The Motor - The Drivetrain's Muscles

What does an electric motor do?

An electric motor converts electrical energy into mechanical torque and motion by letting magnetic fields created by current in copper windings in the stator interact with magnetic fields created by permanent magnets in the rotor. In two-wheeled electric vehicles, almost exclusively permanent-magnet synchronous motors (PMSM) are used, offering high efficiency and precise control, and this motor type exists in several configurations that we will cover below.

Motor types in two-wheeled electric vehicles

In two-wheeled electric vehicles, two motor types (PMSM implementations) are used most often: hub motors and mid-drive motors. Both categories have multiple subtypes, and we will discuss some of them here. The choice affects retrofit complexity, performance, efficiency, cooling, weight distribution, and service needs.

Hub motor

Hub motors are generally easy to retrofit as kits. One drawback of the traditional hub motor is cooling: the stator is enclosed by the rotating rotor (the hub), which makes heat rejection difficult at high power.

Direct-drive

Direct-drive hub motor uses the rotor as the wheel hub and has no gears (ratio 1:1, u = 1).

• Advantages: durability, low noise, regenerative braking capability and resilience at high power.
• Drawbacks: weight

Geared (planetary gear)

Geared hub motor has an internal planetary gear (ratio u > 1).

• Advantages: can be more efficient both during acceleration and at everyday speeds. Mechanical gearing can improve efficiency primarily during acceleration, but also while cruising at typical speeds by keeping the motor near optimal rpm; often compact and light.
• Drawbacks: more wear parts (planetary gear teeth, often plastic) and even harder cooling than direct-drive hub motors, which limits continuous power. Regenerative braking is theoretically possible, but these motors are usually fitted with a freewheel, which does not allow regenerative braking without complicated modifications.

Torque arms - a necessity

Hub motors generate high wheel torque, which creates a reaction torque at the axle, and that requires torque arms to be installed to prevent the axle from rotating in the dropouts or suspension. Without torque arms, the frame interface can be damaged and the wheel can loosen, especially during acceleration or regenerative braking. A torque arm transfers the motor's reaction torque to the frame safely. Read more about torque arms, installation, and safety in our guide on torque arms.

Mid-drive motor

Mid-drive motors are mounted somewhere other than the wheel hub, most often in the frame or under the crank axle. This enables straightforward mechanical gearing, which gives high efficiency across a wide speed range and much easier thermal management. However, they are harder to retrofit because mounting requirements vary much more from frame to frame.

Bicycle mid-drive

A bicycle mid-drive, found on most premium factory e-bikes today, drives the crank directly and therefore assists your pedaling. This provides the mid-drive benefits mentioned above, but also has important downsides. Drawbacks include higher complexity and drivetrain wear (chain and sprockets) on parts originally dimensioned for human power. Components such as chain, cassette, and chainring wear faster and require more frequent service.

Larger mid-drive motors

Larger mid-drive motors (for example QS138, used in platforms such as Stealth Bomber, electric mopeds, and electric motocross builds) often drive the rear wheel directly through a heavier-duty chain or belt system rather than a simple bicycle chain.

• Advantages: very high efficiency, selectable gearing, good cooling, and consequently support for very high power levels.
• Drawbacks: weight and complexity, more drivetrain noise, and higher demands on frame and mounts. Retrofitting a system like this is extensive work.

GBIKE 1: Direct-drive hub motor in a 26 inch rim

The GBIKE 1 kit is equipped with a large direct-drive hub motor in a 26 inch wheel, representing a carefully considered balance between acceleration, top speed, durability, efficiency, and practical installability. The hub-motor choice is based on several technical and practical considerations that together create an optimal system for a hobby electric vehicle in this class.

Thermal capacity and power durability

The motor is dimensioned to handle up to almost 6,000 W of electrical input power during acceleration. Combined with a 72 V system and a sine-wave controller, this provides strong yet smooth and controlled acceleration. The high peak power enables quick launches and good responsiveness.

