On Mon, 24 Apr 2006 21:33:57 -0400, "David L. Johnson"
<
[email protected]> wrote:
>On Mon, 24 Apr 2006 12:17:01 -0700, carlfogel wrote:
>
>> When Kyle and Berto tested the efficiency of hub transmissions, those
>> two models were virtually indistinguishable:
>>
>> http://www.ihpva.org/pubs/HP52.pdf
>>
>> See Figure 9, which graphs averaged efficiency through all gears, and
>> Table 1, from which the figures were taken.
>
>I wonder how noticeable these "small" differences would actually be. Keep
>in mind that 0% efficiency would mean you could not move at all. The kind
>of difference we are talking about is, yes, a percentage or two of
>complete efficiency, but that may be quite noticeable to an experienced
>cyclist. Certainly the difference between the 93% efficiency of a
>derailleur system versus the 90% of most hub gears would be
>noticeable. If you think of it the other way, it is a 7% loss versus a
>10% loss, which means a 40% increase in frictional losses from the
>transmission.
>
>> Riders, of course, can't tell anything directly about internal friction
>> losses. We can only notice misleading noises and estimate how fast we're
>> going for whatever effort we think we're putting into gears that are
>> probably different, while ignoring wind speeds, tire losses, and
>> inflation differences.
>
>I think you can tell something about the frictional loss. If even the
>best systems lose 7% of the rider's input, that puts these losses up there
>near tire rolling resistance and air resistance at lower speeds.
>Certainly you notice a similar decrease efficiency when you get a
>slow leak -- often the feeling of the extra friction is the first
>indicator that you have a tire going flat.
Dear David,
I don't think that we directly notice extra friction at all.
That is, if we put out the same power (darned unlikely in
real life, but let's ignore that), we would notice only a
tiny speed variation or else a tiny change in effort needed
to achieve the same speed.
The variation is extremely slight.
Stick 211.6 watts into this calculator, accept the defaults,
and change the distance to 10 miles, roughly the kind of
ride that Alex mentioned (in perfectly good faith, I'm
sure):
http://w3.iac.net/~curta/bp/velocity/velocity.html
It predicts 20.00025 mph and 29.999618 minutes to cover 10
miles.
Drop the default 95% transmission efficiency to 94% and
calculate again.
It predicts 19.92081 mph and 30.119264 minutes for the same
10 miles.
The speed dropped less than 0.1 mph. The time increased from
1800 seconds to about 1807 seconds (darn, I goofed on two
previous posts--too many decimals, too many parameters).
I doubt that any rider can "feel" that his average speed is
going to add 14 seconds per hour or that one hub gear is
robbing him of 0.1 mph at 20 mph.
To illustrate how far below the likely threshold of our
sensitivity such tiny differences are, play with some of the
other fields on the same calculator. Same 211.6 watts and 10
miles, back to the default 95% efficiency--but let's raise
the temperature from 70F to 78F.
The warmer air isn't as dense, and I get a speed increase to
20.099 mph, about the same amount lost to a 1% transmission
efficiency drop. I think that most riders would be fooling
themselves if they believed that they could feel the speed
increase as the sun rose on a summer morning.
Back to the defaults, but now force the rider to battle a
0.13 mph headwind.
That tiny extra drag pretty much matches the 1% transmission
efficiency loss--the predicted speed drops to 19.92 mph.
Winds blowing more than 10 times that fast are almost
impossible to detect while standing still in the open, much
less bicycling at 20 mph.
Let's try again. Put the rider on a 0.03% grade. That's not
a modest 3.00% grade that rises 3 feet in a hundred feet,
but a 0.03% grade, a rise of 3 feet in ten thousand feet,
climbing a yardstick in about two miles.
That Alpine climb knocks the neutral 20.0 mph speed back
down to the same 19.92 mph.
I doubt that anyone can pedal up to around 20 mph and
accurately report his speed over a hundred yards to within
0.1 mph without a stop watch.
I'm willing to believe that I've punched in the wrong
numbers, goofed up several orders of magnitude, or missed
something else obvious. But until someone provides some
physics and numbers to show otherwise, it looks as if we
still have a tendency to feel differences that are more in
our heads (or in unrelated factors) than in what we think
we're measuring.
Again, I think that we honestly report how things feel to
us. But the kinds of differences that we sincerely feel
between two test rides are sometimes obviously swamped by
ordinary wind variation, temperature, atmospheric pressure,
tire model, tire inflation, bicycle weight, what we had for
breakfast, and whether we emptied our bladders before riding
off.
As for the derailleur versus Rohloff efficiency, let's have
a quick look. At 200 watts, Kyle and Berto found the raw
average efficiencies to be about 94.0% verus 91.5%, judging
by Figure 12. (They also pointed out that just applying the
measuring instrument cost around 2.0 to 2.5%, but let's
ignore that.)
At 94.0% efficiency, we need about 213.9 watts to push the
default rider along at 20.0020 mph for 30 minutes on our 10
mile test ride.
At 91.5% efficiency, speed drops to 19.8001 mph. The 1800
second ride takes 30.303 minutes, an extra 18 seconds. I
don't think that any rider riding by himself without a
stopwatch can tell if he's 18 seconds late after half an
hour of riding.
But anyone can check my notions to see if case I'm wildly
off. Here are two calculators:
http://w3.iac.net/~curta/bp/velocity/velocity.html
http://www.kreuzotter.de/english/espeed.htm
The first calculator (Austin) offers easy transmission
efficiency changes and oodles of decimals. The Kreuzotter
calculator can be used for rough checks by simply changing
the watts--that is, compare speeds for 200 watts and 198
watts to see how little difference a 1% change power change
makes at roughly 20 mph speeds.
Cheers,
Carl Fogel
