Wheel hub motor: future or physics kit? Interview: Martin Doppelbauer

The wheel hub motor is currently experiencing a small comeback – especially in the media. A German start-up wants to further develop the more than 100-year-old technology, which has already failed many, and make it suitable for mass production. Can this work? In an interview: Martin Doppelbauer, Professor of Electrical Engineering at the Karlsruhe Institute of Technology (KIT).

What is a 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 motors, it is therefore 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 traditional drive shaft and a gearbox are no longer necessary. Many well-known companies therefore hoped in the early 2000s, i.e. well over 100 years after the development of the first wheel hub motor, to be able to make their vehicles more efficient. 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 mass production. BMW and tire manufacturer Continental have invested millions in the company. But how much innovation is really behind it?

We asked Martin Doppelbauer, professor for hybrid-electric vehicles at the Karlsruhe Institute of Technology (KIT) and head of department at the Fraunhofer Institute for Chemical Technology (ICT).

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 gain acceptance. Then it was quiet for almost 100 years. Why?

Because there is and was a better alternative. Namely: the central motor. 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 wheel, you are faced with the challenge of the unsprung mass, which makes handling relatively poor. The heat from the brakes plays a role, but so do environmental conditions such as ice and snow to which the engine is exposed.

There is also very little space in the bike. You have to fit the brakes, you have to fit a motor and you have to fit a cooling system. There are even some hub motors that fit the electronics. That surprises me, because electronics and strong vibrations don't really mix.

On the other hand, there is electromagnetics. Power can be generated from torque and speed. For example, an internal combustion engine can generate good torque but not very good speed. This is mainly due to the up and down movement of the crankshaft. This means that the internal combustion engine derives its power primarily from torque.

With electric motors, it's the other way round. Electric motors can rotate at high speeds. A roller rotates, there are two bearings and a shaft – and that's it. This means that electric motors can do around 20,000 revolutions per minute at peak performance.

This is not possible with a 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 one tenth. This means that you need ten times more torque to achieve the same performance. But this only works with a lot of electricity and the corresponding power electronics.

Siemens, Schäffler, VW and Co: All have failed

Since the early 2000s, the wheel hub motor has experienced a small comeback – mainly 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 failure?

I think the appeal is and was to say that you have a lot more space in the car when the traditional engine is removed. But that really only applies to the combustion engine, because it takes up a lot of space. But that's actually no longer the case with electric cars.

Electric motors are much smaller than combustion engines. If you do it well, the electric motor can even be built around the axle – there are already some concept vehicles like that. And if you design it out, the space saved is equivalent to that of a ten-liter bucket at best.

Despite this, many companies have tried to develop a wheel hub motor. But it must be said that all of them have failed – without exception. There is not a single series-produced vehicle with a wheel hub motor. My personal prediction is that we will not see this on a large scale.

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 more efficient and cheaper than ever before. The reasons given are a double rotor principle, the almost complete absence of rare earths and less magnetic material. How realistic does that sound to you?

The double rotor principle has been around for a while. DeepDrive also seems to have implemented it well. However, the website doesn't really tell you much that would be crucial.

The efficiency seems to be good, but not significantly better than what is known from central motors. At the institute, we have therefore designed the weight of a central motor so that it corresponds to that of two of these hub motors. The efficiency is similar.

But I would not support the idea that two wheel hub motors are more effective. The big problem with these direct drives is that if you have to convert the torque via the currents instead of the gears, you will have problems with efficiency – especially at low speeds.

I was also surprised by the statement that the motor needs less magnetic material. Even if that is true, you still need a relatively large mass of permanent magnets – especially since you have two rotors in one motor and also several motors.

The wheel hub motor will not trigger a revolution

BMW has been investing in the start-up since 2021. Continental is putting the brakes on. Why are companies still investing in this technology?

I could imagine that this could be interesting for custom-made products. Perhaps someone would like to try out a similar technology in a motorcycle, for example. I could imagine that, because the space problem plays a different role there.

In many media outlets, one reads about DeepDrive’s wheel hub motor as a “revolution,” a “breakthrough,” or a “milestone.”

It's all about attracting attention. It's not a revolution, of course. 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 the driving behavior?

I can only give a general answer to that, as few details are known. However, it is assumed that the difference is approximately tenfold – in terms of vehicle mass to unsprung mass.

This means that if you have two wheel hub motors, each weighing around 30 kilograms, you can roughly say that this corresponds to around 600 kilograms more weight in the vehicle – in terms of driving behavior.

But what is fundamentally important to me is that 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 are therefore doing well to develop new technologies. It doesn't even have to be clear from the outset whether we need them or whether they are perfect. And from that perspective, I'm happy that there are companies like DeepDrive that simply dare to try something new.

Are there any concrete advantages that a wheel hub motor can have?

There is something called torque vectoring. This means that if I have two wheel hub drives and am driving around a bend, I can turn a little slower on the inside of the bend and a little faster on the outside of the bend. This improves handling.

That’s why the wheel hub motor has no future

The bottom line is: Why won’t a wheel hub motor be successful and profitable in mass production of cars?

Firstly, due to mechanical problems: thermals, unsprung mass, integration into the wheel and installation space. Secondly, due to electromagnetic difficulties. This means that you have to generate power through torque rather than through speed.

This requires many permanent magnets, is complex, expensive and has limits in terms of efficiency. The space savings are more or less balanced out by the electric motor, which is much smaller than a combustion engine.

You have to lay hoses to each engine – back and forth. For this you need a central distributor and communication elements. If one engine fails, the other engine has to stop immediately. Otherwise you get one-sided torque and, for example, you fly off the highway.

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