On Thu, 19 Apr 2007 17:25:56 -0700, "G.T." <[email hidden]>
Quoted message said:
<[email hidden]> wrote in message
news:[email hidden]...
Quoted message said:Quoted message said:
Again, my position is that it takes the same power to push the same
load up the same hill, but pushing our feet around a rigid 350 mm
circle in a crouched position with an overall 1-to-1 gearing
(foot-to-tire-travel) puts our muscles at a considerable disadvantage
(inefficiency, more raw calories) when compared to walking up the same
hill at the same speed (same foot movement covers same distance in
same time).
Don't our feet travel around a rigid 350mm circle any time we're pedaling
our bikes? Your rhetoric makes it sound like we should always be walking to
avoid that 350mm circle.
Greg
Dear Greg,
Physics, not rhetoric, makes it sound as if we should use the awkward
and unnatural circular movement of the pedals, overbend our knees, and
crouch on a bicycle when we want to trade force for distance to do the
same work
The 1-to-8 travel ratio of pedal-to-tire movement in a typical high
gear lets us take advantage of 25-foot "strides" on flat ground. A
very small force on the pedal pushes us a very great distance against
the minor wind resistance and tire drag.
The awkwardness of the pedal circle shows up when you look at the
speed of our feet going around it. It's easy to forget just how slowly
we move our feet at what we consider brisk bicycle cadences on 175 mm
cranks:
mph
rpm foot
--- ----
60 2.47
90 3.70
120 4.92
At 90 rpm, your feet travel around the pedal circle at less than the
speed they move if you're heading down an empty aisle in the grocery
store. Studies keep showing that the faster they circle, the less
power they put out--efficiency drops after around 60 rpm (but other
factors make the popular 80-100 rpm useful).
Uphill we have to fight gravity. Even a gentle slope forces us to drop
gears, shorten our "stride" and work much harder to cover the same
stretch of road.
When a slope is steep enough to require 1-to-1 gearing for the load
and power available, the leverage advantage has vanished, exposing the
disadvantage of the pedal circle over a normal step.
When an ultra-low gearing produces a 1-to-1 pedal-to-tire movement
ratio (20x38, 175 mm crank, 2100 mm 700c tire), your foot must rise
about 14 inches (350 mm) with every stride, which in turn must be
about 43 inches long (1100 mm ).
It's worth pointing out that elsewhere Kinky just mentioned that his
experience was with 25 x 34 gearing. That works out to about 1 to 1.45
pedal-to-tire-movement, meaning that his bicycle tire moved 45%
fruther than his feet on the pedals. Steep as the trail was, he could
still employ considerable leverage.
(More and more, I wonder whether some posters in this thread know what
their overall gearing actually is. If anyone is curious, feel free to
email me crank length, tire size, and front x rear teeth, and I'll be
happy to plug them into a spreadsheet. Until I fiddled around with
such things, I never really realized that a 53x11 with a 175 mm crank
and a 2100 mm circumference tire moves over 9 feet for every foot my
pedal moves.)
To get some idea of the awkwardness, try walking and raising both feet
14 inches with every step with each stride--that's the height involved
in climbing stairs, which at roughly 100% grade are steeper than
anything people are claiming to climb on bicycles.
Since the details can be easily confused, it's worth pointing out that
a normal stair step is about 7 inches, but that your "stride" covers
two steps (14 inches)--left foot on floor, right foot on first step,
then left foot on second step, 14 inches above where it began. The
grade is about 100% (45 degrees), but like the height of the
individual step varies according to the builder.
Again, note that when we run out of gears and the hill gets steeper,
we "stand" up on the pedals in a chimpanzee-like crouch because even
that wretched posture opens our hip angle and lets us use our muscles
more effectively. The closer we get to a running/walking posture, the
closer we are to efficiency.
Human legs work most efficiently in the natural posture and motion.
Anyone who doubts it can try to "walk" up a steep hill (obviously
nowhere near as steep as a stairway) in the crouching bicyclist
posture while moving his feet in the 14-inch high circle that a 1-to-1
overall gearing ratio requires.
Cheers,
Carl Fogel
