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Half step PLUS granny is a touring gearset derived from an old style
racing gearset, the half step.
The theory behind half step is just that, the chainwheels are selected
very close together (like 48/52) so that a front derailleur change
will select a gear that is roughly "halfway" between the next cog
gear.
This provided a very close ratio gearing for racing. The granny
gear was added for touring purposes, it's a very small inner ring
(like 26T) for loaded hill climbing.
The advantage of this gearing is that you can use a very widely
spaced touring cog (like 13-32) and still get a fairly close ratio
set of gears on the middle and outer chainwheels, but have those
real stump puller gears in reserve.
It does put a great demand on the front derailleur, though, you
have to select one carefully.
CdH
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| > Could someone explain to me what a half step and a granny
> are?
There are three basic layouts of gears:
Half-step
Crossover
Alpine
Standard rear clusters are laid out so that the RATIO of one gear
to the next is a constant. The exception is a "straight block"
where the difference from one gear to the next is 1 tooth.
The ratio of the two largest chain rings (only 2 on a 10,12,14 speed)
is chosen in relation to the ratio of the rear cluster:
Half-step Front ratio = 0.5 * Rear ratio
Crossover Front ratio = 1.0 * Rear ratio
Alpine Front ratio = 1.5 * Rear ratio
The Half-step has the advantage that all of the 10/12/14 speeds
are unique and evenly spaced. It's disadvantage is that you
sometimes have to shift both front and rear to get to the
next gear in sequence. This is seldom a problem because being
in a gear a few percent higher or lower than you want for
the short time it takes to shift the front up or down will
hardly kill you.
The Crossover has the advantage that you NEVER have to shift
both front and rear to get to the next gear. Just pick either
one because the result (change in gear) will be the same. It
has the disadvantage that most of the gears are duplicates
and you get only 6/7/8 unique gears.
The Alpine has the advantage that the range of gears is somewhat
larger that a Half-step or Crossover but the shift pattern is
stranger.
That explains Half-step (and its alternatives).
Now to explain Granny. On a triple crankset (15/18/21 speed) the
top two chainrings are often treated separately from the third
chainring. The top two chainrings are chosen as they would be
for a 10/12/14 speed. Since the available range on this (virtual)
10/12/14 speed is not very wide, the lowest gear on those two rings
may be too high for hill climbing (especially with luggage). The
third chainring is then made very small (24 to 30 teeth) to create
a set of hill climbing gears. The small third chainring is called
a Granny (probably because even your grandmother could climb hills
in so low a gear). Often you will hear of triple cranksets being
set up as "Half-step plus Granny". Now you know what that means.
-John A. Wasser
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| This is intended more as "words of wisdom" aka "learnings from the school
of hard knocks" than as a specific answer to your questions.
The gear setup that your question does not include is the typical mountain
bike setup with a 26-36-46 or 28-28-48 chainwheel setup with, perhaps, a
13-32 freewheel. This provides "three complete ranges of gears" is the
usual hype.
Occassionally someone will evaluate what is wrong with all four gear
arrangements for his particular style of riding and search high and low to
do something truly unique with his gearing. AND HAVE PROBLEMS.
A friend of mine serves as an example. He's a spinner, tourist, low gear
specialist. His knees will last 3 lifetimes. But he really likes the
following setup.
A 14-28 freewheel with a 28-38-50 chainwheel. He has often tried to get a
36 into the middle of that. The problem with it is that there is no front
derailleur made that can shift off the middle ring reliably. With the
28-38-50, the problem is shifting up, with the 28-36-50, the problem is
shifting down. The front drailleur cannot get close enough to the middle
ring to move it far enough to make a shift. He likes it, is willing to
live with it, complain about it, etc., so who am I to judge, except to
chalk it up to experience.
ed
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