DIY electric bike: engine


In the last article about controllers, we already noted that three-phase asynchronous motors are used for electric bicycles, and partially touched on their varieties.

Let’s remind: motors for electric bicycles, with rare exceptions, are of two types: central (carriage) motors and motor-wheels.

The first ones are installed in the carriage assembly and have a built-in controller, the second ones are screwed into the rim in place of the bushings and are installed in the place of the rear (less often the front) wheel.

What are the main operational differences between these engines and in what cases should one or another option be chosen?

As I said, it is installed in the carriage assembly, that is, instead of the carriage. Technically, the installation process is as follows: the connecting rods with pedals are removed, the carriage is unscrewed, the engine is inserted, the fixing nuts are screwed in, the wiring is connected.

At first glance, nothing fancy and you just need to find a crank puller and a carriage wrench. But in fact, when installing a central motor, there are several important factors to consider.

Obviously, bicycle frame manufacturers don’t rely on a motor to be mounted on the frame. In this regard, the installation of the motor in place can be hampered by welds, feathers and even just hydraulic lines laid under the downtube.

And if the hydraulic lines can be started up in another place, then it is extremely undesirable to grind the welded seams of the frame. Firstly, this will affect its strength, and secondly, it will lead to the loss of the warranty.

With feathers it is even more interesting: if the star of the motor rests against the feather, you can do two things, each of which will not have the best effect on the quality of the finished result.

The first is to make a spacer ring for the carriage axle. However, it should be noted that if it is wide enough, there may not be enough threads to mount the motor from the back.

Also in this case, it is likely to distort

The second way is to make a new motor star with a smaller diameter. The task is creative and quite serious. You need to choose the right metal, calculate the dimensions and control the quality. But it is obvious that a star shrinking will lead to a decrease in maximum speed.

There are a couple more important things to consider when choosing a center motor.

At start

If not made, each time at start the engine will perform micro-impacts on the down pipe, which will damage the paintwork and deformations.

It often happens that when changing gears

And secondly, organize a chain catcher — either by locking the front derailleur in the desired position, or make a separate chain catcher yourself.

The largest, and, in fact, the only manufacturers of central motors today are Bafang and Tongsheng.

On bicycles with rear shock absorbers (called double suspension), the use of powerful center motors can cause the bike to flex.

This is due to the fact that when the engine is running, the front sprocket mounted on the motor, as it were, attracts the cassette, that is, the bike tends to fold to the right.

This effect is noticeable only as strongly and reliably and accurately as the feather bearings are made. The more the backlash and the higher the power, the more the bike will twist.

On hardtails (bicycles without a rear shock absorber), the stays are welded to the frame, so there is practically no twisting effect.

As its name suggests, a wheel motor is a wheel with a motor pinned into a hub, and now let’s look at the types of motors that are pinned to a wheel motor.

It is the heaviest engine ever installed in a motor-wheel. I immediately focused on weight because the larger it is, the more we shift from the concept of a bicycle to the concept of a motorcycle.

The direct drive motor does not have a gearbox, therefore, the wheel speed is the same as the rotor speed. And here both its advantages and disadvantages of this implementation are hidden.

The absence of a gearbox, on the one hand, tells us about increased reliability, since there are no extra gears that can fail due to overheating and improper operation.

On the other hand, it means

The weight

Just imagine that there is a 10 kg weight in the center of the rear wheel of the bike, which increases the inertia of the entire bike. During operation, this will seriously affect maneuverability.

It happens that a significant weight, in combination with an unqualified installation, leads to a breakdown of the direct drive motor axis. To make a new axle, you will have to find a quality turner with access to quality materials. And then re-pull the cable inside the new axis.

One of the main advantages of direct drive motors is the ability to use

Its main application is braking without using brake pads. But do not abandon the pads altogether — if there is a malfunction in the electrical wiring, it will be impossible to stop the bike.

The second purpose of recuperation is

But the fact is that the battery charge during braking is relevant only in mountainous areas, since the low efficiency does not allow any noticeable charge of the battery on relatively short descents.

In contrast to direct drive motors, as a wheel motor on electric bicycles, gear motors are installed, in which one wheel revolution corresponds to a certain number of rotor revolutions, in accordance with the reduction ratio.

Most often on Aliexpress there are motors with a single-stage gearbox, less often with a two-stage gearbox. The price of the latter is, accordingly, higher. It is better to check the number of reduction stages at once with the seller when ordering.

Obviously, a motor with a 2-stage gearbox will have a higher starting torque, since it will take more rotor revolutions to complete one wheel revolution, which means it will be easier to rotate the rotor.

