Over the past few months, or years actually, my son and I have been making what was originally meant to be a soapbox car, but grew into an electric go-kart. In this short story, I would like to share my experience building this little vehicle, as well as its motorisation during last year’s pandemic lock-down.
To be honest, I have actually never been really interested in cars, apart from a student dream to perhaps drive one day an English sports car from the 1930ies in British racing green, or maybe a Jaguar e-Type. Rather the opposite is the case; cars are a nuisance to me, merely a means of transport I try to avoid as I prefer cycling or taking the train where ever possible. The reason I joined Five some years ago was the company’s mission statement: “shared mobility for urban environments that does not cost the world” which I still stand by as I believe that both fewer cars and shared means of transport are needed to address climate change; vehicle electrification alone won’t quite cut it. Most importantly and urgently, however, human-friendly urban planning will be essential, encouraging avoidance, walking and cycling.
Despite my lack of interest in cars, my son has always been interested in them, perhaps like many boys do; his interest in vehicles in general was neither encouraged nor instilled by his family. He may have mentioned small cars, or seen some toy cars, but at some point he talked about soap box cars; my memory here is blurry. The idea of making a soapbox evoked some childhood memories as I actually did try to make a soapbox as child myself; I have created a chassis but then, due to the lack of suitable wheels, it remained that way and was never finished. I do admit that making a soap box is perhaps the projection of my childhood dream as much as it is a practical project to inspire my son building concrete things (rather than in Minecraft).
Being an engineer, the first action was to work on a design starting with some drawings in order to get an idea of the size of components. Fortunately, a kind neighbour gave us a big role of paper which we could use to draw a full scale top view of the soap box, including the location of the axles and wheels, the steering wheel and the seat for which we used books to get an idea of the height it should be at. After several iterations we had a preliminary design.
Addressing the cause for not finishing my own childhood soapbox, I spent quite some time sourcing the wheels that are a) sturdy enough for a serious soapbox run, b) include axles that can take some lateral force, and most importantly, c) some wheels that can be connected to a differential onto which a chain ring can be mounted for later motorisation. Children push chair wheels did not quite meet the criteria, but I eventually found all components at Samagaga who were very helpful in supplying mechanical drawings.
The second design constraint was that it must be possible to create the chassis with the limited set of tools available available to me: panel saw and an electric drill, plus pencil and ruler. It is easy to make a very complex design with unlimited workshop resources, but quite a challenge to come up with something that can be realised with simple means. The third constraint was that the assembled soapbox has to fit through the garden gate, otherwise we would not be able to take it for a ride. All together resulted in a design based on planed timber planks sourced from our local builder’s merchant.
Even though the frame can easily be fabricated with wooden planks screwed together, any moving parts are more involved. First, there is the steering knuckle which usually uses a kingpin. The simple solution here was a stainless steel bolt and bronze bushes (SimplyBearings) held in a small oak block. The front wheel axles are then bolted onto that block, too, but a little bit behind to create a bit of a caster stabilising the wheels when going straight. Both steering knuckles are then held by a top and bottom oak plank.
The steering arms are glued and screwed onto the top of the oak knuckles. Ideally a steering rack & pinion with tie rods and steering column would have been preferable, but that is a bit complicated. Instead, both steering arms are connected with a track rod to link both left and right wheel to obtain an Ackerman steering geometry. A thin but strong rope is attached at the mid point of the track rod and guided through pulleys to the nearly horizontal, wooden steering shaft where it is wound around several times to create the friction necessary to make the front wheels turn when the wooden steering wheel is turned, with a steering ratio of about 10 (close to a racing car).
In preparation for a motorisation at some point, the rear axle needs to be able to transmit torque from the central differential, both for braking and acceleration. The rear axle pieces from Samagaga needed to be machined on a lath to allow receiving the wheel adapter which a friend helped me with; they are held in pillow block ball bearings from SimplyBearings. A hydraulic disc brake calliper is attached to the frame with the brake disk mounted on the differential. The other side will be holding the chain ring in due course.
Together my son and I marked out the timber planks according to the design and cut them to length using my panel saw. All was assembled with wood screws. Once finished, I used to attach a rope at the front bumper and pull the kart by hand, or at a later stage with my bicycle which was fun.
Cambridge is quite flat and there are no hills; so a soapbox which is intended to go downhill does not really work out. In addition, being able to control the speed seemed to be more attractive for my son. Therefore, during the first lock-down in the UK, I tried to find an electric motor with controller and the rest as well as the batteries. The latter I got easily from Tayna Batteries, a pair of 12V 20Ah Mobility Batteries — Powerline PLC22–12. As for the motor some research led me to L-faster who offer an 24V36V 350W Electric Cart Engine Kit Electric which includes all bits needed, in particular an accelerator pedal.
Once installed, the drive train was complete, at least for going forward.
After using the electric go-kart for a few months the lack of a reverse gear proved to be a bit of a nuisance; also, my son wanted to learn a three-point turn. A project for the second lock-down.
Since the motor is a simple brushed DC motor, reversing the polarity can be achieved with a double-pole, double-throw switch rated for the voltage and current of the motor and installed on the dashboard. What proved a bit more challenging was blocking the free wheel mounted on the differential and holding the chain ring. Since it is made of hardened steel it was impossible to drill a hole into is outer ring and insert a blocking screw. I had to get the angle grinder out and cut off a ring from the free wheel, remove the steel balls, grind the components flat and screw them back on again. A lock nut makes sure that the chain wheel can be driven in reverse.
A snag list is already building up. First, the steering is still not great as the wooden steering shaft is too soft. At some point I will get a hollow steel tube instead and plate mounted ball bearings to hold the shaft. Next, it would be nice to add aluminium cladding to the body to make it water proof, but also to look swish. And, maybe a stronger motor, perhaps twice the power, or a gear box with at least two gears, and a reverse gear. Last, friends keep asking: when will it become autonomous? Well, there are no plans to put the Five.AI autonomous vehicle stack on. Where does that leave the fun driving?