Anscheinend gehen beide. Yer pays yer money and takes yer choice!

http://www.merriam-webster.com/dictionary/caster
cast·er
Pronunciation: \ˈkas-tər\
Function: noun
Date: 14th century
1: one that casts ; especially : a machine that casts type
2or cas·tor \-tər\ a: a usually silver table vessel with a perforated top for sprinkling a seasoning (as sugar or spice) b: a usually revolving metal stand bearing condiment containers (as cruets, mustard pot, and often shakers) for table use : a cruet stand
3: any of a set of wheels or rotating balls mounted in a swivel frame and used for the support and movement of furniture, trucks, and portable equipment
4: the slight usually backward tilt from vertical of the axis of the steering mechanism of an automobile for giving directional stability to the front wheels

http://dictionary.cambridge.org/define.asp?ke...
caster Show phonetics
noun [C] (ALSO castor)
a small wheel, usually one of a set, that is fixed to the bottom (of the leg) of a piece of furniture so that it can be moved easily

I think, though I'm not certain, that most people use castor for a small wheel, and reserve caster for caster sugar (Streuzucker, laut LEO).

That was my first thought, Steve. I automatically thought of sugar with "caster". I was a little surprised when I looked it up to find that apparently both forms are used for the wheels.

Supporting MikeAL and Steve (BE).

Ich kenne es als: "castEr sugar", aber "castOr oil" und nicht andersrum (obwohl die anderen Formen scheinen, auch vorzukommen).

I think I will take castor AND caster (as a synonym).
Thanks a lot for you support. I was just a little confused, because I found both versions ("caster wheel" and "castor wheel").