The biggest thing in electric skateboards for 2016 is in-hub motors. There are several new crowdfunded eSkateboards that are coming down the pike with these new motors. This article will discuss the benefits and drawbacks of skateboard hub motors and try to predict what the future will hold for the industry.
First we need to break down the three different kinds of hub motors used for electric skateboards.
Direct Drive electric skateboard motors
The more common electric motors that can be found on most of the more inexpensive electric skateboards in China are Direct Drive (DD) hubs. These motors are outrunners (the wheel is what turns) and they have a 1:1 motor ratio. Every time the motor does one revolution, so does the wheel.
Advantages of DD Hubs
- Simplicity: The DD design is the simplest you’re going to find. Other than melting the windings or the insulation on the phase wires or overheating there is little you can do to destroy them. Since it is a outrunner there are no brushes to wear down meaning it should run well pretty much forever.
- Nothing to wear out: No clutches, no gears, no pulleys. The only thing that can fail really is the bearings on the motor. The Manta drive shown above has some pretty beefy bearings on both sides of the wheel so that is pretty unlikely.
- You can kick it : That’s right the DD design is the ONLY design that allows you to use your skateboard like a normal skateboard when your battery dies. A well designed DD motor will have very minimal cogging effect so it will have little resistance to being pushed.
Disadvantages of DD Hubs
- Less power than a comparable Geared hub or a Belt drive: Since the geared hubs and the belt driven motors both get to spin a lot faster than the DD motor then you will need MORE watts to make the DD motor feel as powerful as a Geared or belt drive system.
- Heat dissipation: Since the windings are inside the motor, it is hard for heat to work its way to the outside of the hub unless it has ventilation holes. Such a small motor doing so much work is going to get very hot. There is a chance that this will be an excellent application for ferrofluid cooling, but as far as I know, no one is doing that on a large-scale yet. If you have ventilation holes then you have to worry about water and debris getting inside your motor. Only the least powerful skateboards (less than 400W from China like this one) are sealed, everything else has massive air holes in it for cooling.
- The Urethane wheel keeps the heat inside the motor.
Geared Hub electric skateboard motors
As far as I know the only company currently promising a geared hub motor on a skateboard is Stary boards out of Shanghai, China who raised $750,000 on kickstarter to bring their board to market. That might seem like a whole lot of dough, but if you want to develop a brand new technology (geared hub motor) and at the same time deliver well over 1000 Electric skateboards I can tell you they will burn through that cash pretty quick. Geared hub motors are hard to design and their design, while incredibly innovative also has some pretty serious technical hurdles to overcome. I like everything about their crowdfunding campaign except their usage of Lipo pouch batteries which I have had nothing but problems with. Nothing beats the weight to power ratio of Lipos, but cheap Lipo cells or ones that are abused tend to explode without warning. The Hobby King Lipos I’ve experimented with tend to have overrated C rates, overheating and puffing out problems as well as premature failure. I have never been successful at getting more than a few dozen charges out of any Lipo before I got so nervous about it that I recycle them. Another thing that makes me nervous is that I can’t seem to access their website at stary.io right now, although this might be a temporary issue.
The Stary motor design is completely unlike any geared hub motor I’ve ever seen before. The electromagnets on the rotor spin around at high-speed (technically an in-runner setup) which drives gears which cause the outer hub casing (shown above the parts breakout) to spin at a much lower speed than the motor spins. Basically, you end up with two barrels spinning at high speeds in opposite directions supported only on one side with large vents on the outside of the motor. Most geared hub motors for ebikes are completely sealed and one of the biggest problems they have to contend with is heat build up. When you are running a lot of power through an enclosed case essentially you end up with a toaster oven effect pretty quickly.
By contrast, the Stary design leaves the hub wide open to moisture issues and is going to attract any small metal road debris with its very active electromagnets spinning around inside at high speeds. Pair that with a Lipo cell that will probably last a year at most and you’re left with an eskateboard that is probably not as good of a deal as it at first appears to be.
Advantages of Geared Hubs
- More available torque with comparable power levels when compared to DD hubs : This is probably the direction that eSkateboards need to go to be accepted in the mainstream. I’m not convinced that the Stary design is the right direction to move in.
- Marketing : It’s something new and different and people are into that.
Disadvantages of Geared Hubs
- Gears and clutches add complexity and cost to the motor : It’s really hard to build a solid geared hub motor, just order any cheap Chinese Geared hub ebike motor from a non-reputable brand name and see how easy they are to destroy. Bafang has figured out how to do it right, and they pretty much own the geared hub market in China. It’s much easier to knock off cheap clones of DD motors, as the engineering complexity is just not there.
- Holy crap that motor is wide open to the world : Rocks, water, debris is going to make short work of that fancy motor. You’re going to have to trust me on this one. If you do order a Stary board don’t ride it when it’s wet and blow out the motor after every use with 200PSI of compressed air. You will probably double the life of your board.
- Because you need regen to create breaking effects with the geared hub setup (no freewheeling) the board can not be used as a normal skateboard when the battery dies. The hub gearing system will most likely create too much drag to make it any fun, although I’m just guessing here and have NOT tried this board yet.
- Using metal reduction gears is going to make the motor pretty loud. Most ebike motors use nylon or composite gears, metal is a hard substance to keep quiet and to keep properly lubricated.
- The urethane wheel works to insulate the heat of the motor. Heat buildup is a huge problem with small motors like this under high loads.
Belt Driven electric skateboard motors
Pretty much every board currently on the market with the exception of a few cheap Chinese boards that are filtering in through Alibaba and eBay are all Belt-driven motors. Right now the defacto standard that all other boards are judged against is the $1500 Boosted 2000 Dual+ monster. This board has cracked the code and made something that is really fun to ride and still pretty safe. The board is relatively heavy and expensive but from everything I’ve read online, Boosted really stands behind their product. That might change in the future, but for now, they are the yardstick by which other electric skateboard companies are judged.
Advantages of Belt Driven Motors
- You can get the gearing ratio perfect for your application : Every electric motor has a power range that the motor works best at, the trick is matching that optimum speed with the right sized skateboard wheel for that application (generally under 100mm for street use). Using a belt allows you to do just that.
- The motor is slightly more sheltered from road debris as it can be moved up and away from the ground
Disadvantages of Belt Driving Motors
- You gotta replace the belts : Not that often but if you’re commuting with your eSkateboard every day for extended distances the belts will wear out. If your belt breaks on the road and it’s not a dual drive system then you’re going to be kicking it home.
- You can’t push it : If the battery dies your board is not going to roll without a whole lot of friction. More than likely you’re going to bend over pick up your board and carry it the rest of the way.
- On high power boards the motor is still open to the elements : On some of the lower power 24v boards the motor is completely sealed. Every ‘high power’ motors I’ve seen for electric skateboards are almost always open to the elements on one side. This creates the potential for water, debris and metal objects to find their way inside the motor which is going to suck.
It seems to me that the biggest problems that skateboard motors face is cooling. While having DD Hub and Geared Hub motors might move the industry forward, they still haven’t solved this fundamental problem. I believe that with a little engineering ingenuity and some Ferrofluids and rare earth magnets that this problem could be pretty easily licked. In my experience, the Chinese are just not that good at engineering products that work well and seem to last. What they are good at is knocking off other products and selling them impossibly cheap.
What will the future hold for electric skateboards? All I know is that it is an exciting time and that even if the current batch of hub motors fail to live up to users expectations, they are still moving the industry forward in a good way.