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Electric traction motors are the absolute beating heart of electric vehicle technology; just ask Joseph Inglefield, a Senior Technology Consultant at Knightec.
One of the defining characteristics of any vehicle's architecture is its torque path, the pathway through which torque is transferred from the source to the road, propelling the car forwards. For ICE vehicles (Internal Combustion Engine), the source of this torque is the engine itself, which typically requires the torque to flow through several intermediate components: the transmission, the clutch, the prop shaft, the differential, the side shafts, and finally the wheels to the road. With the development of Electric Vehicles (EVs) and electric motors, we are presented with the golden opportunity to redefine the vehicle's torque path and, therefore, the vehicle architecture.
Most electric original equipment manufacturers (OEM) have approached this hurdle by opting for in-board motors as their torque source (i.e., motors mounted inside the vehicle's structural frame, towards the centreline). These motors then usually drive some reduction gearing, connected to side shafts, which drive the wheels. This in-board motor method removes several components from the torque path, simplifying the system, freeing up packaging space, and reducing parasitic losses from friction. However, with the implementation of in-wheel motors, or hub motors as they are also known, the torque path can be simplified even more, simplifying further, and reducing the parasitic losses.
With this in-wheel configuration, the motor is part of the wheel hub assembly, with the brakes system and power electronics all built into the wheel. A method that removes the need for reduced gearing and side shafts, connecting the power directly to the wheel. Surely this would be a more straightforward, more elegant solution, so why is it not a more commonly adopted concept? There must be a reason for the industry’s reluctance to go down this path, so let’s explore the pros and cons of in-wheel motors.
Pros and Cons of In-Wheel Motors
Modern advances in material technology have allowed inboard motors to become smaller in recent years, but still, they are bulky items and require similarly bulky inverters. These all take up space which could be used for additional cargo storage, more seating or even for expanding the battery pack. With range anxiety being one of the critical concerns for today's EV driver, battery-pack expansion is a real selling point of in-wheel motors. An excellent potential space optimisation that is possible is perfectly demonstrated by Saietta, with its in-wheel motor platform. Moving the motors and power electronics out-board to the wheel hubs, removing the gearboxes, transmission, differentials, and side shafts allow the flat battery pack to cover nearly the entire vehicle footprint.
On the downside, we have significantly increased the number of motors and inverters needed to drive the vehicle. EVs usually have between 1 and 3 motors, depending on the type of drivetrain, and only one inverter to regulate the power going to/coming from the motors. Now, instead of one, there are four smaller inverters (one in each wheel) which raises cost and complexity. This brings us to our next point of discussion, the vehicle's unsprung mass.
Unsprung Mass, Handling & Durability
The unsprung mass is any part of the vehicle directly connected to/supported by the wheels (e.g., hubs, brake systems, tyres, rims.) The increase in unsprung mass associated with in-wheel motors is one of the most glaring concerns for many consumers since the more wheel mass, the more they are affected by bumps in the road, and the slower that the suspension dampers can compensate to keep the tyres in connection to the road surface. This can result in poor traction, and in extreme cases (e.g., a low mu surface), this additional weight can pose a safety threat.
Unlike the sprung components (e.g., chassis, transmission, engine), they are not isolated from the harsh impacts and vibrations the vehicle experiences. Motors are relatively delicate machines containing expensive and sensitive parts (rotor coils, stators, magnets). The inherent risk of having the motors inside the wheels instead of inside the chassis is that they are not isolated from these vibrations and shock impacts, making them more susceptible to damage.
Environmental Wear
The positioning of the motors in the wheels puts them at a higher risk of environmental wear. When the engines are mounted in-board, they are protected by the car's shell. Having them on the wheels dramatically increases their likelihood of becoming worn or damaged and, in extreme cases, potentially resulting in loss of drive. The impact with the curb could damage the motor beyond the point of repair, which would be considerably more complex and expensive to replace. After all, this is one of the critical factors which ultimately led Ford to drop the concept of having in-wheel motors on their F-150 EV. Ford preferred to have the motors mounted in-board where they were protected and sealed from the outside elements due to the rugged nature of their vehicle.
Steering & Manoeuvrability
One of the limitations of standard automobiles is their manoeuvrability, especially in tight spaces. The steering angles are restricted by the packaging space of the engine bay wheels and the articulation angle of the side shaft torque. However, this limitation is completely removed with in-wheel motors, freeing up the wheels and allowing them to be steered independently from one another and too much greater angles, as seen in this example of the 360° in-wheel motor assembly concept. Apart from making parking in a tight space a piece of cake, this would create incredible possibilities, especially for the autonomous transport market, that would previously have never been feasible.
The Makka motorbike, the mid-range utility bike from CAKE, is a perfectly proper application for hub motors because these bikes are designed for lower speed, medium to heavy-duty riding, for which the hub motor is ideal. Companies like Verge Motorcycles are taking the idea of the in-wheel motor for motorbikes to a new extreme with their rim motor, removing even the need for a hub. It is also marketed as a quiet, low-maintenance concept, with no oil, gears, chain, or hubs required.
``I think the future of in-wheel motor technology is slightly less certain. While the space-saving aspect would allow for bigger batteries, extending a vehicle range, I believe that battery technology is already advancing so quickly that this extra volume will soon become negligible. I think this additional 10-15% space increase will become insignificant, and any advantages will not outweigh the drawbacks. I still believe that the increased unsprung mass and motor durability will prevent them from being widely adopted for personal vehicular use.`` continues Joseph Inglefield.
In-wheel motors are revolutionising autonomous vehicles and public transport (autonomous inner-city buses, airport shuttles etc.). This dovetails nicely with the rising trend of autonomous driving, the push toward smart cities, and ultimately aiding the energy transition around the world.
Trust, cooperation and consideration for each other are all ingredients for a thriving employee culture; just ask Mari Shöder, nominated for HR Director of the Year 2022.
Read moreElectric traction motors are the absolute beating heart of electric vehicle technology; just ask Joseph Inglefield, a Senior Technology Consultant at Knightec.
Read moreA small and light vehicle whizzes across the narrow country road, the driver has just let the speed drop to take a curve safely. The regulation in the vehicle registers the safe handling and then gives the driver an advantage for the next suitable time to get an increased power in the form of acceleration, faster charging or stronger engine brake.
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