The Electric Robin | Pete Projects

The Electric Robin Reliant

Fig 1: The Reliant Robin, prior to electrification

Whilst I’ve flirted with the idea of buying an electric car, I’ve so far struggled to take the plunge for a number of reasons. I’m used to driving precarious old cars which cost next to nothing aside from the frustration and expense when they break down. For some reason, I’m also a big fan of small cars. Back in my student days, my prides of joy included an old fiesta and a 1984 Austin Mini. I’ll always remember going to buy the mini with my mum who drove us through heavy snow to get to the old lady selling it for £200. I like how easy little cars are to park, the cheap parts and the cheaper road tax. The engineer in me is more impressed by the genius of the original mini (from which most modern cars descend) than the obese Chelsea tractors which parade about these days. Electric cars today are even bulkier. I can’t help but think that given where current battery technology lies, buying an EV today is a bit like buying a mini-disc player 20 odd years back when the iPod was just around the corner … solid state is coming again! this time in the form of battery cells rather than memory, until then, I feel a bit stumped.

Having said all this, I’m still very excited by electric drivetrains, their phenomenal torque, the promise around integrating more renewables with the grid and sticking two fingers to the likes of OPEC and Putin. So, I decided to take the plunge and have a go at electrifying an old car. The first challenge is figuring out the best donor vehicle. My three rules were:

1. The car shall be small

2. It shouldn’t be too valuable as a classic vehicle in its own right

3. It should have an aesthetic appeal of some sort, I had to like it on a deeper level than just any old car to keep me going through the project

My first inclination was towards electrifying an old Mini, however this is now well charted territory with kits readily available making the project feel a bit passe, furthermore, electrifying a Mini would risk breaking my second rule. It would feel sacrilegious to rip out a working A-Series engine from its little subframe given its heritage as great grandmother to all transverse engine arrangements! I love the aesthetics of the Fiat 500 as a small car but these cross the line even further as far as rule 2 is concerned. Then after a bit of scratching around, I came across the 1970s Reliant Robin:

Fig 2: Original advert for the 1970s Reliant Robin

The Robin somehow sat perfectly against the three rules:

1. It's about as small a car as you can get weighing in at 450kg (so that it could be driven on a motorbike licence back in the 70s)

2. They have very little classic car status being somewhat looked down upon (unfairly in my opinion!)

3. From an aesthetics standpoint, I love the lines they achieved on the composite shell (actually quite an engineering achievement for the time), along with the cute mini headlights and ludicrous Wipac rally front under lights, which have been fitted as though the little car harbors an ambition to go off road one day.

The first challenge was getting hold of one. The nearest and best looking example was in Holyhead, Wales, a four hour drive from Kidlington, Oxfordshire, the point of departure with my friend Sabs who encouraged me (against wavering second thoughts) to embark on the expedition one wet bank holiday Monday.

Video of Sabs and I driving to Wales for the Robin

On finally getting up to Holyhead, the car looked on point with its kitsch upholstery and proud little Robin logo on the dashboard (dated 1977, humorously this happens to be the same year that the Voyager space probes left Cape Canaveral). However, stubbornly, it refused to start. After taking the plugs out and playing around with the distributor we found quite a number of problems… the carb was flooding the engine whilst a worn rotor arm was delivering an anemic spark to the plugs.

Video of Sabs and I trying to figure out what's wrong with the Robin

Amazingly, the owner of the Robin worked as a cleaner in a local car museum which happened to have a Reliant Scimitar with the same distributor cap. Two hours later, she came back with a clean looking rotor arm cannibalised from the Scimitar and 20 minutes later the car was running.

Finally we get the car running!

Fearing breakdown somewhere in the remote Welsh hills, we decided against our original plan to drive the Robin back to Kidlington in convoy with Sabs. Fortunately, the owner also happened to be friends with a one-man logistics business who specialised in collecting and delivering eccentric cars to people. One week later he arrived with his trailer bed, especially configured with a third ramp, to deposit the Robin, just in time for me to drive it to work that morning:

Fig 3: The Robin getting delivered via flatbed recovery truck (left) before getting driven to my garage for restoration (right)

With a working donor vehicle in place, the next task was to determine the best electric drivetrain to propel the Robin. There are a number of trade-offs to consider, the biggest being:

1. The size of the battery trading weight against range and acceleration

2. The system voltage trading compactness of driveline against safety

3. Motor power and torque characteristics trading peak performance against energy efficiency alongside whether a gearbox is necessary

To get a better grasp of the numbers involved, it of course helps to get a grasp of the aerodynamics, rolling resistance and weight of the car:

Fig 4: My CAD design of the Robin with key equations governing drive-line requirements

With a drag coefficient of ~0.5, mass of 450kg and rolling resistance of about 1%, I realised I’d need a motor with a peak power of around 30kW to propel the car to 80mph. Going through this process, one realises the immense tradeoff between top speed, motor power requirements and vehicle range for a given size of battery. The following graph shows the driveline power requirements and range of the car against a battery size of 10kWh.

