We got Kyle and his smokin' hot wife, Samantha, out at Craig's ... and the conversation quickly turned to racing's dirty little secret -- peeing in the cockpit!

Busch told TMZ Sports he's "tried like hell" to pull the trigger -- races are 3 HOURS long -- but he can't bring himself to go full R. Kelly on the track.

But KB says some guys got no problem taking a leak mid-race ... and even claims one of his teammates broke the seal BEFORE his race started!