if you have relatively good control, a regular standing curl shouldn't involve the shoulders as much as you're saying, if at all.
Pumpster, what are your thoughts on why cheat curls are so effective?
Delt stress is inherent for many, it's not a question of control or form. Delts are so prominently effected from weights that they often wear out before biceps, which isn't obvious until the end of the set.
Keeping upper arms in a fixed position when the barbell's pulling downwards and out in front of the body actually requires plenty of stabilization, from the delts. It isn't obvious how much both because there's no overt motion and because the burn only becomes noticable towards the end of the set. Just maintaining the arms steady in that context requires plenty of stabilzers, because with weights the pull is both downwards and in front of you, where the weight is held. Not the same with DBs, where the weight's held on each side rather than in front of you. That's part of why i like cable curls, the pull is not in the same directions and doesn't tax the delts in the same way.
Preacher and arm blaster definitely isolate biceps more, it's not maybe. Clearly the upper arms are far more immobile. The only way to get the same effect on standing BB curls is to use an arm blaster or to do curls standing against a wall. Upper arms aren't held nearly as steady with regular curls even when done under control, and trying to keep them steady enlists plenty of stabilizers, in this case delts.
Cheat curls and seated BB curls are better because they take the delts out and because the money part of ROM in curls especially with weights is just after the beginning and before the end of the ROM. Same reason box squats are better than full squats, and why partial range hack machine squats are better-in both cases more effective and less harsh on the joints.