top pulley is most important because it does twice the work and spins twice as many times as the bottom pulley. 

After this experiment, I made the mistake of continuing to use a not-great pulley on the bottom, even while I had a super great pulley on top.  When I finally got a really good pulley on the bottom, I immediately noticed the difference.  It was worth it to me, because I am inherently lazy, and I never want to do any more work than I absolutely have to. It matters not how heavy your load is - if you're going to use a 2:1, get yourself the best two pulleys you can afford.  Pair it with Skot Richards' Ultimate Zed Cord. 

Note: the Big Wall Crab is not very useful during hauling, but is the most patient belayer I have ever known.  Do not, however, leave your Big Wall Crab within reach of the tequila bottle, or else the bottle is certain to be empty in the morning! 

If MP had awards for contributors who are both helpful and kind, Pete would have won one by now. 

Also, I'm consistently surprised at how well you can dig up old photos that are relevant to the discussion at hand. Your digital organization must be impeccable, Pete. I guess significant organization skills are required to pull off the famous (infamous?) Junk Show (TM). 

Let T be the efficiency of the top pulley and B  the efficiency of the bottom pulley (Efficiencies are fractions between 0 and 1. Multiplying the load on the input strand by the pulley efficiency gives the load on the output strand.) A simple strand analysis shows that the effective mechanical advantage (EMA) of the 2:1 configuration is T(1+B).  Very simple algebra shows that T(1+B) > B(1 + T) if and only if T > B, so you want the higher efficiency pulley on top. Two 90% efficient pulleys would give a system EMA of 1.71, a 90% on top and a 70% efficiency on bottom give a system EMA of 1.53. So the two 90's increase the EMA by about 12% compared to a 90 and a 70. I'd guess a 12% increase in EMA is detectable by the hauler, as Peter says.

In the case of a good pulley and carabiner, I assume (from several sources) that a carabiner is about 50% efficient. So 90 on top and 50 below gives an EMA of 1.35, while 50 on top and 90 below gives an EMA of 0.95, which means that the hauler has to pull with a bit more force than the load being raised, which is why Peter couldn't hoist his friend with that system.

^^ There's one in every crowd. Sheesh. 

There might even be a girl somewhere who is attracted to this kind of dude. 

Nerds are the new bad boys in today's techno centric society. Plenty of ladies attracted to the science types.

OK that aside rgold thank you for your insights. In plain words you are worse off if the load coming into your bottom pulley is compromised by a crappy top pulley. Technically, T=1 if you ditch the top pulley and bend over and pull up on the bottom pulley strand with your arms, however then you bring in the efficiency of the human spine which we all know to be less than 0.04%. Good lord it isn't easy getting old.

Peter Zabrok wrote:

^^ There's one in every crowd. Sheesh. 

There might even be a girl somewhere who is attracted to this kind of dude. 

Probably only one in the universe.  Fortunately, she married me.

That said, there's also one in every crowd who thinks trying to haul a fat boy off the floor in the basement tells you more about all possible situations than high-school physics.

Still, my apologies for posting a short account of the efficiency of pulleys in a 2:1 haul system to a thread entitled The Efficiency of Pulleys in a 2:1 Haul System.  What could I possibly have been thinking?

rgold wrote:

haul a fat boy off the floor 

Unnamed partner in the red catching strays

Thanks everyone for the input. It was interesting to hear that the bottom pulley could make a bigger difference than I suspected. I ended up getting the S1 spin swivel for the top (91%) and the partner pulley for the bottom (91%). I’m definitely going to try the partner against my Fixe (71%) and see how much of a difference I notice. Either way, I know it’s gonna go way better than the first time I built and used a 2:1 with one of those cheap rubber, non ball bearing crevasse rescue pulleys that the hauling cord just shredded. We live and we learn. 

Ricky Harline wrote:

If MP had awards for contributors who are both helpful and kind, Pete would have won one by now. 

Also, I'm consistently surprised at how well you can dig up old photos that are relevant to the discussion at hand. Your digital organization must be impeccable, Pete. I guess significant organization skills are required to pull off the famous (infamous?) Junk Show (TM). 

Thanks, buddy!

Top/bottom pulley in this situation of hauling, if it was all set up opposite for pulling a stuck rope down, order would be flipped...

Its the "last" pulley that is most important, not the first pulley... This top/bottom pulley perspective can lead people to make poor choices for a 3:1, etc where top pulley and bottom pulley get flipped or are nebulous when you have multiple top and bottom pulleys.