Razor MX650 Motor Upgrade Guide: Everything You Need to Know

Why the Stock MX650 Motor Holds You Back

The Razor MX650 ships with a 650W brushed DC motor running on 36V. That's enough to move a lightweight rider on flat ground, but it has a hard ceiling — and you hit it fast.

Here's what that ceiling looks like in practice:

• Top speed: ~17 MPH — barely enough for parking lot fun
• Zero hill-climbing ability under any real load
• Brushed motor wear — the carbon brushes degrade over time, increasing maintenance
• No tuning options — what you see is what you get, forever

The good news: the MX650's frame was designed well. The motor mount, chain path, and geometry can all accommodate a much more powerful mid-drive unit — often without drilling a single hole.

What a Real Upgrade Looks Like

A proper MX650 motor upgrade has three core components:

1. The Motor

You're replacing the 650W brushed unit with a brushless BLDC (Brushless DC) motor. Brushless motors are fundamentally more efficient, more powerful, and longer-lasting than brushed units.

For the MX650, the Kunray KR5V V2 has become the go-to choice in the builder community. It's a 72V, 5000W rated motor with a 10KW peak burst capacity — that's roughly 7–15x the power of the stock unit depending on how you count it.

Key specs:

• Rated Power: 5000W | Peak Power: 10KW
• Rated Torque: 8.5 N·m | Peak Torque: 35 N·m
• Max RPM: 8000
• Motor Type: IPM (Interior Permanent Magnet) — same architecture used in Sur-Ron and Talaria motors
• Protection: IP54, built-in KTY84-130 temperature sensor
• Direct fit: Razor MX650, MX500, SX500, RSF650, Indian650 — no frame modification needed

2. The Controller

The stock controller is matched to the stock motor and has no tuning capability. For a 72V brushless upgrade, you need a new controller.

The Fardriver NS18-KR72V100A is the recommended pairing for the KR5V. It handles 72V at 100A line current, and critically — it has built-in Bluetooth, so you can configure motor parameters (current limits, temperature protection, reverse) directly from your phone via the Fardriver app.

3. The Battery

This is often the part people overlook. Your 36V stock battery is completely incompatible with a 72V upgrade. You'll need a purpose-built 72V lithium battery pack with a continuous discharge rating of 60A or higher.

Capacity depends on how long you want to ride:

• 20Ah: ~30–45 minutes of mixed riding
• 30Ah: ~50–70 minutes
• Higher capacity = heavier, but more range

Sprocket Selection: One Decision That Changes Everything

One of the most impactful choices in your build isn't the motor — it's the sprocket.

The sprocket ratio determines how the motor's RPM translates into wheel speed. Get it wrong and you'll either spin out at low speed or never reach your power potential.

Here's how to think about it:

For most MX650 upgrades, start with the 420-10T. It gives you a well-rounded build that's fast on the straight and still pulls hard out of corners. If you know you'll be riding technical terrain or carrying more weight, go 11T. If you want to chase top speed on flat ground, 9T.

Quick rule of thumb: Fewer teeth = more speed. More teeth = more torque.

The chain and chainring in your kit should be matched to whichever sprocket you choose — if you're buying a complete kit, this is handled automatically.

Throttle Options: Match to Your Build Style

The stock MX650 uses a half-twist throttle. When upgrading, you have more options:

Half Twist Throttle — Keep the familiar feel. Compact, precise, works great with the MX650 ergonomics. The best choice if you're keeping the Razor form factor.

Full Twist Throttle — Full motorcycle-style grip. Better for riders who prefer a longer throw and more progressive control feel.

Pedal Throttle — Not ideal for the MX650 format (more suited to go-karts and seated builds), but listed here for completeness.

For the MX650 specifically, half twist is the natural choice — it fits the existing handlebars cleanly and the control feel is familiar.

