The Effect of Weight on Speed

The energy needed to move a weight up a hill increases in proportion to the weight being moved.

For years, many of us have been spending hundreds of dollars to reduce the weight of our bicycles.

Even Lance Armstrong in the Tour de France switched to a lighter frame, components and wheels during mountain stages to reduce overall bicycle weight. The logic behind his move was that a lighter bike will have faster acceleration, lower rolling resistance and requires less energy cost when climbing.

In simple mechanical terms, it takes more energy to average the same speed with additional weight. A valid fact, provided that frame, crankarm and wheel stiffness can be maintained with a lighter machine.

Here are some tips and information on when considering lightweight components and bikes to improve your performance:

The ability to climb efficiently, accelerate quickly out of turns, at the start of a race or to chase another cyclist down is critical to racing cyclists. Because adding weight to a bicycle, wheels and components increases inertia, it will slow down the rate of acceleration. But reducing small amounts of weight on the frame does not affect steady state speed greatly, but rotating weight is different.

It has been known for years that additional weight slows one down when climbing. The energy needed to move a weight up a hill increases in proportion to the weight being moved--in other words, the weight of the bicycle and the cyclist. Less weight on the bicycle means less weight dragged up the hill.

In addition, weight adds rolling resistance to tires. Extra weight causes a greater deformation of the sidewalls and tread of a tire, thus, increasing the rolling resistance. Rolling resistance increases in direct proportion to the weight that the wheel supports, and is equal to about 0.3 to 0.5 percent of the load on the wheel. It can be reduced by lighter loads on the wheel, as well as using larger diameter wheels, smoother and thinner treads and using tires with stronger sidewalls.

Lastly, low weight in rotating components is even more important. To accelerate a wheel or pedal and shoe system, kinetic energy of rotation must be supplied, in addition to the kinetic energy of linear motion. For example, with a wheel, if the weight is mostly concentrated in the rim and tire it would take nearly double the energy needed to accelerate it than an equal nonrotating weight. In other words, one pound added to a wheel or shoe/pedal system is equivalent to nearly two pounds on the bicycle frame.

Also, it should be mentioned that increased weight retards cycling performance far less than aerodynamic drag does. Thus, for example aerodynamic handlebars and disk wheels (which increase weight but reduce wind drag) in a 40 kilometer time trial may do more for a cyclist's performance than using an ultra-light frame and wheels. But, now-a-days lightweight disc and tri-spoke wheels can be built with the same shape and rigidity as a heavy disk wheel of a few years ago.

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