Boost Brompton ebike conversion kit review: the lightweight, affordable route to an electric Brompton

If you’re a committed Brompton convert, which option should you explore if you’re looking to go electric?

I’ve been asking myself this question recently because, no matter how much I enjoy riding my folding C Line Explore (£1,299) around town and further afield, there are some occasions when a little boost would make the Brompton experience that much easier and less sweaty. Chief among these is when the 16in wheels and Sturmey-Archer hub gears that don’t shift under load are faced with a hill.

The natural starting point should, of course, be Brompton’s own range of electric bikes (ranging from £2,980 to £3,499). And yet, for all the British brand’s practicality elsewhere, I can’t shift the feeling that it hasn't quite nailed it when it comes to battery-assisted models.

This belief stems from the system being front-wheel driven, creating a degree of twitchiness, a lack of traction and the occasional wheel spin on slippery slopes.

In addition to that, the front-mounted, click-to-fit battery needs to be removed before the bike can be folded. However, the incoming 20in-wheel, £3,499 Electric G Line from Brompton has a rear-hub motor, along with disc brakes.

Of other electric folding bikes, both the MiRider One GB3 (£2,495) and GoCycle G4i (£3,999) are well-regarded options, but lack Brompton’s unparalleled folding mechanism.

Which brings us to Brompton-specific ebike conversion kits, which act as a cheaper alternative to swapping your existing Brompton for a pricier electric bike.

British manufacturer Cytronex’s well-thought-out kit consists of a new front wheel with a hub motor for £1,145.

This means you can fold your Brompton without having to remove the battery and it offers a lighter 3.65kg weight. However, it’s still front-wheel driven, as is the Swytch Go kit.

Boost's kit is priced at £995.

Boost Brompton ebike conversion kit fitting and setup

Boost was founded by electronics engineer Nick Bailey in 2020.

This is the brand's first Brompton-specific kit and it offers rear-wheel drive. As someone whose mechanical prowess is challenged by assembling an IKEA chest of drawers (and who lives in fear of rear-wheel punctures on my own Brompton), I asked a mechanic to convert the test bike. However, he told me it was a slick and swift operation that I should have done myself.

The Boost kit includes a Brompton rear wheel with a hub motor. This means if your bike has hub gears, they’ll be lost. However, the derailleur is retained, making the bike a two-speed with a motor.

The compact 750g battery fits directly onto the Brompton’s swingarm, neatly occupying the space behind the seat tube. This doesn’t affect the swingarm movement or the fold at all.

Unlike other conversion systems, Boost’s battery stays in place. If you keep your Brompton indoors or under your desk, that’s not an issue, and it charges in three to four hours. The system is pleasingly clean and consists of only one cable running from the battery to the hub.

Boost Brompton ebike conversion kit performance

The Boost(ed) Brompton is a joy to ride. Having the motor in the rear hub reaps dividends when it comes to the feel on the asphalt, and there were no signs of twitchiness or wheel spinning either on wet roads or sharper inclines.

You can toggle between motor modes (Eco, Standard and Turbo) with an optional wireless head unit or Boost’s cycling app. Each gives you a progressive nudge when needed, but nothing that feels excessive.

The Boost system adds close to 2.5kg to the weight of a Brompton.

That extra heft is noticeable when you’re out of juice on the climbs, but not an issue when the motor is engaged. It’s also less than Brompton’s own 4kg to 5kg electric weight gain.

The battery lasted around 30km on my test rides, which falls below the 50km range of Brompton’s electric options and the Cytronex kit.

That means more charging time, but isn’t an issue out on the road, because it’s pretty rare that I ride further than that on my folding bike.

Happily, Boost has made the transition when the motor cuts out at 15.5mph/25kph pretty smooth.

One thing worth noting is that Brompton says the retrofitting of any electric bike conversion kit invalidates the bike's warranty.

However, Bailey says: “Boost will be pleased to offer continuation of the Brompton frame warranty when a Boost system is fitted, and we don’t anticipate any claims.”

Boost Brompton ebike conversion kit bottom line

Boost's conversion kit is a great option for turning your Brompton into an electric bike.

While the range isn't the longest, it's more than enough for urban commuting, and the light weight and affordable price are positives.

Overall, then, this feels like the lightest, cheapest way to convert your Brompton in a practical manner.