There've been a lot of great suggestions in this thread. AS a rule, Southern Rock is great place to start for biker type audiences along with "Bad Motor Scooter", "Magic Carpet Ride" type songs that have been mentioned. Doobie Bros, Santana, CCR type of bands that fall outside of the Country rock genre are also well received. Outlaw country (Wailen, Hank Jr, Charlie Daniels, etc) would also go over well. But yeah, bikers are people and like anyone else, just want good music.
I would tend to lean towards CCR's version of Grapevine vs, Marvin's. I personally prefer Marvin's but for this type of crowd, I'd lean towards rock vs. Motown.