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Mechanical Efficiency



 
 
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  #11  
Old April 21st 17, 04:08 PM posted to rec.bicycles.tech
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Default Mechanical Efficiency

On Thursday, April 20, 2017 at 10:47:53 AM UTC-7, Jeff Liebermann wrote:
On Wed, 19 Apr 2017 12:10:33 -0700 (PDT), wrote:

I have been looking all over the Internet to find comparisons of
mechanical efficiency and haven't found anything that isn't
covered in different units of measure that require multiple
iterations of conversions.


https://en.wikipedia.org/wiki/Bicycle_drivetrain_systems

I also couldn't find anything that is instantly useful. There's
probably a reason why nobody has made such a general study. My
guess(tm) is it's because it wouldn't apply to most useful situations,
power levels, rotational speeds, and other design limitations. A
realistic comparison would include a range of acceptable power
transmission levels, weight limits, and possibly some operational
considerations. On a bicycle, the book
"Bicycling Science"
by David Gordon Wilson
2nd and 3rd editions (they're somewhat different)
http://www.alibris.com/Bicycling-Science-David-Gordon-Wilson/book/17828968
https://books.google.com/books?id=0JJo6DlF9iMC
is a good reference of work for such surveys. The section on power
transmission includes Berg cable drives, toothed belt drives, drive
shafts, etc.

Can anyone here compare the efficiency of:

A very long chain drive as used in a recumbent


Might be something in he
http://www.cyclingpowerlab.com/drivetrainefficiency.aspx
http://www.bikeradar.com/us/news/article/friction-facts-measuring-drivetrain-efficiency-35694/

A hydraulic drive system and


The weight penalty of dragging a plumbing nightmare around might make
this a losing proposition even if it were 100% efficient.

A electric motor


I'm going to assume you're only interested in a human powered
transmition, not building a battery or wind powered bicycle or hybrid.

An electric generator and motor transmission is the mostly the product
of the generator and motor efficiencies, going from kinetic energy to
electric energy and then back to kinetic. A small (1-2HP) permanent
magnet electric generator is probably about 75% efficient while a
similar sized motor is about 70%. There are power/rpm curves for both
that can be used to determine the most efficient operating point, or
the efficiency at some particular power and RPM level. Using my
guess(tm) numbers, that's:
0.75 * 0.70 = 53% efficiency.
In other words, an all electric drive system sucks. On Pg 337 of the
"Bicycling Science" book, the author does a better job of estimating
the electrical system efficiency. He has the following in series:
70% - Pedals driving gears with short chain to generator.
98% - Generator
95% - Controller
80% - Motor driving short chain to wheel
Multiplying these together yields 52% efficiency.

There's also a short section on hydrostatic drives (as in earth
movers), which the author claims maxes out at 80% efficiency and is
therefor useless.

And perhaps there's something I missed?


Well, I would suggest you disclose what you are trying to accomplish
so that I don't need to guess(tm) so much. You also missed other
transmission systems:
1. Perforated metal tape instead of a chain
2. Cable (Snek) drive
3. Berg cable and plastic chain:
http://802.11junk.com/jeffl/pics/Berg/
4. Toothed rubber belt
5. Direct gear drive using lots of spur gears.
6. Drive Shaft
7. Direct drive (Penny Farthing)
8. Ratchet drive
9. Various eccentric gear drives (elliptical crank, reciprocating
pedals, rolling exercise machines, etc).
http://www.elliptigo.com
https://www.streetstrider.com
10. Rowing motion drive
https://rowingbike.com
11. Front wheel pedal drive with hub transmission.
12. Whatever else I forgot.

--
Jeff Liebermann

150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558


I had started this because in a recumbent the chain run is so long that it can't possibly be very efficient. So I then considered a drive shaft. This sounds OK by itself until you start considering the length of the drive shaft and that it would be made so lightly that it would require many bearings along it to prevent flexing.

Then looking at one of the land speed record streamliners they have machined a front wheel drive system. But then you have to design a system that allows the pedals to remain in the same plane as the legs and turn to accomodate the steering. This loses a great deal officiency as well.

Also with streamliners the speeds are so great that you have to make absolutely phenomenally large gearing.

As you could see in that youtube video of the German riding to work the actual average speed of an unsuspended bike also isn't very great. The rough ride slows the bike greatly because the rider cannot retain an efficient peddaling position.