For continuous operation, the motor can reliably propel a rider at 65 to 70 km/h, which is more than enough for a fun and practical hobby electric vehicle. This sustained performance is possible thanks to robust construction and sufficient thermal mass, even though thermal management, as with all direct-drive hub motors, is somewhat limited by the stator being enclosed by the rotor.

Balance between speed and torque

The motor's Kv value and overall sizing are chosen to provide an optimal balance: enough torque for strong acceleration from standstill while keeping top speed practically useful. This is achieved through a carefully selected combination of winding configuration (which sets Kv/Kt), system voltage, and wheel size, together with the inherent advantages of direct drive such as durability and low noise.

Why 26 inch wheels?

GBIKE 1 comes with a hub motor pre-mounted in a 26 inch rim. This choice is the result of a careful balance between multiple factors that affect both performance and appearance.

Because direct-drive hub motors have no gearbox, wheel size becomes the only mechanical gearing in the system. A smaller wheel gives higher gearing, which means the motor rotates more turns per meter traveled. This is beneficial because you get more physical output per electrical input at lower rpm, making the system more efficient. In general, you want as much gearing as practical, and without a gearbox that means a smaller wheel is better. (Read more in Wheel Size and System Efficiency)

At the same time, you cannot go arbitrarily small. The wheel still needs to look proportional and provide acceptable ride feel. 26 inches is the optimal balance: as small as practical without looking odd next to a front wheel up to 29 inches.

In summary, a 26 inch motor wheel gives GBIKE 1 strong efficiency through higher mechanical gearing while still enabling a practical and visually balanced overall setup.

Retrofit practicality as a kit

A key advantage of the hub-motor setup is that it enables straightforward home installation as part of a kit. Unlike mid-drives, which often require extensive frame modifications and special tools, a hub motor can be installed by end users with basic tools and technical understanding. This makes the technology accessible to a wider audience and keeps both installation cost and complexity down.

Reasonable cost and overall balance

Overall, GBIKE 1's motor delivers performance and usability that are well balanced against cost and practical constraints. The direct-drive construction removes wear parts such as planetary gears, reduces maintenance needs, and enables regenerative braking with a compatible controller. For riders looking for a robust, powerful, and installable drive system at a reasonable cost, this large hub motor is a well-considered choice.

PMSM: Kv and torque (Kt)

To understand how a permanent-magnet motor (PMSM) works in depth, a mathematical model is often used. It is described by the formula U = R·I + kₑ·ω. Here, U is the supplied voltage, R is winding resistance, I is current, and ω is motor rotational speed. The term kₑ·ω is called back-EMF and is the voltage the motor generates by itself while spinning. The applied voltage must therefore overcome both resistive losses (R·I) and the motor's own generated voltage (kₑ·ω) to drive current.

Two important constants are often used to describe the motor's characteristics:

Kv (rpm per volt) describes how fast the motor spins unloaded at a given voltage. A high Kv means a motor that spins faster but produces less torque per ampere. A low Kv gives a stronger motor that needs higher voltage to reach the same top speed.

Kt (torque constant) describes how much torque (M) the motor produces per ampere according to M = Kt·I. In SI units, these are related by Kt ≈ 60 / (2π·Kv). A motor with low Kv therefore has high Kt and produces more force for each ampere of current.

Speed and torque

Theoretical top speed is determined by battery voltage (U) and motor Kv value. Unloaded speed can be approximated as ω₀ ≈ Kv · U. If we know wheel circumference (C), we can estimate theoretical speed v₀. Speed then becomes v₀ ≈ (ω₀ · C) / 60. Under load, rpm drops because part of the voltage is used to drive current through the windings (R·I) rather than building speed.

Acceleration depends on torque, given by M = Kt·I. Because torque is directly proportional to current (I), controller current is what determines how quickly you launch. The relationship shows that higher current gives more torque, while higher voltage primarily expands the achievable rpm range and therefore top speed.

For the hub motor in GBIKE 1, this is especially important because there is no gearbox. The motor must be dimensioned to handle the high current (I) needed for high launch torque. The mechanical power (P) delivered by the motor is the product of torque and rotational speed, P = M · ω.