Stromer ST7: For the S-Pedelec with a vehicle weight of 38 kilograms, the manufacturer relies on energy recovery


Stefan Weißenborn

The effect is immediately noticeable: it is unusually tough on the country road down into the valley, where the meadows are in bloom. Normally, a bike would certainly be able to accelerate to 50 or 60 km/h effortlessly.

But the test bike from the Swiss brand Stromer decelerates on purpose: this is the regenerative brake principle. It is intended to noticeably increase the range of the S-pedelec through energy recovery.

The recuperation mode can be switched on via a button, and the bike becomes a small power producer. If there is space in the battery, the test bike also recuperates inconspicuously with every pull on the brake lever: Then the engine brake is used before - if necessary - the disc brakes grab.

Instead of energy escaping unused into the environment in the form of heat at hot-running brakes, it is profitably converted. During recuperation, the function of the motor is reversed: used as a generator, it generates a braking torque like a dynamo, which generates electricity that is fed back into the drive battery.

"This is technologically elegant," says Benedikt Schmülling, professor at the Chair of Electromobility and Energy Storage Systems at the University of Wuppertal about the basic principle. "Regaining" or "recovery" means recuperation translated from Latin.

The idea of using kinetic energy while driving is familiar from hybrid and electric cars. Recuperation is one of its essence, it increases the range. The technology makes the drive more efficient and improves the CO2 balance.

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But when it comes to e-bikes, recuperation seems like a gimmick for nerds – especially since potent batteries are on the market. In view of the low power consumption, there is hardly any money or CO₂ to be saved. And unlike an electric car, a pedelec can be ridden with an empty battery if necessary.

Bikes that are equipped with the technology are still expensive and not always intuitive to use, says expert Schmülling. The idea of capturing energy also makes sense in principle for bicycles and is not new: According to the press service Fahrrad in Göttingen (pd-f), the first large-scale manufacturer to have complete drives with recuperation in its range was the Canadian company BionX in 2008.

It doesn't work with the mid-engine

The Swiss manufacturer Stromer first introduced recuperation to a wheel in 2010. Sensors are now located in both brake levers. "The level of recuperation is slightly different on the left and right and depends on the speed of the vehicle," says Markus Wittwer, engineer from the development department at Stromer.

However, the regenerative brake on bicycles and pedelecs has not yet found widespread use. One of the main reasons is that, due to its design, it can only be implemented in conjunction with rear hub motors, says engineer Schmülling. By far the most common, however, are mid-motors, in which recuperation would only work if there was "a fixed coupling between the pedals and the rotating mass of the wheel – that is, there should be no freewheel". The crank would always rotate when the wheel rotates. Rather impractical.

The Stromer test bike shows what the technology can do. The intensity of the current recovery can be adjusted in five stages. In the highest, as described above, the engine brake is so strong that the bike almost stops when the slope subsides. At this point at the latest, you should switch off the recuperation again - otherwise you will feel like you are on an exercise bike in heavy mode.

How useful it is to recover energy with pedelecs depends heavily on the driving situation. The percentage of energy recovery increases the more hilly the route profile is. "The Bergisches Land, for example, would be ideal," says Schmülling. "You use the support upwards, downhill the recuperation pushes energy back into the battery."

Only: If you as a cyclist are looking forward to a nice descent with free rolling, the technology is a real fun brake. "Recuperation makes sense when braking is necessary," says Stromer engineer Markus Wittwer. For example, on short distances in the city, such as at traffic lights.

Three-quarters of the braking energy can be recovered

This is where a special advantage of the technology becomes apparent: it protects the disc brakes. Schmülling: "They shouldn't wear out at all." Especially on e-bikes, there is no need for maintenance work, which would otherwise be due after about 2000 kilometers, but depending on the quality of the material and the stress on the windshields, it can sometimes be less than half.

According to Schmülling, up to 75 percent of the kinetic energy that would otherwise be released into the environment as heat via the brake discs can be converted during recuperation. However, far less flows back into the battery, since, for example, air and rolling friction still have to be overcome.

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"Ideally, you get back 10 to 20 percent of the energy you used to drive," says Schmülling. In the e-biker currency watt-hours, this means: "So if you were to move a 500-watt-hour battery completely, you could use 550 to 600 watt-hours." Stromer has determined similar values in tests in the Alps. For urban cycles, Wittwer estimates the potential for recovery at 5 to 10 percent. Assuming you can travel 500 kilometers with 120 Wh, that would be 6 to 24 kilometers more, depending on the terrain.

The advertising messages of other bicycle manufacturers even give hope for more: Turbine Bicycles, for example, which installs Zehus rear hub motors with recuperation function, is talking about a 30 percent increase in range "on the flat". Rear hub motor manufacturer Go SwissDrive even praises its energy recovery system as "technology from Formula 1".

Some are getting out of technology

However, both brands and S-pedelec specialist Stromer are very interested in hub motors: Turbine, for example, relies on a retro look with thin frame tubes, so clunky mid-motors would be visually disturbing.

But although recuperation on e-bikes has been tried out for some time and has some advantages, the technology has not yet really caught on. Niche Mobility from Spain is currently trying to bring it forward via crowdfunding, while the Canadian manufacturer BionX went bankrupt – the motors with generator function were prone to errors.

The Swiss e-bike pioneer Flyer as well as the German manufacturer Kalkhoff currently no longer offer recuperation bikes. "We rely on the mid-engine concept," says Flyer spokeswoman Anja Knaus. Kettler, on the other hand, is equipping its Velossi S-pedelec in version 2024.3 with a recuperation-capable rear unit from the Taiwanese engine specialist TDCM for model year 0.

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Schmülling thinks it is likely that recuperation on the pedelec will prevail at some point if the systems automatically work completely in the background and are also cheaper. "Of course, it would be ideal if you didn't even have to think about how it works."

At the moment, very few bikes that have the technology are cheap. An appropriately equipped S-pedelec from Stromer costs the equivalent of over 5000 euros. The Kettler Velossi 3.0, delivery start in March 2024, will cost 6799 euros. The folding bike Vello starts at 3090 euros, with Turbine at 3079 euros. The outlier downwards is Cooper Bikes , which offer their otherwise technically and visually very simple e-bikes with Zehus drive in the current price war for bicycles discounted from 1800 euros.

Useful especially for cargo bikes

The chances could be better than for private drivers in the commercial sector, where heavy duty is at stake: "I suspect that there will still be bikes with recuperation again, especially in the cargo and special bike sector," says pd-f technology expert Arne Bischoff. "In the case of LEVs (Light Electric Vehicle, editor's note), I can also imagine concepts with several wheel hub motors similar to those in the automotive sector."

Last year, a 170-kilogram electric cargo motorcycle with a fast-charging system, recuperation and 100 kilograms of payload was presented at the University of Ulm – intended for delivery services, for example.

In a larger context, however, engineer Schmülling considers energy recovery to be almost necessary: "Five or ten percent savings are also good. In the future, we will have to become as efficient as possible with all devices.«