As you likely already know, the word for "bicycle" in German is das Fahrrad. This is sometimes shortened to das Rad, which is the German word for "wheel."

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Oben rechts ist ein silbernes Rad auf rotem Hintergrund.

On the top right is a silver wheel on a red background.

Caption 30, Bundesländer und ihre Rezepte: Rheinland-Pfalz

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Jetzt waren wir den ganzen Tag mit dem Rad unterwegs.

Now we've been on the go with the bikes all day long.

Caption 46, 48 h in Innsbruck: Sehenswürdigkeiten & Tipps

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The phrase mit dem Rad fahren in the example above can refer to multiple people who are all biking. However, the plural of das Rad is die Räder. 

Vier Räder und ein Lenkrad.

Four wheels and a steering wheel.

Caption 4, Werbung Mercedes-Benz TV Spot mit Roger Federer

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The noun das Rad is used to describe the circular wheel itself. But when there is a bracket, caster, or housing involved, or when you are referring to mounted wheels on the underside of an object, die Rollen is the right word to use. A piece of furniture or a wheelchair has Rollen, not Räder. The two examples that follow refer to rollerskates and a longboard respectively. 

Die Schwierigkeit: sich sicher auf acht Rollen bewegen.

The difficulty: moving confidently on eight wheels.

Caption 17, Tanzen: Auf acht Rollen

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Dadurch haben die Rollen sehr guten Grip auf der Straße.

Therefore the wheels have very good grip on the street.

Caption 32, Longboarding mit Lassrollen

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Additionally, the word for "motor scooter" is der Motorroller, or der Roller for short. 

Further Learning

You'll find many examples with the various meanings of das Rad and die Rollen (also "role" as in an acting role!) on Yabla German.