2 Wheel Regenerative

In this post, I want to examine the state of the art for braking the worlds’ most popular electric vehicles, 2 wheelers. I would like to compare the technical options for 2 and 4 (or more) wheelers, and review some of the lessons learned from 4 wheelers. Finally, we can speculate as to what comes next for electric bikes and braking.

East beats West

The overall market size for motorcycles lags a little behind cars (60 Million vs. 65 Million units in 2019), but what is more prominent is the regional variations. China was the largest car market (21M), with USA (17M) and EU (16M) close behind, followed by Japan (4.3M), India (3M) and Brazil (2.6M). India is the dominant bike market (18.5M), closely followed by China (16.5M), followed by Indonesia (6.5M), Vietnam (3.2M) and the Philippines (1.8M). The EU posted 1.6M units, the USA is 11th on the list (<0.7M). So while car markets are broadly similar across both hemispheres, bike markets are dominated by Asian countries. So the market forces and legislative instruments effecting motorbike requirements are going to be those of India and China, not Europe and America.

Fig. 1: Traffic Jam in Saigon, Vietnam. Image used under CC licence

The Indian backdrop is interesting for a number of reasons. Firstly, motorcycles provide primary transport for the majority of people, either as private 2 wheelers or shared mobility 3 wheelers. 2 wheelers represent almost 80% of total vehicles, whereas cars (both economy and premium) represent less than 15% (source). And even for those owners of cars, motorcycles still represent the fastest urban transport, being able to navigate through traffic far faster than any other method, and so remain a primary commuter option. So for India, getting 2 wheelers electric is the significant challenge.

Next is the political ambition. In the past 4 years, India has declared that by 2030, 100% of new vehicles would be electric. This was quickly followed up with a revised 30% quota by 2030. The target for 3 wheelers and 2 wheelers stands at 100% by 2023 and 2026 respectively. There is still a live debate as to whether this applies to low-power bikes (<125cc) only or all bikes, and whether its phased in for large cities or nationally. As discussed when looking at clean urban air, India has a pressing need to reduce urban air pollution, being home to 6 of the 10 most polluted cities on the planet. The overall direction of travel is clear, and all suggested timelines foresee significant developments in the next few years. But unlike electrification of cars, this change will happen from the lowest price points, upwards.

Taj Mahal scrolling pollution tracker
Fig. 2: Air Quality Information is provided to Taj Mahal visitors. Image used under CC licence.

China has had significant political intervention in the motorcycle market, which has lead to a huge boon in electric two wheelers, and a significant decline in ICE equivalents. First was a ban on motorcycles from cities, which were largely replaced by low powered electric assist bicycles, and a growing variety of electric scooters and motorcycles. The second is a determined effort to support EV production and adoption through a number of fiscal subsidies. As a result, the overall market shrunk, and shifted focus to cheap urban scooters and e-Bikes.

Keep it Simple, Sidecar

It has been said that regen on bikes isn’t worth the trip, there isn’t enough energy to recover, or the systems would be too heavy or expensive to justify the costs. The usual argument goes that aerodynamic loads are higher on bikes (drag is higher, but frontal area much lower than a car, longer discussion here), so recovering momentum isn’t worth it (weight dominates at lower speeds and increases linearly, and aero dominates at higher speeds, and increases exponentially, so there is no hard and fast rule here). And while some of this might be true for a pedal-assist electric bike, it’s certainly not the case as we move up the food chain. And in a world where battery range is both a limiting factor and a competitive advantage, squeezing every last mWh out of the battery is beneficial. As with cars, if the battery is the most expensive component on the bike, its also the most expensive way of increasing the bike’s range. And just like with a car, the cheapest and lightest way to increase the battery size by up to 30% is to refill it on the go.

Regenerative deceleration

The most basic solution for automatic brake energy recovery simply creates negative torque when the throttle is released. Two big advantages of this strategy are that a reasonable amount of regen scenarios can be covered, and very little engineering needs to be done to software or hardware. For a little bit more sophistication, its even possible to put some dead stroke into the pedal or lever mechanism, and engage the eMotor torque a little further. Again, no major change to brake hardware, no messy control logic or brake blending. Cheap, Cheerful, Effective, Efficient.

If we consider how an electric motor generates torque in a braking event, we can describe three distinct phases; a power-limited phase (the left hand portion of the graph, where decreasing RPM is matched with increasing torque), a torque-limited phase (the flat-lined peak of the curve, where the motor is delivering maximum torque) and a torque drop-off phase (where slowing RPM or velocity will lead to a torque drop off).

