What is an Electric Bicycle? In its simplest form an electric, or power assist, bicycle is a conventional pedal-powered bicycle that has had an electric motor attached to it. Early assist bicycles had a self contained gas or electric motor with a drive wheel that rubbed against the tire to help propel the bike. Modern electric bicycles are designed ground up to house motors and batteries to be far more efficient. As the price of fuel has increased over the past few years, so have the available electric bike options. Bikes on the market today range from conventional bikes modified with kits, to hybrid commuting machines, to futuristic designs that are more motorcycle than bicycle.
How Does it Work? Electric bikes are designed to help overcome terrain and limit the amount of stress the rider experiences. The riding experience varies greatly depending on the throttle system the bicycle is equipped with. There are 3 main types of throttles being used on electric assist bikes: speed sensors, torque sensors, and conventional twist throttles.
Twist Throttles (Power On Demand): Similar to a motorcycle or a scooter, the twist throttle is mounted to the handlebars and the rider merely turns the throttle to apply power assist. These throttles can be used with or without the rider pedaling. Additionally, twist throttles tend to offer decreased range from the two options, due to the constant and static load against the battery.
Torque Sensors (Power Assist or Pedelec): A torque sensor is a system built into the front crank of the bicycle. It measures how hard the rider is pushing against the pedals, and automatically increases the motors power to compensate. This system helps the rider accelerate from a stop and helps level out hills, all while preserving as much battery life as possible. It also allows for the greatest safety margin—the assist is directly tied to your pedal action. Torque sensors offer the best available range along with a riding experience closest to a normal bike.
Speed Sensors: Somewhere between a throttle and a torque sensor, the speed sensor monitors that the rider is actually turning the pedals and then turns on the motor. Unlike the torque sensor, the output of the motor is not variable instead it operates at full power all the time. The speed sensor does require the rider to pedal at all times to gain any assist. Like the throttle, the speed sensor puts a constant load against the battery, leading to a lower range.
Every electric bicycle offers a charger that allows you to replenish the battery when the bike is not in use. These chargers can range from simple 110v power adapters, like a laptop computer would use, to elaborate fast chargers that require industrial-type power service. A normal 9Ah battery should charge in around 4-hours.
Regenerative braking, like on a hybrid car, is available on very few electric bicycles for several reasons. First, on a two-wheeled vehicle, your forward momentum is your stability. The regeneration system introduces significant drag against the rear wheel that can compromise your stability under braking. Other systems rely on a pedal mounted generator to charge the battery while you are riding. This essentially transfers the energy you would use riding up a hill to a less strenuous time of the ride—essentially negating any net assist.
Assist speeds can vary between different manufacturers. Many limit the power assist to speeds under 15mph, allowing for compliance with the federal electric bicycle guidelines. Bikes with torque or speed sensors will allow the rider to pedal faster than the assist speed.
How Far Can it Go? The manufacturers published range on an electric bike can be misleading. Range is usually a function of battery capacity and motor power, though the range of your electric bicycle can vary significantly based on how and where you are riding. All battery packs on bicycles are rated in Amp-Hours (Ah). Ah is a number that represents the capacity of the battery—how many gallons are in the tank, so to speak. It is important to note that the voltage of the battery can also help increase the range. Switching from a 26-volt to a 37-volt battery can bring gains of nearly 30% in range.
Significant gains in range also come from using the pedals and gearing of the bike as much as possible. Even in situations when you do not need to pedal, using the bicycles transmission, shifting gears, and adding your human power helps keep battery power in reserve for when you really need it. Some power assist bicycles have multi-mode assist settings that vary the total motor output based on a preset assist level. Some manufacturers publish a range based on the lowest assist setting, sometimes as low at 10%!
When choosing an electric bike understand that there is a zero sum relationship between speed, range, and weight. This means that higher assist speeds and long range require much larger batteries and a heavier bike. Conversely, a lighter bike will have a lower speed, or range. The best manufacturers have built their bikes with a good balance between speed, weight, and range.
Batteries. Battery technology is ever evolving and recent advances have allowed for some significant breakthroughs in the functionality of electric bikes. New lightweight battery designs have slashed the overall weight of the bike—but do so at a much higher price. Below is a brief overview of the primary battery chemistries used in electric bikes. This list is by no means exhaustive, but it does cover the most common configurations.
Lead Acid batteries are the oldest, and least expensive, option available. Identical to the battery in your car, these batteries are made from lead plates suspended in a wet bath of acid. Originally developed in the 1850’s, lead acid batteries are very inexpensive, but are also extremely heavy. Lead Acid batteries have the lowest energy-to-weight ratio of any electric bike battery. These batteries can be found on the lowest price-point electric bikes on the market, though the low price comes with the highest total weight of any electric bike.
