Is an eBike Conversion Kit Legal in the UK?
Guides • UK Law • Road Legal Builds Is an eBike Conversion Kit Legal in the UK? The Complete 2025 Guide By George — Qualified
Guides • Conversion Kits • Buying Advice
How to Choose an eBike Conversion Kit: Hub Motor vs Mid Drive, Wattage & Battery
Quick answer: The right conversion kit depends on three things — your frame, your terrain, and your intended use. Hub motors are simpler and more affordable. Mid drives are better for hills and longer range. Wattage determines performance ceiling and legal status. This guide walks through every decision in order.
About this guide: I have fitted both hub motor and mid drive conversion kits at the All4eBikes Cardiff workshop for the past four years — over 150 mid drive builds and a significant number of hub motor conversions across a wide range of bikes and use cases. The advice here is based on what I have seen work, and where I have seen people go wrong.
Contents
- 1.The first decision: hub motor or mid drive?
- 2.Hub motor kits — what they are and who they suit
- 3.Mid drive kits — what they are and who they suit
- 4.Understanding wattage — what the numbers actually mean
- 5.UK road legality — what this means for your choice
- 6.Choosing the right battery
- 7.Frame compatibility checklist
- 8.Use case matcher
- —FAQ
The First Decision: Hub Motor or Mid Drive?
This is the choice that shapes everything else. Hub motors and mid drive motors are fundamentally different systems, not just different power levels. Understanding which suits your situation is worth doing before you look at any specific kit.
| Hub motor | Mid drive | |
|---|---|---|
| Where the motor sits | Inside the rear wheel hub | At the bottom bracket, driving through the chain and gears |
| Hill climbing | Good at lower wattages; excellent at higher wattages | Excellent at all power levels — uses the bike’s gears |
| Range efficiency | Moderate — fixed ratio drive | Better — motor operates in its efficient range across terrain |
| Drivetrain wear | Low — motor doesn’t touch the drivetrain | Higher — motor power goes through chain and cassette |
| Installation complexity | Moderate — wheel and cable work | Moderate to advanced — BB removal, cable routing, programming |
| Frame compatibility | Very wide — fits almost any bike with a rear dropout | Narrower — depends on BB shell type and size |
| Ride feel | Motor push from the rear wheel | Natural pedal feel — power through the cranks |
| High-power options | Available up to 8000W | BBSHD at 1000W is the practical ceiling |
| Typical cost | Lower to moderate | Moderate to higher |
Hub Motor Kits — What They Are and Who They Suit
A hub motor conversion replaces your rear wheel with a motor-laced wheel. The motor drives the bike directly, independent of the gears. Installation involves fitting the new wheel, mounting the battery and controller, and connecting the display and pedal assist sensor.
Hub motors are the better choice when
- You want a simpler, more affordable conversion
- Your bike has a non-standard or press-fit bottom bracket that prevents mid drive installation
- You want minimal drivetrain wear — the motor doesn’t touch your chain or cassette
- You need high raw power — hub motors scale to 1500W, 2000W, 3000W, and beyond for off-road and private land use
- You want a discreet conversion that doesn’t alter the visual profile of the bike
- The terrain is mostly flat to moderate
Hub motors work less well when
- Your terrain is consistently hilly — a fixed-ratio hub works harder on climbs than a mid drive using the gears
- Maximum range per charge is a priority
- You have a rear suspension bike — the motor and cable routing become more complex
- You want the motor to feel integrated into how the bike pedals
From the workshop: Hub motor kits tend to suit riders who have a specific, relatively flat commute and want to convert an existing bike without spending more than necessary. The conversion is clean, the ongoing maintenance is low, and for flat terrain the system performs very well. On hilly routes — and most of South Wales qualifies — a mid drive becomes the better investment.
We stock hub motor kits from 250W at 36V up to 8000W at 72V. Browse the full conversion kit range to see what’s in stock. The 250W kit is road-legal as configured. Everything above 250W is for off-road and private land use only.
Mid Drive Kits — What They Are and Who They Suit
A mid drive motor replaces your bottom bracket and drives through the bike’s existing chain and gears. This means the motor benefits from your cassette on hills — in a low gear on a steep climb, the motor works less hard to move you at the same speed, in the same way your legs do.
Mid drives are the better choice when
- Your terrain is hilly or mixed — the gear advantage makes a real difference
- You want the most natural pedal-assist feel
- Range per charge matters — mid drives are typically more efficient
- You want a road-legal build that still performs well
- You are building a capable e-MTB or a long-distance commuter
Mid drives work less well when
- Your frame has a press-fit or non-standard bottom bracket shell
- Your budget is tight — mid drive kits and batteries cost more than a basic hub motor setup
- You want very high raw power (above 1000W) — hub motors are the better option at higher wattages
- You want minimal installation complexity
The most popular mid drive conversion at our workshop is the Bafang BBS02B, fitted on everything from steel commuter hybrids to aluminium hardtails. For the vast majority of UK riders, it offers the best balance of performance, road legality, and long-term reliability. See our full Bafang mid drive guide for a detailed comparison of BBS01B, BBS02B, and BBSHD.
