Biopace
From Wikipedia, the free encyclopedia
 |
This article or section uses first-person or second-person inappropriately or excessively. Please edit this article to use the more formal tone expected of an encyclopedia, per Wikipedia's style guidelines. (help, talk) |
Biopace is the name of Shimano's ovoid bicycle chain ring that is designed to help overcome the "dead zone" where the pedals are up and down and riders have little mechanical advantage.
Contents |
[edit] With respect to elliptical
Biopace is a patented non-round chainwheel design made and licensed by Shimano. To a casual glance they resemble elliptical chainwheels, but on closer examination they turn out to be the exact opposite of the classical elliptical design. The product of extensive research and computer-aided design, Biopace chainwheels have the small radius engaged when the cranks are horizontal, the large when they are vertical. This is because the Biopace design is based on a dynamic analysis of the motion and momentum of moving cranks and legs, unlike the static, geometric analysis that produced classical ellipticals.
[edit] How they work
The key to understanding the biopace chain ring is thinking of the power that your legs transmit to the wheels; i.e. how fast you can pump energy into the wheels. Every cyclist wants to increase his power. There are two fundamental ways to do this: increase the force of your legs on the pedals, or increase the distance over which your legs exert that force in a given time lapse (i.e. make your feet move faster).
The first point is understood quite naturally. Without changing gears, if you want to go faster you instinctively push harder on the pedals, and you do indeed go faster.
To illustrate the second point of making your feet move faster, consider changing gears while maintaining the same speed along the road with a higher cadence. If you are capable of maintaining the same force on the pedals with your feet, the higher cadence increases the distance that your feet move per second, thus increasing your power and your speed along the road. It might be easier to understand if you consider changing gears so as to pedal with a lower cadence. Most cyclists would instinctively push harder on the pedals to compensate, but if you resist this urge you will find yourself cycling slower with this lower cadence.
Because it is uncomfortable pushing very hard on the pedals, experienced cyclists instinctively select a gear that gives a higher cadence. So, why not simply select ever higher gears so as to pedal ever faster in order to increase your power? There is a limitation to the cadence that you can comfortably maintain. In order to follow the circular motion of the pedals moving at a constant rotational speed, your feet must constantly accelerate, first down, then backwards, then up, then forwards. This acceleration of your feet in itself requires your muscles to exert force just to move your feet, without even considering making the bike move. As you try to pedal faster and faster, the smaller muscles in your legs reach their limit and are not strong enough to accelerate your feet at the speed that you are asking of them. These weaker muscles, then, are the limiting factor to using a higher cadence. It is in fact the horizontal motion of the pedals, while the cranks are more or less vertical, that is difficult for the body. The difficulty is even more evident when pedaling with greater force, such as for climbing with the small chain ring.
What if we could somehow reduce the acceleration of the feet during this horizontal motion?; what if we could allow the feet to move more slowly during the horizontal phase? This would no longer be the limiting cadence, and you would be able to pedal with a higher cadence before your muscles have difficulty, and you would be able to provide more power to your wheels and ride faster along the road! This is exactly what the biopace chain ring does.
By making your pedals move slower (compared with a circular chain ring) during the phase of horizontal motion, your smaller muscles are more able to move your feet at the selected rotational speed, and are able to maintain their force pushing on the pedals. As already discussed, this greater force means that greater power is transmitted to the wheels during the phase of horizonal foot motion.
By making your pedals move faster during the phase of vertical motion, your larger muscles are allowed to push on your pedals over the same distance as usual, but in a shorter time. (That is the same as pushing over a longer distance in the same time.) This greater rotational speed in the vertical phase means that greater power is transmitted to the wheels during the phase of vertical foot motion.
Thus, during each phase of the rotation of the pedals, the biopace chain ring transmits more power to the pedals. It does this by arranging things such that, at each phase of the rotation, the pedals turn at a speed that is more comfortable for the particular muscles in your legs that are being used at that phase. As a rider with biopace chain rings, you have the choice of riding your bike a little faster, or just relaxing and enjoying the same speed as usual but a little more comfortably.
[edit] Where are they now
In practice however Biopace chainrings have become out of fashion and Shimano has stopped their production.
It should also be noted that within the bicycle industry the term "biopace" has come to represent any new product introduced claiming to revolutionize a certain aspect of cycling, yet on closer inspection is poorly made and doesn't perform as advertised.
