Hub Motors

A hub motor puts the whole drive system inside one of your wheels, with no brackets, chains or extra gearing hanging off the frame. This page explains how they work: the difference between direct drive and geared hubs, and the motor physics that decides how fast, strong and efficient a hub motor will be.

If you are choosing a motor for your own bike, see our Hub Motor Options guide. For why we build around hub motors in the first place, see Why Hub Motors Are Awesome. And for the full technical deep dive, watch the two live presentations Justin gave on the topic for the SPEZI bike show:

Direct Drive Hub Motors

Slick and discreet, the hub motor is one of the most common drive methods for not just ebikes, but scooters, solar cars, and many other light electric vehicles. With a hub motor conversion, there is no need for external mounting brackets and drive chains to support a motor and transmission. Instead all of this is contained inside the wheel which mounts on your bike like any other wheel. There are two basic categories of hub motors: direct drive and geared.

The direct drive hub motor is about as simple as things get. Imagine taking an electric motor, but holding the axle and letting the body of the motor spin. Now lace a bicycle rim onto this spinning motor body and there you have it, no moving parts other than the wheel itself.
Typically these are radial-flux Brushless DC (BLDC) machines that have an array of permanent magnets on the inside surface of the hub. The stator windings are attached to the axle, and the hub is made to rotate by alternating currents through these windings. In an older brushed DC hub motor, the magnets are on the axle, and the windings are actually spinning on the inside of the hub. A carbon brush transmits electricity to the spinning windings via a commutator plate.

The one disadvantage of the direct drive scheme is that they are usually large and heavy for their power output. The reason for this is that the wheel speed is quite low, around 200 rpm. The power density available from an electric machine is directly proportional to the speed between the magnets and the winding, so in order to get adequate power and torque output the motor has to be large. In a geared transmission the motor is often spinning over 3000 rpm, and hence a much smaller motor can deliver the same power.

One way to increase the relative speed between the magnets and the windings is to move to a larger diameter motor. You could see this with the old BionX hub, which had a radius about 50% larger than a typical Crystalyte hub of the time, yet was quite light and narrow. However, there are mechanical complications to making the motor too large in diameter while keeping the weight down, so a practical trade-off has to be made.

Common to all direct drive hub motors is that they are always mechanically engaged. This is both a good thing and a bad thing. It is good because it makes regenerative braking possible. Any direct drive hub motor can do regen, but only with a motor controller that supports it, and many budget controllers either don't or don't do it well. All of Grin's controllers, including the Phaserunner and Baserunner, support proportional regen. Our Regen page covers how it works and what it's worth. Always engaged is bad because it means that you are always overcoming the rolling resistance of the motor even when you are not using it. Depending on the symmetry and quality of the motor, this additional drag torque can range from imperceptible to feeling like you are always riding with a flat.

Geared Hub Motors

As you can imagine, a geared hub motor has gearing inside it to reduce the high speed of a fast and efficient motor to the low speed of the wheel. From the outside, a geared hub motor is usually smaller in radius but wider than a direct drive hub. On the inside, they can take a variety of different forms although most frequently they have an outrunner motor that drives the center of a planetary gear set linked to the rotor.

The geared hub concept takes the weight advantages of a transmission drive and packages it into the simple looking and easy to install hub motor. These typically weigh about 50% less than an equivalently powerful direct drive machine, and they put out a lot of torque for their size. The geared hubs we carry today include the Bafang G310 and G311, the Shengyi SX1 and SX2, eZee, and the GMAC.

Most geared hub motors have a freewheel (a one-way clutch) inside. That means there is very little rolling friction to spin the wheel when you are not using the motor, but it normally rules out regenerative braking. There are exceptions: the GMAC has a locked clutch so it can do regen, eZee offers regen versions of its hubs, and Grin's new Freegen hub freewheels with zero drag when you pedal yet still does regen with your brake lever.

These advantages of the geared hubs have to be weighed against their downsides. There are many moving parts which are prone to wear, they generate some audible noise, and with less mass to soak up heat they reach their thermal limit sooner on long, steep climbs.

Almost every geared hub motor is single speed. Some people argue passionately that ebikes should have multiple transmission ratios so that the same 500 watt motor can both haul you slowly up a 15 degree hill and zip you along on the flat while always operating around the peak efficiency zone, but so far the cost and complexity of multi-speed hubs have kept them rare. Grin's new Magic Drive takes a different approach: it is a rear geared hub that works like an electric CVT. Your pedals drive the planet gear carrier instead of the wheel, so you pedal at one comfortable cadence at any speed, with no gears to shift.

Technical Refresher on Permanent Magnet Machines

Hub motors for electric bicycles are almost universally permanent magnet machines, either brushed or brushless. Permanent magnet motors have the highest power density of any common motor type, and they are also the easiest to understand. Both brushed and brushless motors have identical performance characteristics so the discussion below applies equally to both types.

As a thought experiment, imagine hooking up a DC motor to a power supply while holding the shaft locked. As you increase the current from the supply, the torque on the motor shaft would increase as well. The relationship is linear, twice the current yields twice the torque.

Now, disconnect the supply and connect a voltmeter across the motor leads instead. With the motor stationary, you will see zero volts. But start spinning the shaft and you'll read a voltage, directly proportional to the speed that the shaft is spun. This is referred to as the back-emf voltage, and is also expressed by a simple relationship V = K2 * RPM. If you derive expressions for K1 and K2 from elementary physics, it is found that they are directly related to each other because they both result from the same flux interactions between the magnets and coils. With the rotational speed given in RPM, then K1 = K2 * 2Pi/60. If you know the torque-current relationship, then you also know the voltage-speed relationship and vice versa.

