Well Castor gras and castor sec were made by the aboriginals. They trapped beaver and used their fur to make these. Well castor gras is a fur coat (made from beaver skin) they put the smooth, downy...
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Use a shopping cart for an example, the rear casters (wheels) are fixed, the front casters are steering casters.
Pollux
The next time you go to your local Shopping Center have a look at the wheels under the shopping trolley. That is a castor wheel.