The Brompton is simply one of the best folding bikes in the world.
From being an idea conceived in a London flat by engineer and designer Andrew Ritchie back in 1975 it has become one of the world’s most popular folding bikes.
It’s success is based on the fact that not only does it fold quickly down to a very compact size of 60 x 59.5 x 28 cm (around 100 litres volume) but it is also an incredibly strong bike that rides pretty well too, given its small 16” wheels.
There are a number of electric options around, most notably Brompton’s own version.
As this was rolled out from 2017 there have been many other retrofit options developed in the preceding years and newer options like the Swytch, for the more budget conscious, continue to hit the market.
Here we take a look at four options we have tested first hand, all of which have their own pros and cons which we detail.
Brompton have developed their own high quality but pretty conventional solution.
A slim geared hub motor sits in the front wheel.
And a 312Wh battery easily slots onto the headset area in the space where a bag mounting block usually sits.
In place of the luggage mounting block the electric version has a small ‘black box’ that is part of the battery mount which acts as a kind of junction box for the hub motor and LED lighting wiring as well as housing the controller.
The battery features integral USB port for charging small devices on the go.
There’s lots to like about this design.
The battery removes very quickly at the push of a button, cleverly leaving you with two lighter items to carry rather than one heavier one (14.8kg for the six speed bike and 3.46kg for the small case that contains both battery and charger).
It looks a very smart design with the electric cabling very neatly routed and the front wheel easily removable in the event of punctures etc via a ‘screw and pull’ connector in the power feed cable.
LED lighting cable runs are similarly neat and tidy and come with an automatic setting.
Mudguards are included in the price and it’s available with two or six gears and a choice of two colours.
Compared to the other motors I tried it’s one of the most powerful out there and the real big plus is the bottom bracket-housed torque sensor that introduces the power as you step on the pedals.
Torque sensing is the system all the better quality e-bike motor systems use, so its mere presence is reassuring.
It avoids the haphazard feeling power delivery of more budget systems that use a simple crank motion detecting system.
The biggest downside is that the system can’t be retrofitted to any of the many thousands of Bromptons out there.
In use the biggest downside is the lack of a power control button on the handlebars, meaning you have to lean over the bars to change power levels and light settings – far from ideal.
It really begs out for a wireless handlebar control.
Another major drawback is that the system limits your luggage carrying options somewhat.
Note though there is a bag upgrade rated at 20 litres storage that gives some storage space either side of the battery but it still falls short of the 25 litre T bag that fits on non-electric Bromptons.
The option of a rear rack would increase luggage capacity but doesn’t seem to be offered by Brompton as an option, though again you can get non-electric versions fitted with one.
The power delivery did feel a little unpredictable compared to high quality mid-drives like Bosch however, with occasional unpredictable surges or dead spots, possibly due to the fact the torque sensor is only activated by pressure on the left hand pedal (better systems have both pedals activating the system).
The motor is the most powerful of all the models tested here and it has the biggest battery as a standard offering, though that comes at the expense of making it one of the heaviest and most expensive options.
£2595 – 2895 depending on gearing, colour and battery bag options
Swytch were founded in 2017 but have very recently launched a Brompton specific retrofit kit with a lightweight handlebar mounted battery (our supplied prototype battery didn’t have a capacity rating but we believe the final production version will have a 180Wh capacity battery of the same physical size as this prototype).
The motor is a conventional geared hub motor but narrow enough to fit into a Brompton’s narrow 74mm front fork width without any fork stretching.
The whole kit adds some 3.4kg to the weight of an unassisted Brompton.
This gave a 14.81kg e-bike weight, composed of removable battery of 1.78kg and a very manageable ‘carrying weight’ of 13.03kg for the rest of the bike.
Our test bike was the B75 budget-priced (US$995 / £745) ‘limited edition’ Brompton very much in keeping with the competitive introduction price of the kit of US$650 / £499.
Swytch’s big selling point is the ease of conversion which involves three (hopefully) simple stages; swap over the tyre and inner tube from the bike you want to convert to the Brompton-sized Swytch hub-motored wheel supplied and fit it to the bike, clamp on the battery to the handlebars and the pedal sensor to the bottom bracket area, arrange, secure and connect the cables and away you pedal.
All the components are pretty standard budget e-bike budget components but put together in a nice clean design.
Our observations on the ‘quick and simple’ installation were that it is generally just so but you need to be aware of a couple of points.
The rims are Swytch’s own and not Brompton’s so you may need to do a little experimenting with which ETRTO 349 tyres fit most easily.
We also had to pay attention to the cable run (cable-tied in place) between the handlebar-mounted battery and the hub motor to ensure it doesn’t catch on the handlepost hinge release mechanism.
The riding experience is generally good, with a nice amount of power up hills and a decent top assisted speed of around 15mph on the Euro spec version we tried.
There is also a US spec with nominal 20mph max assisted speed though this has a larger 252Wh battery mounted on the luggage block and costs more.
We particularly liked the small and unobtrusive design of the pedal motion sensor which pushes into the left hand side of the bottom bracket recess (though this costs extra compared to the standard clunkier and more obvious looking standard issue sensor).
There is some noise that is perhaps a little more than you would expect.
Quoted range of 50km / 30miles looks rather optimistic to us as we achieved around 20 miles mainly on the flat riding in power level 4 out of 5 most of the time.
That’s about par for the course rather than being exceptional for a battery this size.
