how to use bicycle wheels as pulleys for an exercise device
- Thread starter johnlvs2run
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frenchyge said:
That sounds good. Thanks.
frenchyge said:
Would 4 pulleys with 90 degree turns would have the same resistance as 2 pulleys with 180 degree turns?
- - -
CMI has a bearing model of their pulley, which has a higher efficiency, albeit for double the price.
johnlvs2run said:
Ummm.... seems I misremembered that little fact from long, long ago. For a free-rotating wheel you can ignore the contact angle.
johnlvs2run said:
For rescue pulls where a string of multiple pulleys is required, a few percentage points make a decent difference compounded through the system, but personally, I think you could ignore pulley efficiency for a single-pulley setup. Nylon rope sliding over a carbiner is 50-70% efficient, depending on who you listen to, so I'd reckon that any decent diameter pulley on bearings is going to be in the 90's.
Some of those rope pulleys in the link that Felt Rider provided above look pretty good, but others seem pretty pricey again. What's your budget? Is this more of a science project or are you spec-ing equipment for patent and production?
frenchyge said:
What I'm wondering is how much force is lost based on efficiency, and how efficiency is determined.
I have sent an email to cmi-gear with this question.
Below $100 would be good. This is a personal project.
I had a nice conversation with an engineer at concept2 this afternoon, and might order one of their skiergs in the morning.
They are using only 1 inch plastic pulleys.
A plus is that I'll be able to take the top off, which means I could remove the 4 pulleys and attach 2 of the cmi pulleys to the rafters.
Where are you getting your fan (resistance) unit?
I was curious about motion this morning because I emulate a similar movement in the gym using a compound pulley system on the cable cross over.
I am a strength athlete using lower repetitions than you will use and I doubt I can go much over 60 pounds x 10 reps so the amount of resistance for that type of aerobic exercise must not be much downward force. I tried a quick Googling of typical downward force on ski poles, but did not come up with anything. But even with the pulleys on the gym equipment are not heavy duty. They are nylon with bearings and vinyl coated small gauge steel cables that are handling a weight stack up to 120 lbs.
I also experimented this morning with a really cheap option. I tried out a resistance band (rubber band) and threw it over the top of an elevated cross bar (chin up bar) and used the same motion. That seemed to work as well, but that particular band was too light. I could not locate the heaviest band in the gym, but I could tell if a person was in a pinch to get started training that would be a really cheap option and not much fabrication involved. It may not be exactly the same motion, but close.
I was curious about motion this morning because I emulate a similar movement in the gym using a compound pulley system on the cable cross over.
I am a strength athlete using lower repetitions than you will use and I doubt I can go much over 60 pounds x 10 reps so the amount of resistance for that type of aerobic exercise must not be much downward force. I tried a quick Googling of typical downward force on ski poles, but did not come up with anything. But even with the pulleys on the gym equipment are not heavy duty. They are nylon with bearings and vinyl coated small gauge steel cables that are handling a weight stack up to 120 lbs.
I also experimented this morning with a really cheap option. I tried out a resistance band (rubber band) and threw it over the top of an elevated cross bar (chin up bar) and used the same motion. That seemed to work as well, but that particular band was too light. I could not locate the heaviest band in the gym, but I could tell if a person was in a pinch to get started training that would be a really cheap option and not much fabrication involved. It may not be exactly the same motion, but close.
Felt_Rider said:
From concept2 or skierg (same company).
Below is a photo of their relatively new skierg. Also they have three protypes in the works.
Yes, the resistance is quite low. The fan and monitor are the same that they use for their rowing machine.
Interesting. What is the diameter of the pulleys?
That definitely works!
An advantage of either a skierg, or rowing machine conversion, is the concept2 performance monitor,
which gives instant feedback every stroke. Also there is a yearly ranking 10 distances on their site.
Yes, good point.
johnlvs2run said:
The link below is similar to what I was using this morning to check motion. With the multi pulley system the motion is nice and smooth through the whole range, but a single pulley would be just as nice. No need to get that complex.
The pulleys have plastic coverings, but I am sure they are about 4" dia. as I looked at them closely this morning.
[URL="http://us.commercial.lifefitness.com/content.cfm/dualadjustablepulley"]Link
[/URL]http://us.commercial.lifefitness.co...=resource&rid=645509&lid=430217&colrid=430214
johnlvs2run said:
I'm speculating from the CMI site and this, that pulley efficiency is force output divided by force input under some standard (unlisted) pull speed. On the CMI site the large double/multiple pulleys are shown with efficiencies in the 185 - 215% range which is what leads me to believe it's based upon force rather than work.
Funny that on the CMI link above (here), the RP101 is shown with no bearing and a 93.3% efficiency, while the RP102 has a bronze bushing and 91.4% efficiency. Both the models with sealed bearings are 95.6% efficient.
As Dave said above, for a workout device efficiency isn't terribly important. The machine readout can simply be set to back out the gross efficiency of the machine itself and display the actual output of the athlete.
Felt_Rider said:
Do the 4" pulleys have a snappy return?
The C2 guy was surprised when I said a larger pulley would be more efficient.
They are currently using 1" plastic pulleys.
frenchyge said:
CMI has not yet returned any answer to my questions.
The RP101 has a glass sheave, the RP102 has a bushing, the RP 103 and 104 have bearings.
The glass sheaves do not last as long.
Here is a link about pulley characteristics, and a link about pulley construction.
Why would pulley efficiency not be important?
For example a carbiner at 50-70% would seem to make a major difference to the force and power output (not sure how much) as compared to a pulley with 93.3% efficiency.
The monitors are not adjustable to account for any loss from the pulleys.
