Introduction: Electric Microkart With Independent Suspension
This Instructable provides an overview of the design and construction of a small electric go-kart or microkart with brushless motors, lithium batteries, independent suspension and a composite deck chassis, built from new components for under $1500. By keeping the design as light as possible, this microkart maximises performance from 6.5hp and its tiny size exaggerates the 40mph top speed.
SKIP TO BUILD STEP 1 - DRIVE TRAIN or continue reading below for more background info about the project.
My overall goal was to produce an electric go kart for under $1500 that showcases current motor and battery technology, is suited to on or off road use, great to drive and challenges a few of the standard approaches to kart design along the way.
Some friends are building electric gokarts by taking a sprint kart (fixed gear racing gokart) and replacing the 2 stroke engine with an electric drive train. This is a quick way to make an electric kart, but unless you already have a racing kart you need to first buy a kart which would blow my price goal. I also wanted to further develop the go kart concept in a few other areas including size, chassis, suspension and independently driven wheels.
Size - to keep weight as low as possible, provide greater options for racing in carparks and indoors and make for easy transport in the boot of a car rather than a trailer, this design miniaturises the already miniature, hence the name microkart. This will extract maximum performance from the small electric drivetrain.
Chassis - structurally racing gokarts are flexible enough to lift or at least reduce the pressure on the inside rear wheel when cornering to counter the effect of a solid rear axel. In this design the rear wheels are driven independently with a motor for each wheel so the chassis will not need to flex for cornering and all four wheels can remain on the road. In addition the electrics are mounted inside the chassis instead of on top for protection and a cleaner look.
Suspension - go karts don't usually have suspension which is fine on a smooth kart track, parking lot or in a warehouse, but to find more places to drive it, improve performance and to provide a smoother ride off road, the microkart is designed with suspension. While it does appear to go against one of the principals of the go-kart which is keeping the design as simple as possible, the challenge is to do it cheaply and simply using easily available components.
Independent drive - ultimately I want to produce a 4wd gokart to maximise acceleration on and off road with each wheel independently powered by its own electric motor. To reduce costs and complexity (avoiding CV/universal joints at the front) I will be driving the rear wheels. Independent drive should extract more grip than a solid axel as well as saving the weight of the axel. It also opens opportunities for the future for developing a traction control system.
So on with the build....
Step 1: Motors
One third of the gokart budget, $500 was allocated for the motors and controllers. Higher power motors designed for electric bike use were chosen as providing the optimum combination of power, price and packaging. Two 2000W or 3hp brushless electric motors from Cyclone Motor were purchased online for $159USD each and two brushless electric motor controllers from Golden Motor for $95 each. The motors included a 7 tooth sprocket for standard bike chain. A little over the $500 budget (particularly as the prices don't include delivery costs) but well priced considering the cost was less than $100 per hp.
Getting the power to the rear wheels uses a standard bike chain driving a recycled bike sprocket on each rear wheel. See the step on wheels for more information on mounting bike sprockets on the rear wheel hubs.
Electric Motor Source Guide
There are three main groups of electric motors that are practical options for a go kart.
1. Radio control plane or car brushless motors.
The RC community were great innovators in making their own high power small electric brushless motors for RC aircraft and now there is a huge range of options of ready made motors and controllers. They provide the highest power to weight ratio electric motors available and are very cost competitive, but they are not the most durable, are typically very high revving thus need higher gear ratios for karts, need to be de-rated for use in go-karts to avoid overheating and they are not well suited to start stop or slow speed operation. Hobby king is a good starting point to find a high power RC brushless motor. Instructables user Teamtestbot has a good primer on using RC hobby motors here https://www.instructables.com/id/The-New-and-Improv...
2. Electric bike motors
These are also cheap and readily available, particularly in the 200w to 500w range to suit e-bike regulations. They are provided in kits that are easy to wire up and they are designed for higher torque and slow speed operation. While 200w to 500w would be OK for a kids gokart, there are a few sources of e-bike motors up to around 5kW or 7hp. Cyclone Motor, Golden Motor and Kelly Controls are suppliers of higher power e-bike motors.
