Quick answer: an electric bike conversion kit is compatible only when the wheel or bottom-bracket size, dropout or frame fit, brakes, battery position, voltage, controller and connectors all suit the donor bicycle. Check these items before ordering; wheel diameter alone is not enough.
This UK-focused checklist helps you assess a bicycle before choosing a hub-motor or mid-drive kit. It is for riders comparing an electric bike conversion kit, a cycle conversion kit or components for an electric bicycle conversion. If any frame, fork, brake or electrical measurement is uncertain, ask a qualified cycle mechanic or the kit supplier to confirm it.
Electric bike conversion kit compatibility at a glance
| Check | What to confirm | Why it matters |
|---|---|---|
| Donor-bike condition | Frame, fork, wheels, headset, brakes and drivetrain | Motor and battery mass place more demand on the bicycle |
| Wheel size | Tyre sidewall and rim marking, ideally the ETRTO size | A 27.5-inch, 29-inch or 700C label does not describe every combination |
| Dropout or bottom bracket | Internal width, axle type, material and shape | The motor must seat correctly without forcing the frame |
| Brake type | Disc or rim brake, rotor mount and caliper clearance | The conversion wheel must work with the braking system |
| Battery space | Mounting area, cable clearance and removal path | The battery must fit securely and remain accessible |
| Electrical system | Motor, controller and battery voltage, current rating and connectors | Incorrect combinations can damage components or create a safety risk |
| Pedal sensor | Crank, bottom-bracket and sensor clearance | The PAS disc and sensor need space and correct alignment |
| Intended use | Public-road EAPC or private-land build | UK legal requirements affect the suitable configuration |
1. Start with the donor bicycle
Inspect the frame and fork for cracks, corrosion, impact damage or previous repairs. Check that the headset, wheel bearings, spokes, tyres, brakes and drivetrain are in good working order. A motor and battery add weight and can increase the loads transmitted through the wheel, axle and brakes. Service or replace unsuitable parts before fitting a kit.
2. Confirm the wheel and rim size
Read the size printed on the tyre sidewall and, where possible, use the ETRTO number. This normally appears as two numbers such as a tyre width followed by the bead-seat diameter. It is more precise than relying only on names such as 27.5-inch, 29-inch or 700C.
- Match the conversion wheel's rim diameter to the donor bike and suitable tyre.
- Confirm that the rim width is appropriate for the planned tyre.
- Check the valve type and rim tape before assembly.
- Allow clearance around the tyre, mudguard, frame and fork throughout the wheel's rotation.
The cycle conversion kit range includes common 27.5-inch, 29-inch and 700C options, but complete fit still depends on dropout, brake and frame measurements.
3. Measure the dropout width and inspect the axle fit
For a hub motor, measure the clear distance between the inner faces of the dropouts with the original wheel removed. Check the axle slot width, dropout depth, cable exit and any derailleur, cassette or freewheel requirements.
The motor axle should seat fully and squarely. Do not spread, file or force a frame or fork simply to make a kit fit. Aluminium and carbon parts require particular caution. Follow the bicycle and kit manufacturers' guidance. A correctly specified torque arm may be required where the kit supplier calls for one, but it cannot make an incompatible or damaged dropout safe.
4. Match the braking system
Identify whether the bicycle uses rim or disc brakes. For discs, confirm the rotor mounting standard, rotor diameter, caliper clearance and alignment. For rim brakes, the conversion rim needs a suitable braking surface and compatible width. After installation, centre the wheel, align the brakes and test lever feel, pad contact and stopping performance before riding.
5. Plan the battery position
Make a cardboard template using the battery's published dimensions, including its rail and the clearance required to slide it on and off. Place it in the intended position, turn the handlebars, compress the suspension where relevant and move the pedals through a full rotation.
- Check bottle-boss spacing and the strength of the mounting area.
- Leave room for cables, connectors and the battery key.