Understanding Wattage — What the Numbers Actually Mean
Wattage is the most misunderstood spec in the eBike conversion market. Here is what the numbers actually mean in practice.
| Wattage | Real-world performance | Road legal in UK? | Typical use case |
|---|---|---|---|
| 250W | Solid assist on flat to moderate terrain. Feels like strong tailwind. Struggles on sustained steep gradients with heavier riders. | ✓ Yes (if PAS only, 25 km/h cutoff) | Urban commuting, flat routes |
| 500W–750W | Noticeably more torque. Handles hilly routes comfortably. Mid drive at 750W programmed to 250W continuous remains road-legal. | ⚠ Requires programming for road use | Hilly commuting, mixed terrain |
| 1000W–1500W | Strong performance. Hub motors at this power are off-road only. Mid drive (BBSHD) can be programmed for road legality. | ✗ Off-road only (hub motors) / ⚠ Programmable (BBSHD) | Off-road, cargo, demanding builds |
| 2000W–8000W | High-performance off-road and private land use. Not road-legal by design. | ✗ Off-road and private land only | Trail riding, private land, speed builds |
The number on the box is not the whole story: A 750W Bafang BBS02B programmed to 250W continuous output delivers better torque and hill-climbing ability than a native 250W motor — because the motor is physically larger and more efficient at lower outputs. This is why most of our workshop road builds use a BBS02B rather than a BBS01B, even for commuters who want full road legality.
UK Road Legality — What This Means for Your Choice
A converted eBike is road-legal in the UK as an EAPC (Electrically Assisted Pedal Cycle) provided three conditions are met: maximum 250W continuous output, pedal assist only, and speed cutoff at 15.5 mph (25 km/h). A bike that doesn’t meet these conditions is legally a motor vehicle — requiring a licence, registration, MOT, and insurance.
For a full breakdown of the rules, including which kits can and cannot be made road-legal: see our complete UK eBike legality guide.
Key point for kit choice: If your use is primarily on public roads, your kit options are either a 250W hub motor or a programmable mid drive (BBS01B, BBS02B, or BBSHD) correctly configured. High-wattage hub motors (1000W+) cannot be made road-legal and should only be chosen if off-road or private land use is your primary intent.
Choosing the Right Battery
The battery is as important as the motor. An undersized battery limits performance, range, and motor longevity. An oversized battery adds unnecessary weight and cost. You can browse the full eBike battery range to see all voltage and capacity options. Here is how to match them correctly.
1
Match the voltage to your motor
Most hub motor kits and the Bafang BBS range run on 36V, 48V, or 52V. The motor and battery must match voltage. A 48V motor requires a 48V battery. Running the wrong voltage damages the controller.
2
Choose capacity (Ah) based on the range you need
As a rough guide: 10Ah at 48V (480Wh) gives approximately 25–40 miles of assisted range, depending on terrain, rider weight, and assist level. A 17.5Ah battery (840Wh) gives approximately 45–65 miles. Hillier terrain and heavier riders reduce range; flatter routes and lighter assist extend it.
3
Choose the mounting style based on your frame
Downtube batteries mount to bottle-cage bosses and are the easiest to remove for charging. Triangle batteries sit inside the main frame triangle for better weight distribution and higher capacity. Rear rack batteries suit step-through and utility bikes. Check your frame before choosing — not all styles fit all bikes.
4
Consider discharge rate (C rating) for higher-power kits
High-wattage motor kits (1500W and above) draw high current under load. Your battery must be rated to supply it. A battery with a low discharge rate will sag in voltage under load, reducing performance and battery life. For 1500W+ builds, look for batteries with a continuous discharge rating of at least 30A.
| Motor type | Voltage | Minimum capacity | Recommended capacity |
|---|---|---|---|
| Hub motor 250W | 36V | 10Ah (360Wh) | 13Ah (468Wh) |
| Hub motor 500W–1000W | 48V | 13Ah (624Wh) | 17.5Ah (840Wh) |
| Hub motor 1500W–3000W | 48V or 72V | 17.5Ah (840Wh) | 20Ah (960Wh+) |
| Bafang BBS01B | 36V or 48V | 10Ah | 13–17.5Ah |
| Bafang BBS02B | 48V (52V for performance) | 13Ah | 17.5Ah |
| Bafang BBSHD | 48V or 52V | 17.5Ah | 20Ah |
Frame Compatibility Checklist
Before buying any kit, run through this checklist for your specific frame.
1
Rear dropout width (for hub motors)
Most standard bikes have a 135mm or 142mm rear dropout width. Check your bike’s dropout spacing matches the motor wheel you are buying. Most hub motor kits are supplied for 135mm dropouts — 142mm thru-axle frames require a different wheel build or an adapter.
2
Bottom bracket type and width (for mid drives)
Bafang mid drives require a threaded English (BSA) bottom bracket shell. BBS01B and BBS02B need 68mm or 73mm width. BBSHD needs 100mm. Press-fit shells (PF30, BB86, BB92) and threaded non-BSA shells (Italian, BB30) are not directly compatible.
