Surely you must be joking. The history of mountain biking is dirty beyond belief. Even examining only the winners of men's XC world championships, without even getting into all the other violations from other riders we have;
- Jerome Chiotti (never busted, but admitted it publicly on his own initiative years later because he couldn't live with it, was stripped of title)
- Filip Meirhaeghe (busted for EPO)
- Christophe Dupouey (eventually committed suicide after being found out as part of a performance enhancing drug trafficking ring)
- Michael Rasmussen (pulled by his own road team from TdF '07)
- Roland Green suspended a couple of years after his world titles, although perhaps technicality for failing to file a TUE/Therapeutic Use Exemption on time with the UCI, for a medication that he had previously received permission to use, not as serious a violation)
Considering that there haven't been that many different world champions in the past 15 years (some multiple winners), this is an awfully high percentage that have been tripped up.