The Pros and Cons of Using In-Wheel Motors in Electric Cars

By · May 08, 2013

The Toyota ME-WE concept, with in-wheel motors for better packaging

The Toyota ME-WE concept, with in-wheel motors for better packaging.

There are many differences between gas and electric cars. One of the most intriguing is the possibility for an EV to fit motors inside the wheels.

We've seen many concepts with in-wheel motors, including the Toyota ME-WE, which debuted in Paris a few weeks ago. However, no production car is available with this architecture yet, but it's certainly worth a closer look. Think of the great possibilities: more room for passengers and the elimination of all transmission losses. The motor turns and the car moves without any inertia. It sounds so simple, but it's not. There are several challenges with in-wheel motors (IWM). The first one, obviously, is size.

The Toyota ME-WE concept, with a perfect flat floor, no transmission bump

The Toyota ME-WE concept, with a perfect flat floor and no transmission bump.

There's not much space inside a wheel, and it's impossible to give it all to an electric motor. A reducer is also needed. An electric motor can easily go at more than 10,000 r.p.m., but no car on the road can have its wheel turn that fast. The limited space then implies that an IWM will make less power than a motor under a front hood. A second implication, which receives less thought, is that to be powerful despite the small size, an IWM will need the best available magnets. That means the very expensive neodymium ones (a rare earth material).

It's possible to buy an electric scooter with an IWM, but one that makes very little power. A car requires much more, and the special case of three-wheelers set aside, it also requires at least two in-wheel motors—say, for both rear wheels. That raises the cost issue again. Two small high-tech motors will never be cheaper than a single larger low-tech motor.

Another common expense for a car manufacturer is the development of a differential. There are many off-the-shelf parts for this in a conventional architecture, but none can be used with IWM. The device—which in a curve makes the inside wheel turn at a lower speed than the outside one—requires software. All modern cars have an electronic differential, stability control and traction control, but those systems would work differently with IWM, and few engineers have any experience in that field. It's especially tricky when you add regenerative braking into the equation.

Finally, there's the problem of unsprung weight. Racers have been using light alloy wheels for decades. Heavier wheels would be a return to the past. With today's technology, manufacturers can make a complete in-wheel motor that weighs less than 100 pounds—including the tire on a 16-inch rim. So that problem appears to be manageable.

A Mercedes hybrid bus which was used at the Davos Economic forum, it had in-wheel motors

A Mercedes hybrid bus which was used at the Davos economic forum, it had in-wheel motors.

Reinventing the Wheel

But the biggest issue, by far, is this one: building a car with in-wheel motors requires a clean sheet approach—a new start with a new way of thinking. For example, every car on the road has a hood, but there's no need for one in a car with IWM. Some engineers are forecasting that IWM could appear in hybrid buses before they show up in cars.

If you think way off into the long-term future, most car people believe that in-wheel motors have tremendous potential. The technology is not cheap yet, but it could be if it was mass-produced—with the idea that many different models could have the same motors. The car's body, its look, its battery and the software that controls the in-wheel motors would be individualized for each model, but the motor hardware itself would be the same. Well, luxury cars, of course, would still get cooler shinier wheels.


· · 5 years ago

Laurent one of your statements requires clarification here: "...All modern cars have an electronic differential, stability control and traction control...."

If a car has in wheel motors, the differerential control is at best a software construct.. There are no differential gears as opposed to all Teslas for instance that MUST have them since there is only one motor.\

ON the weight issue, its up to the manufacturer's choice as to use a direct drive hub motor, or use a highspeed screamer and reduction gearing.

· · 5 years ago

Ok, this wasn't meant to be pick on Laurant week, but, "....A second implication, which receives less thought, is that to be powerful despite the small size, an IWM will need the best available magnets. That means the very expensive neodymium ones ...."

If they use your aforementioned gearbox at each drive wheel, I see no reason why a high speed induction motor, with NO permanent magnets at all, couldn't be used. Tesla has made (or have had made for them) several thousand of these very small motors, and I don't hear many people complaining about the lack of acceleration of a Tesla, and they only have one motor!

· · 5 years ago

A very interesting overview of wheel motors, Laurent. I especially like the Toyota ME-WE concept, which like of reminds me of a Mini Cooper with suicide doors that's crafted from Play-Doh. This car has recycled plastic body panels (kind of like the Think City had) and a bamboo floor. I love it!

As for the hub wheels, check out what Protean is doing . . .

There are certainly some challenges to tackle, but this really would be the bee's knees for the EV of tomorrow.

· · 5 years ago

I hate to tell you this Laurent, but your article completely missed the whole point of using in-wheel motors. They are considerably more efficient and you didn't even mention it. According to Protean, their in-wheel motors can yield a 30% increase in efficiency, compared to a centralized motor. So, a car like the LEAF could have a 100+ mile range, from the existing battery. This increase in efficiency would be huge for EVs, since cost and range are two of the things holding up sales.

