MUNICH — "You hear them before you see them ..."

Holger Köhler grumbles at the increasing arrival of so-called e-bikes, bicycles that have motors to help supplement the riders' pedal power. Those on a cycling path who are overtaken by an e-bike hear the hum of the motor well before its rider passes. But when Köhler was cycling up the hill towards his village recently, he was surprised to see a beautifully crafted racing bike pass him when he expected a bulky e-bike, which tend to be unusually ugly.

It seems as if the motorized two-wheeled industry is trying to conquer the very last niche of the bicycle market. Manufacturer Haibike, for example, developed racing bike Xduro, which weighs in at a hefty 41.9 pounds. This is the so-called S-Pedelec, a bicycle fitted with an electrical motor that can reach speeds of up to 27 mph. An insurance registration number is marked underneath the saddle, and a bulky battery is attached to the down tube which in turn powers the motor situated within the chain rings.

For those who ride actual racing bikes without electrical help, such as Köhler, this bicycle is a sacrilege. "It's not sports equipment," he says.

But it could be of interest for many commuters if it were fitted with a mudguard and a carrier. The number of Pedelecs, S-Pedelecs and e-bikes sold in Germany is rising steadily. The number of bikes with built-in tailwind has risen from 110,000 sold in 2008 to 480,000 in 2014, according to Germany's Bicycle Industry Association. And the industry is banking on the trend to continue. In the medium-term, sales of 600,000 e-bikes per year are expected, and that would be equivalent to a 15% share of the market even though e-bikes cost between 500 and 2,000 euros more than their non-motorized equivalents. The standard Xduro costs 4,700 euros, whereas a model with improved breaks and more sophisticated design can cost up to 8,000 euros.

Expensive? No problem

The steep prices don't seem to be a barrier to sales. "The electrical bike has lost its reputation as a cheater's bike," says Anja Smetanin of the ecology-minded German Transport Club. Even though the first e-bikes on offer were city and trekking e-bikes, there is much more variety now. Mountain bikes with batteries and electrical motors, for example, make longer bike tours possible and enable cyclists to conquer steeper ascents.

For small businesses and bike couriers alike, the transport system of choice is the heavy duty bicycle with electrical motor because delivery tours via bicycle are quicker than navigating city traffic in a car. Parents put their children in a children's bike trailer on weekends and are glad that, thanks to the extra energy delivered by the battery, they can cycle down to the local pub and beer garden without too much effort.

So-called fatbikes, a type of mountain bike with particularly broad and thick tires, allow the cyclist to cycle through sand or loose snow. The e-bike motor enables them to travel longer distances despite more friction.

Some environmentalists are banking on the e-bike boom "to support the changeover from normal cars to environmentally friendly alternatives for everyday activities," says Anja Smetanin. Nearly every second car trip of 3.1 miles or less could be made instead with a bicycle. This measure would reduce carbon pollution by almost five million tons a year, according to the German Environment Agency.

But critics wonder if this is enough to convince car-loving drivers to switch over. They also worry that cyclists who up until now were using carbon-neutral means of transport might switch to e-bikes. That would mean converting nonpolluters into polluters.

And what about the risk of accidents? The insurance company Allianz released statistics showing that 10% of cyclists killed in accidents last year were using e-bikes (39 of 396 cyclists). Police couldn't identify a higher risk of accidents among Pedelec users. Munich police says accidents involving e-bikes represent only 1% of the average 2,700 annual bicycle accidents.

But bicycle lobbyists such as the General German Bicycle Club (ADFC) warn that the cycle lanes in cities weren't designed to accommodate more and faster cyclists. Andreas Glas of the ADFC says that broader cycle lanes or specially marked lanes are an urgent necessity.

"You are already going fast at 12.4 mph on heavily trafficked inner city lanes," says Andreas Gerstner, an e-bike expert. Pedelec cyclists can go up to 15.5 mph and S-Pedelec cyclists even faster than that. So he suggests that beginners take courses like the ones offered by German Traffic Control to get used to the bike's driving performance before revving your two-wheeler up a mountain range.