Reader Paul Pontin ‘goes electric’ with this tiddler.
Over the years I’ve built several one-off motorcycles and, like most of us, lusted after more power, but I was struggling to find the old ‘joy of riding' that I used to have. Living in South East England I figured the density of modern traffic had simply extinguished the excitement. I started hankering after the simpler life, something altogether smaller, nimbler and more fun! Crawling to work one morning through the grind of suburban traffic in the car, I’d been overtaken four times in two miles by cyclists. I then had an epiphany: mopeds aren’t such a daft idea! Back in the day riding a moped could be genuinely terrifying, as urban traffic was a hell of a lot faster than it is today. Nowadays being limited to around 30mph wouldn’t be so bad, but poor acceleration would. That’s when the idea for this project was conceived. In an attempt to interest the kids in mechanics I had already cobbled together a few off-road electric vehicles, which was enough to ascertain that the 250w limit legally imposed on electric bicycles is absolutely no fun at all. What I had in mind was a moped, just like we grew up with – but powered by an electric motor. Three major objectives for the conversion were considered: it should try to retain the aesthetic of the donor bike (electric bikes often look like a cross between a motorcycle and a dog kennel!) and it should be light, fun and cheap. So that was the idea – it took three years before it was realised! I eventually acquired a knackered 1993 Honda MT50-SJ from ebay for £127. I wanted an MT50 as the twin side frame rails gave space in which to house the vulnerable batteries, with collision protection being afforded by the chassis configuration. The initial idea was to keep the frame and running gear largely unchanged, build an electric motor assembly that was a ‘drop-in’ replacement for the MT50 engine and then stuff all the batteries and electronics under the original seat and tank. I had just semi-retired and I was looking forward to this project as a gentle run down from full time work in electrical engineering. Gentle? Ha! The conversion ended up being way more complex
So you’re semi-retired and decide to turn an old moped into an electric moped. Where do you even start?
than originally planned. It involved modifications to the frame, plus the manufacture of one-off fibreglass moulds to produce a tank, rear mudguard and seat unit. All the running gear was either modified or rebuilt. In addition, nearly all the ancillary bits required fabricating from scratch. Around 250 individual metal parts had to be machined along with all the surface finishing, zinc plating and painting. I even set up an anodising plant in my summerhouse because the cost to get the mass of parts done professionally was prohibitive. By the end the bike was almost entirely custom built. Ultimately the project took over 2500 hours to complete. Full-time work was never this hard! Mid-mount electric motors do exist for motorcycle use; but they are big, heavy and rather expensive, so I started hunting around for something else that could be pressed into service. The solution came from an unlikely source. The motor was originally intended to power large scale radio controlled planes; such motors feature a huge power-to-weight ratio but, unsurprisingly, are also highly stressed. Specifically built to spin a propeller, they are blessed with a bountiful air supply to keep temperatures under control. Producing up to 6kw (8hp) and weighing in at just 1.8kg the potential was just too good to pass up. These motors possess virtually no safety margin and will eagerly consume huge currents for a second or so before literally bursting into flames. Equally, the higher the voltage it is fed, the faster it spins until it centrifuges itself into oblivion. It’s like having an internal combustion engine that just keeps making more insane power until it either melts or blows up! On any electric drive system the gearing used to reduce motor speed down to rear wheel speed is critical. Hours were spent trying to get the voltage and gearing right. Electric motors have an efficiency curve; rule of thumb – they are at their most efficient at around 80% of no-load speed. Any inefficiency appears as heat, so if you drop into the inefficient speed region then the tiny motor soon heats up and POOOF! Without a propeller to keep it cool I had to come up with another plan. The answer came in the form of a 3in fan unit (designed to pump air from the bilge of a boat) fitted into a machined piece of toilet waste pipe. A two-stage reduction system was employed to gear the 7800rpm of the motor down to the rear wheel. The first stage is similar to the primary drive of a normal engine and employs a chain to drive a lay-shaft that sits behind the motor. The lay-shaft transfers drive over to the opposite side and then via the secondary chain to the rear wheel, just like a normal engine. The motor had to be heavily modified before it could be used. Magnetic position sensors were fitted inside the motor which facilitate much better timing of the power to the stator coils, allowing some fancy electronics to avoid exploding the motor. A temperature sensor mounted to the coils is used to limit things if the electronics get it all wrong. The motor was fitted with relatively small bearings, being all that’s required to spin a nicely balanced propeller. A large outrigger bearing had to be mounted on the drive side to take the considerable additional radial load imposed by a chain drive. All of these components were mounted between 8mm thick aluminium engine plates, which were laboriously hacksawed to shape. Spacers between
