I've reviewed a couple of Brompton's own electric assist models: the 'standard' 6-speed build (now called the C-Line Electric and available in 4- and 12-speed versions only) and the part-titanium, slightly more refined, P-Line Electric. Whilst there was plenty of motor power on offer, they both lacked any ability to control power levels via a handlebar control and by today's standards they are starting to look heavy (current quoted weights are 17.4kg and 15.6kg respectively). Enter the Boost for Brompton kit which weighs only 2.5kg in total as against the 5.5kg or so of the official Brompton electric system. 

I looked at the 'full size' Boost kit back in 2022 and liked it (Boost have since dropped the Lavita prefix). Like the original, Boost for Brompton features just three main elements that make it the most minimalist kit out there: a rear hub motor, a battery and a wireless handlebar controller. 

Boost for Brompton, by Richard Peace

The big difference is that the Brompton version has a smaller battery (126Wh) that is fixed to the frame and not designed to be removed by the rider on a daily basis.  
 

Spec and fitting

The main point to note before you think about converting your cherished Brompton to electric is that the hub motor obviously replaces any hub gears, so in effect it's designed to run as a single speed, 2-speed or 4-speed. If you own a hub geared Brompton variant, the Boost may well not be for you - but the good news is that Brompton are moving in the direction of derailleurs. 

Original 2-speeds came into being around 2002 with the introduction of the unique-to-Brompton mini 2-speed derailleur and their own design 4-speed derailleur was introduced in 2021. In other words, there should be a reasonable amount of choice of second-hand Boost-compatible bikes. 

Of further note is the fact that Brompton say after market conversions like this void the warranty on the bike; however, Boost say will honour any remaining warranty period on your Brompton, should there be any.  

Brompton's own electric variant uses the front carrying block for the battery, so it compromises carrying ability, especially if you have an existing bag you want to use (some systems like the ARCC kit will incorporate their front-mounting battery into a Brompton bag). In contrast, the Boost system puts a small battery at the forefront of the rear triangle, which is a much neater and better balanced solution.     

Whilst Boost say the kit can be fitted by the end user, they also say: "We encourage customers to visit one of our partner shops for an install (we have over 100 across the UK). That way customers can be sure their bike is in good condition ready to be converted, and should have a seamless experience..."

I was sent a ready-converted 2-speed with a weight of 14.3kg. This model, pre-conversion, should weigh around 11.4kg meaning you'd expect the test bike to weigh around 13.9kg. However the test bike also had a non-standard telescopic seatpost adding a few hundred grams, so the claimed 2.5kg kit weight looks about right, making it a truly lightweight kit. Boost say you can add it to the titanium-framed Brompton T-line which holds the prospect of a 10kg e-folder!

The motor looks very similar in design to that employed in the full size Boost kit, though we assume it's a little narrower to allow for the narrower-than-standard rear forks of a Brompton (112mm vs 130mm-plus on standard size larger frames). 

There is an online video that shows fitting and it looks straightforward enough. Any minor difficulty might be caused by the closeness of the hub motor shell to the chainstay on the non-drive side, but spacing washers should cure that if it turns out to be too close for comfort on your particular Brompton.

The small capacity 126Wh battery is bolted permanently to the frame and Boost say it is, "a bespoke item, with the casing 3D printed in Silverstone and assembled with our proprietary electronics here in the UK."

The final element, the wireless handlebar display, remains (as far as I could see) just the same as on the original Boost. It has a large clear LCD readout with two buttons underneath and also features a backlight. It removes from the mount easily and the mount itself is fixed onto the bars via a rubber O-ring - more on which in a moment.
 

The ride

The ride performance mirrors the original Boost almost exactly and I echo my comments there. "General impressions out on the road were very positive. Once you've gained a little speed the power is applied very smoothly. It kicks in less than a second after you start pedalling and stops even more quickly when you stop the cranks." There is, however, probably more noise than your average hub motor. 

And as with the full-size kit, the main performance idiosyncrasies relate to the fact that power delivery increases automatically as road speed increases. So below 4mph there is little or no assistance and it gradually ramps up to give maximum assistance above 12mph. 

This applies whichever of the two power levels you are in (Eco or Boost) - i.e. you get more power in the high power setting but it still ramps up as road speed increases. There is also a Super-Boost mode which gives even more power for around 20 seconds after pressing a display button, before turning itself off automatically. You can use this to override the slow power ramp up, though this leads to a lot of button pressing, which can be fiddly.

The end result is you can storm up a hill at 12mph but hit a steeper section where speed drops below this and the assistance will drop too. After my test of the full size kit - which had the same rather frustrating characteristic - I suggested the unit could be reprogrammed to provide the same level of assistance whatever the road speed, but this doesn't seem to have happened. To echo the original comments on the full size kit, it would have transformed my standard hill climb test result from an okay one into a much better one.

My test bike was fitted with a 54-tooth chainwheel and a larger rear sprocket of 16 teeth, so it should be an easy matter to improve the climbing ability on steep hills a fair bit by reducing the size of the former and increasing the size of the latter, allowing for easier human pedal input when speed slows. 

With that very small 126Wh battery on board, I deliberately made a point of turning the power off during our range test when the going was sufficiently easy so as to preserve battery capacity. On a flattish course with only a few moderate hills, I achieved a range in excess of 24 miles. That was with the power in the lowest Eco level on the ride out, which was into a headwind and on a sometimes rather muddy railpath. The return leg had the power switched off most of the time with a slight following wind. This was possible as it's pretty easy to pedal without power on the flat. 

As I was changing the power levels rather more than when I tested the full size kit, I noticed that the O-ring fixing the display onto the bars meant the display could slip around. This meant it was a fiddly operation to change power levels sometimes - especially when turning the power off, as you need to hold the button down for several seconds. 

A clamp style fixing that anchors the display firmly in position would be a big improvement on any future version of the Boost for Brompton kit. I also noted the backlight only stayed on a short amount of time before switching itself off - so of no real use for nighttime riding.
 

Value and summary

Boost for Brompton is a superbly neat kit and of the myriad choices out there, its minimalist design means it is the one that keeps the Brompton as near to its original concept of an easy and quick-to-fold machine. But there are a few caveats.

Firstly, the battery can't be removed for charging - you have to take the bike to the charge point - and this means it remains part of the folded bike's weight too, unlike most other e-Brompton options. It's also significantly pricier than the original Boost, which has a bigger battery (perhaps because of that bespoke 3D-printed battery case). But for me, living in a very hilly area, the biggest drawback is that unusual power delivery that effectively limits low speed power unless you want to be constantly pressing the Super Boost button.  

If you already have a compatible Brompton or can get hold of a keenly priced second hand model it could still stack up financially against the official Brompton Electric, though it inhabits the same price territory as the more sophisticated and easier to use Cytronex Brompton which we've reviewed in both it's 'regular' and T-Line variants.

Boost for Brompton is a great concept and one of the neatest pieces of e-bike design out there and a 'version 2' could surely address many of the above points and make it a very fine kit indeed. 

boostbike.uk