Step-by-Step: What the Upgrade Process Looks Like

This isn't a line-by-line wrenching guide — for that, refer to the KR5V installation manual and the official Fardriver wiring video on YouTube. But here's the high-level sequence so you know what you're walking into:

Phase 1: Gather Everything Before You Start

• KR5V V2 motor + Fardriver NS18 controller + sprocket/chain/chainring + throttle
• 72V lithium battery pack (not included in the motor kit)
• Basic hand tools: metric socket set, screwdrivers, wire stripper, crimping tool
• Heat shrink tubing and electrical tape for wire protection
• Chain breaker tool (to size the chain to your build)

Phase 2: Remove the Stock Components

1. Disconnect and remove the stock battery
2. Remove the stock motor (typically 4 mounting bolts)
3. Unplug the stock controller and throttle
4. Remove the stock sprocket and chain

Phase 3: Install the KR5V V2

1. Mount the KR5V V2 to the frame using the existing motor mount holes — it's a direct fit on the MX650
2. Install your chosen sprocket on the motor shaft
3. Route and tension the chain (5–10mm of sag is ideal)
4. Mount the Fardriver NS18 controller in a secure, ventilated location on the frame

Phase 4: Wiring

This is where most people slow down — but it's simpler than it looks.

Standard forward rotation wiring:

Connect the throttle signal wire, battery positive/negative, and power the controller. If the motor spins the wrong direction on first test, you can either swap two phase wires or enable reverse in the Fardriver app — the app method is easier and doesn't require rewiring.

Phase 5: Configure via Fardriver App

Download the Fardriver app (iOS/Android), connect via Bluetooth, and set:

• Temperature protection limit: 60°C warning / 65°C cutoff
• Line current limit: Start at 60A, work up gradually
• Reverse: Enable if needed (for left-side motor mounts)

Phase 6: Test and Tune

• First ride: low speed, listen for unusual noises
• Check motor temperature after 5 minutes of riding — should be well under 50°C at moderate load
• Gradually increase throttle use as you build confidence in the setup

What to Expect After the Upgrade

The difference is immediate and significant. Here's what builders consistently report:

Performance:

• Top speed jumps from ~17 MPH to 50–60 MPH depending on sprocket, gear ratio, and battery voltage
• Hill climbing goes from "barely possible" to "aggressive and confident"
• Acceleration is in a completely different league — the 35 N·m peak torque is felt from the first twist of the throttle

Handling:

• The V2's reinforced 17mm shaft eliminates the flex that older conversion motors sometimes showed
• Weight distribution stays similar since the motor mounts in the same location as stock

Maintenance:

• Brushless motors require significantly less maintenance than the stock brushed unit
• No brushes to replace, no commutator to wear — the main ongoing tasks are chain tension checks and occasional bolt torque verification

Common Questions (Answered Directly)

Do I need to modify the frame? No. The KR5V V2 is a direct bolt-on for the MX650 frame. No drilling, no welding, no modification required.

Can I reuse the stock battery? No. The stock 36V battery is incompatible with a 72V motor system. You need a new 72V lithium pack.

Is this street legal? In most jurisdictions, no. This build exceeds the power and speed limits for street-legal electric bikes in most countries. Treat it as an off-road and closed-course vehicle.

How hard is the install for a beginner? If you're comfortable with basic mechanical work and aren't intimidated by electrical connectors, this is manageable as a first build. The wiring is color-coded and straightforward. Plan for a full weekend the first time.

What if the motor spins the wrong way? Enable reverse in the Fardriver app. Takes about 30 seconds.

What's the motor temperature limit? Do not exceed 65°C (150°F). The KTY84-130 temperature sensor lets you set automatic cutoff through the controller — use it.

Ready to Build?

The KR5V V2 Complete Kit includes everything you need on the motor side: motor, Fardriver NS18 controller, your choice of sprocket (420-10T, #35-11T, or #35-9T), matched chain and chainring, and your choice of throttle.

Add a 72V battery and you're building.

→ Shop the KR5V V2 Complete Kit