Some Thoughts About E-Bike Control

By: 
John Allen

 

As of 2020, new battery technology and electronics have led to practical e-bikes -- machines powered by both the rider and an electric motor. E-bikes are increasingly popular. People ride them for different reasons. Some cyclists are aging, or out of shape, and want to keep up with friends on group rides. Other cyclists are transporting children or bringing home shopping.  Some have a commute to work which is a bit too long or hilly for the time available, or want to arrive without having worked up a sweat. I’m sure that there are other reasons too.

And so, I am moved to ask: how does the addition of an electric motor affect control of a two-wheeler? What is the difference if power is applied at the front or rear wheel, or at the cranks?  

Safety concerns.

There are three main safety issues when e-assist is added to or designed into a bicycle:

  • The bicycle can just go faster.  Greater speed increases the potential for crashes, and their seriousness. There has been a spike in fatalities to older people in the Netherlands riding e-bikes.
  • Related to this, infrastructure designed for slow bicycling (or not even well for that) works poorly for people riding at 20 miles per hour or more. Yet people tend to think “it’s just a bicycle” and maintain the same poor riding habits. My hair stands on end with YouTube videos I’ve seen of people riding e-bikes at speed in the door zone of parked cars, etc.
  • Different options for motor location and control raise special safety issues, and this article will describe them in more detail.

Rear, Mid and Front Drive

An e-bike may be powered directly through the front or rear wheel, or from the crankset (“mid drive”). A front or rear hub motor is an easier retrofit to an existing bicycle than a mid motor, because no moving parts other than the wheel need to be replaced or modified. The battery may be placed along a frame tube or on the bicycle’s rear rack. Only an electrical connection to the controls and battery is needed, the battery is even incorporated into some popular hub motors, with wireless control. But an integrated system, designed from the ground up, provides different and often better options.

Rear Hub-Motor Control

Differences in rear hub-motor control can be subtle under tame conditions, but sometimes, not so subtle.

I once rode an e-bike with a rear hub motor. Starting out in a low gear nearly reared the bicycle up like a horse. Evidently, the bicycle had a torque sensor which applied power from the motor in proportion to the torque at the rear wheel --  higher torque in a low gear signaled the motor to produce excessive power. With a rear hub motor and more than single-speed gearing, torque in proportion to pedal force requires more sophisticated control, with an additional speed sensor at the rear wheel, or a torque sensor at the cranks.

A rear-hub-motor e-bike can have derailleur gearing, can coast with the pedals stationary, and can be programmed to initiate regenerative braking (charging the battery) when the rider turns the pedals backwards, also allowing backpedaling to position the pedals for starting and stopping -- unlike with a coaster brake, which prevents the pedals from spinning backwards. Regenerative braking is, however, rather weak due to the slow rate at which batteries can charge: the bicycle should have front and rear handbrakes .

Mid-Motor Control

Torque with a mid motor and torque sensor is directly proportional to pedal force, and is also compatible with a conventional gearing system that requires reducing force on the pedals while shifting.  A mid motor will wear the chain and sprockets, though, and can pose complications with shifting unless the gearing is integrated into the motor assembly.  

Front-wheel Drive

The main advantage of a front hub motor is that it is an easy retrofit. It has to be throttle-controlled unless there is a separate sensor at the crank or rear wheel. The motor’s mass makes steering less responsive.  

A cyclist in the conventional riding position can stand and shift body mass forward and backward with each pedal stroke so almost all the weight is on the rear wheel during the high-power part of the stroke, increasing traction when the most power is being applied. This advantage is retained with a torque sensor and rear-wheel drive, but lost with a powered front wheel.

The driven wheel can skid when powered and the surface is slippery. A rear-wheel skid is usually recoverable, but If the front wheel skids, you can’t steer to balance and you fall.

No torque sensor?

Less versatile and responsive than torque sensing  is crank rotation sensing alone. This activates the motor if the pedals are turning, but he motor supplies the same amount of power all the way around the pedal stroke. The level of power production is steadier, and a motor can produce more power overall, but power production is not interrupted when the rider "soft pedals".  With a mid-motor then, all gear shifting is under power unless the rider stops pedaling -- impossible with derailleur gears. 

Some e-bike motors are controlled only by a throttle. Throttle control is most practical with a single-speed drivetrain or a hub motor, because reduction of power while shifting gears is not intuitive as with pedal control or unassisted pedaling. With a throttle, the fingers can't be kept in position over the brake lever to shorten reaction time, and so the throttle should be on the side with the rear brake lever, the one less useful for situations that require a quick stop. The usual twist-grip throttle is practical only with flat handlebars.

One “feature” of throttle control is that the bicycle can be powered without pedaling. The rider isn’t getting any exercise, which is obvious. Take it or leave it… But also, to another road user used to seeing bicycles that require pedaling, a throttle-controlled e-bike can appear to be slowing down – a confusing mixed message. Probably brake lights should be required in connection with throttle control. There is certainly enough energy stored in the battery to power them.

Summary

There are many ways to add electric assist to a bicycle. I hope to have clarified some of the differences, and issues with them. I have a good background in the science of this, though I’ll admit that my e-bike riding experience is limited. Please feel free to respond to this article with your comments.