Career
He rode for Priority Health from 2006 to 2009. He came fourth in the UCI Road World Championships time trial in September 2009, and had signed to join UCI ProTour team Garmin–Transitions the following season. In December 2009, a urine test taken at the 2009 USPRO Time Trial Championships the previous August, in which Zirbel came second, tested positive for DHEA. Zirbel denied having taken the substance knowingly, but he nevertheless voluntarily accepted a two year suspension, and immediately announced his retirement.
His suspension was reduced in March 2011 to 18 months, which had already been served, in recognition of assistance given to the United States Anti-Doping Agency.
He suggested that, despite earlier comments about retiring, he would seek to resume his racing career. He maintains that he ingested the banned product inadvertently.
In 2011, Zirbel rode for Jamis–Sutter Home. On May 28, 2011 he placed 2nd in the United States Pro time trial championship, behind David Zabriskie.
He joined Optum–Kelly Benefit Strategies for the 2012 season.
