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Our Own Conversion of a Brompton

Please see the review below of our own Bespoke e-bike Brompton conversion.  Review written by Peter Eland, former editor of Electric Bike Magazine.

The Bespoke’d Brompton 

With an ‘official’ electric assist system still not available for the Brompton folding bike, third party systems have sprung up to meet demand. We tried one from conversion specialists Bespoke E-Bikes of Cumbria. So how does it ride – and fold? 

Despite a number of rumours over the years, the manufacturers of the Brompton folding bike have not yet come up with an ‘official’ electric assist system. That’s left the field open, and there are several third-party systems on the market. Most place the battery in a removable front bag, but the conversion reviewed here, from Bespoke E-Bikes in Cumbria, places the battery on the Brompton’s main frame. 
With any of these conversions, any warranty remaining on your Brompton will be voided: the bike’s manufacturers don’t endorse using it with electric assist. Nonetheless, users have been converting their machines for many years now, and I’ve not yet heard of any significant issues. 
The Bespoke E-Bikes conversion uses many parts from kit supplier Conv-E, with a special motor to fit the Brompton’s narrow forks, modified cabling and a custom-fitted battery mount. It’s available with either a thumb throttle or a full-length twist grip. Note that the twistgrip shown on our review bike is an older model: an updated version is now used. 
This isn’t a kit you can fit yourself: Brompton owners will either need to get their bike to the Grange-over-Sands factory, or use the ‘three way courier service’: Bespoke E-bikes send out a Brompton-sized box into which you pop your folded bike. They then arrange collection, and finally return it to you after the conversion. This costs £85, including insurance up to £2000 for each leg of the journey. 
This aside, the basic conversion costs £875. Additional options include upgrading the forks by milling off the Brompton’s unlovely ‘tube ends squashed flat’ standard dropouts and replacing them with proper brazed-in, cast dropouts, and with the fork ends resprayed black. As any front motor system will put additional stress on the dropouts, this might be a reassuring upgrade. If you want to upgrade the tyres to a more robust model, Bespoke E-Bikes can fit the Schwalbe Marathon Plus for £33 each. There’s a one-year warranty on the conversion, although the battery cells have a two-year warranty.
Options planned for the future include offering a full bike service as part of the conversion, providing battery packs colour-matched to the bike’s frame, adding a rotation sensor ‘pedelec’ control system, and possibly developing a kit which it is possible for customers to install themselves. 
Our review bike was a one-off in that it came with a custom, leather-wrapped battery pack to match its Brooks saddle, and it had also been used for some development work. So please excuse any cosmetic blemishes: otherwise it’s a good representation of a ‘production’ conversion. 

The Brompton’s distinctive appearance is surprisingly unaffected by the conversion: perhaps on our bike the very contrasting colours of main frame and battery pack helped keep it unobtrusive. The cables, although there are quite a few of them, all cling neatly to the frame. 
Let’s start at the battery: this is the latest Conv-E unit, using Samsung cells and providing 324 Wh (26V, 9Ah). A rather lovely illuminated blue pushbutton turns it on or off. The aluminium casing also holds the control electronics, so the only connections required are to the motor and throttle unit. These are made via high-quality plugs (made by Binder of Germany, apparently) onto which Bespoke E-bikes add white markers for easier alignment. The ‘spare’ connector is for the charger. 
The battery is mounted to an aluminium bracket, which is fixed to the Brompton’s frame via two threaded mounts. Bespoke E-Bikes drill the frame accurately and use threaded inserts; if ever the electric system is not required, a bottle cage would fit on instead. They are, they say, confident that these holes do not compromise the strength of the frame. 
Releasing or securing the battery is, as with the Conv-E, achieved using a small Allen key. While it doesn’t have the security of a full lock, it’s more than enough to prevent opportunistic battery theft. It might be an idea to offer a more keyring-friendly Allen key than the T-handled one provided. Anyway, once the battery is secure it’s rock solid. The plastic loops on the frame tube in front of battery, incidentally, are to hold the plugs in place when the battery is removed.
Up front, there’s a 250W geared hub motor, built with stainless steel spokes into a good quality double-walled rim from Sun Rims. A connector lets you remove it for puncture changes (two cable ties need cutting too). Finally for the electrical systems, the throttle unit on the right handlebar comprises a twist grip and three-level battery voltage display. There are no cut-outs on the brake levers, nor any pedal sensor, so it’s purely throttle-controlled. 
Overall weight will depend on which Brompton model you start with, clearly. On ours, all-steel and with three-speed gears and no rack, overall weight was 14.4 kg without the battery, and 16.75 kg with it. Not too much for many riders to lift one-handed onto a train! 

