LOW VS. HIGH GEARS AND GEAR RATIO
MAY 31, 2005
You wouldn't happen to have
run into an easy schematic that explains gear ratio for
bicycles. One that makes it both easy to understand and,
more important, easy to remember.
That is, when you’re in high
gear, is it easier to get up a hill, or faster to get down
one?
John Y.
John:
I find that gears confuse average
folk like no other mechanical aspects of bicycles. Let’s
see if I can keep from making things worse.
First, what about this "high"
and "low" gear stuff? One way to think of it:
low = slow, high = spry (fast). (Lame, I know—but stick
with me.) So whenever you should or must go slow, like when
starting from a stop, going up a hill, or biking into a
headwind, you use a lower-numbered gear. Whenever you want
or have to go fast, like when going down a hill or biking
with the wind behind you, you use a higher-numbered gear.
Also, the inner gear (both front
and rear) has the lowest number.
If you wanna see all this, have
a look at the section on gears in my book, Urban
Bikers’ Tricks & Tips. As with everything
in my book you’ll get it explained with pictures: how
a bike’s chain looks on different gears, in different
situations.
OK, now let’s talk science.
Many bikes have different sets of gears in front (part of
what we call the chainring) and in back (part of the cassette
or freewheel). Why would you want your bike’s chain
on, say, gear 1 in front and gear 5 in back, or gear 2 in
front and gear 1 in back? Answer: If you wanna go the slowest
with the easiest pedaling, use the combination that gets
you lowest gear ratio. (More on that in a second.) If you
wanna go the fastest, use the combination that gets you
highest gear ratio. For going at medium speeds use the combination
of gears that ends up with gear ratio somewhere in between
the highest and lowest.
How do you know the gear ratio?
You gotta use math. For each front gear and rear gear you
have to count the number of teeth on the gear, stick that
number into a computational table, and based on your bike’s
rear wheel diameter either do some calculations or use the
ones somebody else has done. All very esoteric, but shown
pretty well on the GeRZ
Web page.
You might get a kick outta trying
this on your bike: Find the gear ratio for the combination
of your lowest front gear (the smallest one) and the lowest
rear gear (the largest one). This tells you, more or less,
how steep a hill your bike can climb easily. A gear ratio
of 16 means you can pretty much go up a wall. In comparison,
for many average bikes with three gears in front and seven
in back the lowest gear ratio comes to around 21.
Mr Bike
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