Electric Assistance

e-Bromptons, Pedelecs, conversions, converters and technology

Under Construction - This section of the Brompton wiki is being updated to include options to add electric assistance to a Brompton


Popular conversion using a Tongxin front hub motor. Shown fitted to a M3L
Popular conversion using a Tongxin front hub motor. Shown fitted to a M3L

Firstly this is a brief overview of why you might choose to add electric assistance to your Brompton. Please follow the links below for more detailed information about pedelecs and ebikes.

Often known as Pedelec this involves adding a small electric motor to a bicycle. Usually an ebike is distinguished from a pedelec in that it uses a throttle control only to bring in electric assistance as required. Pedelecs have a sensor that detects the rotation of the main crank which then engages electric assistance. Some pedelecs have both systems of control. Although pedelecs/ebikes are available to buy ready designed and built it is also possible to convert existing bikes either with the use of a kit or building a kit from the component parts.There are several ways of doing this i.e. adding a small electric hub motor built into the front/back wheel or an electric motor added to the crank/BB assembly. To add assistance to the main crank usually involves a specially designed frame. Brompton themselves had planned to release a model prefitted with electric assistance (known as Project X) but at the time of writing, this has been delayed again with no announcement of when it will be available.

With the Brompton a hub motor can be fitted in the rear wheel but this is not covered here as the weight added to the swinging rear frame is problematic, as is spreading the rear forks to accommodate these wider motors and incorporating any associated gearing.

These pages will deal specifically with adding electric assistance to the Brompton by incorporating an electric hub motor into the front wheel.

NOTE: You will probably invalidate any warranty on your Brompton by fitting an electric assist kit and do so at your own risk.

For adding assistance to other bikes please checkout the links below.

Reasons for adding electric assistance to your Brompton


  • If you are not a strong rider, getting older or have some kind medical condition that hinders you, then this will transform your riding habits enabling you to commute more often and or ride for longer distances.
  • You can add as little or as much assistance as you want so that you you do not arrive at your destination all hot and sweaty.
  • Providing assistance in very hilly areas!
  • In hot weather allows you to pedal lighter for all or the last minutes of a ride so you are less sweaty after indoors - cycling tend to not make you sweat when riding on the flat but the removal of chilling breeze and putting on indoor work clothes can begin a build of sweat 10-20mins after indoors. A motor can reduce this.
  • Assistance when riding in windy conditions.
  • Provided added assistance when you have a lot of luggage or are pulling a trailer.
  • Kits based around the fork can be added seasonally, e.g. add motor in summer remove in winter.

Another advantage of adding electric assistance is that you do not require vehicle registration, road tax or insurance providing you remain within the legal limit i.e. for the UK max assist of 15mph (speed of bike assisted when not pedaling) and install a motor of no more than 250W power. Obviously you can ride faster than 15mph but the motor must only assist up to 15mph. Note I am still uncertain of the exact requirements of the law re motor power. It use to be no more than 250W but it may have recently changed to be 200W (to line up with European law). In addition use of throttle only assistance may also be illegal. In practice even a motor specified at 200W will draw over 300W under load e.g. up a hill. Anyway more recent details can be found here UK Rules. Note also you must be aged 14 or over to ride an electric bike.


  • Getting the conversion done for you can be quite expensive, in some cases as much as the new cost of a Brompton at around £700-£1000 dependant on battery size.
  • Will add a minimum of 1.4kg to the carry weight of the bike, or up to 6kg overall (bag and bike) dependent on motor and battery size choices.
  • Will compromise the fold unless installed correctly.
  • Requires modification to the front forks.
  • Adds complication to the Brompton in terms of changing the front tyre and servicing/routine maintenance.
  • Will likely invalidate any Brompton warranty.
  • Adds drag when it is not being powered. Will slow you when speed is naturally high (stronger cyclist, downhill). See drag comparison with other Brompton wheels.

Conversion Options

  • Complete conversions - By far the easiest but most expensive. Many of the kit providers will do the conversion for you using one of their kits. Obviously you will need to take your Brompton to their premises or pay postage both ways plus an additional cost over and above the kit price to fit it. Some of the kit providers listed below also sell new Bromptons with their kits retro fitted for you.

  • Kits - Again vary in cost dependent on if you buy a kit already installed into a set of front Brompton forks which you just swap out with your existing forks or the more common kits which include a motor ready installed in a new wheel rim.

Video of fitting a kit already installed into a new set of Brompton forks.

This is a very nice video, produced by Freedom EBikes, showing the complete detail of fitting a kit in which you modify your own forks and then fit the new wheel and associated electrics and controls to the Brompton. The process is very similar for many of the wheel kits provided and if you buy the components individually and make up your own kit.

  • The final option is to build your own kit from component parts - By far the cheapest but hardest you source all the component parts individually, usually direct from China, and then build the conversion kit yourself. This can be very time consuming, frustrating and will require high level of engineering and technical skill including the ability to build a wheel. Note some of the Chinese suppliers will provide complete kits with a motor ready built into a wheel. However the quality of the wheel build is often variable and for 16" wheels they often use a 16" (305) rim which of course is too small for the Brompton. The links below provide step by step accounts of people who have done this. One of the main advantages of the complete DIY route is that you can buy spare parts and maintain the conversion well beyond the 6-12 month guarantee period that many suppliers provide with their kits and conversions. However this DIY option is not for the faint hearted!