3
Battery mounting options
Does your frame have bottle cage bosses on the downtube for a downtube battery? Does the main triangle have enough clearance for a triangle battery? Measure the internal dimensions of the triangle and compare with the battery specs before ordering.
4
Cable routing
External cable routing (cables running on the outside of the frame) makes installation straightforward. Internal routing (cables inside the frame tubes) is possible but adds significant complexity. Factor this into your DIY vs workshop decision.
5
Wheel size
Hub motor wheels are pre-built to specific sizes. Confirm your bike’s wheel size (26”, 27.5”, 700c / 29”) and ensure the kit matches. Fat tyre bikes (4” tyres) require specific wider-hub motor variants.
Use Case Matcher
1
Flat urban commute, want road legal, minimal fuss
Hub motor 250W 36V + 36V 13Ah downtube battery. Simple, affordable, road-legal out of the box. Best for flat to mildly undulating routes under 15 miles each way.
2
Hilly commute, want road legal, want proper performance
Bafang BBS02B 750W + 48V 17.5Ah downtube battery, programmed to EAPC spec. The right answer for most South Wales riders. Handles any gradient, road-legal when set up correctly, and a comfortable 40–55 mile range.
3
Off-road trail riding, private land, maximum fun
Hub motor 1500W–3000W + 48V or 72V triangle battery, or Bafang BBSHD + 52V triangle battery. Which is right depends on your frame. BBSHD for bikes with 100mm BSA shell; hub motor for everything else. Both are off-road and private land only.
4
Cargo bike or very heavy rider
Hub motor 1500W–2000W for rear-drive torque, or Bafang BBSHD on a compatible frame. Both handle sustained heavy loads. Hub motor is simpler; BBSHD uses the gears for greater efficiency on varied terrain.
5
Not sure — want expert advice on your specific bike
Message us on WhatsApp before you order. Send us your frame make and model, your intended use, and how far you ride. We’ll give you a straight recommendation in minutes. We do not benefit from recommending a more expensive kit if a simpler one is genuinely the right answer.
Visit us: 2 Beresford Road Lane, Cardiff, CF24 1QU
Hours: Mon–Fri 11am–6pm • Sat 11am–3pm • Sun closed
Hours: Mon–Fri 11am–6pm • Sat 11am–3pm • Sun closed
FAQ
What is the difference between a hub motor and a mid drive conversion kit?
A hub motor sits inside the rear wheel and drives the bike directly, independent of the gears. A mid drive motor mounts at the bottom bracket and drives through the chain and gears, meaning it benefits from your bike’s cassette on hills. Hub motors are simpler and more affordable. Mid drives offer better hill climbing, greater range efficiency, and a more natural pedalling feel — at higher cost and installation complexity.
What wattage eBike conversion kit do I need for hills?
For hilly terrain on a road-legal build, a Bafang BBS02B 750W programmed to 250W continuous is the most effective option. The motor’s torque (approximately 120 Nm) handles sustained gradients far better than a native 250W motor, while remaining fully EAPC-compliant when correctly configured. For off-road use with no legal requirement, a 1500W or 2000W hub motor gives excellent hill performance at lower cost.
How far will I get on one charge?
Range depends on battery capacity, terrain, rider weight, and assist level. As a general guide: a 48V 13Ah battery (624Wh) gives approximately 25–40 miles of assisted range on mixed terrain. A 48V 17.5Ah battery (840Wh) gives approximately 40–60 miles. Hilly terrain, heavier riders, and higher assist levels reduce range; flatter routes and lighter assist extend it.
Can I fit a conversion kit to any bike?
Hub motor kits fit almost any bike with a standard rear dropout. Mid drive kits (Bafang BBS series) require a specific bottom bracket shell type and width — threaded English (BSA) at 68mm, 73mm, or 100mm. Press-fit shells and non-standard frames need adaptors or are unsuitable. If you are unsure whether your frame is compatible, send us your bike details on WhatsApp before ordering.
Should I fit the conversion kit myself or use a workshop?
Both are valid options. DIY suits riders who are comfortable with bicycle tools, have done bottom bracket or wheel work before, and have time to research the programming step. Workshop fitting makes sense if you want the motor correctly programmed for UK road legality, if your frame is carbon or has internal routing, or if you rely on the bike daily and cannot afford downtime. We support both approaches at All4eBikes — supply only or full workshop build.
What battery do I need for a 48V conversion kit?
A 48V battery in the correct capacity for your use. For most commuting builds, a 48V 13Ah or 17.5Ah downtube battery is the practical choice — enough range for most journeys and removable for indoor charging. For performance or high-wattage builds, step up to a 48V 20Ah or a 52V pack for better performance under load. The battery voltage must match your motor and controller — do not mix 48V and 52V components without confirming compatibility first.
Not sure which kit suits your bike and riding style? Message George on WhatsApp with your frame details and a description of your typical ride — we’ll give you a direct recommendation.
How to Choose an eBike Conversion Kit
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