Then I considered hydraulics. In fact this probably weighs very little more than either the long chain with the double gearing system or the front drive system with the machined hub containing a 1:4 gearing.

After all - the disk braking system weighs in only slightly more than standard rim brakes including the disk most of which are steel.

But the efficiency is the problem. For the speed you absolutely must have as little loss as possible.

Electric drives are readily available but the weakness and the loss of efficiency is through the generator and the speed multiplication systems.

So I'm still in the design state of the drive system before even worrying about the shell. I can see that the aerodynamics of the present generation of streamliner can be easily improved. Apparently they are doing their testing in very low speed wind tunnels and are relating that to the higher speeds.

And the weight of the entire streamliners are WAY out of hand. While aerodynamic drag is important over about 30 mph, you have to get up to the speed before good aerodynamics is significant. And you can burn up all of your energy getting there with the weights I'm seeing. That is the main reason they appear to accelerate so slowly and not because of faulty gearing.
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  #13  
Old April 21st 17, 06:12 PM posted to rec.bicycles.tech
Jeff Liebermann
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Posts: 4,018
Default Mechanical Efficiency

On Fri, 21 Apr 2017 08:08:06 -0700 (PDT), wrote:

I had started this because in a recumbent the chain run is so long that
it can't possibly be very efficient.


I beg to differ. Long chains are more efficient than short chains.

The loss of efficiency in a chain drive is mostly from the friction
losses when the chain link pin is rotated inside the sleeve. Only the
upper part of the chain loop is under tension. Only the links that are
rotating from the straight line chain, to the crank and the freewheel
have pins that are rotating and therefore add friction. In other
words, for large diameter gears, only two links on the entire chain
have rotating pins. For smaller diameters and tighter bends, perhaps
2 more pins.

In addition, the tension on the chain is distributed only over the
upper part of the chain loop. For example, if you a pulling with 100
lbs of tension on the upper part of the chain loop, and there are 10
links (and 20 pins) in the upper part of the chain loop, then the
pressure per pin is:
100 lbs / 20 pins = 5 lbs/pin
However, if you use a much longer chain, as in a recumbent, with 50
links (100 pins) between the two gears, the tension per pin is:
100 lbs / 100 pins = 1 lbs/pin
Since frictional losses increase with surface pressure, the longer
chain would have LESS friction than the shorter chain because there is
less pressure on the pins and sleeves. Of course, a longer chain
would weigh more, but that's another calculation and is not directly
involved in the efficiency calculation.

--
Jeff Liebermann

150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #15  
Old April 21st 17, 06:18 PM posted to rec.bicycles.tech
DougC
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Posts: 1,276
Default Mechanical Efficiency

On 4/21/2017 10:33 AM, Frank Krygowski wrote:
On 4/21/2017 11:08 AM, wrote:


I had started this because in a recumbent the chain run is so long
that it can't possibly be very efficient.


I don't know of any reason that a long chain should be much less
efficient than a short one.

Chain inefficiencies (small as they are) come from the friction as the
links bend around sprockets, and friction as rollers roll on and off
sprocket teeth. Some of the bending friction is between side plates
exacerbated by lateral angles, and long chain runs tend to have smaller
lateral angles. They may be more efficient than short chains.

Welp,,,, I would presume that a longer chain would tend to require more
idlers to route a circuitous path than a short one would.

Using plain bicycle chain still tends to win out most of the time tho.
It's just easier and cheaper to use than anything else. Home-builders
can make sprockets any size you want with just a drill press and any 2-D
CAD program. Any of that other stuff is way more complicated to try to
fabricate.

I have pondered the same problem too however: with a conventional
bicycle drive train, even with triple chainrings you run into the
problem of not having enough width in the drive ratios... You want to be
able to crawl in a granny gear when you need to, but you still want to
be able to pedal at a comfortable cadence at 35 - 40+ MPH.
,,,,,,,
I know how most do it: they just use a normal 2x9+ setup with the widest
ranges they can get, and they sacrifice the low end for a bit higher
end. Some bikes also use compound gearing, with a second rear hub +
derailler after the first, but that gets messy fast.

