Building your own electric skateboard is extremely enjoyable & also very challenging. One of the most complicated & very important elements of building a quality, high performance eSk8 at home is the Drivetrain or sometimes referred to as the Propulsion System or Powertrain
The Drivetrain of a an electric skateboard is the group of components that delivers power from the motor to the wheels. This excludes the motor that generates the power. So basically the Drivetrain of an electric skateboard consists of four key parts.
- The Motor Pulley
- The Wheel Pulley
- The Drive Belt
- The Motor Mounting Plate and Truck Clamping Parts, referred to as the Motor Mount
The four parts listed above when grouped together with the motor, which provides the power or torque to move the Drivetrain components, is technically called the Powertrain Or Propulsion system. The motors with the best power to weight ratio for building an electric skateboard are Brushless Outrunners, that’s why they use them on RC Planes!
Knowing the Revolutions Per Minute (RPM) of a Brushless Outrunner motor is critical to designing your Drivetrain, I will explain how to calculate that further down. Without knowing how fast your motor spins you cannot determine the required Gearing Reduction Ratio and therefore cannot build a system to reach your desired top speed. As the RPM of your motor is determined by the voltage being supplied from the battery we must also discuss the battery in this guide. Fortunately most motors have the kV (and RPM) rating noted and for this guide that is enough info to make our propulsion system.
We must also know the Diameter of the wheel you intend to use, With the diameter we can calculate the circumference of the wheel. Or you could just measure the circumference… ***C=2πr blah blah…***. This tells us the distance travelled per revolution.
!!! STOP !!! This is getting way too technical! OK so I promise no Algebra or Maths required.
So now we know about the Six Key elements of a propulsion system that must work together to give you the performance you desire. Motor Pulley, Wheel Pulley, Drive Belt, Motor, Battery & Wheels
Out of these six parts there are five variables that need to be determined so you can design & build your Drivetrain.
- Gearing Reduction Ratio.
- Motor KV.
- Battery Voltage.
- Wheel diameter.
- Desired Top speed.
The top four variables are all related and if one is changed the top speed of your propulsion system will also change. That is why its is generally easiest to determine your desired top speed then work backwards to determine the others.
SPEED KILLS - YOUR ELECTRONICS
Now lets just be realistic for a minute! How fast do you really want to travel on your powered-popsicle-stick? My balls a fairly large & I have bombed some narly hills in my time, but on the flats, in a suburban street, where shit can just appear out of nowhere, I start to get scared around 50kms/h. Or about 31mph - At this speed you are travelling at 13.8 meters every second. So I think this should be your absolute maximum for safety & longevity of components.
But It is all about perspective… take for example if you ever get to ride on a race track that is open and smooth without unexpected obstacles appearing from behind cars 50 might seem a little slow now. If you want to go faster than this it becomes much more complicated to build a reliable system and probably needs to be discussed in a separate thread.
Personally I aim for about 45km/h which tends to be about 41km/h in the real world due to losses & resistance and gives you plenty of torque when you have a dual drive, generally speaking lower top speed = more torque.
So you want to travel at 45km/h?
Good choice! Now lets work out how to make this happen. I will assume you don’t own any parts to make your electric skateboard. I will also assume your are on a tight budget as was I when I built my first eboard.
The best thing to do is eliminate some of the variables! - Time to go shopping! I suggest buying the wheels, battery and motor first. This will make it easier to calculate the other variables…
Wheels As you know when it comes to wheels it is the diameter that is important. Most commonly people use wheels between 80mm & 90mm diameter. 83mm Diameter is very common, mainly because you can buy nice wheels that have a hollow core that is perfect for passing bolts through & securing the wheel pulley. Like these wheels.
You can go bigger or smaller wheels, however if you go too far outside this diameter range you will encounter some constraints that will make the system more expensive, more difficult to build or reduce performance.
For example; The bigger you go in wheel diameter, without changing the drivetrain, the more your top speed increases but that is at the expense of torque. Lower torque output is not great, you want as much torque as possible. So if you must increase speed the best way is to increase voltage as the torque stays the same. Increasing the wheel diameter or increasing motor pulley increases speed and also increases current draw. If you must have bigger wheels or higher top speed you must design the drive train specifically for it.
The smaller you go in wheel diameter, without changing the drivetrain, the lower the top speed & the more torque you get! But there is a limit of how small you can go. Mostly it has to do with ground clearance and pulley diameter limitations. You need to be able to fit batteries & motors under the deck. If the wheel is small the wheel pulley also must get smaller, because if it doesn’t it will get damaged from contacting the ground. But you can’t just keep making the wheel pulley smaller as this reduces the number of teeth & the reduction ratio you can achieve between the drive & driven pulleys.
QUICK NOTE ABOUT PULLEY TEETH PITCH
There is a way to have a smaller pulley & also maintain high reduction ratios. The most common tooth pitch for electric skateboards is 5mm Pitch. Tooth pitch is the measurement between two teeth. So if I had a pulley with just 10 teeth at 5mm pitch the circumference would be 50mm. If I had a pulley with 10 teeth at 3mm pitch the circumference would be just 30mm. So what this means is you can use a smaller pitch & increase the number of teeth and have a pulley that is actually smaller in circumference. So I could have a 15 teeth pulley at 3mm pitch with 45mm circumference. This is advantagous as you can have much larger reduction ratios and have the pulleys fit onto a smaller wheel. But there is a trade off! the smaller teeth can’t transfer the same amount of torque as the big teeth so you can get belt slipping issue. Running with 5mm pitch wheel pulley and keeping the wheel diameter at 83mm is enough to ensure you get good reduction ratio & good ground clearance, the maximum number of teeth I recommend for the wheel pulley on an 83mm wheel is 36 teeth. As the amount of teeth increases so does the diameter of the pulley making it closer to hitting the ground which is bad.