Fig. 3: Torque blend of a regen brake event

With reference to our cheap and cheerful throttle-off strategy above, there are a number of issues with following the maximum eMotor torque profile above. First, if there is strong throttle-off regen coupled with low grip, the tyre can lose traction, with no obvious recovery available to the rider. Second, if we don’t manipulate friction brakes, the deceleration performance is non-linear, and difficult to control. Last, if we have a sudden drop-out of eMotor, then the rider must attempt to compensate, but may not have sufficient actuation capacity. While the possibility of a full power failure of the eMotor may be remote, thermal performance restrictions of inverters and battery packs are relatively common, especially in aggressive riding. And of course, its possible the battery is relatively full, so low levels of charge can be accepted. So a cheap and cheerful strategy must aim for a constant torque level from the eMotor, to give the rider a predictable performance and acceptable safety. But this means a lot of free watt-hours left on the shelf.

To get an idea of where to draw the line for better brake energy recovery, we can have a look at how hard a bike brakes. In cars, data collections suggest that 95% of braking is within 0.3g, and 99% of braking is within 0.4g. This informs a lot of regenerative system layout, where 0.4g regen is considered state-of-the-art for 4 wheelers. For motorcycles, it is easy to imagine that the same traffic conditions apply, as they move generally in the same traffic patterns. But several studies of cars vs. bikes show that motorcycles tend to travel about 10% faster than cars on the same road (studies from New ZealandBelgium and Serbia) , and therefore must brake harder for the same traffic restrictions. For regenerative braking, then, this suggests that the distributions for deceleration are a little higher, and so a greater proportion of energy is available at higher deceleration rates.

2 wheels good, 4 wheels better

For 4 wheelers, to achieve this higher level of regenerative braking, it is necessary to manipulate the hydraulics, so that the changing levels of eMotor torque can be blended with a suitable level of friction braking. This also means a beefier actuation system, so that the hydraulic pressure can be quickly deployed if eMotor torque drops away. For the majority of the last decade, this beefier brake system meant higher unit costs, albeit justified by the battery savings. But innovation in control systems meant that boost, regeneration and modulation could be accomplished in one new component (so-called 1-Box system), and this lead to an overall cost decrease, as the function increase. So much so, that 1-Box systems are fitted to ICE cars, to save unit cost. While battery prices have fallen dramatically in the same timeframe, it’s hard to resist a system that costs less, weighs less and delivers more.

BMW K100
Fig. 4 : BMW K100, the birth of 2 Wheeler ABS. Image used under CC licence.

For large bikes, where ABS systems are already required, this presents an interesting parallel. If we take a moment to consider the adoption of ABS on both platforms, while cars had a couple of decades head start (the 1988 BMW K100 is credited with bringing ABS to 2 wheels), but the technology is widely deployed in both sectors now. System complexity has risen on both fronts over the years, as hardware integration has led to smaller and lighter packages. The regenerative technology requirements are very similar and the major Tier 1 suppliers are already offering solutions for cars that can scale to bikes. And considering that large electric bikes now come with battery packs twice as big as PHEV cars, it seems difficult to imagine that increased regen won’t be a significant trend here, as the sector develops. 

But what for the large market for small electric bikes? Battery sizes are significantly smaller in these bikes, and Lead Acid is still used at the bottom end of the market. We need to differentiate between brake systems installed. The cheapest bikes employ the simplest setups, typically a rear hub motor, combined with cable-actuated drums front and rear, and often lead acid batteries. Next will be hydraulic set-ups with a Combined Brake System, where one actuation lever will actuate both wheels. And finally, some bikes in this sector include an ABS system. Starting with the easier situation, the ABS systems mentioned above will easily find home in this sector of the market, once unit cost becomes close to competitive with existing systems. 

For bikes with CBS systems, it’s a slightly higher hill to climb. There is no parallel to regen systems on 4-wheelers to bridge the innovation gap, and smaller installed batteries means a smaller regen budget to pay for more complex components (once they become widely available). So this leaves some space for a more simplified system, which could be delivered for a unit price close to an existing CBS system. In this arena, manipulating the hydraulics via mechanical rather than mechatronic means is most likely to achieve the cost and functionality targets, and could bring a unique solution to the problem, rather than supplant something from cars. Innovation in this sector has a fairly large audience already, and as battery ranges increase, the economic imperative for better efficiency here will grow.

Fig. 5: A simple electric 2-wheeler

In the arena of simplified cable pull brakes, while the technical challenges for brake actuation are not insurmountable, the cost benefits are more likely to preclude significant uptake here. Range and efficiency are still important here but purchase price is the dominant factor, so simple solutions with minimum calibration will be the order of the day.

Looking towards the coming years, it would seem clear that trends in battery pricing and energy density will see electric bikes in all price categories increasing range. And given what we know of the main motorbike markets, it is difficult to predict that EVs won’t take an increasing share over the next decade. It would seem obvious that the competitive advantages of efficient electric powertrains for 4 wheels will be as compelling on 2 wheels, and so an increase in the contribution from brake energy recovery.