IIRC, the efficiency of a rope pulley system is related to the ratio of pulley diameter to cord diameter. IOW, those efficiency figures on the climbing pulley site assumes a 10 or 11mm diameter climbing rope. Run it with 4 or 5mm Perlon accessory cord and the pulley is potentially much more efficient. But the efficiency is also one of those diminishing returns sort of things, if you get 95% efficiency out of a certain pulley diameter with a given type of bearings it doesn't necessarily become 97 or 98% when you double the sheave diameter, at some point the bearings are the limiter and you'll never reach 100% efficiency.johnlvs2run said:
So it's possible that for all practical purposes their 1" plastic pulleys yield the same overall pulley loss as you'll see with the fancier pulleys or at least limit the losses to the point where it's not measurable on the rowing erg meter.
-Dave
daveryanwyoming said:
What effect does the cord diameter have?
They are using a 3mm cord, as they feel the heat buildup is less than with a larger one.
So a 4" pulley at 95% would have the same loss as a 1" pulley at 95%?
A reason for me looking into the pulleys is that the skierg has a 32 to 41 percent loss of watts as compared to the rowing machine, though both have the same fan and internal system.
The differences that could cause this are (1) the skierg being vertical, (2) the 4 pulleys, and (3) the different exercise motion.
Bend radius of the cord relative to diameter of the cord. Get your hands on a big ships hawser or gym climbing rope and bend it around a 1" pulley sheave and then try the same thing with a 5mm piece of accessory cord. Think about what the individual fibers in the cord are doing as they flex and straighten around the pulley sheave and then think about why the larger diameter cords heat up more...johnlvs2run said:
I'm not saying it definitely would or wouldn't, the point is that you can't just look at the sheave diameter in isolation as it depends on the cord you'll use it with and the bearings in the pulley. That and it's possible that the 4" is say 95% efficient, the 1" is only 90% efficient but the difference is below the display accuracy and resolution of the rowing ergometer. IOW, is the difference big enough to matter?
For example a Powertap's advertised accuracy is +/- 1.5%, at 300 watts that's approximately +/- 5 watts yet many folks using that tool try to pin their FTP down to the watt when the instrument isn't that accurate or hope to measure the power savings they gained on a local climb by switching out to some titanium bolts. Tough to do when the measurement instrument and overall test environment lacks both the precision and accuracy to measure differences that small. Same is likely true for your pulleys, easy enough to test if you had the erg load unit and all the pulleys to play with but a bit hard to predict without detailed and accurate specs for all the components.
Yeah, hard to say without doing some additional testing with the different components.
-Dave
Effect of cord diameter.
Dave, thanks much for your detailed explanations.
I've ordered a skierg today, which should be here in 8 to 12 days.
It turns out the internal workings of the skierg are different than the rowing machine, another possible cause of the huge difference in watts.
daveryanwyoming said:
Dave, thanks much for your detailed explanations.
I've ordered a skierg today, which should be here in 8 to 12 days.
It turns out the internal workings of the skierg are different than the rowing machine, another possible cause of the huge difference in watts.
Felt_Rider said:
I won't be satisfied with the skierg until the 32 to 41 percent loss of watts as compared to the rowing machine is resolved.
It was a little disconcerting this afternoon to see that the internal workings are different.
I am inquiring to C2 if they have verified the internal resistance is the same.
Provided the resistance is the same, there's a good chance that I'll see about bypassing the 4 pulleys with 2 of the climbing pulleys on the rafters.
Other than resistance, I would do this as I feel it's a better design.
In the case that the skierg will not be satisfactory, then I will return it (last resort) and get a rowing machine to convert.
johnlvs2run said:
Are you saying that you'd expect to see approximately equal capability to produce power in a downward poling motion as you would in a horizontal rowing motion? I'm no skier, but it seems to me that the leg extension plus lat pull would be a much more powerful motion than a downward rotating shoulder motion. Couldn't that explain the difference in wattage?
frenchyge said:
Yes.
Mostly I expect to see similar - or less (if the poling motion were more difficult) - resistance from the skierg as there is in the rowing machines.
The fastest rowers are tall, with long torsos, long arms, and comparatively short legs.
The skiing motion is primarily body weight and rotation of the hips.
There is much more shoulder rotation in rowing... so... no.Couldn't that explain the difference in wattage?
I agree that the difference in movement could be a factor, and included it in the 3 possibilities, now 4 with the internal differences.
However, differences due to body mechanics are not known.
The differences that are known are (1) the 4 pulleys, and (2) the internal workings.
The 2nd might be even greater than the 1st, as the rowing machines internal bungees go around pulleys at each end.
The skierg internal bungees are wrapped a dozen times round a spool.
It seems to me that every one of those would be adding resistance to the exercise.
All of this leads me to think a rowing machine conversion, with pulleys attached to the rafters, could very well work better than the skierg.
Due to lingering questions, now with the internal mechanics, I have cancelled the order for the skierg.
For best efficiency, get a bike wheel with a powertap hub and hang it from your garage rafters using a roof rack wheel holder like Dave linked to early on. Then wrap a cord/cable around the rim to your pull handles, and a bike chain around one of the small cogs to a heavy weight like the bumper of your car. As you pull the cord, the car will bounce up and down a few inches on the springs. The powertap will record second-by-second torque + power between the rim and the cogs, and let you download it all to a computer for your training journal, etc. Plus you can create other metrics to help 'optimize' your poling stroke like a snappy pull with high peak torque vs. a slower pull where torque is applied over a greater range of the stroke, left vs. right arm balance, different poling cadences, etc.
PM me for the address to send the royalty checks once this ski-training device takes off.
PM me for the address to send the royalty checks once this ski-training device takes off.
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