Handlebars are the bar you hold and steer a bicycle with. Grip the handlebars, drop handlebars, let go of the handlebars.
The shape decides the riding position and therefore everything about how a bike feels: drops for speed, flat for control, swept back for sitting upright. A handlebar moustache is named for the shape, and it is one word in that use.
1nounhandlebarsThe bar you hold and steer a bicycle with.
“He gripped the handlebars tightly as the bike raced down the steep hill.”
handlebars is a part.
handlebar comes from handle + bar.
Etymology from Wiktionary, CC BY-SA