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.

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.

"The actual crash I’ve got a memory blank. I’ve obviously hit the ground hard and cracked my helmet. The first thing I remember is standing up on McCarrs road and looking around," he said. "It was a bit of a war story."

He said the bunch of cyclists on the ride, organised by the Manly Cycling Club, are all highly experienced and know the road well.

"We ride on that route very regularly, like weekly, so we all know the course very well," he said. But at 5am "it’s quite dark when we’re riding ... there’s no marking there of that obstacle".

He couldn't understand why someone would erect a makeshift speed hump on a road through the national park – although graffiti scrawled on a nearby sign suggests it's targeting noisy vehicles.

"It’s in an area of national park, there’s no housing nearby. I’m at a bit of a loss," Mr Holland said.

"It would seem it’s a misguided message to motorists. But how they’ve decided to communicate that message is really irresponsible and had major consequences, and could have had much worse major consequences," he said.

"For them to spend the time to concrete a speed bump on a backroad … is … I’m dumbfounded. I guess it’s the sort of mentality that lights a bushfire and the impact that has on people. You just can’t explain the stupidity of the behaviour."

Richard Bjorkmann, another cyclist injured in the crash said the route was very popular with local cyclists and he rode through it about three times a week.

"There would be probably thousands of cyclists going through there on a full-day basis. It's one of the most popular bike areas as well in northern beaches ... it's secluded, there's no traffic lights," he said.

Mr Bjorkmann called police when he got home, but said he was disappointed to be told they were unlikely to take any action "unless someone comes forward with evidence".

Mr Holland said he just wants people to think before they act.

"Someone sabotaging a road with a major obstacle has bad consequences," he said.

A spokeswoman for NSW Police said she could not find a record of the call.

with Rachel Clun