In addition to the high starting torque, geared motors have another advantage over direct drive motors — they weigh several times less (about 2 kg). Accordingly, the load on the spokes, on the rim is lower, the weight of the finished electric bike is lower, and the maneuverability is higher.

Of course, gear motors are also not without drawbacks, the main of which is

In order not to overheat the motor, you will either have to constantly monitor its temperature, or simply use a controller with thermal protection, as is implemented, for example, in

The second disadvantage of a geared motor is the inability to use recuperation. This is due, in fact, to the presence of a gearbox. But let’s figure it out — is it really that important

If we consider a heavy vehicle, such as, for example, a trolleybus, tram, electric locomotive, electric car or a heavy electric bike (more likely even an electric motorcycle), then yes, this is saving brake pads and recharging the battery.

But if we are talking about

Firstly, it is much more efficient to save battery power by using the assistant mode (pedaling assist), since the degree of assistance (and therefore the battery consumption) is determined by the cyclist himself.

Secondly, good disc brakes are sufficient for a light e-bike, as opposed to a heavy e-bike.

Thirdly, as I wrote above, the use of recuperation for recharging the battery is advisable only in mountainous areas, with long descents.

If the central motor rests against the lower tube of the frame during operation, then the motor-wheel rests against the dropouts with its axis, and if no additional reinforcement of the dropouts is applied, the motor axis will turn even at low power.

To avoid this, on all motor wheels (with the exception of very low power in combination with a steel frame), so-called

Torkarms are made of steel, put on the motor axis and attached to the frame. For low-power engines, one torkarm is used, for powerful ones — two, one on each side of the motor.

It seems that everything is clear and simple, but there are also some

Firstly, the thickness and grade of steel from which the torcs are made are of great importance. If the metal is not hard enough and the engine is powerful, the axle can turn.

Secondly, manufacturing precision. The tighter the torcharm sits on the axis of the motor, the lower the likelihood of it turning, and, accordingly, the higher the permissible load.

Thirdly, during installation, the torches must be strictly perpendicular to the motor axis. If a misalignment is allowed, when tightening the nut, tension will arise, which can lead to either the axle jumping out of the dropout, or to its breakage.

Fourthly, the torcarm must be securely bolted to the dropout or frame feather. The best option is to use the threaded holes designed to fit the trunk or fenders.

Insufficiently rigid fastening of the torcarm (some amateurs fasten it with a plumbing clamp) will lead to its loosening and breakage of such fastening, and as a result — turning the axis, damage to the cable and, most likely, to its replacement (re-pulling).

Due to the fact that different manufacturers do not always adhere to a common standard in the production of motors, it is best to purchase a motor

You should also pay attention to

In addition, a long cable will need to be laid and secured somewhere, and if it becomes necessary to remove the wheel, it will pull the dismantling of these fasteners along the entire length to the connector.

In the photo of the direct drive motor, which I gave above, the power wires are brought out separately from the wires from the position and speed sensors. This is inconvenient, since during installation and dismantling, it becomes necessary to disconnect each wire separately.

Moreover, such a connection is

In order for the cyclist to be able to see the temperature of the motor on the display, if necessary, several conditions must be met. First, obviously, the display must have such a function.

Secondly, the motor cable must have an appropriate wire (usually white). And thirdly, of course, the temperature sensor itself must be installed in the motor.

The manufacturer could run a white wire in the motor cable to measure speed, but not install a temperature sensor. In this case, you can do it yourself, as I have already

The central motor, in fact, takes over the function of the cyclist — pedaling, that is, the moment of force is transmitted to the wheel through the chain, and it, like the cyclist, is easier for him to work in the lower gear of the rear derailleur and harder on the high gear.

Such a motor is very convenient when passing a track with steep climbs — it switched to a lower one and it calmly pulls you up, went to a horizontal section, switched to an increased one — and accelerated.

In addition, a bicycle with a central motor has a more correct weight distribution, due to which it has increased maneuverability. This is very noticeable when participating in competitions like

However, the wheel motor has its own

Firstly, the motor-wheel is easier to install, it is enough to make sure that the frame is not axially fastened, but QR (quick release, quick release, with an eccentric). Although for axial fastening it is possible to make

Secondly, on the wheel motor, you can continue to move even if the chain or rear derailleur fails. And during operation, accordingly, the wear of these components is lower.

Thirdly, the motor-wheel with a two-stage gearbox has a high enough torque to drive on rather steep climbs, so that in everyday use it is practically not inferior to the central motor.

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