Rotating Bodies are Good

copyright Brian Miller

Acceleration is very important; it is critical for street riding. A rider needs to quickly scoot ahead of, or away from danger. A machine that can accelerate well can also decelerate well. Rapid deceleration is pretty good for avoiding danger also.

Much more valuable than reducing overall bicycle weight, acceleration improvements are made by reducing rotational weight. Reducing weight farther out toward the circumference of a rotating member helps best. Reducing weight near the rotational center has negligible effect. As always the compromise with durability will determine how much weight ends up at the outer edge of that rotating system.

Lightweight spoke nipples, rims, tubes, and tires will make the greatest benefit to acceleration and stopping.

Rotational weight liabilities also include heavy things at the edges of the entire bicycle system like the saddle and handlebars, especially the ends. Here, the rotational axis is the line the tires draw on the ground. Reducing the weight of these lumps on the outskirts of the bike seems to benefit acceleration. Adding or removing weight near the center, for example lighter crank spindle or wheel axle, has little affect on acceleration/deceleration.

Human power generation is similar to internal combustion power generation. Pushing on longer crank arms, a human can generate good torque, or turning force, good for pulling weight or going up a hill. The drawback being that beneficial high rpm cannot be achieved. Something about the centrifugal force of pedaling a large circumference causes the legs to go sloppy and power generation is reduced. It follows the example of the lever and the movable fulcrum in basic physics.

Using shorter cranks, a pedaler, or engine, will produce less torque, but will produce more horsepower. This is because a higher rpm can be achieved. The human body, like the motor, works more efficiently at this faster pulse. More power is made per unit of exertion. To go faster easier, a rider should try to achieve higher crank rpm.

Reducing weight at the pedal end of the crank arm aids acceleration and allows higher rpm. Reducing weight at the center axle of the crank has no effect. Stop bragging about your titanium BB spindle, you poser.

The converse of this reduction of inertia is to add weight at the edge of rotating bodies to conserve momentum. Heavier tires will reduce top speed but allow a rider to coast farther. Heavy pedals will inhibit high rpm but will benefit a steady pace.