chainring/cog size and efficiency

[Andrew Lee]

I was always under the impression that for a given ratio that a larger chainring/cog combination
would be more efficient assuming all else equal (chainline straight...). Thus a 52/13 would be more
efficient than a 44/11, though both give ratios of 4:1. The graph showing the efficiency of an XT
44/32/22 11-32 drivetrain in 15 different gears seems to imply the same.
http://www.selbst-machen.de/speedhub_vs_xt.gif In this graph, they don't show any duplicate
ratios, but it is clear that as the chain was shifted onto larger cogs, the efficiency went up
(looking at the graph on top with the blue, green and yellow dots from left to right). When
they switch down to the 32 chainring from the 44 or 32 to 22, the efficiency takes a big drop
and it looks like if they had chosen to duplicate or com close to duplicating the same ratio
in the 32 as the 44 ring, the efficiency would have been lower in the 32. This all fits in
with my previous thinking. A study by Spicer of Hopkins University (overview of it at
http://www.jhu.edu/news_info/news/home99/aug99/bike.html ) seems to confirm this - larger rear
cogs are more efficient. However, they found that higher chain tensions mean higher
efficiency... something that they didn't expect and couldn't explain (seems backwards from
what you would expect with friction losses?). They did not compare different chainring/cog
size combination of the same ratio that I can tell from the web site link. A later study base
on Spicer's data found this:

A googled post from alt.rec.bicycles.recumbent:

>However, in an article in the fall 2000 "Human Power" called "Efficiency of bicycle chain drives:
>results at constant velocity and supplied power"
John
>and Claire Walton did an analysis comparing chain and sprocket efficiency
at a
>constant supplied power and vehicle speed.
>
> Using data from the previous Spicer HP article, they found that at constant power and vehicle
> speed, the efficiencies were 92% for the 11
tooth,
>90.5% for the 15 tooth, and 88.5% for the 21 tooth.
>
> Another cycling myth (of many) bites the dust!
>
> Tailwinds (and nine tooth sprockets) Rich Pinto Bacchetta Bicycles

This confuses me. The implication is that the higher chain tension from using a smaller chainring
allows it to be more efficient at a constant power and speed (same gear ratio?) Without actually
seeing the Walton's paper, it seem like maybe they are pulling something out of Spicer's data that
is somewhat suspect. It seems to directly conflict with the first graph that I referenced too... but
I don't know how that graph was made (does it say in the sidebar? I don't read German.) and under
what conditions... constant power?

What do you all think of this? The reason that I was thinking about this was because I was comparing
the benefits of a smaller set of rings for an all around bike... 48/36 12-27 vs. 52/39 13-29 for
example. I know the actually differences would be miniscule, but it's fun to think about. Lighter
for the smaller setup, but less efficient (if I am right about larger rings being more efficient). I
kind of think that if there is any measureable efficiency gain with the larger rings, even 1/2%, it
would overcome that slight gain in weight. Of course, it would also eliminate the need to find a 110
bcd double, which are less available nowadays. Why don't cyclocross bikes have gearing like 52/44
14-29 (availability of 14 starting cogs nowadays?).

 

[Andrew Bradley]

Andrew Lee <whatsupandrewathotmaildotcom> wrote:

><snip>A study by Spicer of Hopkins University (overview of it at
>http://www.jhu.edu/news_info/news/home99/aug99/bike.html ) seems to
confirm
> this - larger rear cogs are more efficient. However, they found that
higher
> chain tensions mean higher efficiency... something that they didn't expect and couldn't explain
> (seems backwards from what you would expect with friction losses?).

Confusing web page!

Speaking of chain tension they show a picture of somebody lifting the chain off the ring with
fingers. Do they mean "resting" chain tension?

Assuming that's not what is meant, with the same chainring you can only have a higher chain tension
_and_ a larger sprocket by increasing the resistance. Combined with lower RPM (to maintain the
constant power) this could be more efficient, for one thing it would seem there might be less energy
dissipation in the "unloaded" part of the slower wrapping chain.

> > Using data from the previous Spicer HP article, they found that at constant power and
> > vehicle speed, the efficiencies were 92% for the 11
> tooth,
> >90.5% for the 15 tooth, and 88.5% for the 21 tooth.
> >
> > Another cycling myth (of many) bites the dust!

> This confuses me.

Yes it is confusing and they could be missing the point entirely. The "myth" lives on for the
moment. What they may have found is that big gears make for drivetrain efficiency. But it's surely
not the lack of teeth on the sprocket that causes the effect. (A bigger chainring might have been
even more efficient)

The main consideration in cadence is the rider, not the drivetrain, cadence needs to be a
constant in the investigation of drivetrain efficiency. If the researchers haven't understood
that, they've goofed.

Maybe someone who has read the papers can enlighten us.

Andrew Bradley