BILL BOSTON CYCLES

Bottom Bracket Ht.
Home Up Feedback Contents Search Links

 

Up

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:

What type 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.

Riding style

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 bracket.

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.

Lower bottom 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. 

Seat Tube Angle

 

 

Back ] Home ] Up ] Next ] 

Copyright © 2003 BILL BOSTON CYCLES
Last modified: July 09, 2014