The owner of a 35-year-old bike shop on San Francisco’s busy Valencia Street is driving a neighborhood crusade against an unlikely enemy: the city’s plans for a protected bikeway outside his store.

Valencia Cyclery owner Paul Olszewski wants to torpedo the barricaded lane idea, which he says would take 14 parking spaces out of the 29 on his block, along with a center turn lane that doubles as a loading zone. He spent hours last week hoofing up and down the 2-mile artery, knocking on every merchant’s door from Market Street, on the north end, to the southern boundary at Cesar Chavez Street.

The consensus, he maintained, was clear.

“I canvassed over 100 merchants, and 99.9% of them are objecting” to the protected bike lane, Olszewski said Monday morning as he worked in his shop, a storefront packed into the bottom floor of a baby-blue Victorian in a crowded strip of restaurants and apartment buildings. Nearby, the owner of a stationery store was printing flyers for an open house-style meeting Monday night at City College, where critics mingled awkwardly with bike advocates.

What had begun as scowly opinion-spouting in the neighborhood has become a searing conflict, with each side charging forward to present its vision for a bustling San Francisco street.

City officials want to install barricades and shift parking along the full length of Valencia Street to separate cyclists from traffic. Under the proposed redesign, engineers would line the existing green and white bike path with posts, concrete mounds and railings, all set against a row of parked cars. The agency has already set posts along the first segment, from Market to 15th Street, and officials expect to start the southern section, from 19th Street to Cesar Chavez, as soon as next month. They would finish the final strip, from 15th Street to 19th Street, sometime next year.

Bike advocates have long pressed for a bulwark on Valencia, and they have backing from City Hall. Mayor London Breed directed the San Francisco Municipal Transportation Agency to speed up production of safe bicycle infrastructure at various locations, and she’s cited Valencia as an essential north-south corridor that could use a protective buffer.

“Safety is the No. 1 concern,” said Zane Williamson, who bikes down Valencia each day to get downtown from his home in Noe Valley. The route requires him to jostle among SUVs, delivery trucks and ride-hail drivers who are angling for a place to pull over — a maze of obstacles that intimidates his wife and daughter, who refuse to ride there.

“Right now there’s a finite amount of space on the road, and cars pretty much own it,” Williamson said. “We’re just asking for a small percentage back.”

But the changes would take up nearly half the curb space on Valencia. Some 53% of the existing curb would remain, divided among metered parking, passenger and commercial loading, blue handicapped zones and short-term parking, said Erica Kato, a spokeswoman for the transportation agency.

Olszewski is unwilling to accept that trade-off.

Customers already orbit the blocks to find spaces to park, he said. If the number of available spots is cut in half, he believes many of them would shop elsewhere.

“Last Saturday a fellow came by with (his friend) and two kids, and bought an entry-level bicycle,” Olszewski recalled. “As I was walking him to his car I asked how long it took him to find parking. He said he was circling for 45 minutes — and that he was about to give up and drive to Dick’s Sporting Goods in Serramonte.”

The shop owner let out a frustrated sigh. “We’re a family-(oriented) bike shop,” he said. “We sell a lot of entry-level bikes. Inevitably, people drive here.”

Other merchants are also fuming, and several showed up to the meeting in a packed City College classroom on Monday to vent their frustrations. Cyclists also flocked in, many with helmets dangling from their backpacks.

Rather than have everyone shout at a lectern, staff from the SFMTA accepted people’s comments on small cards.

Naj Daniels, a secretary from St. Mark Baptist Church, stood in a corner with other members of the congregation. Many of them have no choice but to drive, Daniels said. Some are elderly and others come from as far away as the East Bay to the small brick church on Valencia.

“I’m not against bicyclists being protected,” Daniels said, providing a disclaimer similar to that of other opponents — including Olszewski. But they insisted that any bike lane design take everyone’s concerns into account.

For most, that means keeping the parking. For some merchants that means preserving the aesthetics of the street: Olszewski distributed a set of talking points that decried “the steel ‘tube’” that would be created if vehicles are moved to the center of the roadway “without anything to break up this view.”

Cyclists clamoring for the barricade are equally passionate. Several have called for a boycott of Valencia Cyclery, and many are skewering Olszewski on Twitter. Someone even formed a “Valencia Pro-Cyclery” parody account to stream messages in support of the bike lane plans.

It didn’t take long for the emails to start pouring in, Olszewski said. And they showed just how corrosive the situation had become.

“They’re saying we’re in favor of more cars in San Francisco, that we’re in favor of global warming, that we don’t care if women and children on bicycles get killed,” he said. “This is assassination politics.”

He’s proposed a compromise: reflective bumps to delineate the bike lane, a sign on each parking meter warning motorists to watch for cyclists when opening their doors, and something like speed bumps at intersections to force cars to slow down.

Jose Ibarra, owner of Ibarra Brothers Printing, the stationery store making signs for the meeting on Monday, offered his own suggestion: protect the bike lane during rush hour, but remove the barriers between 9 a.m. and 5 p.m. and later in the evening, when people flood into restaurants and bars.

No way, say cyclists. To some, Valencia Street has become a battlefront in the larger movement to reclaim San Francisco streets from automobiles. While city engineers pursue plans for the new bike lane, other officials are calling for more monumental change. Malcolm Heinicke, chair of the SFMTA board of directors, recently pitched the idea of banishing cars from Valencia Street altogether. The notion of a car-free Valencia entices some cyclists, but others say they’d be content — for now, at least — with a wider, safer bike lane.

“This street has to serve a lot of different needs,” said Alexandra Sweet, a transportation planner who bikes along Valencia every day with her 2-year-old in tow. She often gets cut off and pushed into traffic by drivers making a hasty right-hand turn or pulling into the bike lane.

She favors the planned bike barriers, and wants to see a street where any parent would feel comfortable letting a 5-year-old ride a bike. That, she said, “would mean we’ve succeeded.”

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan