Day #5: Rest Day in Manchester

On Wednesday I had a rest day, so I'll just pontificate on the topic of gears...

1. Gears and Chains (lesson #1)

If you want one wheel to drive a second wheel, you can press them close together, so by friction one follows the other:

The first problem with this is that the wheels need to be pressed together really hard or else they will slip:

and if they are pressed too hard they will be difficult to turn.

This problem can be solved by adding teeth to the wheels. The teeth interlock and when one wheel turns, the other follows:

We've made gears! Now they don't need to be pressed together at all!

But, there is a second problem... when the gear on the right turns clockwise, the interlocking gear will turn counter-clockwise:

We can fix this by adding a third gear... every odd numbered gear will turn the same way:

Connecting two gears with a chain, keeps them from slipping _and_ allows both gears to turn clockwise, and the gears can be far from each other:

2. Gears and Chains (lesson #2)

A crank is an arm for turning a gear:

If two gears are connected by a chain and both are the same size and have the same number of teeth (size and number of teeth usually go together), then, when the gear on the right turns 1 full revolution, the left gear will also turn exactly 1 full revolution:

Turning the crank once pushes 20 teeth's worth of chain around, which causes 20 teeth's worth of rotation of the second gear...

If the front gear with the crank has 20 teeth, but the back gear has only 10 teeth, what do you think will happen? Yes, the back gear will go around twice!

What if the front gear has 20 teeth, but the back gear has 40 teeth? Yes, the back gear will only go around half of a rotation!

The front gear pushes around 20 teeth's worth of chain, but that's only enough to turn the gear with 40 teeth through half a revolution.

So, we can see that by using different size front and back gears we can change how many times our wheel goes around for each time we pedal our crank... this will be useful in getting up hills :-)

There will be more info on gears later... sorry :-)