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Tension not Compression

It's easy to think of the spokes as columns supporting the wheel and helping it retain its shape. But, the "support" that the wheel receives is created by pulling the spokes towards the center of the wheel (tension) rather than pushing out from the center (compression). If you've had the occasion to hold a spoke that was removed from a wheel, you've probably noticed how flimsy it is. You could bend one in half without too much effort. However, if you tried to pull one apart you would not be able to. The "pulling" of the spokes toward the center of the hub is what gives the bicycle wheel its strength.

Paolo Salvagione RealMedia Clip
Bicycle maker Paolo Salvagione discusses how bicycle wheels get their strength.

So just how strong are bicycle wheels? "Wheels, from what I remember, can hold about 400 times their own weight on a regular basis and they won't collapse until roughly 700 times their own weight, which makes them one of the strongest man-made structures on the planet," explained bicycle maker Paolo Salvagione.

Pneumatic Tires

The pneumatic (or air-filled) rubber tire is something we take for granted today. Almost every type of bicycle wheel has a pneumatic tire on its rim. The development of the pneumatic tire was an important landmark in the development of the modern bicycle. Prior to its invention in 1888 by John Boyd Dunlop, bicycling was a bumpy and somewhat uncomfortable experience. Tires were made out of leather (and later solid rubber) attached to a wood or metal rim. The air-filled tire brought with it a smooth, comfortable, and stable ride. It's no surprise that it also helped make bicycling more popular.


 Paul Doherty RealMedia Clip
Paul Doherty talks about the difference between "road" and "mountain" tires.

 Are your tires fat or thin?

Depending on the type, your bicycle has either fat tires or thin tires. Most road bikes and touring bikes have thinner tires, while mountain bikes have big fat tires. Each type of tire has been adapted for the surfaces they ride on.The road tires are inflated to 100 or even 120 PSI (pounds per square inch). A firm thin tire on the asphalt surface won't flatten much. The less the tire flattens out on the bottom, the less surface area is in contact with the road. Less contact in this case means less friction, and more speed. This is why keeping tires properly inflated is so important.  

Wide and fat mountain bike tires flatten out more on a hard asphalt surface. However, on a dirt trail, a mountain bike tire "floats" on top of the rough surface. A thinner road tire would cut deep into the dirt, forcing the cyclist to pedal her way out of a hole.

It is easy to imagine a pneumatic tire "flattening out" on the bottom as it rotates. But surprisingly, steel train wheels on a steel rail experience the same effect. The temporary flattening-out of the wheel, as well as sinkage on the contacting surface, is what leads to "rolling resistance." This term is used to describe how much energy is "lost to the road" as a wheel moves forward. Tires with low pressure traveling on soft ground tend to have higher rolling resistance. This one of the major reasons why road racing is a faster sport then mountain biking.

Although you can't tell from this image of a locomotive, a steel wheel will "flatten out" on a steel rail. All wheels must overcome this "rolling resistance."
This mountain bike tire is made of natural rubber, which some believe grips the trail better than synthetics. TIre Treads

Tire treads

The treads of mountain bike tires can affect performance. Rough or "knobby" treads grip dirt trails better, but create greater friction on smooth roads. Smooth tires grip smooth roads better, with less resistance, but slip on dirt trails. Mountain bike tires manufacturers produce a variety of different patterned nobby treads. While cyclists have different preferences, there has been little scientific support for one tread performing better than another.


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