April 19, 2015

The Longboard Has Gone Too Long Without a Motor

As previously indicated,  my longboard (can it be called that?  It's tiny) recently sprouted a battery pack and motor.

I started out with this board.  Nick crufted the deck out of a dumpster a while back, and I got an extremely cheap set of wheels and trucks for it.  The construction is interesting.  The deck is made from a 1/8" thick sheet of wood laminated on each side with a very thin layer of aluminum.  The resulting deck is extremely light and flexible, which makes the board really smooth out bumps (which are rather abundant on Cambridge roads and sidewalks).

Most electric longboards out there are stiff, because you generally don't want your batteries and electronics bending.  Alternatively, some electric boards have a small battery at one end, motor and controller at the other end, and nothing in the middle, allowing the board to be flexible.  This requires a full-sized longboard and/or a small battery though.  I wanted to keep the flexibility of this board, even though to get reasonable range the entire underside would need to be packed with battery.

My solution was the bendypack, which is the only part of this build that's anything new.  The rest of the electrical system is typical diy-electric longboard stuff, with a HobbyKing motor, motor controller, and remote control system.

The key to the flexible battery pack is its corrugated enclosure.  The corrugations allow the pack to flex at the gaps between rows of cells.  The inter-cell connections run against the deck, which is very close to the neutral bending axis of the board and battery assembly.  This means that the wires barely have to stretch as the pack flexes.

To make the corrugated casing, I cnc milled a foam mold on the MITERS cnc mill.

The mold was milled in three parts, because of the size limitation of the mill:

Two layers of fiberglass were vacuum bagged over the mold:

The foam was dissolved out with acetone. It left some color behind though:

The case fits 18 26650 A123 26650 cells.  These are arranged in a 6s3p configuration, for a nominal 19.8 volt, 6.6 amp hour pack.

Cells were soldered together with grounding strap to minimize the thickness of the connections:

The pack was insulated from the deck with a sheet of rubber.  Around the edge is a much softer synthetic foam, to act like a gasket and seal off the pack against the bottom of the deck.

The pack was screwed to the deck with a bunch of 2-56 screws around its perimeter.  These screws were tapped directly into the bottom of the deck.  The material of the casing is very thin, so the screw heads alone would have pulled through the fiberglass.  I 3d printed a series of oddly-shaped washers which were glued to the casing to distribute the load.  The washers aren't shown here:

The motor-wheel-pulley system is nothing too special.  The 42 tooth HTD-5mm pulley on the back wheel was downloaded from this thread on endless-sphere and 3d printed.  Sounds a bit sketchy, but there are 6 M5 bolts going through it to hold the wheel hub, and the load is spread across a bunch of teeth.

The motor pulley was CNC milled out of some square 7075 bar stock:

I much prefer split collar style clamping mechanisms to set screws, so I manually machined some slots in the pulley, and drilled and tapped a hole for the clamping screw.

The super cheap trucks had no square faces or cylindrical surfaces, so I stuck if in the lathe and turned a small section down.  We don't have a steady rest for the MITERS lathe, so I used the old drill chuck, which has smooth jaws, with lots of oil:

I CNC milled a two-part motor mount, with about a centimeter of adjustment room for tensioning the drive belt.  The motor is a SK3 5065 236 Kv, left over from a project of Nancy's a couple summers ago:

The mount was temporarily fixed in place with a big set screw, and tack welded to the truck by Mike.  The 9mm wide HTD 5 timing belt was borrowed from Ava's motorized ripstik.

I shoved an RC airplane controller (160 Amp dlux HV) and rc receiver into a 3d printed case, and glued that to the battery pack.  I don't really want this to be the permanent solution, but it works well enough that I probably won't change it until something breaks

Right now the "switch" is just the battery's deans connector.  Also not ideal, and it will get replaced with a proper switch eventually.

Belly up:

The board is geared for a no-load top speed of just over 22 mph, so realistically can probably get to about 20.  I very rarely max it out though - with a board this short, it's just too easy to hit a rock and go flying.  I haven't measured the range exactly, but I'd estimate it at 7 or 8 miles, although this will depend on how hilly and windy it is.  Riding back and forth from East Campus to MITERS 3 times (.7 miles each way) drained about 3.5 amp hours from the battery pack.  In terms of power, I've watt-metered the board at a bit over a kilowatt peak out of the battery pack during acceleration.