This project uses two 2kW or 3hp brushless electric motors from Cyclone Motor. These are ideal size and well priced to suit the project. Cyclone use Kelly controllers for their motors, and while I have found Kelly controllers to be fine on previous projects, Golden Motor cruise controllers have a good range of features at a better price, thus we are using two 48V 50A continuous (100A peak) controllers from Golden motor on this project.
3. Small electric car motors such as etek type motors often sold for small electric car conversions
These are ideal for a go-kart conversion using a larger gokart frame and suppliers such as Motenergy which has replaced Etek motors is a good example. These motors are typically 10hp plus. The disadvantages are that the motor, controller and battery are all going to be more expensive and while they will fit a regular gokart frame well, they could start to be a bit bulky for a microkart. EVPower.com.au supply a 10kW Motoenergy motor for $615 or a 30kW Motenergy for $918 excluding taxes and delivery and a controller is going to cost you a similar amount.
There are a few other sources such as old forklift motors, industrial starter motors but these tend to be hit and miss to source, and can be heavy and / or may not be suited to continuous use in a gokart.
Step 2: Batteries
Lithium batteries have come down in price in the last few years and in my view are the only suitable choice for an electric gokart. They combine high power densities, light weight, high current outputs and large number of recharge cycles with reasonable price. While they are still more expensive than lead acid batteries, they are now cheaper per charge cycle if you take the battery life into consideration.
The battery pack for the kart comprises 15 Headway cells, weighs 7.2kg (23lb), has a nominal voltage of 48V and a maximum discharge current of 150 amps. The cells are connected in series using connector tabs, screw terminals and plastic frame that were supplied with the cells.The battery pack is matched to the electric motor to ensure we can get the maximum output from the motors without drawing too higher current and provides a reasonable range.
There is not a lot to say about connecting up the cells to make the battery other than to be very careful not to short the pack out with your spanner. These cells will deliver a higher current than their rated maximum when shorted out, making big sparks and melting things. Connecting the battery management system (BMS) is a little more tricky as each cell needs to be connected to the board and getting it wired up wrong will destroy the BMS board. Follow the wiring diagram for the BMS (which may vary slightly between models) very carefully and triple check it wired correctly before connecting the power.
Lithium Battery Selection
There are a few different lithium battery formats available including pouch cells (common in RC models), large format prismatic (usually used in car conversions), 18650's (this is the AA of lithium cells and is typically found inside the 18v Li-on battery drills and larger cylindrical cells such as the headway cell.
Pouch cells can be ok, but are more prone to damage and it's harder to be sure of the quality. They can be the cheapest option, but probably the riskiest. The large format cells are not suited unless you are racing enduros as they do not have as high peak discharge so you will end up with a pack that has much greater range (or kWh) than you need for a the required peak current capacity. 18650's and Headways have similar power density and are both suitable, but the headway cells are a lot larger so you will need a lot less of them, making for simpler battery pack construction. With 18650 cells as well as connecting them in series to achieve the correct voltage, you will need several sets in parallel to deliver sufficient discharge current and pack range.
There are a couple of different size Headway cells. We selected the 15Ah 40152 cells which are 3.2V, 15Ah capacity (1 hour discharge rate) and can deliver a peak current of over 150A. At $30 each the 48V battery pack requires 15 cells and costs $450. The battery pack also requires battery management and a charger, so the total pack cost was around $600. All up the battery pack weighs 16kg or 35lb. All of the battery pack components including cells, connectors, battery management and charger were purchased through EV Power's webstore at http://ev-power.com.au/webstore/index.php/12v-life...
Step 3: Ply Deck Chassis
The compact nature of the electric drive train and lithium batteries allows an innovative approach with the gokart structure, securing all the components inside a thin hollow deck.
The design is basically a thin hollow box, just deep enough to fit a layer of batteries. Hollow boxes are strong and rigid compared to tube chassis so this also allows the use of other materials to build the chassis instead of the usual steel tubes.
We are using a ply top and bottom and pine webs and edges. The size of the kart being very small has been tailored to allow the top and bottom panels to be made from a single standard sheet of ply. Using ply and pine keeps the costs to a minimum and allows very simple fabrication.
The material for the chassis came from a single sheet of 9mm ply that was about $25 at the hardware and edging and bracing was machined up from pine offcuts.
The seat which is also shown in the chassis photos is an old recycled plastic school chair.