Suppose now the motor is connected to a voltage supply, but the shaft is left to spin freely. Initially, there will be a large current flowing through the motor as given by Ohm's Law. I = V/R where R is the winding resistance in Ohms. This produces a correspondingly large torque (T=K1*I) causing the shaft to accelerate. As the shaft spins faster and faster, the back-emf voltage increases. This reduces the current through the windings because there is less available voltage to push the current, so I = (V - emf) / R. The motor shaft continues to accelerate until the back emf voltage is nearly equal to the supply voltage. At this point, the current has been reduced close to zero, just enough to produce a torque that overcomes the frictional forces on the motor.

The torque-speed relationship between these two extremes is a simple straight line, given by the equation w = V/k - R*T/k2. Notice that there are only two parameters needed to characterize this 1st order model of a motor, 1) the motor constant K, and 2) the winding resistance R. For a given K, the smaller the resistance R, then the less the speed drops when the motor is loaded, and hence the more powerful the machine. The motor constant K determines both how much voltage is required to make the motor spin at a certain speed, and how much torque it will output for a given current.

If you take the same motor and wind it with twice the number of turns of copper, then k will double, so the motor will require twice the voltage to reach the same speed, but it will only need 1/2 the current to output a particular torque. Notice that the power input (V*I) for a given torque and speed is the same in both cases. The power lost in the copper is the same too, since twice the number of turns means twice the length at half the wire area, so 4 times the resistance. Power loss goes as I²R, so halving the current is exactly canceled by a 4-fold increase in R. The important point here is that rewiring the motor for a different K value does not change the motor's fundamental performance in any way, provided that the same total amount of copper is used.

A third property that is important to characterizing a permanent magnet motor is the torque needed to simply turn the shaft. This is often called the cogging or drag torque and results from both the strong forces between the iron poles on the rotor and the magnets, and the induced eddy currents caused by the moving magnetic field. On direct drive hub motors, this torque is quite substantial, between 0.3 and 1 N-m, and it increases with the motor speed. Proper engineering and tight manufacturing tolerances can reduce this torque, but it is always a lot higher in permanent magnet motors with iron poles than any other type of machine.

The amount of power put into a PM motor is equal to the battery voltage times the current. The amount that is converted into mechanical power is equal to the back-emf voltage times the current. The conversion efficiency from electrical to mechanical energy is therefore simply the ratio of the back-emf voltage to the applied voltage.

When the motor is unloaded, it spins up to a point where the back-emf voltage is quite close to the battery voltage, and so the conversion efficiency is almost 100%. However, all of the mechanical energy produced is used to overcome these internal cogging losses, and no power is available as useful torque on the shaft. The efficiency from the perspective of output power to input power is 0%. As the motor is then loaded, the speed goes down so the electro-mechanical conversion efficiency goes down, but a significant percentage of torque produced is now available at the output shaft, so the overall motor efficiency increases. Load the motor further still, and then the losses from the cogging torque become insignificant compared to the I²R losses in the copper, and the efficiency starts to roll down again.

If you take two motors with the same K and R values, but different cogging torques, then the one with the lower cogging torque will exhibit higher efficiency. But this efficiency peak is only in the low power end of the graph, under heavy loadings the two graphs converge. For a respectable power density, a PM motor should typically be run at about 80% of the unloaded speed, corresponding to roughly 80% efficiency. Whether or not the motor has 95% efficiency at low powers is pretty insignificant in terms of the amount of energy it will draw on an actual trip, so don't base a motor decision on its claimed peak efficiency.

You can see all of this in action on our Motor Simulator, which plots the torque, power and efficiency curves of real hub motors at any speed. And for why a motor's watt rating tells you so little, see Motor Power Ratings.

Motor Windings: Why Turn Count Doesn't Change Torque

Most hub motors are made in several windings: the same motor with a different number of turns of copper around each stator pole. Our All-Axle motor, for example, comes as a slow 8 turn, a standard 6 turn, and a fast 5 turn. The classic example is the Crystalyte 400 series from the early days of ebikes, sold as the 406, 408, 409 and 4011, where the last digits are the number of turns: a 406 has 6 turns, the 4011 has 11 turns.

The torque that is produced by one of these motors varies in direct proportion to the total current flowing around each pole. So a 406 motor with 15 amps flowing through the winding has a total of 90 amps around the pole. The 409 motor would need just 10 amps to have 90 amps around the pole and hence the same torque output.

One false and oft-repeated conclusion is that therefore the 409 is a higher torque motor than the 406 because it can produce the same torque with fewer amps, or likewise more torque with the same amps. This is not the case. All 400 series motors can deliver exactly the same torque at exactly the same efficiency. The lower winding count motors just need more current to do this, but because they have fewer turns of a shorter length of heavier gauge wire, they can handle high currents with minimal loss. To use a concrete example, let's compare a 404 with a 408. The 408 has twice the number of turns as the 404, so the copper wire in the windings has 1/2 the cross sectional area and twice the length, for a total of 4 times the winding resistance of the 404. For a given torque output, the 408 needs only 1/2 the amps, but because it has 4 times the resistance the net electrical loss (I2R) is exactly the same.

Another consequence of having a larger number of turns around each stator is that the voltage induced in the winding by the passing magnets is increased in direct proportion to the number of turns. So for instance, at a certain speed of rotation, the 408 motor will produce twice the back-emf voltage as a 404 motor rotating at the same RPM. To power a motor, the battery pack voltage needs to be greater than the back-emf voltage, and so the 408 motor needs twice the voltage to spin at the same speed as a 404. However, it will only draw 1/2 the current, and you can see that the net power input (Volts * Amps) remains the same.

In practice, this means you choose a winding for the speed you want from your battery voltage and wheel size, not for torque. A faster winding in a small wheel and a slower winding in a large wheel can end up at the same road speed. The Motor Simulator lets you compare windings side by side.