The Protean in-wheel motors have 110 horsepower and 590 foot-pounds of torque. Two of these motors would make for a very fast EV. They weigh only 68 pounds each, including the inverters, controllers and brake componenets. Most important, they come with the control software, so an auto manufacture doesn't need to figure that out. This negates most of the negatives you mentioned.

If in-wheel motors are safe, durable and reliable, they seem like a no-brainer to me.

· · 5 years ago

Are there any high-speed trains that use in-wheel motors? Efficiency there would also be beneficial yet unsprung weight would not be such an issue. Also not an issue would be the location of the brakes as they are usually shoes over the wheels.

· · 5 years ago

Stability and traction controls are mostly the work of software on big sedans, and I'm not talking of EVs. There's no gearbox in each wheel, just a reducer.

About Protean, their 30% efficiency gain is between a standard gas sedan and a hybrid version equipped with their system.

· · 5 years ago

SIM-Drive is another example of in-wheel electric drive. They use a motor in all four wheels, so an advantage is that you can have four wheel drive without a transfer case; and you can have traction control as well.

Gears are at best 98% efficient, so any gain in efficiency by using reduction gears, need to offset at least that loss.

There is a third way, that splits the difference between a conventional EV drivetrain and in-wheel motors, and that is twin motors each with direct drive shaft connection to the wheels. This has been done in the FVT eVaro and the Delta E4. The eVaro had custom motors drive both front wheels, and it had inboard disk brakes; reducing the unsprung weight to less than a conventional system. The E4 uses YASA motors, and these two vehicles have massive torque. And the SIM-LEI, SIM-WIL, SIM-CEL, and eVaro, and the E4 are among the highest efficiency EV's around.

Yes, this still requires a software solution to do the differential function, and they take more space than in-wheel motors . The advantages are that there is only the U-joints and CV-joints friction losses, and with inboard brakes have lower unsprung weight than conventional setups.

The eVaro uses *only* regenerative braking to stop the vehicle, and the friction brakes are only needed to hold it still and as backup in an emergency. Since it has front wheel drive, it can get more regen because the front wheels on any vehicle provide about 70% of the braking power. The SIM-Drive cars can get regen from all four wheels.


· · 5 years ago

"is that to be powerful despite the small size, an IWM will need the best available magnets. That means the very expensive neodymium ones (a rare earth material)."

I would have to agree with Bill on this one.

I call that BS. IWM doesn't require high cost Neodymium magnet motors. AC induction motors will work just fine. Especially if you put 1 per wheel. A typical car with a 50 HP induction motor per wheel will easily provide sufficient performance. And a 50 HP induction motor can be made pretty small if you increase the frequency and number of the poles.

"Two small high-tech motors will never be cheaper than a single larger low-tech motor."

Well, NOT necessarily. For example, a 150 HP low tech motor will have less torque at the high end and require a gearbox. But 4 small motors each with each wheel (30, 30, 45, 45 hp each) can cost less than a 150HP low tech motor with a gear box.

The expensive part is the controller. motors on each wheel will require slightly more complext circuits to control them. But at much lower power rating, those components will be much cheaper. ICs that can handle 150HP will cost magnitude more than ICs than handle 30 or 45 HP.

Another thing to point out is efficiency. Low tech motors will require more permanent magnet which will cost more b/c of the magnets... High tech motor can just go Induction route and bypass many of the expensive REMs...

· · 5 years ago

Thank you for the post. On the weight issue, its up to the manufacturer's choice as to use a straight drive hub motor, or use a highspeed screamer and reduction gearing.

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· · 5 years ago

Thank you for the post. On the weight issue, its up to the manufacturer's choice as to use a straight drive hub motor, or use a highspeed screamer and reduction gearing.

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· · 4 years ago

Bret F is right. Four wheel drive car with built in motor is 25% more efficient in average than single centralized motor. Some +/- little percent variation is possible vs. type of electric motor. Actually asynchronous squirrel-cage motor has lowest efficiency and torque and replacement it by AC synchronous motor with soft steel rotor may increase the difference up to 30%. Actually synchronous motor should be modified for possibility to work with zero speed rotation. After that mechanical brake is not needed with 1 ton/foot of torque per each motor and acceleration is limited by tire traction and battery power only. The battery power limit is possible to avoid by using ceramic multilayer capacitor as power source, but it is more acceptable in serial hybrid configuration. Another benefit of in-wheel motor is more efficient kinetic energy recuperation. If power train with centralized motor, differential, and reducer is returning back to the battery 50% of energy that was spent for acceleration in-wheel motor configuration is returning 78% of energy at the same conditions. Don’t forget also what high speed rotating motor is gyroscope too. And its kinetic energy is proportional to square speed of rotation. Actually gear mass of reducer should be added to the rotor mass of motor during any calculation. What it means? Manufacturers are reducing car mass by the usage of aluminum and carbon fiber parts. As results car price is grooving. And at the same time 10% of energy is wasted for gyroscope acceleration/deceleration.
I have several questions about electrical car and myths about it.
Will be battery price reduced when electric car will be in mass production?
I am not sure. One car battery is equal to 1000 laptops battery by capacitance. Do we have enough lithium for batteries production? Regular Lead-accede battery has increased price twice at last 10 years.

