According to SimGuruSnelson, the 8-Second Rider Showtime Achievement was a discrepancy on the site, and is no longer a valid Sims 3 Achievement. The Achievement has been removed from the site as of today. Snelson has also confirmed that they do plan on adding new achievements “soon”.
Thanks for bringing the 8-Second Rider Achievement discrepency to our attention. It is not a valid game achievement and has been removed from the web. Apologies for any inconvenience or frustration this may have caused you and all Simmers. Though, for me personally, I could watch my Sims ride that bull all day long and not get tired of it! We are absolutely still planning on introducing new Achievements on a regular basis. Coming soon!
8-Second Rider (Silver) *Requires The Sims 3 Showtime
This achievement has been removed and is no longer a valid achievement.
Description: Ride the Mechanical Bull at the Crazy Cowboy or Crazy Cowgirl setting 5 times in a row without getting Pwned