Fallen Nassau Police Officer Geoffrey J. Breitkopf so loved riding a motorcycle in his spare time, he once winded his way through the vast open spaces of South Dakota and Wyoming on an eight-day trip with a cop buddy, Nassau Police Medic Bill Youngson.

Breitkopf, a veteran tactical officer who was killed in the line-of-duty from friendly fire in Massapequa in 2011, was top of mind for Youngson and a contingent of about 20 other Nassau police officers Tuesday as they began a 300-mile journey south to Washington on the Police Unity Tour, a charity ride that raises money for the national memorial honoring fallen police officers.

“Whether you’re on a motorcycle or a bicycle, it’s that being free feeling, and that was Geoff,” said Youngson, as he geared up to participate in this year’s ride.

Hundreds of police officers — minus their dress blues and instead rocking spandex shorts and helmets — descended yesterday morning on the grounds of the National September 11 Memorial & Museum in lower Manhattan, to set off for the ride.

Bagpipers from the Port Authority Police Department played, a color guard and a police officer sang the national anthem in a send-off ceremony that also included the reading of the names of police officers from departments in New York and New Jersey, who were killed in the line of the duty. The names of officers who were killed on Sept. 11, 2001, or whose deaths have been directly linked to the terrorist attics, were also read aloud.

The ride, which began in 1996, spans four days and concludes Friday at the National Law Enforcement Officers Memorial and Museum in Washington, D.C., where thousands of other police officers on bikes, and the family members of the fallen, will converge.

The tour raises money for the upkeep of the museum and memorial, which is inscribed with the name of every law enforcement officer killed while on duty in the country. This year organizers estimate the ride will raise in excess of $2 million.

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Breitkopf, 40, a member of Nassau’s elite Bureau of Special Operations, was killed by friendly fire from a Metropolitan Transportation Authority police officer at a Massapequa crime scene.

MTA Officer Glenn Gentile shot Breitkopf, who was in plainclothes and carrying a gun, because he thought he was an armed suspect, officials have said. Then-Nassau District Attorney Kathleen Rice deemed Gentile not criminally responsible for Breitkopf’s death.

Acting Police Commissioner Thomas Krumpter, who last year biked the tour, said in a statement:

“Although I am not able to participate and ride with the Police Unity Tour this year, I wish them a safe trip on their journey in support of all law enforcement officers and their families that have made the supreme sacrifice.”

Nassau Lt. Chris Ferro, commanding officer of Narcotics Vice Squad, was riding Tuesday for the fourth time. On the back of his bicycle were the photographs of two Nassau police officers killed in the line of duty in October 2012: Police Officer Arthur Lopez, 29, who was gunned down during a traffic stop near the Nassau-Queens border, and Joseph Olivieri Jr., 43, a Nassau highway patrolman who was struck by an SUV while at an accident scene on the Long Island Expressway.

“It’s a bigger cause than oneself,” said Ferro, who investigated Lopez’s killing and called it his “most important case.”

Darrell Fuller, of Queens, was convicted at trial of first-degree murder and sentenced to life in prison without the possibility of parole in Lopez’s killing.

Ferro added: “I have many friends and colleagues who gave the ultimate sacrifice whose names are engraved on the memorial wall in Washington, D.C. To me it’s in their memories, their families’ memory. Their families will know we’ll never forget them.”