Was the bike an enclosed streamliner? If they mentioned 'teardrop' in the article,it suggests that there was a body on the bike.
At Bonneville,the MC streamliners have drag coefficients on the order of Cd 0.11.
The flow is completely attached with the exception of the skids,minor wheel openings,ducts,and tail area,where the tailpipe and parachute tubes are located.
If the tailpipe was forward on the 'fuselage' the exhaust would be dumping into a lower pressure area compared to the wake,but would upset the boundary layer,potentially triggering flow separation which would cause a drag increase.
Could you dig up that article? It would be nice to scrutinize it.
