Grin Tech

Grin Tech Brompton Folding Electric Bicycle Conversion Kit


Grin Tech

Grin Tech Brompton Folding Electric Bicycle Conversion Kit


SEB has been electrifying the world famous Brompton Folding bike for some time now. Finally a super simple and relatively  easy to install kit is available!

Brompton's are among the most exquisite of compact folding bicycles, but also among the more challenging to convert cleanly into electric assist. The front fork uses tiny 8mm dropout slots and a narrow 75mm hub width, the rear tire folds into the bottom bracket eliminating most mid-drive options, and the triple fold adds challenges to the cable routing and component location. But once properly outfitted, they make excellent electric bikes. Portable, compact, easy to travel with, and a delight to ride.

Motor Options

At the moment, we have two direct drive hub motor options for the Brompton forks: the tiny Crystalyte NSM motor and the larger Crystalyte SAW20 motor.

The small NSM motor is recommended if you want the most discrete package, and/or if you expect to fly with the Brompton bike where every ounce of weight savings helps to stay within the 23kg checked baggage limit. It offers a gentle 250 watt power level, which makes riding a snap but definitely requires pedal assistance going up steeper hills, and as a result it can have poor efficiency when loaded down.

We had Crystalyte produce us a narrow 20mm version of their SAW front hub motor in order to provide a motor option that still fit in the Brompton fork, but was larger in diameter for better torque and peak power output. This motor weighs more than the NSM and is less discrete, but it also can climb hills with better ease and efficiency. We recommend it when you require higher levels of assist than the NSM can provide, and where the final bike weight is less of a concern.

Both motors have fast 13 rpm/V motor windings with a standard 36V battery pack. Full performance under different load conditions is available from our motor simulator, by choosing the Clyte SAW2013 or Clyte NSM13 option from the drop-down list, and creating a 16.7" custom wheel size.   

SAW20 vs NSM Comparison
Weight 2.4kg 3.2kg
Diameter 154 mm 184 mm
Motor KV 13.9 RPM/V 13.6 RPM/V
Phase Resistance 0.41 Ohm 0.24 Ohm



You also have the option to choose whether you want a comprehensive V3 Cycle Analyst device or a simple 5-LED display console.

The eZee LED console gives a basic indication of your remaining battery charge, and the up/down arrows let you set 5 different power levels. It is a great option for an introductory throttle controlled ebike setup where a simple non-technical display is all that is needed.

The V3 Cycle Analyst has a much more complete technical readout of your speed, battery voltage, amp-hours used etc. With the V3 Cycle Analyst, you can not only tune your throttle behaviour, you can also configure the bike for automatic pedalec assist modes with a PAS or Torque sensor so that you don’t need to press a throttle for power.

If you opt for a V3 Cycle Analyst display, then you have the option to add pedalec control of the Brompton without needing to use the throttle for power. Both the simple PAS sensor and the more elaborate torque sensors can reduce thumb fatigue while riding and lead to a pleasant automatic assistance.

The 12 Magnet PAS sensor installs on the left crank and simply senses when you are pedaling. You then configure the Cycle Analyst to put out a baseline power whenever you pedal, and use the throttle for when you want to override that with manual control.

The TDCM torque sensor takes this up a notch by sensing not just cadence, but also the force that you are applying to the pedals. This allows the Cycle Analyst to regulate the motor power in proportion to your pedal effort. As you ride harder, you get more assist. However, the installation of the torque sensors is considerably more involved than the split disk PAS sensor, involving a replacement of your bottom bracket and drilling a cable exit hole in the bike frame. So we only recommend this option to those with greater familiarity with bike mechanics.

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