The Limiting Factor for Electric Motorcycles is No Longer The Battery. It’s The Motor.
The modern electric vehicle is ten years old. A decade has passed since the first Tesla and Nissan electric cars, Zero Motorcycles and Pedego bicycles hit the streets. In that time the electric vehicle (EV) has gone from curiosity to mainstream consumer product.
Initially, the public’s scepticism for EVs was centred around the limitations of range and the life span of the batteries. Those pioneering e-bikes, electric cars and motorcycles all suffered from a trifecta of business misery: high cost, low performance and limited usefulness. But spurred on by billions of dollars in research and advancements derived from the tech industry, the EV battery has nearly caught up to most consumer expectations, increasing in storage capability by about 10% per year while constantly getting cheaper.
Most financial institutions and think tanks around the world predict continued double-digit growth of EVs for the next decade at least as the economic and performance case for electric vehicles close to that of combustion counterparts. Among all vehicle types, the e-bike is leading the charge in terms of raw numbers. The humble bicycle, electrically motorized, is fast finding itself at the vanguard of personal mobility.
( above : 1 kW electric motor stators all wound up and ready for installation )
However, the electric motorcycle is constrained. Everyday hundreds of millions of people depend on powered two-wheelers (P2Ws) for basic personal transportation, making it by far the most common vehicle type for powered individual mobility in the world today. 98% of those P2Ws are small motorcycles and scooters, with engines under 200cc and making less than 10 kW (14 hp).
As this global fleet of P2Ws begins to transition to electric power, the big challenge will become the motor, not the battery. Battery price and efficiency continues to shrink, but copper, magnets and the volume needed to provide sufficient, reliable motors is not.
Existing electric traction motor technology has changed slowly since the introduction of semi-conductor power controllers and brushless motor designs in the late 1970s. On large, high output EVs such as cars, more has been done to advance performance and increase specific power density, but the same cannot be said in the lower output classes.
( above : not much room to play with… patent layout for a Kawasaki electric motorcycle illustrate the limited space allowed for electronic drive motorcycles )
Electric motorcycles present different challenges than electric cars. Motorbikes are highly space constrained and sensitive to changes in the centre of gravity. Internal volume for components, such as batteries, charger, a motor and control electronics, is limited to what can fit between the two wheels and the rider’s legs. Massive or weighty parts, such as batteries and electric motors, must be placed as close as possible to the centre of the roll axis or the vehicle risks being unstable and difficult to use.
The battery, being the largest and heaviest component must take priority of placement. This typically leaves the motor to be squeezed as closely as possible in the space remaining, usually towards the rear. This arrangement exposes the electric motor’s key weakness, vulnerability to heat, as the battery hides the motor from cooling air whilst adding it’s own heat to that generated by the motor itself.
( above : DC hub motors with different output — not variable sizes )
The solution for low cost brushless DC motors, the kind that drive electric motorcycles and bicycles, is to add a mechanical fan to draw in some air, or simply make them bigger. The larger the motors main components are, the more material there is to absorb heat away from the copper windings that are most likely to fail in the face of high temperatures.
Brushless DC traction motors are facing a thermal limit. Unless low cost, practical heat management technology becomes available, electric motorcycles market development will suffer.
Most low cost electric bikes slow to a crawl when under heavy use, to avoid excess heat build up. Higher speed and performance models rely on forced air-cooling, large heat sinks or water cooling. While these solutions solve the heat issue, they add complexity, part count, and tremendous cost to very price sensitive vehicles. For the electrification of the world’s motorcycle fleet to take place, prices of electric powertrain parts must continue to drop while at the same time performance limitations must be eliminated.
( above : a heavily over-loaded gas powered Honda Cub plies the roads without hesitation )
Today there is a market for 350 million workhorse motorcycles in the 5–15 hp range. These vehicles, like the one in the picture above, have to run in high altitudes of the Tibetan plateau, in elevated ambient air temperatures, and endure sustained hill climbing whilst loaded with occupants and cargo. These vehicles have to perform in these conditions for years, whilst not costing more than $2000 USD.
At present, electric motor technology can provide any two of the three main criteria needed : high specific power density, low cost and reliability. Solving the heat management problem is therefore the highest priority, if the electrification of the world’s P2W fleet is to take place.
The past decade has been exciting for EV enthusiasts. The range, performance and competitiveness of battery-powered cars and bicycles has exceeded even the most optimistic predictions of 2009, largely due to tremendous innovations in chemistry and software. But electric motorcycle growth is stalled.
It shouldn’t be. Managing heat is a hardware issue. An engineering problem. If there is anything the motorcycle industry has proven in a century and a half, it is their hardware ingenuity.
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Michael Uhlarik is an international award-winning motorcycle designer with 20 years of experience creating bikes for Yamaha, Aprilia, Piaggio, Derbi and many others. A veteran motorcycle industry planning consultant and part-time industrial design lecturer, he co-founded electric motorcycle startup SURU in 2016.
He lives in Nova Scotia.