The Brompton has a well-deserved reputation as the folding bike of choice for train travellers: the package is neatly rectangular, with the oily bits in the middle, stands up well, and it holds together securely as you carry it. All those virtues are retained with the Bespoke conversion: as you’ll see from the pictures the only difference is that you need to twist the saddle slightly to clear the back of the battery as you drop the seatpost down. The dimensions of the folded package are barely affected. If you removed the battery before folding, there’s even less difference. 
The folded bike remains quite ‘luggable’ – certainly for me as a fairly strong rider. I preferred to leave the battery on (one less item to juggle) but if you have a bag handy, it is a useful chunk of weight to remove from the bike before you have to carry it any distance. Of course, like most Brompton commuters you’ll probably just wheel the bike along the platform anyway, and only fold it when you’re about to get on board. 

So is it all worth it? On the evidence of my test ride, I’d say it definitely is. One of the limitations of the Brompton, with its narrow rear triangle, is that many conventional gearing systems won’t fit, and most riders stick to the standard three-speed hub gear, as was fitted to this bike. But a limited gearing range means you either end up with a low top speed, or gears not low enough for good acceleration from a standstill. The Brompton is geared for a decent top speed, so it can be a strain when setting off – especially uphill. 
And that’s where the conversion kit comes in! It addresses exactly this weak point: a twist of the throttle as you set off ‘tailwinds’ you past the straining stage, and you can then just pedal away and conserve battery for later. It’s great for traffic lights, for stop-start traffic in general, and in urban conditions I really found it made the ride much more enjoyable. It’s also good to arrive less sweaty – by helping with acceleration the kit reduces exertion considerably. 
If you take this to extremes, though, and let the motor do all of the work, the smallish motor shows its limitations. On the flat and in still air it would only pull me (a fairly heavy rider, admittedly) up to around 13 mph, some way short of the 15 mph legal cut-out limit. Steeper hills definitely need pedal power to help it out. So it’s not a ‘pull me along’ bike – more ‘help me accelerate’ and ‘take the sting out of hills’. Used in this way, the battery pack should have more than enough capacity for the vast majority of commuting journeys. 
The motor does have the low whine typical of geared hubs when in operation: it’s audible on a quiet bike path, but not at all in traffic. With the motor off I couldn’t detect any noticeable extra rolling resistance.
The motor was powerful enough to skid the front wheel several times during the test. The front wheel on a Brompton is relatively lightly loaded, and if you pull on the bars to accelerate uphill, for example, it’s not hard to lose traction. It was never a serious problem, though – just minor shifts to your weight, or easing off the throttle for a moment, are enough to stop any skidding. 
The twist grip on the test bike had been set so that it would provide palm support when the throttle was full on – with it in the ‘off’ position, the bulge which gives the palm support digs into your hand. As I rode with it more off than on, I think I would have preferred the thumb throttle option, so that my wrist could remain in one position, with a supportive non-twisty grip under it! 

This is a simple but effective conversion for the Brompton. Look elsewhere for torque sensing, brake cut-outs or sophisticated displays; instead we have a purely throttle-controlled motor, achieved in a way which doesn’t compromise the fold and seriously enhances the bike’s usability in urban cycling. 
Just how well the electric motor worked to cover for the Brompton’s relatively limited gearing was something of a surprise. It’s just the right amount of power, too, to take the strain without throwing off the handling or encouraging you to burn through the battery by not pedalling at all. 
This Bespoke offering will inevitably be compared to competing Brompton conversion systems; the key difference perhaps is that it has a frame-mounted battery, rather than placing it in a front bag as many competitor systems do. Unless you want the bag anyway it would, I suppose, be just one more thing to remove before folding, and then one more thing to carry. With the Bespoke design, you can carry the whole machine in one hand, leaving the other free. That said, a bit of extra weight in a bag right over the front wheel is no bad thing for front wheel traction. Both layouts have their merits. 
I found little to complain about in the implementation of the conversion, barring my preference for a thumb throttle. I will make one suggestion, though. That leather-clad battery looks so much more lovely than Conv-E’s standard shiny aluminium that it really should be an ‘official’ option! 
Peter Eland