Step 4: Suspension & Wheels
The microkart rides on front and rear independent suspension with unequal length wishbones at the front and multilink suspension at the rear.
While gokarts usually have no suspension to keep things simple, we now have access to cheap ebay components and fabrication techniques that makes it easy to build a full suspension kart. The plan is to develop a tiny gokart wishbone suspension setup that can be cnc cut from 12mm aluminium plate although for prototyping and fine tuning the suspension geometry the parts have been fabricated from steel tube and threaded rod. Joints are 16mm round tubes with nylon bushes or 10mm spherical joints. Spherical joints of this size can be found on ebay from $5 to $10 each.
The front suspension uses unequal length wishbones with the top wishbone 165mm long and the bottom wishbone 190mm long measured between centres. The wishbone arms are symmetrical but the top and bottom mounting brackets are offset by approximately 20mm to give a caster angle of 9 degrees. The kingpin and axle are fabricated from 16mm steel rod with a kingpin angle of 8 degrees and a scrub radius of 72mm.
Caster - angle of the kingpin viewed from the side
Kingpin angle - angle of the kingpin viewed from the front
Scrub radius - distance between the centre of the tyre and the projection of the kingpin onto the ground.
The rear suspension uses a multilink arrangement that combines a trailing arm that pivots around the motor axis to keep the chain a fixed length with equal length transverse links that control the wheel camber similar to wishbones.
The transverse links are simply cut from threaded rod and are 250mm long between centres. These linkages keep the wheel and tyre vertical.
The trailing arm pivot is fabricated from two pieces of 2.5mm plate and doubles as the motor mount. One of the plates is a backing piece and the other is a large diameter ring that pivots about the motor axis. One end of a square hollow tube is welded to the ring and the other end is welded to the axle.
Being light, the kart only needs small springs. To keep it simple I'm using rubber in compression. The large rubber bump stops sold at the hardware as door stoppers give about the right size and spring rate and are easy to fit so I'm using those to start with. They don't allow the fully suspension travel to be utilised so I will need to make some custom springs as mountain bike spring/shock units typically have a spring rate that is far to high. The solution may be a longitudinal torsion spring made from a recycled torsion bar from a small car.
There are many suppliers of racing kart wheels and tyres however pricing for a set of four wheels starts from $400. To keep costs down, wheels and tyres have been sourced from the local hardware and the cheapest way to get them was to buy a couple of hand trollies with pneumatic tyres and steel rims. These are available for $30 - $40 each and the wheels come fitted with ball bearings (although quality of these in dubious) to suit a 16mm axle. The wheels are 10" diameter (250mm) which is the same as the racing kart tyres, although they are much narrower. The rims are 4" or 100mm diameter and the tyre designation is 4.10 / 3.5 - 4
Step 5: Steering
The steering is made by finding two steel pipes about 20mm in diameter where one will slide and turn smoothly in the other. The larger tube is welded to the front suspension mounting bracket at a comfortable angle for the steering wheel. The smaller tube has a 90mm arm welded to it.
100mm long steering arms are welded to the kingpins on the front wheels. The angle of the steering arms is determined with the wheels at full droop (ensuring steering arms do not foul on the chassis) and takes into account Akermann steering principles.
When turning the inner wheel turns through a tighter radius than the outer wheel. To achieve this the steering arms are aligned from the wheel pivot to a point at the centre of the rear axle. If the steering arms are behind the front wheel, the arms will be converging. If they are in front of the front wheel, the arms will be diverging.
If you have a sharp eye for detail, you will notice that in the rush to complete the kart in time to enter the Wheels Contest, I have arranged the steering arms so that the kart steers in the opposite direction to the steering wheel. It is perfectly driveable and after a wobbly start, it was interesting to see how quickly you adapt to 'opposite steering'. I will have to fix this up by changing to trailing steering linkages on the wheel uprights or placing the steering arm above the steering column rather than below.
Step 6: Testing and Development
With the looming dealing for the "Wheels" and "Batteries" contests, I've uploaded the instructable before I've had time to do any real testing and its been pouring with rain here for the las few days, so not ideal for testing an electric kart. I'm looking forward to giving it a run against my mates chinese fun kart powered by a 6.5hp Chonda.
UPDATE: 19th July 2014, This is a not too serious project, so here is a seriously unserious movie of "The Little Gokart" to finish with.