Nickel Metal Hydride (NiMh) were brought to market in the late 1980’s as the first high-performance rechargeable battery. Offering a much better energy-to-weight ratio than Lead Acid, NiMh batteries became a practical solution for the large battery packs needed for electric vehicles. More recently, NiMh cells were used for the battery packs on most first generation hybrid cars—significantly driving up the cost of the raw materials that make up the battery. Advances in lithium battery technology have replaced NiMh in the majority of electric bicycles, although it is still available in some models.
Lithium Ion batteries represent the current standard in electric bikes. Packing the best available energy density into an extremely lightweight package, Lithium Ion batteries have fostered a revolution in electric bike design. Due to this new technology electric bike weights have slipped under 50lbs, all while ranges have nearly doubled. Lithium batteries are not without their pitfalls. Due to their construction, most lithium batteries are susceptible to overheating due to over-charging. To prevent this, and to ensure long-term reliability of the battery, each lithium pack should have a battery management system (BMS) installed. Some manufacturers will omit the BMS as a cost saving measure. Lithium batteries can also come in several other chemistries. One, Lithium Polymer, is a flexible cell allowing for the battery to be made in nearly any shape. The other, Lithium Iron Phosphate (LiFePo4), is a relatively new technology. LiFePo4 batteries do not have the same temperature concerns that Lithium Ion do.
The location and integration of the battery is a key feature of any electric bike. Some models hang the battery from the parcel rack, almost as an afterthought, while others mount the battery in the centerline of the frame—or even hidden inside a frame tube. Battery installation is more than just a matter of good looks. The battery represents a significant part of the bike’s weight, and its installation can affect how stable the bike is to ride.
Motor and Control Systems. Much like batteries, there are many different motor and controller systems being used on electric bicycles today. Early and some low-price models use a chain or belt drive and an externally mounted motor. This type of design allows the installation of the motor without having to design a special frame. Most current models use a brushless hub-centric motor mounted into the front or rear wheel. These motors are very efficient, require no maintenance, and often have a reduction gear integrated in them.
Like horsepower in cars, motors are rated in watts—and there are many ways to measure (and manipulate) that rating. Look for bikes that have a continuous wattage rating, it may be lower but it is a more accurate representation of the performance of the motor. Additionally, some motors will use a reduction gear to help boost the performance of the motor. A 250w motor with a reduction gear can perform as well as a 450w non-geared motor.
Electric bikes are available in both front-wheel-drive (FWD) and rear-wheel-drive (RWD) configurations. RWD bikes tend to be easier and safer, to ride as most of your body weight is carried on top of the drive wheel. Though, the RWD motor location limits the number and style of gearing the bike can use. With FWD bikes, the motor can cause the front tire to skid in slick conditions causing difficulty with steering.
Electric motors have the ability to deliver 100% of their power instantaneously. To make the bike safe to ride, a controller is installed to act as the brains of the bike, and to control the acceleration of the motor. Controllers can have additional features installed in them, such as: multiple assist speeds, light controls, battery diagnostics, or a trip computer.
Frames. Good electric bicycle frames are not all that different from conventional bike frame designs. Usually made from steel, chrome moly, or aluminum, electric bikes are available in hybrid, mountain, cruiser, folding, and tricycle styles. The right style for you depends largely on what you are comfortable riding. Be cautious of how the material the bike is made from affects its weight. Some of the less expensive steel framed bikes can approach 100lbs, while a lithium battery equipped aluminum frame can be as low as 40lbs
The low bar step-over bike frame, conventionally referred to as a ladies bike has been growing in popularity with men recently. Painted in gender neutral colors, the step-over bike frame is easier to get off and on, and historically has been popular with Europeans of both genders.
Folding bikes have come a long way in the past few years. Early folders were ‘clown bikes’ with tiny wheels and single speeds that never stayed together. Modern folding bikes have full size 20” wheels, full transmissions, and well designed folding hinges and latching systems.
Security. Your electric bicycle is a significant investment, and good security measures will help it remain yours. A large part of the expense of an electric bike is the battery and motor—and securing them to the bike is a primary concern. The battery should be removable to facilitate charging away from the bike, but still secure enough that it cannot be stolen. Second, where on a conventional bike quick release hubs are a convenience, on a power assist bike they can facilitate theft of the motor. Conventional axle nuts are the preferred installation.
Some electric bikes come with integrated locking systems as standard equipment. Some of these systems feature a wheel lock that is physically bolted to the bike which will immobilize the bike. You can insert a chain to lock the bike to a nearby pole.