The wheel hub motor is currently experiencing a bit of a comeback – especially in the media. A German start-up wants to further develop the over 100-year-old technology, which has already caused many failures, and make it suitable for the masses. Can this work? In the interview: Martin Doppelbauer, Professor of Electrical Engineering at the Karlsruhe Institute of Technology (KIT).
What is a wheel hub motor?
A wheel hub motor is a motor that is integrated directly into the wheel or wheels of a vehicle. In contrast to classic engines, this is also referred to as a decentralized drive system. A wheel with a wheel hub motor combines an electric drive and mechanical brakes – sometimes power electronics are also added.
Due to the decentralized arrangement in the wheel, a classic drive shaft and gearbox are unnecessary. Many well-known companies therefore hoped to be able to make their vehicles more efficient at the beginning of the 2000s, well over 100 years after the development of the first wheel hub motor. But they all failed.
The Bavarian start-up DeepDrive has been working on the technology again in recent years and wants to make the wheel hub motor suitable for the masses. BMW and tire manufacturer Continental have invested millions in the company. But how much innovation is really behind it?
We have Martin Doppelbauer, Professor of hybrid electric vehicles at the Karlsruhe Institute of Technology (KIT) and head of department at Fraunhofer Institute for Chemical Technology (ICT), asked.
The first wheel hub motor
BASIC thinking: Hello Mr. Doppelbauer, in 1900 Ferdinand Porsche presented the Lohner-Porscher, an electric car with a wheel hub motor, at the World Exhibition in Paris. However, the technology failed to catch on. Then it became quiet for almost 100 years. Why?
Because there is and was a better alternative. Namely: the central engine. If you take a closer look at the technology of the wheel hub motor, there are two challenges. One is the integration into the wheel and the other is the electromagnetics.
When integrating it into the bike, you are faced with the challenge of unsprung mass, which makes the handling relatively poor. The heat of the brakes plays a role, but environmental conditions such as ice and snow to which the engine is exposed also play a role.
There is also incredibly little space in the bike. You have to accommodate the brakes, you have to accommodate an engine and: you have to accommodate a cooling system. There are even some wheel hub motors that house the electronics. That surprises me, because electronics and strong vibrations don’t actually mix.
On the other hand, there is electromagnetics. Power can be generated from torque and speed. For example, an internal combustion engine is good at generating torque but poor at producing speed. This is mainly due to the up and down of the crankshaft. This means that the combustion engine derives its power primarily from torque.
With an electric motor it is exactly the other way around. Because the electric motor has a good speed. A roller rotates, there are two bearings and a shaft – and that’s it. This means that electric motors can achieve around 20,000 revolutions per minute at maximum performance.
This is not possible with the wheel hub motor, as it can only rotate as fast as the wheel. Depending on the size, this is around 1,500 revolutions per minute – a maximum of a tenth. So for the same power you need ten times more torque. But this only works with a lot of electricity and the corresponding power electronics.
Siemens, Schäffler, VW and Co: They all failed
Since the beginning of the 2000s, the wheel hub motor has experienced a bit of a comeback – especially because electric cars have become significantly more efficient and relevant. Honda, Siemens, Schäffler, Volkswagen and Ford have tried out the technology. Why are so many well-known companies investing in a technology that is apparently doomed to fail?
I think the appeal of it is and was to say that you have a lot more space in the car when the classic engine is out. But that actually only applies to the combustion engine, as it really takes up a lot of space. But that’s actually no longer the case with electric cars.
Electric motors are much smaller than internal combustion engines. If you do it well, the electric motor is even built around the axle – there are already some concept vehicles out there. And if you design it out, the space gained is at best equivalent to that of a ten liter bucket.
Nevertheless, many companies have tried a wheel hub motor. But you have to say it like it is: everyone failed – without exception. So far there is not a single production vehicle with a wheel hub motor. My personal prediction: You won’t see this on a large scale either.
The wheel hub motor from DeepDrive
The German start-up DeepDrive has recently been making headlines. The company claims to have developed a wheel hub motor that is said to be more efficient and cheaper than ever before. The reasons given are a double rotor principle, the extensive avoidance of rare earths and less magnetic material. How realistic does that sound to you?
