Keirin is a Chinese compound meaning 'compete wheel'. The characters used to write keirin are . Keirin makes little sense until you compare it with its friends keiba 'compete horse' or 'horse racing' and keitei 'compete boat' or 'boat racing'. Keirin in fact refers to competitive cycling events.
While words like keirin are most naturally written with Chinese characters, which show their component meanings, for some reason the industry itself often uses hiragana or katakana to write keirin. For example:
This is helped both by the large number of strokes needed to write . Moreover, the reading kei is a less familiar reading for the first character than kyō. As a 'special' reading, there is perhaps a greater temptation to write it in kana.
Despite the high frequency of in public notices, etc., a Google search on the Internet (September 2003) finds that the Chinese-character version is by far the most common.