Haw long to wait for battery charging infrastructure? It will take a lot of time, but before it electricity production should be doubled.
Is fast charging OK for battery life? Actually no, battery life may be reduced up to 2 times. Or if some type of battery may accept it, its price will be extremely high.
Is electricity cheap? Yes of course, today. But I am not sure about tomorrow. Gas tax is 20% at least in the final gas price. Who should compensate money loss to government? Actually it should be electric car owners, because they have avoided gas tax by switching to electric car.
What is the solution for all questions those mentioned above?
Looks like everything may be solved by serial hybrid car. On board energy production is needed. What kind off step is preferable? Take low speed turbocharged diesel engine and add sterling cycle engine for exhaust gas energy utilizing. 64% of total fluid chemical energy will be transmitted to mechanical energy. And we will have also enough thermal energy for in-cabin heater. For the next step transform mechanical energy to electric and distribute it between in-wheel electric motor (or other type of power train that does not have reducer and differential). In some cases don’t need to transform AC to DC and back to AC. Some generator/motor configuration allows doing it W/O ICE rpm manipulation. And the electric transmission may be more efficient than mechanical for torque transmission from ICE to wheel. Total fuel to wheel energy efficiency may be in the range 55~57%. It is almost similar to modern electric car wall to wheel efficiency. With a coal electric plant efficiency and energy transmission loss, modern electric car is twice less efficient than mentioned above serial hybrid. And environment pollution is 3 times more at least.
Just let me convert Cadillac Escalade to serial hybrid as an example. Gas mileage is 57mpg HW and 85mpg city. Low rolling resistance tire may reduce gas consumption to 68mpg HW and 113mpg city. It means you need to visit a gas station one time every 1700 – 2000 miles at the same gas tank capacity. Service interval may be 25k miles if you don’t have any problem with tire pressure verification and adjustment. Why SUV is selected? It was popular 15 years ago and only rising gas price has reduced its popularity. Also many saving is bigger on high consumption car. At 15k yearly mileage saving is $3000. Why Escalade? Everything should comparable not only by the size and by horsepower too. Actually it does not have 2MW power output at 65mph, but its 400hp is bigger than has Honda CRV.
What about family sedan like Toyota Camry? MPG is 82/136 HW/city with regular tire and actually should be improved with low resistance tire. Also a car body design that is similar to GMC EV1 may have 130/175 HW/city. In this case gas mileage price is $0.03 per mile. It is less than have now Leaf and model S. Switching to natural gas may reduce cost of mile twice.

What to do a person who like a pure electric car but does not like annoying frequent charging? It is possible if you have city only driving and ready for subcompact with reduced power consumption and actually weight and size. Subcompact car like “Smart ForTwo” may have power consumption 40Wh/m in city mode and 71Wh/m in HW mode. Solar panel on the roof may provide daily energy that is enough for 40 miles at the summer time. And at the winter time wall charging is needed once a week.
Actually in the electric car a direct wheel drive is more efficient than a centralized motor drive system. It may be in-wheel motors or motor to wheel connection through a hub. It is giving a lot off benefits especially in serial hybrid configuration. Also proper on-board power generation is required for the serial hybrid. It is including high efficiency generation (low speed turbocharged diesel or common rail ICE) and co-generation (Sterling type of engine or similar that is utilizing exhaust gas energy of ICE).
Now existing electric and hybrid cars those are in production have to big price/quality ratio. Used hybrid and electric cars resale value is going to 0~20% after 7 years of usage, especially for electric car where battery replacement is needed and original battery price was up to 40% of new car price. Potential customer does not see any many saving. Anyway biggest benefit which has now electric car is carpool access. When price/quality ratio will be reduced and True Cost to Own will be comparable to the conventional car, customers will stay in-line for the new hybrid and electric cars.

· · 2 years ago

This post is old but...

@vdiv · 2 years ago

"Are there any high-speed trains that use in-wheel motors?"

I'm no engineer but I do know I've never seen a train turn a corner that wasn't a few hundred meters in diameter. My conclusion from this observation is that trains don't turn sharp. Therefore they don't have a differential (like a car). Therefore there is no gain to be had by putting a hub motor on the wheels. Instead motors are spread across axles/bogeys.

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