According to the Readers Digest Great Encyclopaedic Dictionary
(1964) in the section SPORTS AND GAMES : CYCLING a bonk wagon
is defined as follows;
Vehicle in a road race which carries riders' clothes and riders who have retired in the race.
This will be amusing to any British readers as since the 1980's the word "bonk" has become synonymous with "shag", i.e it means the same as "fuck" without being taboo. The usage was popularised by the Sun newspaper.