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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?

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up vote 6 down vote accepted

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.

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So if I don't want to lift that heavy, I can go with 30mm and maybe even strengthen my grip? – Baarn Sep 25 '12 at 18:18
I wouldn't advise it. Honestly, having to grip a larger bar would probably do more for your grip than having to fight rotation. (See also: products like "Fat Gripz" which increase the bar diameter as a grip exercise.) – Doc Faustus Sep 26 '12 at 14:13
Thanks for the advice, I'll check them out. – Baarn Sep 26 '12 at 14:16
@Informaficker When doing exercises that rotate the bar (cleans, snatches), you most certainly don't want to be working your grip at the same time. If you're going to buy equipment, a bar is the place to spend good money. – Dave Liepmann Oct 30 '12 at 20:58

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.

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