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Post by jrblack on Nov 1, 2017 17:52:20 GMT -7
I'm very new to this game: just mounted my RPTC hangboard and installed two eyebolts with Petzl Fixe pulleys (https://www.petzl.com/US/en/Sport/Pulleys/FIXE).
My 12 yr old son weighs 102.4 lbs fully clothed and wearing a harness. I put 90 lbs on the other side, leaving him with a 12.4 lb net weight... making his hangs pretty dang easy!
But he discovered he could just hover in mid-air: the 90 lbs seemed to outweigh him. I can only assume this is because of friction in the system?! The pulleys should run freely under this load, right (193 lbs total load).. they are rated to much much higher.
I'm worried about getting consistent statistics from this system once I start training. I don't much care about absolute statistics (for example, in order to compute the removed-weight-versus-bouldering-grade spreadsheet figures... not that important to me). But I do care about getting the same weight removed today and tomorrow, under the same setup. And I don't want it to be easy to "cheat" the rig (by doing a slight pull-up and then "settling into" the counterweight to take advantage of favorable friction).
Ideas? Suggestions?
The problem decreases with lighter loads. And I doubt I'll be putting more than 100lbs net load on the pulleys for my own workouts.
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Post by daustin on Nov 1, 2017 18:42:25 GMT -7
Are the pulleys new? I noticed this after a couple years, seems like my pulleys had gotten a bit gummed up with chalk, a couple squirts of WD40 did the trick.
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Post by jonfrisby on Nov 1, 2017 19:30:16 GMT -7
I keep a scale near my hangboard and weigh myself with weight attached. I use that number for resistance rather than bodyweight which fluctuates anyway.
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Post by MarkAnderson on Nov 1, 2017 19:54:44 GMT -7
Yes, there is friction in the system, which increases with load. In my estimation, the effective weight removed is pretty consistent between workouts. The biggest thing to consider is that if you are stepping up the resistance by some fixed quantity each workout (such as 5 lb per workout), it's probably not truly linear. In other words, in my estimation/experience, the difference between [Body Weight - 30 lb] and [Body Weight - 20 lb] is probably more than 10 lb. How much more is hard to guess, but easy to measure (as Jon suggested).
Incidentally, I thought about running some simple experiments and writing a blog post about this, but I figured very few people would care.
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Post by MarkAnderson on Nov 1, 2017 19:57:06 GMT -7
I keep a scale near my hangboard and weigh myself with weight attached. I use that number for resistance rather than bodyweight which fluctuates anyway. One question about this....
As the OP suggested, with my rig if you pull up and then sag into the counterweight, the pulleys take up more tension and the effective load on your arms diminishes noticeably. Do you experience this, and do you account for it in the way that you weigh your resistance? (and if so, how? I think it would be hard to do, but I've never tried it)
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Post by Chris W on Nov 1, 2017 19:57:50 GMT -7
I would be interested in it.
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Post by jrblack on Nov 1, 2017 21:18:46 GMT -7
Are the pulleys new? I noticed this after a couple years, seems like my pulleys had gotten a bit gummed up with chalk, a couple squirts of WD40 did the trick. They are brand new. And I thought about WD40, but the hangboard is in my bedroom and the stuff smells so bad...
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Post by jrblack on Nov 1, 2017 21:25:14 GMT -7
I keep a scale near my hangboard and weigh myself with weight attached. I use that number for resistance rather than bodyweight which fluctuates anyway. One question about this....
As the OP suggested, with my rig if you pull up and then sag into the counterweight, the pulleys take up more tension and the effective load on your arms diminishes noticeably. Do you experience this, and do you account for it in the way that you weigh your resistance? (and if so, how? I think it would be hard to do, but I've never tried it)
Mark, I think it would be interesting to see what the differential is between an "honest" hang and one where you "sag" into the counterweight, measured across various counterweights. I suspect it worsens as counterweight increases and as spacing between pulleys increases. As a corollary I suspect using a single pulley induces far less friction than using two. (Though I love your two-pulley setup that gets the weights out of your way for doing the hangs!)
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Post by scojo on Nov 1, 2017 22:42:23 GMT -7
Are the pulleys new? I noticed this after a couple years, seems like my pulleys had gotten a bit gummed up with chalk, a couple squirts of WD40 did the trick. They are brand new. And I thought about WD40, but the hangboard is in my bedroom and the stuff smells so bad... If they're new, my guess is WD40 won't help at all. The amount of friction you noticed seems pretty standard for a pulley system setup. I don't think variation in the friction would be significant from workout to workout, so I wouldn't worry about it.
