i discussed the smith machine aspect on another board one time and did a little research on it. basically if it is a counter weighted machine the bar will be anywhere from 15-25 lbs but this is not the total issue. even after you add weight the counter weight is still there. but how to think of it is like this..........
say you have 5lbs of counterweight to the 45 lb bar. obviously this will make 45 seem really light. now add two wheels to it and the counterweight does not remain as effective. now if you were to put 300 plus pounds of weight on the bar the counterweight becomes almost obsolete and the bar that is 45lbs starts to factor more towards its actual weight. at what point these #'s work at what % i do not know. some really smart guy probably has a formula but i am not that guy.
hope this makes sense and doesn't seem like babble.