I have always used 自転車 to express my use of a bicycle, when out of nowhere, my word-of-the-day drops チャリ onto my lap, saying it means the same thing. So now I'm left to wonder: what's the difference between the two, and which has the more common usage?
To refer to bicycles:
- 自転車: The formal word for bicycles. It includes so-called 電動自転車 (electric-assisted bicycles), but does not include motorcycles with petrol-powered engines.
- チャリ: A common slang term for 自転車. It's an abbreviation of チャリンコ which means the same thing. By the way, according to this, the etymology of チャリンコ is not clear. You can use チャリ when you talk with your friends, but it's safe to avoid this in formal business settings.
- バイク: Some people who are keen on motor sports use バイク to refer to bicycles, like English speakers do. Some compound nouns such as マウンテンバイク and ロードバイク always refer to specific types of bicycles, too. But most Japanese people use バイク as the synonym for オートバイ (motorcycles).
FWIW, I personally have always stuck to 自転車. When I was a teenager, maybe more than 90% of my classmates used チャリ, though.
自転車 can refer to any kind of one/two/four-wheeled human-powered transport, including sport bikes (and apparently even some motorized bicycles but NOT motorcycles, which are often confusingly called
チャリンコ are only used for the standard one-speed city/shopping bicycle.
ちゃり is short for ちゃりんこ which is slang for 自転車.