Re: compact crank cassette options

On Wed, 19 Sep 2007 17:08:13 -0600, carlfogel@xxxxxxxxxxx wrote:

On Wed, 19 Sep 2007 12:49:02 -0700, "Scott G." <scottg@xxxxxxxxxx>
wrote:


And even though I'm more of a grinder than a spinner, and love to sprint, I
haven't found a need for an 11 back there. The 12 works just fine, for me.
Besides, it's a taller gear than I used to race with (52x13).

--Mike-- Chain Reaction Bicyclesc

Eddy used a 52/13, Rando types used 46-28 with a 13-26 in back.
The English all raced Sturmey FMs, 4 speed medium ratio.
Either everybody was a lot slower in the old days or people have
gotten a lot stronger, must be the drugs. ;-)

11 & 12 are less efficient than a 13 cog, according to Frank Bertos
tests.

Dear Scott,

It's a little more complicated than that.

Briefly, real testing shows little practical difference with
derailleurs when looking at the highest gearing.

Here's Berto's chain efficiency graph for a real Shimano wide range
triple, showing 15 selected gears for 44-32-22 x 12-16-20-26-34. I
added the gear ratios and color:

http://i6.tinypic.com/4kn6wxu.jpg

(It's figure 11 from http://www.ihpva.org/pubs/HP52.pdf, but that
takes forever to load, has no captions, and is hard to figure out. It
was a 9x3 27 speed, but Berto didn't test the rear 14, 18, or 23.)

As you can see, the highest gear (44x12) is the rightmost red data
point, gear number 27.

The rightmost red 44/12 turns out to be right at average efficiency,
not the lowest efficiency.

For chart fiends, note that each rear gear (the five colors) shows the
same pattern of a three-point curve for its combination with the
22-32-44 front rings, with the middle point always lowest for the
middle 32 ring. Just trace any color's 3 data points from left to
right to see what I mean.

One reason for this unexpectedly good performance by the smallest cog
is that the little 11-tooth is paired with the biggest and most
efficient 44-tooth front sprocket.

Another reason is that the chainline for the 44-12 is better than
cross-chaining on many other combinations on the wide triple.

Most of all, at the same bicycle speed, higher gearing means lower
chain speed, which in turn means greater chain tension at the same
power, which is the biggest factor in chain efficiency at these
levels. In the highest gear, 44/12, the chain moves slowly at high
tension and efficiency.

Of course, few riders putting out only 80 to 200 watts care much about
such tiny efficiency changes--and that's what these are, small
differences of less than 10 more watts lost out of 200.

At 200 watts, for example, the best combination is the 44/20, 95.9%
efficient, producing a theoretical 191.8 watts.

At the same 200 watts, the 44/12 is 93.7% efficient, producing a
theoretical 187.4 watts, only 4.4 watts less.

Even that difference of 191.8 watts versus 187.4 watts is deceptive
because in real cycling, human legs put out the 200 watts, not an
electric motor.

At the same bicycle speed of 20 mph, the 44/20 spins frantically at
116 rpm, while the 44/12 cruises along at a gentle 70 rpm. Bending
your knees 60% faster to go the same speed will waste more watts than
the chain efficiency will save.

Another way to put it is to imagine where a 22-32-44 would sit on the
chart if we replaced the 12 with an 11 or a 13. The whole curve isn't
likely to move up or down as much as the difference between any other
sets of 3 data points for the rear cog, since they're all for
increases of 4, 6, or 8 teeth on the rear.

Chain efficiency tends to be more theoretical than practical in
ordinary bicycling.

Cheers,

Carl Fogel

An improved version of the colored and captioned Berto graph:

http://i19.tinypic.com/4zux068.jpg

CF
.

Relevant Pages

  • Re: compact crank cassette options
    ... it's a taller gear than I used to race with. ... Here's Berto's chain efficiency graph for a real Shimano wide range ... The rightmost red 44/12 turns out to be right at average efficiency, ... few riders putting out only 80 to 200 watts care much about ...
    (rec.bicycles.tech)
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