Easy Riders

First Off The Grid

• Power
  • 15hp (11kW)
• Torque
  • 190ft.lb (260Nm)
• CO2 per mile
  • 0 kg

Jason Momoa’s impossible dream

© HBO Max. On The Roam: Season 2

Jason Momoa came to us with another dream: if we engineered a hub motor into the rear wheel of some of his beloved 1920s Harley Davidsons, could he cruise emissions-free through town and then open up the twin cylinders outside the city limits? Would this be physically possible? Would it enhance the ride? Nobody knew – but this is part of the adventure with Jason.

Like the Rolls Royce Phantom II before it, this project raised significant technical issues – though of an entirely different order to the Phantom. Could we find a motorbike hub motor with enough torque to move a Harley? Was there enough room to integrate batteries and the rest of the electronics and crucially, could we make the motor and the original engine – technologies manufactured a full 100 years apart – work together?

The FD and JD models represent the pinnacle of Harley Davidson’s “F-head” (inlet-over-exhaust) models, before the introduction of the flathead in the 1930s. They have a massive 74 cubic inch (1,200cc) capacity and belong to an era of hand-controlled riding. Riders have to operate a hand-shifted three-speed transmission and a foot-controlled rocker clutch, while managing the spark timing and carburettor mixture directly from the handlebars. They also have no front brakes and a dual “heel and toe” rear brake – the rider had to choose between an internal shoe (smoother) and an external band (more stopping power but prone to “grab”). So there is plenty going on!

Key challenges in adding a hybrid drive to a 1920s Harley Davidson

The first issue was sourcing a suitable motor. The only motorbike hub motor in the world with enough torque for a Harley is designed and made by our friends at Maeving, and happily they agreed to supply us. The initial idea was to house the batteries in one half of the fuel tank. This meant a custom-made battery and a need for cooling, as the tank is directly above both cylinder heads. We took this design concept a long way, but in the end opted for the simple approach – use the Maeving batteries and house them in panniers. This proved to be a successful approach – the panniers are a little deeper but otherwise the same dimensions as original panniers that were attached to one of the bikes, and two batteries can be used which gives the bike considerable range (more than 50 miles) on electrons-only.

We converted three bikes for Jason:

• 1921 Harley Davidson Model FD – this was a “spares bike” and has a seized twin cylinder 61 cubic inch petrol engine and no gearbox, so it is EV only
• 1924 Harley Davidson Model JD – twin cylinder 74 cubic inch 20 horsepower 50 ft.lb petrol engine – now plug-in hybrid
• 1927 Harley Davidson Model JD – twin cylinder 74 cubic inch 20 horsepower 50 ft.lb petrol engine – now plug-in hybrid

For the bikes with working engines, we redesigned the Maeving motor to accept a chain, so the petrol engine can still drive the rear wheel. We also added a disc brake for more stopping power. The motor was built into a replacement rear wheel and everything else was located in panniers that clamp to the original rear bike rack.

A whole new experience

So how do they ride? Sublimely! Exactly as Jason wanted. A simple switch on the handlebars enables you to select petrol only, EV only or both together. The throttle twist grip accelerates both petrol and EV, and if you run them both together you have seemingly limitless torque available. Running on EV only means you can enjoy the cruise and talk with your fellow-riders. A whole new experience!

But the big revelation is starting the petrol engine. Put it on the stand, run up the back wheel on EV, and then drop the clutch – or do the same on the fly. Simplicity itself and a step-change in accessibility for these grand old bikes – kick-starting them can be a major obstacle to ownership for some.

This experience can be yours too!

[ Feature image: © HBO Max. On The Roam: Season 2 ]