Becoming a Mountain Biker
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6. The right size?
Of course, the quality of components on your bike is
important, but not as important as the right size of the
frame. It is even more difficult to determine the right
size than a beginner can imagine, since there is quite a
confusion in this area.
How is the size indicated?
The size of a bike or better the size of a frame (of the
bike) is usually given as the length of the seat tube in
inches (in). The seat tube is a part of the frame
between bottom bracket (BB), where the pedaling axle is,
and the seat post. And here comes the confusion -
manufacturers measure the seat tube in two different
ways. It can be either "center to center"
(from the center of the BB to the point, where the top
tube is welded to the set tube), or "center to
top" (from the center of the BB to the top of the
seat tube). There is usually one size difference between
these measurements, meaning: 17 center to center = 18
center to top. The size of a bike is usually indicated
via a sticker on the seat tube.
How to check the size?
Stand over the bike and you should have 7 to 10 cm
clearance between your crotch and the top tube. This is
your approximate size.
And why do manufacturers indicate the size of their
bikes by the length of the seat tube? Because it is the
easiest to try when buying. All other dimensions of the
frame are related to the seat tube then, e.g. seat
angle, top tube length, etc. Unfortunately people are
not always in standard proportions and it can happen
that two bikers of the same height ride bikes of
different size - one can have longer legs than the
other. This clearly illustrates the fact that sizing
according to the seat tube length is really just
approximate.
The length of the bike
Very important, if not even the most important, is also
the length of the bike, i.e. frame. And it can be
measured in many ways as well. The exact length of the
top tube is not the same as the length of the frame
itself, since mountain bikes have a sloping top tube. So
the term effective top tube is used to indicate the
horizontal length of the frame, which is simply
horizontal distance from the middle of the head tube to
the middle of the seat tube or seat post (see the
picture on the side).
How to check the length?
Sit on the bike and position yourself as if you were
riding - lean forward with hands slightly bend at the
elbows. Look in direction of the front hub, and if it is
hidden behind the handlebar then the length of the bike
is more or less correct. If the hub is visible
completely and is in front of the handlebar, closer to the
rear wheel, then the bike could be too long and the size
of the bike too big. And if the hub is visible on the
other side of the handlebar then the bike could be too
short and the size of the bike too small. Of course it
is best to do this check while riding for real.
You can adjust the length of the bike by moving the
saddle fore and aft to a certain degree, since the
saddle position is also related to bottom bracket and
pedaling. Another way to accommodate incorrect frame
length is by changing the stem to shorter or longer
one, again to a certain degree, since you should stay
within 90-130 mm lengths in order to avoid steering
abnormalities. If you need a stem outside these limits,
then you'd rather change the frame size.
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