Battery options vary in type, size, weight, capacity and cost. Many of the kit suppliers include options for purchasing batteries of different capacities. One thing to consider before selecting a battery is the maximum distance of commute or how far the typical ride will be. If for example you have charging facilities at work you may not require a large capacity battery. Unlike a traditional non folding bike which you do not carry, it is more important for the Brompton to ensure you select the lightest weight battery/capacity size that you need for your regular journeys given that you may have to carry it. The distance you can commute on your battery before it runs out is also highly variable dependent on how much you pedal assist and how many hills you need to climb. More modern Li-Ion or LiFePO4 batteries also perform better if you do not run them flat every trip. As a rule of thumb you should ensure that the size of battery you select will have at least 25% of its capacity left before recharging.

The battery can be the most expensive component of the system. Battery quality and life is again highly variable. Some battery detail can be found below but please follow the links provided at the end of these pages for more in depth detail.

Main component parts of kits and what is required individually for DIY conversions.

Motor - There are quite a few motor options available for the Brompton. Most motors in use are brushless and are either geared or non geared. These pages concentrate primarily on the Tongxin (probably the most popular used for Brompton conversions) and very small lightweight Keyde brushless geared front hub motors. You can read up on other motor options by checking out the information links at the bottom of the page.
Sample 1.4kg Keyde Goldant and 2.0kg Tongxin slim 80mm wide front hub motors direct from China. Can be supplied with 28h or 36h drilled flanges.
Sample 1.4kg Keyde Goldant and 2.0kg Tongxin slim 80mm wide front hub motors direct from China. Can be supplied with 28h or 36h drilled flanges.

Typically when looking at front wheel hub motors on the manufacturers web site you will see them specified as 36V/24V, 100mm, motor for V, disk or roller brakes. When buying direct from China it is essential to specify the motor correctly to work with the Brompton i.e. 36V, 260RPM, 80mm wide motor for V brakes with 36H or 28H hub flange drilling.

  • Voltage - Usually 36V or 24V. 48V motors are available but usually for higher powered applications.
  • RPM - Different "Rotations Per Minute" are required for different sized wheel rims. Typically the smaller Brompton 16" (349) wheel requires a higher RPM. In the case of the Tongxin brand, their highest one is 260RPM. Lower RPM motors can be used, but these will top out at a lower speed on the smaller 16" rim. A 260RPM motor will spin a 16" (349) rim at around 14-15 MPH under load, at full power.
  • Width - Front wheeled motors usually come in 100mm widths and more rarely in 80mm widths. Obviously with the Brompton's narrower fork width, the 80mm wide ones are more suitable and easier to fit.
  • Brake type - For Brompton you want to specify V brakes as the other types like disk and roller brake are not suitable.
  • 36H or 28H - This is the number of holes drilled in the hub motor flange and need to match up with the type of rim you are fitting it into.
  • Sensored/Sensorless - This is the way the motor is controlled using hall sensors or not. The motors here are typically sensorless, easily distinguished by the three wires that come out of the motor. More details about sensored/sensorless motors are available on line.

Wheel Rim - Kits usually come with the motors prebuilt into a Brompton 16" (349) rim. Most kits use a Sun CR18 rim as usually the motor is drilled with a 36h flange. Many users find the Sun rims difficult to fit and remove tyres from. Motors supplied with 28h flanges mean that you can then use Brompton rims.

Motors built into rims, either radial or cross one (X1)  spoke patterns can be used.
Motors built into rims, either radial or cross one (X1) spoke patterns can be used.

Controller - To be completed

Throttle, Pedelec, Brakes - To be completed

Battery - To be completed

Weight considerations

For a Brompton that has to be carried a lot adding a motor can be a disadvantage as explained earlier. However with a little thought and planning like placing the electrics and battery into the front bag and using a lighter weight battery to match your common ride distance, the weight of the bike can be reduced and spread between both hands to carry bike and bag. For this conversion the battery is in one of the rear pockets of the C bag connected to the controller which is in the other pocket. Both bike and bag are then connected using an umbilical chord.

M2L-X fitted with a 1.4kg Keyde motor. Carry weight of the bike is 11.5kg the same as a stock M3L
M2L-X fitted with a 1.4kg Keyde motor. Carry weight of the bike is 11.5kg the same as a stock M3L

Umbilical wire to bag.
Umbilical wire to bag.


More information on all things ebikes and pedelecs

UK Pedelec Site
Endless sphere
UK Gov Rules



Freedom eBikes
Nano Electric Bikes
Electric Transport Shop

Component Kit Parts


Keyde Goldant Motor
OutRider Tonxin/Nano Motor


General ebike parts

Technical Links

Sheldon Brown Wheel Buiding


I am happy to answer questions (where I can) if you want to contact me on **jerrysimonsimon at hotmail dot com**