- Keep the battery away from tyre, chainring, crank and suspension movement.
- Consider how added mass affects handling and lifting.
- Use a secure mounting system designed for the battery.
6. Match voltage, controller and connectors
The battery's nominal voltage must match the controller and motor system specified by the manufacturer. Confirm the battery-management system, continuous discharge capability, controller current requirement, connector type, polarity and charging arrangement.
Never connect components based only on wire colour. Similar-looking connectors can have different pin assignments. Use the supplied wiring diagram and verify the specification with the seller before energising the system.
7. Check the pedal-assist sensor and controls
Confirm there is enough clearance for the pedal-assist sensor and magnet disc, that the crank can be removed with the correct tool and that the sensor can be aligned securely. Plan positions for the display, brake cut-off sensors, throttle where supplied and cables. Wiring should not pull when the handlebars turn, rub sharp edges or sit where a tyre, chain or suspension component can catch it.
8. Choose hub motor or mid-drive fit
A hub motor concentrates compatibility checks around the rim, dropout, axle, brakes and gearing. A mid-drive replaces the crank-area drive unit, so bottom-bracket shell width, diameter, threads, chainline and frame clearance become central.
Read our Bafang mid-drive vs hub motor guide for the practical differences. The conversion-kit installation guide shows the typical fitting sequence.
9. Check UK road-use requirements
Under the current GOV.UK electric-bike rules, an electrically assisted pedal cycle must have pedals that can propel it, a maximum continuous rated motor output of 250W and electrical assistance that cuts out at 15.5mph.
A completed bicycle outside those conditions can be treated as a motorcycle or moped for public-road use, with approval, registration, tax, insurance, licence and helmet requirements. Higher-powered conversion kits should not be used on public roads or cycle paths as though they were ordinary bicycles; use them only where legally permitted.
10-point pre-purchase checklist
- Photograph the bicycle, dropouts, brakes, crank area and battery position.
- Record the frame and fork material and inspect their condition.
- Write down the tyre and ETRTO markings.
- Measure the internal dropout width or bottom-bracket shell.
- Confirm axle, cassette or freewheel and gearing requirements.
- Identify the brake type, rotor mounting and clearance.
- Template the battery and removal path.
- Match motor, controller and battery voltage and current requirements.
- Confirm every connector and accessory included.
- Choose a configuration appropriate for UK road or private-land use.
What if a kit does not fit?
Stop rather than forcing the part. Recheck measurements and specifications, then contact the seller with photographs and dimensions. A different wheel size, axle standard, battery case or motor type may be required. Structural frame or fork modification should not be used as a shortcut.
For budgeting, see how much it costs to convert a bike to an e-bike. For the complete project, read how to turn a regular bicycle into an electric bike.
Compatibility FAQs
Can any bicycle take an electric bike conversion kit?
No. Compatibility depends on condition, frame or fork material, wheel or bottom-bracket dimensions, dropouts, brakes, battery space and electrical requirements.
Will a 700C conversion wheel fit a 29-inch bicycle?
They can share a bead-seat diameter, but that does not prove complete compatibility. Rim width, tyre clearance, dropout spacing, axle, brakes and frame design still need checking.
Can I use a battery I already own?
Only if its voltage, discharge capability, battery-management system, polarity, connector, mounting and charger are compatible with the controller and motor.
Is a 250W conversion kit road legal in the UK?
Motor rating alone is not enough. The completed bicycle must meet all EAPC conditions, including functioning pedals and assistance that cuts out at 15.5mph.
Should I use a professional installer?
Professional fitting is sensible when you are unsure about frame, fork, brakes, electrical compatibility or wheel building. Inspect and test the completed bicycle before road use.
Ready to compare options? Browse electric bike conversion kits for common UK wheel sizes, then match every measurement above against the product specification before ordering.
Last reviewed: 9 August 2026. This guide is general technical and legal information, not a substitute for manufacturers' instructions or professional inspection.