I actually just asked SAA about this for a JHB-Nairobi flight. In the end I'm flying Kenya Airways ($80 for a bike), but in case useful to anyone else, this is what SAA customer service said: On our direct SAA flights, you are entitled to 1 additional piece at 23kg for your Sporting equipment at no additional charge. This allowance excludes SA Airlink, SA Express, code share and star alliance partners.
