[Phys-L] Re: gear-ratio activity

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

John Denker wrote:
> Hi Folks --
>
> Recently there was some discussion of how to teach
> about gears and gear ratios.  This prompted me to dust
> off my spreadsheet that analyzes the gear-ratios of a
> bike.
>
> There are two versions (for two different bikes):
>    http://www.av8n.com/physics/bike.xls      (21-speed)
>    http://www.av8n.com/physics/bike2.xls     (27-speed)

SNIP

> That means that this bike, which is nominally a 21-speed
> bike, really has only 11 speeds (separated by ten steps).

SNIP

********************
Hi,

        That a bike has functionally fewer gears than in
theory is not much of a suprise.  It is usually
recommended that the large front gear should not
be used with the larger rear gears or the small
front gear be used with the smaller rear gears
because of the angle the chain must run.  On a 21
speed this eliminates maybe 6 "speeds."

        I know people who biked across the country and
they carefully tested their gear ratios on various
grades and road conditions and had the gear
clusters change several times to get the gears to
match themselves.  I think they wanted smaller
differences in the climbing gears.

        I cycle to work and watched my fellow cylcist.
Many do not downshift for stops and have to
standup hard on the pedals to get started.  I
suspect many do not shift very much.  The guys in
cycling cloths and shoes are no better at shifting
than the average student.

Thanks
Roger
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