Battery Voltage The battery is a fairly easy choice, it is often determined simply by your budget, the more you spend the more watt hours they tend to offer. It will be no less then 6S and max of 12S (so lets say a range of 24 to 48 volts for simplicity). For this exercise lets say you go with 6S (24V) If you want to read more about choosing a battery check this Guide about battery ‘C’ rating
Motor kV Rating The motor purchase is important. Get it wrong and it will be a problem that you can’t easily fix. You can buy wheels & pulleys cheaper than you can buy motors, what I mean is you are better off experimenting by changing out some pulleys then buying a stack of motors to determine what one gives you the preferred balance of torque & speed. You generally need to buy a motor between 200kV & 300kV but this is not always true. Remember the wheel diameter, battery voltage & motor kV all work together to determine the top speed you can reach. So today lets go with 245kV, this a fairly common rating and readily available from hobby shops.
Now we know the Wheel size, Battery Voltage, Motor kV rating & Top speed. 83mm Wheel Diameter 245Kv Motor 6S Battery (24v for simplicity) 45km/h
With this information now you can more easily calculate your pulley sizes and determine the final reduction ratio. As we want to achieve a top speed of 45km/h we can define the last variable, the gearing reduction ratio, to get the desired top speed we want.
There are some important constraints in drivetrain design, when using pulleys & belts, that must be carefully considered. Firstly there is a minimum amount of teeth you should have engaged between the pulley & the belt. It is called teeth-in-mesh and you should have 6 teeth or more in mesh if you want to achieve the maximum torque transfer rates specified. Your ability to transfer maximum torque will be diminished if you have less than 6 teeth engaged. So it can still work just not to its full potential, so maybe the belt will skip over some teeth.
15 to 36 teeth is the ideal configuration to achieve maximum torque transfer when using 5mm pitch pulleys
Second constraint is your center distance ‘C’, it is best to have the shortest possible center distance. A shorter ‘C’ means less leveraging forces being applied onto the motor mounting hardware, it can help to reduce vibrations and minimizes the span of the belt. Basically the shorter it is the less chance the mount will flex, warp, bend or snap which is important when you are frequently accelerating & braking hard.
However you cannot make it too short because you also need your motor to have some clearance from the truck hanger so you can make a turn. It is also worth noting that the longer your center distance the more the belt will wrap around the pulleys so it can actually result in more teeth in mesh, but it is probably better to keep your center distance short and go with a larger pulley to obtain more teeth in mesh.
I recommend something around 60-70mm Center Distance. It’s short & also offers good clearance for motors up to 63mm barrel diameter. However If you wanted the motor to be mounted out the back of your electric skateboard you will need a longer center distance to give the motor additional clearance from the baseplate & pivot cup.
You must also design your motor mount with a belt tensioning system, a method to increase or decrease the center distance, this is also useful if you like to change your motor pulley to modify performance. If you install a larger pulley to increase top speed, you want to be able to use the same belt, so you will need to be able to decrease/increase centrer distance enough to accommodate the size of your motor pulleys. With a set of motor pulleys you can fine tune your performance quickly & easily with very little cost.
When it comes time to choosing the pulley sizes for your system the easiest way to make the calculations is to use the CENTER DISTANCE DESIGNER You can enter the variables such as, Pulley teeth, Center Distance & desired Reduction ratio.
HOT TIP: The general theory of efficient drive train design is to aim for the highest possible reduction ratio, whilst keeping the motor in the 8000 - 9000rpm range where the motor losses are at minimum. So the most efficient propulsion system on paper that would work within the constraints of an electric skateboard would look something like this Drive Pulley: 17 teeth 3mm Pitch Driven Pulley: 60 teeth 3mm Pitch Battery Voltage (V): 42.0 V Motor Rounds per Voltage (kV): 215 kv Tire Diameter: 3.25inch (83mm) Final Reduction Ratio: 3.5:1 Motor rpms: 9 030 rpm Top Speed: 39.74 km/h (24.74 mph)
This setup above is using a 3mm pitch pulleys & belt, you would need to use a 15mm wide belt as a minimum to transfer the torque. However in real world testing under braking load this Drivetrain design will possibly skip/jump teeth as it is at the limit of its torque transfer capabilities. This can lead to reduced service life of the belts, Meaning ya’ belts get shredded! On dual drive this system it can work slightly better, but only if the rider is under <70kg. Any heavier and the braking torque load is to high for the belts.
So the moral of the story is sometimes the most efficient build on paper is not actually the best choice in the real world, in this case I would prefer to lose some efficiency on paper to ensure drive train reliability is better. So for me I am sticking with 5mm pitch
But what if you need to change something? Different wheel sizes? Different Battery? How do I calculate everything together?
What you need is a calculator that you type everything into & it spits out your answer! Well fortunately they exist. So as promised there will be no Algebra!
Basically you enter your motor pulley teeth, wheel pulley teeth, battery voltage & Motor kV and it spits out your speed.
As you can see below the parts we chose only got our top speed to 38km/h
So your home work for the day is to reply in this thread explaining what you would do to increase your speed to 45km/h
REMEMBER THESE KEY POINTS
- KEEP REDUCTION RATIO AS HIGH AS POSSIBLE
- HIGH SPEEDS (OVER 50KM/H) EQUALS HIGH CURRENT DRAW & CAN DAMAGE ELECTRONICS
- DUAL MOTORS = MORE TORQUE NOT MORE SPEED
- HIGHER VOLTAGE IS ALWAYS BETTER