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Well the question title pretty much says it.
While browsing the web for dumbbells I stumbled over the supposedly cheaper 30mm bars and the 50mm bars that all seem to be ball beared.
What is the purpose of the ball bearings in the bars and when should consider buying these?
The ball bearings allow the weights to rotate without the bar in your hand needing to rotate. As you move a weight in most exercises, unless you have amazing form, some amount of rotational energy will be transfered to the weight. This is true in Olympic lifts even if you have perfect form.
If the weight was not allowed to move freely, you would need to counteract that rotational energy in order to keep your grip on the bar. So instead of the bar rotating in your hand, a bar with ball bearings allows the weight to move and the bar to remain still.
From some of the videos I've seen, you want dumbbells that have the weight free from the handle that you hold onto for certain lifts, like the snatch. This allows you to throw the weight and not have to spin the bar in your hand.