And none of these is still really convenient to use, in that they do not
place the ratios in a mathematical order. Ideally you would want a
couple of button-levers marked [shift up] and [shift down], and it would
go through all the ratios in order, from one end to the other. With an
automatic car transmission for example, it shifts 1-2-3-4-5
automatically when accellerating. You don't need to TELL it what gear to
use.....

I've seen this same question asked about Di2 setups: why is there still
two levers? Since a microprocessor could easily figure out the
next-highest gear or next-lowest gear, and switch the front & rear
deraillers to reach it. You should just need ONE lever, or two
buttons.... But this only seems to be stubborn tradition; riders are
used to mentally managing the front and rear deraillers separately. So
that was the way Shimano decided to control it.
  #16  
Old April 21st 17, 06:18 PM posted to rec.bicycles.tech
[email protected]
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Posts: 3,345
Default Mechanical Efficiency

On Friday, April 21, 2017 at 10:12:39 AM UTC-7, Jeff Liebermann wrote:
On Fri, 21 Apr 2017 08:08:06 -0700 (PDT), wrote:

I had started this because in a recumbent the chain run is so long that
it can't possibly be very efficient.


I beg to differ. Long chains are more efficient than short chains.

The loss of efficiency in a chain drive is mostly from the friction
losses when the chain link pin is rotated inside the sleeve. Only the
upper part of the chain loop is under tension. Only the links that are
rotating from the straight line chain, to the crank and the freewheel
have pins that are rotating and therefore add friction. In other
words, for large diameter gears, only two links on the entire chain
have rotating pins. For smaller diameters and tighter bends, perhaps
2 more pins.

In addition, the tension on the chain is distributed only over the
upper part of the chain loop. For example, if you a pulling with 100
lbs of tension on the upper part of the chain loop, and there are 10
links (and 20 pins) in the upper part of the chain loop, then the
pressure per pin is:
100 lbs / 20 pins = 5 lbs/pin
However, if you use a much longer chain, as in a recumbent, with 50
links (100 pins) between the two gears, the tension per pin is:
100 lbs / 100 pins = 1 lbs/pin
Since frictional losses increase with surface pressure, the longer
chain would have LESS friction than the shorter chain because there is
less pressure on the pins and sleeves. Of course, a longer chain
would weigh more, but that's another calculation and is not directly
involved in the efficiency calculation.

--
Jeff Liebermann

150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558


Huh? These long chains do not have straight runs and in fact near the back have a rather shorter run to the cogs which always puts them in more of a sharper angle to the cogset. In other words - you are always in a more "cross-chained" angle.
  #17  
Old April 21st 17, 06:40 PM posted to rec.bicycles.tech
Doug Landau
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Posts: 1,424
Default Mechanical Efficiency

On Friday, April 21, 2017 at 10:12:39 AM UTC-7, Jeff Liebermann wrote:
On Fri, 21 Apr 2017 08:08:06 -0700 (PDT), wrote:

I had started this because in a recumbent the chain run is so long that
it can't possibly be very efficient.


I beg to differ. Long chains are more efficient than short chains.

The loss of efficiency in a chain drive is mostly from the friction
losses when the chain link pin is rotated inside the sleeve. Only the
upper part of the chain loop is under tension. Only the links that are
rotating from the straight line chain, to the crank and the freewheel
have pins that are rotating and therefore add friction. In other
words, for large diameter gears, only two links on the entire chain
have rotating pins. For smaller diameters and tighter bends, perhaps
2 more pins.

In addition, the tension on the chain is distributed only over the
upper part of the chain loop. For example, if you a pulling with 100
lbs of tension on the upper part of the chain loop, and there are 10
links (and 20 pins) in the upper part of the chain loop, then the
pressure per pin is:
100 lbs / 20 pins = 5 lbs/pin
However, if you use a much longer chain, as in a recumbent, with 50
links (100 pins) between the two gears, the tension per pin is:
100 lbs / 100 pins = 1 lbs/pin
Since frictional losses increase with surface pressure, the longer
chain would have LESS friction than the shorter chain because there is
less pressure on the pins and sleeves. Of course, a longer chain
would weigh more, but that's another calculation and is not directly
involved in the efficiency calculation.


