So... What is bottom bracket height and how
does it affect fit?
Bottom bracket height is is
the height of the centerline of the bottom bracket axle above the ground and
should be determined by your need for pedal to ground clearance.
Bikes designed for road
racing and short course races will require a higher bottom bracket height
because the rider needs to pedal through turns and can not tolerate the pedal
striking the ground. The sharper the turn, the greater the lean angle and the
higher the bottom bracket needs to be.
Bicycles designed for
touring or time trials can tolerate a much lower bottom bracket height because
they either donít have to pedal through sharp turns or the courses are mostly
straight line events. When selecting a bottom bracket height, the following
things should be considered:
of event will the bicycle be used for?
Touring, time trials
and most triathlons do not require a high bottom bracket because the
course is mostly a straight line with little if any high speed maneuvering
required. With 170 mm cranks, a 10 Ĺ
inch bottom bracket height is a good starting point.
Road racing will
require a slightly higher bottom bracket due to the need to pedal through
some turns. 10 ĺ inch bottom bracket height is a good starting point if
you are using 170 mm cranks.
Criterium races are
conducted on short courses with many sharp turns. While most riders will
not pedal all the way through the sharpest turns, pedaling quite deep into
the turns is required to remain competitive. A bottom bracket height of 11
inches or higher may be required for these races depending on riding style
and rider confidence. It is wise to explore the capabilities of your bike
with caution as a pedal strike can be a very painful event.
Only you can determine
what style of riding is appropriate for you.
If you are a road
racer and you do not feel comfortable pedaling deep into turns, you
probably wonít do it and may not need a high bottom bracket.
If on the other
hand you spend a lot of time touring or on fast club rides and are
aggressive in your cornering, you may want to consider a higher bottom
Length of cranks to be used
Longer cranks will
of course require the use of a higher bottom bracket to provide the same
ground clearance while cornering.
On the other hand,
shorter cranks will afford more ground clearance at the same bottom
bracket height or allow you to lower the bottom bracket.
Type of pedals
The newer style of
clipless pedals have a lower profile and will provide more ground clearance
than most of the older style pedals that are used with toe clips
Effect on rider position
Like crank length, bottom
bracket height has a profound effect on rider position.
A higher bottom
bracket height raises the saddle relative to the handlebars. This will
in turn require the top tube / stem length be shortened to compensate.
brackets will lower the saddle relative to the handlebars and allow the
use of a longer top tube / stem combination.
Things to watch out for
Many manufacturers in an
effort to add a smaller size to their line, will raise the bottom bracket
height. While this does in fact shorten the seat tube, this practice leads to a
higher center of gravity. An excessive bottom bracket height will produce a corresponding
rise in saddle position relative to the handlebars, requiring a shorter top tube
/ stem length for proper fit.