Fig 5: Power requirements of the Reliant Robin against speed

The aspiration on top speed has a big impact on everything to put it mildly! If I’m content to meander around at ~50mph, I can comfortably get away with a 10kW motor and get nearly 70 miles of range on a battery which weighs 88kg (about the weight of one of my friends in the back seat). If on the other hand, I want to bomb it up the M6 at 80mph, I’d struggle to get more than 25 miles on the same battery. 10kWh is a small battery by EV standards to say the least, a Hyundai Ioniq has a 40kWh battery with a stated range of about 155 miles under normal driving conditions (probably a lot less at 80mph).

Fundamentally, I had to decide how important range is for this project. Let’s say I wanted to get to my hometown of Galashiels from Oxford, stopping off at Tebay services (Cumbria) along the way (~250miles), I would need around 50 kWh of battery weighing in around 400kg (doubling the weight of the car!). To accommodate a monster like this, the braking system, suspension (and arguably chassis) would all have to be uprated. The car would be so heavy that my little 30kW motor would yield pathetic acceleration, unless I uprated this as well to something around 60kW+. With such a large motor, I could no longer use the original driveline components (prop shaft and rear differential) which were originally spec’d for the little 40 horsepower (~30kW) engine… I decided therefore to set my sights on a much more modest/realistic range, getting round the Oxford ring road once on a single charge was more than enough and why on earth would I consider a terrifying drive all the way to Scotland in a Reliant Robin in any case? This locked down the decision to go for a 10kWh pack + 30kW motor. A 96V system would keep things reasonably safe from a DIY perspective whilst reducing the size of cables and motor windings compared to a 48V system.

Having figured out the high level spec for the main driveline, I now needed to get into the details of the specific components. I found a Chinese manufacturer (Green Motor Tech) who provide full drivetrain solutions alongside (helpfully) detailed torque, speed and efficiency maps.

Fig 6: Three phase 30kW motor specified to propel the Robin

A critical factor in coupling the motor to the drivetrain is to understand whether an extra gear stage (or indeed multiple gear stages) are necessary. The data provided on the motor allowed me to develop a numerical simulation of the vehicle which could answer the questions around gearing whilst providing the likely acceleration profile of the vehicle once electrified.

The following diagram shows the Matlab Simulink model created which simulates straight line driving with foot to the floor. The simulation makes use of key data from the Robin (such as drag area, tyre dimensions and weight) alongside the motor performance maps provided by Green Motor Tech. I programmed the motor map into a lookup table which allows the simulation to track the changing motor shaft power as the vehicle changes speed. This should hopefully provide a fairly accurate idea of the response of the vehicle as it accelerates.

Fig 7: Numerical simulation of the Robin including drag, rolling resistance and torque, speed map of the motor

The following graph shows the output of the simulation which was setup to model the vehicle accelerating as hard as possible for 30 seconds:

Fig 8: Vehicle straight line driving performance predicted by simulation

The results indicate that 0-60mph should be achieved within a respectable 7 seconds or so with direct drive to the rear differential. The top speed of the vehicle maxes out at 80mph (in keeping with the original top speed of the 850cc Robin engine). Given that this can all be achieved with direct drive, I can get rid of the 4 speed gearbox as well as the engine and simply couple the motor directly to the prop-shaft to rear differential. Consequently the conversion to electric should be mass neutral meaning no need to up-rate the brakes or suspension components. Whilst the range will have roughly halved compared to the old petrol setup, the acceleration will have doubled, a perfectly acceptable exchange given that the Robin is more of a village car than a touring vehicle.

At the time of writing, I'm waiting for delivery of the battery and motor to fit to the vehicle, however going through this exercise on paper alone has given me a new level of respect for engineers trying to design small electric vehicles with today’s battery technology. Whilst this is a bit of fun for myself to get to and from the shops, I look forward to the day when next generation, solid state battery technology, will allow us to bring back an electric version of the Robin, capable of getting me from Oxford to Carlisle on a single charge whilst weighing less than a tonne!