just do your other bicep exercises that work each bicep head individually, then burn out at the end by curling just the olympic bar by itself. if it's the amount of weight that's the problem, that should solve it, but some people just don't feel comfortable curling a straight bar. could also be your grip. if you're using this exercise to put on some mass to your biceps, generally you have to go heavier than usual and stick to a normal grip (about shoulder width), so don't worry about cheating, just make sure it feels right and works the muscle at the same time. if you have to cheat with the last few reps, go for it, but make sure you are working the muscle without hurting your tendons. if it still doesn't feel right then you still have quite a few other bicep exercises to try.