David K. van Keuren was a bighearted man who had a big laugh and big quadriceps.

The laugh came with him from Wisconsin, and it rang out in all directions no matter where he was. The leg muscles came from his relentless bicycling.

He was forever on his way to or from somewhere, and he got there on his bicycle, rain, shine, sleet or snow. He never turned down an invitation, even if he could make only a short appearance at a dinner or a party or a fundraiser, his friends said.

Rae Johnson, a fledgling artist who knew him from a local bicycling group, said he showed up early at her first exhibit with a single red rose, which he gave to her, and then asked which art piece was her favorite. She pointed one out. He immediately bought it, and "it made all the difference for me," she said.

Her husband, Mark Hathaway, met van Keuren on a ride through the Maryland countryside. Hathaway struggled up one of the hills, only to find van Keuren at the top, waiting for him "with this giant smile on his face, kind of a smile like, 'Isn't it great to be out here on a beautiful, sunny day?' " Hathaway said.

Later, when van Keuren stopped to help him with a flat tire, he cheered up the frustrated cyclist. "He understood important things about life. It wasn't about flat tires," Hathaway said.

That's why van Keuren always carried spare parts and stopped to help if someone else ran into trouble on the road.

On the morning of March 26, as he biked the 1300 block of South Capitol Street on his way to work, the 53-year-old historian was struck by a dump truck and died at the scene. No charges have been filed.

The news of his death reverberated through the local bicycling community; the gay community; the Naval Research Lab, where he worked as a historian; and the wide circle of friends that van Keuren had nurtured during his 18 years in the District.

He was remembered with an informal and a formal memorial service in Washington, a church service in Wisconsin and a memorial ride yesterday on a route known to bikers as "Thursday Thunder," which winds through Rock Creek Park up through Potomac.

Friends at the services spoke about thank-you notes he had sent that brought them to tears. Kai Henrick Barth recalled being a graduate student at a professional history of science conference where almost everyone criticized his research. But van Keuren befriended him and urged him on.

Van Keuren, born and raised in Wisconsin, had been a quiet boy who read voraciously and was involved in everything except sports in high school, said his older brother Michael. He took up serious bicycling while attending college in Eau Claire and earning his master's degree at the university in Madison.

But he was not an only-in-Wisconsin youth: He went to London at age 16 and backpacked around Europe at 22. He had a doctorate in the history of science from the University of Pennsylvania and started working for the Naval Research Lab in 1986.

"He really didn't come into his own until he came to Washington," his brother said. "He really needed the big city."

He didn't need a car or a television. Between work, the gym, the countless AIDS rides, regular training runs with the Cycling Fools biking group, a writing group, a reading group, classical music concerts, art openings, sailing excursions and many afternoons whiled away over good coffee and good conversation with friends, van Keuren had a full schedule.

"Occasionally, he would talk to me in French," said David Michael, a musician who was one of his oldest friends in Washington. "Now, I don't speak French," but that never stopped van Keuren.

A week before the accident, he and a co-editor, Helen M. Rozwadowski, had sent the proofs of his third book, "The Machine in Neptune's Garden: Historical Perspectives on Technology and the Marine Environment," to the printer.

For a time, van Keuren would get up early and help out at Food and Friends, a Southeast Washington kitchen and pantry for people living with HIV/AIDS.

"He liked giving gifts, but he didn't like receiving gifts," said his sister, Marina Nelessen.

He spent one two-month period on a scientific expedition aboard a Russian ship in the Arctic Circle -- and managed to get a phone connection to call his mother on her birthday.

No one is perfect, and van Keuren could be a curmudgeon, complaining to friends if he had not taken a trip in a while or if the weather was bad. He had strong opinions and did not shy away from disagreements, his friends said.

"But you never thought he was a jerk for having those views," friend David Michael said. "You always respected and liked him because he respected people."

Historian David van Keuren, 53, was an avid member of the Cycling Fools bicycling group.