Wheels never stop turning

Something's been bothering me and I'm hoping someone out there can help.

I've been thinking about it for a couple of days and still can figure it out.

Here's the problem:

 

Do you know what a unicycle is?

I'm sure you do. It's a bicycle with only one wheel. If you ever went to the circus you've probably seen the clowns or jugglers on one. Maybe even on the high wire. 
On a unicycle, you're able to go forward or backward. Both ways, correct?

I have a bicycle. I may not use it that much, but I have one. Someone correct me if I'm wrong, but you can't go backwards on a bicycle. You can only pedal forward.

 

Why do we call something that goes both ways a 'uni' cycle and something which only goes one way a 'bi' cycle? Shouldn't it be the other way around?

 

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