The double rotor principle has been around for a long time. DeepDrive also seems to have implemented this well. However, you don’t really find out much on the website that would be crucial.
The efficiencies seem to be good, but not significantly better than what is known from central engines. At the institute, we have therefore designed the weight of a central motor so that it corresponds to that of two of these wheel hub motors. The efficiencies are similar.
But I wouldn’t support the idea that two wheel hub motors would be more effective. The big problem with these direct drives is that if you have to convert the torque via the currents instead of via the gearbox, you have problems with efficiency – especially at low speeds.
I was also surprised by the statement that the motor should need less magnetic material. Even if that’s true, you still need a relatively large mass of permanent magnets – especially since you also have two rotors in one motor and several motors.
The wheel hub motor will not trigger a revolution
BMW has been investing in the start-up since 2021. Continental provides the brakes. Why are companies still investing in this technology?
I could imagine that this could be quite interesting for custom-made products. Maybe you want to try out similar technology in a motorcycle, for example. I could imagine that, since the space problem plays a different role there.
In many media you can read about the DeepDrive wheel hub motor as a “revolution”, a “breakthrough” or a “milestone”.
It’s all about attention. Of course it’s not a revolution. I can’t imagine mass production in the passenger car segment.
Unsprung mass
Keyword “unsprung mass”: DeepDrive’s wheel hub motor is said to weigh around 32 kilograms. What influence does the weight of a wheel hub motor have on driving behavior?
I can only give a general answer to that, as few details are known. However, one assumes a factor of approximately ten – in relation to the vehicle mass to the unsprung mass.
This means: If you have two wheel hub motors weighing around 30 kilograms each, you can roughly say that this corresponds to around 600 kilograms more weight in the vehicle – in terms of driving behavior.
But what is still fundamentally important to me: There is currently strong competition in the electric car sector. China, for example, is relying on classic electric drives. Electric cars make up almost 50 percent of new registrations there.
As Europeans and Germans, we would therefore do well to develop new technologies. It doesn’t even have to be clear from the start whether you need them or whether they are perfect. And from that perspective, I’m happy that there are companies like DeepDrive that just dare to try something out.
Are there specific advantages that a wheel hub motor can have?
There is so-called torque vectoring. This means: If I have two wheel hub drives and drive through a curve, I can turn a little slower on the inside of the curve and a little faster on the outside of the curve. This improves driving behavior.
That’s why the wheel hub motor has no future
Bottom line: Why won’t a wheel hub motor become established and worthwhile in mass production of cars?
On the one hand, due to mechanical problems: thermals, unsprung mass, integration into the wheel and installation space. On the other hand, due to electromagnetic difficulties. This means: You have to make power through torque instead of speed.
This requires a lot of permanent magnets, is complex, expensive and has limits in terms of efficiency. The space savings are virtually offset by the electric motor, which is much smaller than an internal combustion engine.
Because you have to lay hoses to each engine – back and forth. For this you need a central distributor and also communication elements. If one motor fails, the other motor must stop immediately. Otherwise you get one-sided torque and fly off the highway, for example.
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The article Wheel hub motor: future technology or physics kit? In an interview: Martin Doppelbauer by Fabian Peters first appeared on BASIC thinking. Follow us too Facebook, Twitter and Instagram.
As a Tech Industry expert, I believe that wheel hub motors are a future technology that has the potential to revolutionize the automotive industry. These motors offer numerous benefits, including increased efficiency, improved performance, and reduced maintenance compared to traditional internal combustion engines.
While some may view wheel hub motors as simply a physics kit, I see them as a cutting-edge technology that has the potential to transform the way we think about transportation. With advancements in electric vehicle technology, wheel hub motors are becoming increasingly popular as a clean and efficient alternative to traditional engines.
In my interview with Martin Doppelbauer, I would discuss the potential of wheel hub motors to disrupt the automotive industry and the importance of investing in research and development to further improve this technology. I would also highlight the numerous benefits of wheel hub motors, such as increased energy efficiency and reduced emissions, and discuss how they can play a key role in the transition to a more sustainable transportation system. Overall, I believe that wheel hub motors are a future technology that has the potential to drive significant innovation in the automotive industry.
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