A homemade bump in the road apparently aimed at "noisy" motor vehicles has left a group of early morning cyclists in a mangled heap.

A group of about 30 experienced racing cyclists were riding along McCarrs Creek Road in Ku-ring-gai Chase National Park near Church Point about 5am on Thursday when the front rider struck the "speed hump", sending five of them - travelling at up to 45km/h - to the ground.

Graffiti near the obstacle suggests it's aimed at motorists.

The most seriously injured rider, Tony Holland, was set to fly to Italy to compete in the UCI World Masters Road Championships on Sunday - an event he'd been working towards for the past year.

He checked out of hospital on Thursday afternoon with concussion, broken ribs and a broken collarbone, his competition plans destroyed along with his bike.