So you are saying that losses from routing the lower run of the chain through tensioners and idlers are negligible?
  #18  
Old April 22nd 17, 04:13 AM posted to rec.bicycles.tech
Jeff Liebermann
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Posts: 4,018
Default Mechanical Efficiency

On Fri, 21 Apr 2017 20:03:26 -0400, Frank Krygowski
wrote:

On 4/21/2017 1:12 PM, Jeff Liebermann wrote:
On Fri, 21 Apr 2017 08:08:06 -0700 (PDT), wrote:

I had started this because in a recumbent the chain run is so long that
it can't possibly be very efficient.


I beg to differ. Long chains are more efficient than short chains.

The loss of efficiency in a chain drive is mostly from the friction
losses when the chain link pin is rotated inside the sleeve. Only the
upper part of the chain loop is under tension. Only the links that are
rotating from the straight line chain, to the crank and the freewheel
have pins that are rotating and therefore add friction. In other
words, for large diameter gears, only two links on the entire chain
have rotating pins. For smaller diameters and tighter bends, perhaps
2 more pins.

In addition, the tension on the chain is distributed only over the
upper part of the chain loop. For example, if you a pulling with 100
lbs of tension on the upper part of the chain loop, and there are 10
links (and 20 pins) in the upper part of the chain loop, then the
pressure per pin is:
100 lbs / 20 pins = 5 lbs/pin
However, if you use a much longer chain, as in a recumbent, with 50
links (100 pins) between the two gears, the tension per pin is:
100 lbs / 100 pins = 1 lbs/pin


Not true, Jeff. The tension in the chain is a constant The load on
each pin in the free upper span is exactly the same, and it doesn't
change if the chain is longer, i.e. has more pins. A normal chain with
a 100 pound load has 100 pounds on each pin, no matter how long.


Argh. We went through this exercise a few years ago in this newsgroup
when I allegedly made the same mistake. Am I wrong again? (I just
hate it when that happens).

The way I look at it is that if I replace each link in the upper part
of the chain loop with a spring scale, methinks the deflection of each
spring (a measure of the force) would be the pulling load divided by
number of spring scales.

Or, if we break the chain and put one spring scale between two
adjacent links, the measured force will be equal to the applied load.
However, if we break the chain in two places, methinks the measured
force will be half the applied load. If the force were equal to the
applied load on each link, I would expect the two spring scales to
also indicate a force equal to the applied load, which I don't believe
is the case.

Now if you had two chains side by side sharing the same 100 pound load,
those pins would each have 50 pounds on them.


Yep, here we agree.

To use an electrical analogy, you're confusing a series situation with a
parallel situation.


The electrical analogy would be each link in the chain is a resistor.
The applied load is the voltage. If the resistors are all equal, the
voltage across each resistor would be equal to the applied voltage
divided by the number of resistors.

I'll admit that I might be wrong about all this, but at this moment,
it seems correct to me.



--
Jeff Liebermann

150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #19  
Old April 22nd 17, 04:31 AM posted to rec.bicycles.tech
Jeff Liebermann
external usenet poster
 
Posts: 4,018
Default Mechanical Efficiency

On Fri, 21 Apr 2017 10:40:30 -0700 (PDT), Doug Landau
wrote:

So you are saying that losses from routing the lower run of
the chain through tensioners and idlers are negligible?


Worse. I was ignoring the losses from tensioners, idlers, grease, pin
rotation caused by a slight chain droop, inertial loading from a
longer and thus heavier chain, chain acceleration lag, etc. I assumed
that the original question was about a simple power transmission
system, not the complex mess that such systems inevitably evolve into.
Something more like this test fixture and a practical bicycle:
http://cdn.mos.bikeradar.imdserve.com/images/news/2012/11/06/1352163122826-1476emv18vmdi-630-80.jpg
The additional losses can be tested separately and included later.

You're correct that tensioners, idlers, etc are important. For a
chain drive that is about 95% efficient, small frictional losses might
reduce the 95% efficiency rather significantly. However, I don't
believe we were trying to nail down a single number for a chain drive
efficiency. As noted in the "Bicycling Science" books, the efficiency
will vary somewhat depending on speed, conditions, maintenance,
gearing, etc. What I believe was of interest was comparing the
efficiencies of a drive chain, hydraulic drive, and an electric
generator-motor drive. From my readings, these will show drastically
different efficiencies, where the effects of tensioners, idlers, etc
will not make much difference in the comparison.

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 




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