Mid-Drive vs. Hub Motor: An Engineer's Guide to Choosing Your E-Bike Drivetrain

If you are in a rush to start your build, here is the high-level engineering breakdown of mid-drive vs hub motor systems.

While e-bike kits exist for both, the choice fundamentally alters the mechanical dynamics of your bicycle.

To truly understand the mid-drive vs hub motor debate, we must look at where the force is applied relative to the wheel.

Hub Motors: The Direct Approach

A hub motor replaces the standard hub of a bicycle wheel. It applies torque directly to the wheel (or via internal planetary gears). Because it bypasses the bicycle's drivetrain (chain, cassette, derailleur), it is mechanically simpler in terms of power transmission.

From an engineering standpoint, this is a distinct advantage for reliability. If your chain snaps, a hub motor can still drive the bike home. However, it suffers from a fixed gear ratio. The motor must spin at a speed relative to the wheel. On steep hills, the motor RPM drops, pushing it out of its optimal efficiency curve and converting more battery energy into heat rather than motion.

Mid-Drive Motors: Mechanical Advantage

A mid-mount motor sits at the crank (bottom bracket) and drives the chainring. This is the key differentiator: it utilizes the bicycle's existing gears.

Just as a car needs to downshift to climb a hill, a mid-drive allows the motor to spin at a high, efficient RPM even while the wheel turns slowly. This variable gear ratio means a 750W mid-drive can often out-climb a 1000W hub motor because it can leverage mechanical advantage rather than brute force. This integration makes custom motor setups using mid-drives the gold standard for performance tuning on variable terrain.

When evaluating hub motors for e-bike kits, you will encounter two distinct subtypes: Geared and Direct Drive (DD).

Geared Hub Motors

These contain internal planetary nylon or composite gears.

• Pros: They provide higher torque at low speeds, freewheel with zero resistance when the battery is dead, and are generally smaller and lighter. They are excellent for a stealth e-bike build because they can hide behind a disc brake rotor.

• Cons: The internal gears are a wear item. They cannot do regenerative braking (usually) because of the internal clutch.

Direct Drive (DD) Hub Motors

These are essentially large electromagnets with no moving internal parts other than the bearings.

• Pros: Near-infinite durability and the ability to use regenerative braking (turning the motor into a generator to recharge the battery and save brake pads). They are capable of handling massive power for high-performance DIY builds.

• Cons: They are heavy (often 6kg+), large, and suffer from 'cogging' resistance when pedaling without power.

Engineer’s Note on Unsprung Weight: Adding a heavy hub motor to the rear wheel increases 'unsprung weight.' This negatively impacts suspension performance, making the bike feel sluggish over bumps compared to a standard wheel.

Mid-drives, such as the Bafang BBSHD or Tongsheng TSDZ2, are favored by technical enthusiasts.

The Efficiency King

By operating near optimal RPM regardless of road speed (assuming the rider shifts gears correctly), mid-drives run cooler and squeeze more range out of a battery pack. For 1000w motor upgrade projects intended for off-roading, a mid-drive is often the only viable option to prevent motor overheating on long ascents.

The Maintenance Trade-off

The major downside is the stress placed on the drivetrain. A human produces perhaps 100-200 watts continuously. A mid-drive can output 750W to 1500W bursts directly into a bicycle chain designed for human power. This leads to accelerated wear on chains, cassettes, and derailleurs. If you choose this route for your custom motor setups, invest in e-bike specific chains (like KMC-e series) and steel cassettes.

Center of Gravity

For handling, mid-drives are superior. By locating the motor and usually the battery in the center and low on the frame, the bike handles more like a traditional mountain bike. This is crucial for technical single-track riding.

For the garage hobbyist, the installation process is a major factor in the mid-drive vs hub motor decision.

Hub Motor Installation

Difficulty: Moderate. The main challenge is ensuring the axle fits your dropouts. Most hub motors use oversized axles (12mm or 14mm with flats filed to 10mm). You often need to file dropouts or use torque arms—thick metal braces that prevent the powerful motor axle from spinning out and destroying your bicycle frame. Wiring is generally simple: run the cable from the wheel to the controller.

Mid-Drive Installation

Difficulty: High. You must remove the bicycle's bottom bracket, which often requires specialized tools and significant force. Compatibility is tricky; you need to measure the bottom bracket shell width (usually 68mm-73mm, but sometimes wider on fat bikes) to buy the correct kit. You also need to ensure chainline alignment is correct, often requiring spacers or offset chainrings to prevent the chain from dropping.

Toolbox Essentials:

• Crank puller

• Bottom bracket tool

• Lockring wrench

• Soldering iron (for connector swaps)

Scenario A: The 15% Grade Hill

• Mid-Drive: You shift the bike into the lowest gear (largest cog). The motor spins fast, the wheel turns slow. The bike climbs the hill effortlessly at 8-10 mph without overheating.

• Hub Motor: The motor is forced to spin slowly. Efficiency drops to 50% or less. The rest of the energy becomes heat. Unless it is a high-power system, it may bog down or overheat.

Scenario B: The Flat Commute

• Hub Motor: Shifts are unnecessary. You have instant power. If you snap a chain, you can still throttle home. The ride is quiet and smooth.

• Mid-Drive: You must shift gears as you accelerate. You cannot shift gears while the motor is under full load (crunching gears), so you need a gear sensor to cut power momentarily while shifting.

For flat, urban environments, the simplicity of the hub motor often wins. For dynamic terrain, the mid-drive reigns supreme.

When budgeting for e-bike kits, look beyond the sticker price.

Upfront Costs: Generally, hub motor kits are cheaper. A decent direct-drive kit can be found for $250-$400 (excluding battery). Mid-drive kits like the Bafang series usually start around $400-$600.

Long-Term Costs:

• Mid-Drive: You will replace chains and cassettes significantly more often. A mid-drive ridden hard might need a new chain every 500-1000 miles.

• Hub Motor: Extremely low maintenance. However, if the rear wheel has a flat tire, changing the tube is a nightmare compared to a mid-drive bike (due to the heavy motor and wiring connected to the wheel).

If you are building a budget DIY conversion, a hub motor offers the best bang for your buck.

As an engineer, I don't believe in a 'best' motor, only the correct motor for the application. Here is my final recommendation based on your specific needs:

Choose a Mid-Drive Motor If:

1. You ride off-road or in hilly areas: The mechanical advantage is non-negotiable here.

2. You want a balanced ride: Essential for mountain biking and technical handling.

3. You are technically proficient: You are comfortable with drivetrain maintenance and shifting gears properly.

Choose a Hub Motor If:

1. You want a bulletproof commuter: Reliability and redundancy are key for getting to work.

2. You are on a budget: Get more power for less money.

3. You want a stealth e-bike build: A small geared hub motor is almost invisible to the untrained eye.

4. You want regenerative braking: Only available with Direct Drive hubs.