I found this by Chad Waterbury:
When asked about the best biceps exercise, my answer hasn't changed in ten years: the rope climb. You only need to visit Muscle Beach in Santa Monica — a place where there are enough ropes to excite a family of gymnasts — and observe the biceps development of guys who climb them on a consistent basis.
First, from a biomechanics standpoint, the rope climb forces your arm to pull from a position that's close to the midline of your body. This overloads the elbow flexors more than the upper back muscles.
It's been said that the chin-up won't build big biceps if your back is strong. I can't say I agree, but I understand the argument. The solution, however, is simple: make the movement a biceps-dominant exercise through the law of biomechanics.
Any time you pull with your hands close together, there's no way your back can take over. And when you pair that with the fact that your hand is in a neutral position to target the brachialis (a key upper-arm booster), you've already got yourself a killer exercise.
Second, the rope requires your gripping muscles to work with ferocious intensity. In fact, there's no better exercise to boost your gripping strength than climbing a rope. Indeed, there's a direct correlation between your gripping strength and your biceps mass.
Third, since your upper back is also helping your efforts, some of the burden is taken off your elbow joints. Plus, it appears the biceps will grow only when the supporting (upper back) muscles are strong enough to handle the added girth.
so it seems my suspicions about it being more of a biceps move than an upper back move were correct. I'll use it in place of curls for a while and see what happens