Bike Geometry: Frame AnglesFeb 26, 2001 Write an essay on this topic.
Popular Products in Sport and OutdoorThe Bottom Line Besides frame angles, there are so many tangibles and intangibles that affect ride quality. It is important that each bike be judged by the total sum of its parts.
Frame angles provide partial information about ride quality of a bicycle. It is a fallacy to judge a bike simply on frame angles. Just about every little angle and dimension can affect ride quality. Frame angles are an easy way to begin bike comparisons since this information is usually readily available from the bike manufacturer.
The angles for discussion are the head tube angle and the seat tube angle. The head tube is the tube where the fork and stem rotate and the seat tube is the tube where the seat post is inserted, in front of the rear wheel. The angles of these two tubes will help determine where the riding position will be and how body weight will be distributed over the bike.
HEAD TUBE ANGLE
Most head tube angles vary nowadays from about 69 degrees (when 90 degrees is perpendicular to the ground) to 75 degrees. Generally speaking, a steeper your head angle (closer to 75 degrees) yields quicker and more responsive the steering. Responsiveness can be explained by the following:
1 - A steeper head angle allows for a more weight-forward riding position, placing the body and head closer to the front of the bike, allowing the rider to see the road ahead sooner.
2 - A more upright head angle allows the front wheel to be tucked down under the rider, thus easier to steer.
3 - With the rider's weight more aligned with the turning angle, the bike will turn with more subtle weight and shoulder shifts rather than large arm movements.
SEAT TUBE ANGLE
Seat tube angles vary from about 71 to 75 degrees. As head tube determines steering position, seat tube determines power transfer position. Pedaling and power transfer is more efficient when the power of the weight and legs are in a more direct line above the drive train or, pedals and crank arms. There is also less distance to travel when the rider stands to pump the pedals for out-of-the-saddle situations.
THE ANGLE EQUATION
Beginning with the steepest angles, road racing bikes have the steepest, averaging 74 degrees for both head and seat tube angle. Racers want responsive, nimble and efficient machines, more geared towards speed, less geared for comfort. A steep head tube angle means that most of the rider's weight will be evenly distributed between legs and shoulders. A racer has to be in good shape to ride with more weight on the shoulders with eyes focused more on the road in front rather than the scenery beyond.
Touring / recreational road bikes:
Steep angles allow for an efficient ride. You can get racing angles in this category as well. In this category however, you do get the option of getting a slightly slacker head angle for a more upright position, better for taking in the scenery, and a little less busy in the steering department. In a more upright position, more weight gets transferred to the butt, which may be more comfortable as well.
Off-road racing bikes, usually have 72 degree head angles and 74 degree seat angles. This seems to be the winning combination for most production race bikes. These angles have been borrowed from proven road racing geometry. With these angles, a bike will be responsive and efficient but will also be busy in the cockpit and might be scary to some to handle at high speeds, on let's say a downhill jeep road. Off road, there are more opportunities to get thrown over the handle bars. A slacker head tube may yield a more stable, predictable ride. Another winning combo is 71 degree head angle and 73 degree seat angle. This yields the best of both responsiveness and stability. When analyzing these numbers, one should consider the majority of the type of riding one plans to do. Will one need a climbing bike? Will one need a technical trail bike? Will one need a downhill or free ride bike? Downhill bikes tend to be slack in both head and seat tube angle so that riders can more easily and naturally shift their weight over the real wheel for traction and better, lower center of gravity.
The frame angles will help determine other important dimensions that affect ride quality such as chain stay length, top tube length, and overall length, or wheelbase. It is always good to know the details of what you are getting with your bike but it is important to judge your bike on overall quality. There are so many tangible and intangible factors that affect ride quality so each bike should be judged by the total sum of its parts. It is a blessing that bike manufacturers design their bikes with slightly different angles. This allows us to find a bike that best suits our riding needs. Ride your friends' bikes, ride the bikes on the showroom floor. Most importantly, get out there and ride.
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