the plates kept everything rigid and holes drilled in the plates lined up with the original engine mounts on the frame. The idea was to just swap out the MT50 motor and bolt in the electric replacement. Even the final drive sprocket is in the same place as the original, meaning suspension and chain alignment is unaffected. Primary and secondary chain-cases were fabricated from welded sheet aluminium to keep everything neat. The completed motor weighs 6.2kg, saving a tidy 16.2kg from the the original MT50 engine which weighs 22.4kg! Next, the bike had to be loaded up with batteries. Lithium polymer batteries were used – again intended for radio-controlled planes. Producing huge currents with minimum voltage sag, these make for a very punchy power delivery. Range was set at around 15 miles enabling cost and weight to be minimised. The range may seem pathetic, but the aim was purely to see if making a functioning road-legal moped from such parts is even possible. I could extend the range later if required. There are two battery boxes. The front one sits between the frame rails on top of the engine plates holding a total of eight battery packs, four on each side. The other, mounted to the frame rails under the rear mudguard/seat assembly holds a further four battery packs. An electrics plate was attached to brackets welded to the top frame rail. Forming the lid of the front battery box this plate keeps everything rigidly mounted to the chassis and houses the speed controller, contactors, fuses etc. Running at the battery voltage (nominally 55.5V), this wiring handles all the high-power traction duties. Lastly, a top plate is fitted to hold all the low voltage electronics, including logic to monitor the batteries and protect them from under or over voltage conditions, as well as a slew of electronics to monitor, control and data-log all the various protection signals required to stop the dreaded motor annihilation. Hooking this lot together was a major headache as there are multiple processors and it all had to be designed, built and programmed specifically for this set-up. Getting the aesthetics right was probably the hardest aspect of the design. I tried to use the original tank, seat and rear mud-guard, but in the end none of them were right, so I decided to mould my own from fibreglass. I am still not entirely happy, but having tried five different designs for the tank and three for the rear guard, I had to stop somewhere! The process for the tank was to first make a plug. In my case a wooden version of the final design was made from MDF and steam-bent plywood. Any surface imperfection in this will appear in the finished moulding so a great deal of time was spent filling, sanding, painting and generally getting it as smooth as possible. Next, a split female mould of the plug was taken. The mould needed to be split into two parts as the angles on the tank mean that it would not release cleanly from a one-part mould. Finally, the split mould was bolted together and fibre-glass laid up on the inside. When it has cured and the mould is unbolted the finished tank can finally be removed. Similar moulds had to be made for the seat and the rear mudguard. The front mudguard was the only bit of bodywork I purchased: £5 from Kempton autojumble! To finish the seat I had to cut and laminate various bits of foam to get the required density and contours and then cut and stitch the seat fabric. That took ages! So what is it like to ride? On paper a dream: the torque available at 0rpm is about the same as a 2016 Honda CBR125R at 8000rpm. This means that whenever you touch the throttle it wants to wheelie! All fantastic fun when I was off-road, but I had to change the gearing and dial the torque back
when I nearly performed an involuntary loop while executing a feet-up U-turn! I am working on a programmable throttle ramp to fix this, but for the time being it is fine and better than it was. It is nimble too: kerb weight is 75.5kg vs 90kg for the original, ensuring the lightness/fun goal has been met. The aftermarket rear shocks are reasonably well damped, while the original forks are the weak link. Thicker oil and some preload spacers for the springs may help. All the foot controls have been removed to save weight and make everything as slim as possible. The rear brake has been modified to allow cable operation from the left hand lever: it’s like a modified push bike! I have the power set to a maximum of around 3.8kw (5hp). This gives a speed of 30-35mph with plenty of power to go uphill without dropping speed. In normal flat running it averages around 1.8kw which keeps the motor temperature down: these power settings keep it within the DVLA requirements for moped classification. Power delivery is the addictive part of electric bike ownership; having all that torque and no gears has no equivalent in a petrol engine. I’ve had motorcycle riding friends laugh at it, but once they ride it they understand! There are very few DIY electric converted bikes in the UK and the range makes it pretty useless for anything other than popping to the shops. I’ve not done this to make money: the rewards are gained from solving technical problems, making things with your own ingenuity and owning something truly unique. If you fancy a challenge, think of a totally ridiculous one-off build… and give it a go!
Yes, it’s an MT50, captain, but not as we know it. Paul Pontin has boldly gone where few have gone before!
The original machine as bought from ebay.
All the electric gubbins fitted to the bike.
ABOVE: Just some of the bits fabricated for the build!
BELOW: Still an MT50 at heart! Well...
Completed engine assembly tray with front battery boxes.
1 4 6 2 5 7 3 8 THE BUILD: 1/ Machining a secondary sprocket. 2/ Drilling the engine plates. 3/ Detailing on the primary drive. 4/ Making the mould for the seat. 5/ Shaping the plug for the rear mudguard. 6/ Here’s the tank split-mould. 7/ Motor and fan-cooling arrangement. 8/ An amazing 8bhp from this!