Well I've seen a few people looking for rear sprockets and not having much luck. When I bought my X-Challenge I bought it with a 14tooth front sprocket and a 49tooth rear and it rips. The sprocket is made by Sprocket Specialists but I'm not sure if you can get them from their site. My buddy Wayne at South Sound BMW mentioned I think that they had these guys make them a stack of these sprockets. So you might want to contact the parts guy at South Sound. your wheelies may vary.