Time Pressure
At this point we only have a couple of days left to finish this project. We’re working hard to pull all the parts together.
Haydn and I have made a couple of prototypes of the motor and gear housing on solidworks and then in cardboard.
Currently, Nick is machining alloy wheel rims, with John the workshop technician running the CNC Haas machine to produce the Urethane wheels.
James is currently working on redesigning the hand controller.
Jason has designed a packing spacer to fit under the front trucks, to make the board level. Just waiting on access to a Modela to machine it.
Damien has bought some larger trucks from Australia, and bought a new board deck from trademe. We’re still waiting for both to arrive.
Hubless Wheel Material and Components
The components and materials for the hubless wheels are all sorted.
We had three options for the bearing sizes which are:
Bore - OD - Width
90 - 98 - 30mm $125.00ea
95 - 103 - 30mm $95.50ea
100 - 108 - 30mm $75.15ea
The prices for the bearings are quite expensive, mainly because it’s not that common to use these bearings for large applications, they’re mainly used for smaller applications but we’ve decided to go for the middle sized bearing.
The materials we decided on is Urethane and Aluminium. We chose the Aluminium because it’s got a excellent strength to weight ratio, helping to keep the overall weight of the board down. It’s going to be for the inside ring, it’ll be cut out on the modela and then finished on the manual lathe. Most common skateboard wheels are made out of Urethane with a hardness rating of between 75a and 100a typically. It’s a hard rubber type material, although not too soft so we can’t use it on the CNC lathe. The Urethane we have obtained is 125mm OD x 250mm length with a hardness of 90A so it’ll be perfect for our wheels.
Batteries again.
We need 36V worth of batteries to power our board.
We want a good weight to power balance. NiMH batteries could be the solution to this problem. Sourcing these batteries is not a problem, finding a 36V charger for these batteries is. Ideally, each individual cell should be charged equally and separately. A 36V charger could damage the cells.
For the purposes of demonstrating our board we think we may make a bank of 30 1.2V AA NiMH cells, totalling 36V, and charge each cell on a charger before each use. This means we can have a very discrete, small, battery pack. This is also a very cheap option. At 3000mAh we can still get almost a third of the power and distance of a commercial off-road board.
James, Nick and I spent the afternoon trying to test the new motor controller. Unfortunately, it wasn’t compatible with our donor board’s wireless receiver, so we’ll need to get another one from Greenskate. Without a working receiver we weren’t able to tell the motor to ‘go’.
We borrowed a regulated power supply from Peter, the technician, and connected it to our donor motor to see if it could tolerate 36v under a load. We slowly turned up the voltage to 30V 2.5A, and could smell our motor burning. We applied a ‘load’ by gripping and putting resistance on the moving wheel. The motor then started drawing more current (3A+). The motor then slowed itself down as it got too hot, which means that we probably melted the coils inside.
We’re now on the lookout for a new motor.
Nigel Vining, founder of Greenskate
Greenskate.co.nz has, so far, been our point of reference for electric skateboards in NZ. They almost exclusively are the only online retailers of electric skateboards, outside TradeMe.
I bought a motor controller from Greenskate. Nigel Vining, founder of Greenskate, agreed to meet up with us in Wellington City to give it to us and talk skateboards.
Nick, Damien, Jason and I met Nigel in town. He told us about how Greenskate started. At Christmas in 2007 Nigel saw an electric skateboard and really wanted one. He said he began experimenting in a similar way to us. He even had the same electric scooter at one stage. Electric skateboard retailers in NZ were too expensive and he soon found that it was more cost effective to buy a batch of ten from China and keep one for himself. The other 9 sold quickly, and later people were demanding parts for them. When Nigel developed a Li-Poly battery pack, other people wanted that too. Now Nigel runs Greenskate with two friends supplying the NZ market with electric skateboards and parts.
I tried to see if I could juice up the 12V kids skateboard by running 24V from the scooter through the motor controller. The motor went hard for a few seconds on one burst. On the next burst of speed, there was a loud pop and smoke started pouring out of the motor controller. If you look closely in the close up photo you can see smoke. One of the capacitors on the circuit board blew up. If we want to get more power out of the motor we need to get a meatier motor controller. I bought a 600W motor controller from greenskate.co.nz.
A scan of the needle roller bearings specifications, and of course like every other component of the board, these are expensive, especially at the size we’re looking at, having a 100mm bore. Although it’s not necessary to have them this big, it’s just an aesthetic choice, decisions to be made….
Frank Schmitt came up with this board, of similar design to ours. It uses a hub motor for a rear drive wheel.
Project can be found in it’s original context at http://frankschmitt.org/projects/powerboard or on his project blog http://3wdm.blogspot.com/
Images taken from Schmitt’s flickr page.
Batteries
Our skateboard requires a pretty decent output power, and a motor will draw a lot of current.
We need some decent, compact batteries that are high capacity and have a high energy density.
At the lowest level are SLA, Sealed Lead Acid, batteries like your car battery. They’re cheap, low maintenance and high capacity, but are heavy and very bulky.
At the top of the range are LiFePo4, Lithium Iron Phosphate, batteries. They’re very high capacity and very small and compact. They have a great amount of energy for such a small package. They require a special charger to charge each individual cell equally.
Ranking in between SLA and LiFePo4 batteries are NiCd, NiMh, Li-Ion & Li-Po. Any one of these could be an adequate compromise.
The price different in these is huge. A set of SLA batteries (at the lowest level) for our board would be about $250. A set of LiFePo4 batteries (at the best level) would be about $700.