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Post by aikibujin on Nov 2, 2017 6:05:33 GMT -7
I'm very new to this game: just mounted my RPTC hangboard and installed two eyebolts with Petzl Fixe pulleys (https://www.petzl.com/US/en/Sport/Pulleys/FIXE). On Petzl's web site, that pulley is listed with a 71% efficiency, which is pretty bad for a pulley. You can get much higher efficiency pulleys (95% for the pulley linked below) but honestly that's overkill for hangboarding. As long as you're getting consistent friction that's probably good enough for training purposes. Up till last month I was using some cheapo pulleys from Home Depot and they worked fine. www.petzl.com/US/en/Sport/Pulleys/RESCUE
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Post by climber511 on Nov 2, 2017 6:57:09 GMT -7
Pulleys have friction - which will vary with the pulley. There is also something called I think (if my memory serves me) "static hysteresis" which in English comes to a sort of "break out" force or the force it takes to overcome the friction and start movement
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Post by jonfrisby on Nov 2, 2017 7:44:38 GMT -7
I keep a scale near my hangboard and weigh myself with weight attached. I use that number for resistance rather than bodyweight which fluctuates anyway. One question about this....
As the OP suggested, with my rig if you pull up and then sag into the counterweight, the pulleys take up more tension and the effective load on your arms diminishes noticeably. Do you experience this, and do you account for it in the way that you weigh your resistance? (and if so, how? I think it would be hard to do, but I've never tried it)
OOPS - forgot about all the damn engineers on this forum!!! I set up my system so that I'm just lifting my feet off the ground rather than pulling and sagging into the hang. I think this mitigates the issue you're referring too, but don't have the technical expertise to really know. It's also only 10-40 pounds of weight removed. The closer the counterweight is to zero, the more accurate the numbers, in my experience.
For what it's worth, my measurements generally indicate that I lose 5-10% of the counterweight assistance, rather than gaining 10% as OP has experienced (so if I have 25lbs on the counterweight, my (BW - CW) = closer to BW - 20 or 22lbs. This is when standing on the scale, so once I'm in the air, I don't know whether resistance changes.
This whole conversation is easily solved by a Zlagboard if anyone's got one?
ETA: Mark/Aikibujin's points about consistency, are, IMO the most important factors.
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Post by MarkAnderson on Nov 2, 2017 8:12:45 GMT -7
OOPS - forgot about all the damn engineers on this forum!!! LOL For what it's worth, my measurements generally indicate that I lose 5-10% of the counterweight assistance, rather than gaining I bet if you pulled up and sagged you would find the opposite.
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Post by jrblack on Nov 3, 2017 8:31:15 GMT -7
So I just ran a very basic experiment with my setup (which is basically Mark's vanilla two-pulley setup).
Rig: two Fixe pulleys, 31" spacing, mounted overhead. No hangboard used whatsoever.
I weigh 170.2 lbs with clothes and harness. Using a digital scale under one of the pulleys, put 50 lbs (plus negligible weight for webbing and a biner) on the other side.
Rising up into the rig, scale showed: 137.6, 136.8 and 139.9 lbs (avg: 138.07 lbs). So the system effectively removed 32.13 lbs from me
Sagging down into the rig, scale showed: 100.0, 98.2, 95.4 lbs (avg: 97.87). So now the system removed 72.33 lbs from me.
The sagging down is pretty technique-dependent: you want to sag until just before the pulleys roll. Knowing where that point is requires getting used to it for a given system. But I worry that when you're tired while training, you might subconsciously start doing this and confuse your proficiency at cheating with actual strength gains.
Not sure how to best address this problem other than just trying to be diligently honest in your training. A 40 lb advantage on a hangboard is ridiculously huge.
John Black (Professor of Computer Science, College of Engineering, CU Boulder... but not really an "engineer!")
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RichF
New Member
Posts: 27
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Post by RichF on Nov 7, 2017 18:21:25 GMT -7
I've definitely noticed this effect before when I used the gym's pulleys. They usually have cheap hardware store pulleys. I have nice sealed ball bearing pulleys (~95% efficient). If I sag a bit on the gym pulleys, the weight drops off noticeably. I'd like to think that I can limit that by not sagging, but when I'm really pushing my limits I can't seem to help it.
Unfortunately you bought "sheave" pulleys, when what you really want to keep yourself honest are ball bearing pulleys.
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