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Castor wheel I used 4 7/16" inch Carbon Steel Bearing Balls (aprox. 11mm of diameter). http://www.ebay.com/itm/50-PCS-11-113mm-7-16-inch-G16-Hardened-Carbon-Steel-Bearing-Balls-/390642299507? You can use any diameter, just change diam variable.
Tetrawheel
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Tetrawheel
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