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Crotch Clearance
For most bicycles used for road riding (and track
as well). The top tube height should be high enough to provide between 3/8 of
an inch and 1 inch of clearance to your crotch in bare feet. Now you might say
that 3/8 inch is not near enough and for some that may be true. For most of us
however 3/8 inch is fine for several reasons
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You
will be wearing shoes.
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You
will seldom if ever have both feet on the ground at the same time while
riding. If you do, you are not riding, but just holding up your bike while
asking for directions or reading a map.
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When
you are standing over your bike with one foot on a pedal, your pelvis will
be rotated and the clearance will be increased substantially.
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How Top Tube Height
affects overall frame design and fit
As
top tubes on most road bikes are or should be horizontal, the top tube height
effectively sets the handlebar position in the vertical plane.
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With
the old style quill type handlebar stems, the handlebars can be positioned
between 3 and 4 inches above the top tube.
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The
new style handlebar stems provide about the same range with the top of the
handlebars at about 3 inches above the top tube when they are mounted
horizontal and about 4 inches when reversed. This of course will vary with
the length of the stem with shorter stems providing less rise and longer
stems giving more rise. It should be noted here that unlike the quill type
stem that provides a range of heights between 3 and 4 inches, the new
style provides either 3 or about 4 inches though some adjustability can be
achieved by using spacers provided the steering tube has not been
shortened.
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As
you can see, any change in top tube height will affect the difference in height
between the handle bars and saddle as the saddle position is determined by leg
length and crank length.
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Higher
top tube heights will provide a more upright riding position and allow a
longer top tube length / stem combination.
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Lower
top tube heights will provide a lower shoulder position relative to the
saddle and require a shorter top tube length / stem combination.
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As
long as the top tube height is high enough to provide crotch clearance in
the 3/8 to one inch range, stem height and length can be varied to
compensate.
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Crotch
clearance greater than one inch will have an adverse affect on riding
position and top tube length. |
Things to watch out for!
If
you see a road bicycle with a sloping top tube, ask yourself, " What was
the manufacturer trying to do?" If it is a bike in a smaller size, and the
obvious answer is to provide a lower stand over height, BEWARE. The
minimum top tube height with 700c wheels is 30 inches above the ground while
maintaining an adequate head tube length to support the fork and head bearings.
It is possible to reduce the stand-over height by several inches by sloping the
top tube, in most cases the top tube will still be too long, or the the head
tube angle will have been reduced severely to provide adequate front wheel
clearance when the top tube was shortened (see AccuTrax
to find out more about this). To better understand building bikes to fit riders
instead of a standard components, see Those Bikes
with Small Front Wheels.
Crank
Length
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