The fact the power is delivered by motion sensor rather than torque sensor means some inbuilt inefficiency and also means the power kicks in after a short delay rather than immediately you begin pedalling.
We also found that you have to route the power feed cable carefully.
Though not the most sophisticated in terms of power delivery, this is certainly one of the lightest front hub motor options out there and is also very cost-effective and pretty easy to retrofit.
US $650 or £499 (larger capacity 252Wh kit with 20mph assist limit available for US$750 / £599)
The Nano retrofit kit from the eponymous UK firm is based around a small, light and rather ingenious front wheel hub motor produced by Chinese firm Tongxin.
The Brompton kit certainly merits the Nano epithet, because at 2.3kg it is one of the smallest, lightest motors available.
Although fitted with a conventional pattern sun-and planet gear system, the Tongxin is very unusual in employing friction rollers rather than gear cogs to transmit the drive.
The Nano is one of the few hubs that will fit into the narrow forks of the popular Brompton folding bike, as it comes in a narrow 80mm version, though forks will still need spreading a little (it’s therefore compatible only with steel, not titanium forks).
Nano supply a special tool to help widen the forks slightly and the fitted motor does not affect the folding ability of the Brompton.
There is also a retrofit service in the UK if you don’t fancy doing the task yourself (pricing link here).
A small controller velcros neatly onto the side of the frame.
The battery is housed in the front bag, with copper contacts making the connection with the motor once the bag is in place.
Battery choice is between 144Wh powertool batteries from Bosch or Oregon or a bigger 418Wh generic Chinese battery adapted to Nano’s unique ‘battery in the bag’ system. More detail on battery choice here.
The ‘battery in the bag’ is a brilliant bit of design.
It means that you can remove the Brompton bag (they come in a bewildering variety of sizes and designs) with the battery remaining inside and have two fairly easily carriable packages when the bike if folded.
And unlike Brompton’s own system you can utilise any Brompton bag you like, preserving the bike’s full carrying capacity.
What’s more it will retrofit to Bromptons with rear racks too.
The choice of large or small capacity batteries also means this is the most flexible system out there; we loved the fact the small batteries can be added to.
If you need to only undertake a short journey using a single battery the kit adds only a little more than 3kg to the overall weight of the bike.
Two of the smaller batteries can be taken on flights too, generally not possible with bigger batteries.
The other big plus is the offer of different power control systems, including a throttle controller (you still need to pedal to get the motor to work, but this option gives great control for stop-start type riding in busy traffic).
There is also a motion pedal assist system that means power kicks in on pedalling, perhaps more suited to longer tours where you don’t want to have a thumb on the throttle control for long periods.
There is also the option of full lighting powered by the e-bike battery. Again this can be fitted for you or by the customer (good bike DIY skills required here).
Performance wise there is loads of torque available up the hills.
A little more complex to fit than the Swytch and the top assisted speed lags a little behind other models due to the high torque gearing (on our particular model at least – you would have to enquire at Nano if differently geared versions are available giving more top speed at the expense of hill climbing ability, if that’s what you want).
Wiring runs are longer and more complex than other systemsand possibly a little more prone to damage (though our long term test model has only suffered one fault, damage to the copper strips on the mounting block and these are easily replaced).
Velogical is an ingenious German engineering firm which, after developing the highly efficient and tiny Velogical rim dynamo (see our review here), went on to apply the principles in reverse, so to speak, to develop the extremely lightweight Velospeeder e-assist rim drive system.
Whilst the original design has been around for a while, Velogical produced a Velospeeder kit version for the Brompton folding bike in 2017, made to fit only models with rear rack.
The company say the kit can be self-fitted by a careful and competent bike mechanic.
The Velospeeder Brompton kit features two small motors that are spring tensioned against the rear wheel rim, with a crank motion sensor kicking the motors into action on pedalling.
They’re powered by a small frame-mounted battery which sits on top of the Brompton frame in the angle with the seatpost.
There’s also a three-point switch on the handlebars (on / off / wet weather mode), which on my test bike came in the shape of a three-speed cable shifter, though the wet weather mode wasn’t enabled and so couldn’t be tested here.
Velogical say the motors are not meant to give mountain bike-style power, but more of a helping hand – the kit is claimed to add around 2kg in total weight, including the battery, making it the lightest retrofit kit available, the motor itself weighing only 500g or so.
As the kit came pre-fitted on the test Brompton we couldn’t weight the system independently.
The test bike came with a 95Wh battery, though bigger capacities are available. Estimated range of our version was around 10-15 miles.
Extremely lightweight and keeps the balance of the bike nicely too. Beautifully engineered.
Over moderately hilly countryside the kit gives a real boost to performance, especially once you reach around 8mph, though it should be noted we didn’t test it in really wet weather which can prove to be the Achilles heel of some friction drive kits.
Can be retracted from the rear wheel to give totally unassisted riding.
As the batteries used here are from the world of radio-controlled models and are consequently relatively cheap to replace, extra range with several extra batteries is an affordable option, and larger capacity options are available.
Available in different max speed assist options, 23, 28, and 35kmh.
Undoubtedly the system’s big weakness is the lack of raw climbing assist up very steep hills, but for places with only occasional steeper hills – such as London or Paris – this is a real performance booster with little extra weight penalty when compared to just about every other electric assist kit out there.
And as with any friction drive you may encounter problems in very wet weather.
There is some noise and it will be clear you’re on an electric assist bike.