Electric Bobby Car Build

This is the electric motor upgrade build (not a chronological log) of a Bobby Car using parts from two Hoverboards/Swagboards. The end result will have four motors, one per wheel, two Xbox 360 potentiometers as throttle and brake triggers on the stock steering wheel, and two push buttons to turn on and off each controller separately (e.g. FWD, RWD, AWD). Inside the Bobby Car all the equipment resides, e.g. the controllers, battery, and cabling. In addition to elements to improve the strength of the base.

Most of the front wheel steering assembly had to be replaced by new 3D-printed parts. The same is true for the rear wheel base as it was replaced with an aluminum mounting plate.

The controllers was flashed to with a new alternative firmware developed by Larsmm, which is a fork of NiklasFauth original base. This firmware provides different four driving modes with different speed, acceleration and features, and smooth acceleration and breaking ramps, an a cool turbo ramp!

• Electric Bobby Car Hoverboard Upgrade - research page
• Electric Bobby Car CAD package - all the parts in Inventor IPT, STEP and STL formats

Base preparation

Main equipment bay

A slot had to by cut out of the bottom to allow internal access and functions as the main equipment bay. It measures 78 x 148 mm, just enough to fit the controllers and battery. Rounded corners to minimize stress areas.

In the final stage it is covered by a see-through Lexan plate.

Rear wheel slots

Two rear indentations were cut out to make space for a wooden structure which later would be used to mount a 12mm aluminium wheel plate. These were cut just at/below the stock axle hole and are at the same height as the new front wheels when installed, so the base would be level.

To judge the right cut height, it is an advantage to first prepare the rear wheel base and use it for make an educated approximation.

To remove sharp edges and to release stress areas, the edge was finished with a heat gun.

Rear wheel structure

The internal rear wood structure consists of one cross beam and six smaller wood pieces stacked on top of each other. The load of the driver will be nicely distributed to the wheels and ground, easily increasing the max. driver weight to over 100kg!

Each wooden piece was fastened to the parent piece using wooden screws, making sure to use a different screw pattern for each layer to avoid conflicts.

Mounting the wooden structure to the plastic base wall was done by nine 4 x 30mm wooden screws (pilot holes pre-drilled), linking each layer of the internal structure. Using washers to distribute the load as wide as possible. If the wheel touches the head of the screws, file them down a bit.

File:Electric bobby car build base rear mounting structure3.jpg

Rear wheels

All the stock parts of the Bobby Car front and rear wheels was removed, only a few parts of the steering was reused. Two Hoverboard wheel brackets and screws were also reused.

• Rear wheel parts included in the CAD package - all the parts in Inventor IPT, STEP and STL formats

Wheel base plate

As much of the driver's weight will be over the rear wheels, the wheels had to be mounted on an strong 200x70x12mm 6061 aluminium plate. Slots and screw holes were routed and drilled/tapped out, respectively, using regular home/garage gear. Additional screw holes were added for future expansions, like a hook for a trailer, photo interrupter sensor, and lights.

• The design of the rear wheel plate can be downloaded here

The plate was prepared by outlining the design on surface. To cut out the wheel axle slots, a Bosch router with a HSS straight two flute Ø8mm milling cutter was used (see drawing for dimensions). I did 1-2mm cutting depth at each pass and multiple finer passes to end up at the final smooth finish. Used WD40 and low RPM on the router to avoid overheating the cutter.

The screw holes needed to be drilled with a pilot hole of Screw Diameter x 0.9 = Drill bit size, e.g. M8 x 0.9 = 7.2mm, or the closest one available (7mm). Then using a taper tap to gradually cutting out the required threads.

Holes to fasten the plate to the wood structure was counter-sunk for a flush finish.

The plate was cut to length with a jig saw.

Installing plate

Once the wooden structure and plate was ready, pilot holes were drilled (to avoid cracking) and the plate was installed using 6 x 45mm counter-sunk screws. This provided a very stabel platform to mount the wheels.

Mounting wheels

Both wheels are mounted to the base plate by sandwiching the axle between the stock Hoverboard bracket using the existing four M8 x 30mm screws.

Axle cover

The axle and motor wires are directly exposed and could be damaged. A 3D-printed cover (include in the CAD package) screws in place over this area.

File:Electric bobby car build wheels rear cover2.jpg

Wiring

The wiring coming from the motor was feed through the base plate and then inside the base through a small hole just underneath the front end of the wheel plate.

The wires were protected from abbreviation by a cable sleeve.

Front wheels

The front wheel assembly was modeled and new parts designed to incorporate the motors into the stock front wheel structure. Wheel clearance height was increase/raised by 10 mm to all the bigger radius Hoverboard wheels to fit (155mm vs 170mm).

All parts were designed/3D-printed (ABS) with strength as one of the main principles, primarily using long through screws and 70% infill to compress and distribute, respectively, much of the stresses experienced on the parts.

• Front wheel parts included in the CAD package - all the parts in Inventor IPT, STEP and STL formats

Full aluminium CNC parts or metal support inserts would be a future upgrade here.

Steering wheel assembly

The assembly consists of four main parts and six screws along with six nuts to keep it all together. See the BOM for full break-down of the screw selection. A skateboard bearing lessens some of the pressure and tear on the upper steering join.

File:Electric bobby car build wheels front assembly5.jpg

Cross bracket

Both wheels are kept in alignment by the stock front cross bracket. This is screwed onto the base frame. The bracket had to be reinforced using eight M4 x 30mm screws which connect to a counter plate with the nuts inside the base, instead of the stock two threaded plastic screws. No washers on the four corner screws, for space reasons.

Wiring

The wiring coming from the motor was feed through a small hole just in the middle of steering range motion to minimize cable stress/bending.

The wires were protected from abbreviation by a cable sleeve.

Steering

Much of the steering assembly was redesigned from ground, except for the steering wheel itself. Much work went into figuring out how the throttle and brake trigger would mount and wiring would be routed. Addition base reinforcement was also needed.

Rod bending

The new rod was based on the stock dimension and bend, just longer on one end to put the steering wheel at a comfortable position.

Using a Ø10mm stainless steel rod, one end was heated using a simple butane camping gas stove to make it pliable. Stuck it in vise and bent it to the approx. radius. Then put the entire bend into the vise and squeeze it into the right dimension, using the stock as a template. Took almost 2 hours.

File:Electric bobby car build steering rod3.jpg

Rod length and holes

To make the steering comfortable, the steering rod had to be lengthened by approx. 20 cm.

Try the rod on the Bobby Car with the steering wheel next to it to find the correct steering height. Mark the spot and cut the rod. To allow for different steering wheel heights, I made three different mounting holes. This also requires a through hole in the plastic on the steering wheel.

To keep the rod from falling through on assembly, an outer 140 x Ø32mm white PVC tube keeps the steering wheel at the correct position and protects the wiring going to the controller.

Base rod support

As there will be a lot of load on the steering rod, additional support had been added to the top and bottom side where the rod protrudes through the plastic base.

The top and bottom support brackets were fastened to the base using slotted M4 x 25 mm mushroom screws and nuts.

Throttle and brake

Using NiklasFauth's method for the throttle and brake levers by mounting them on the front side on the steering wheel. A special 3D-printed holder keeps the trigger, potentiometer and support board in place. All put in place on the steering wheel using hot glue. An upgrade here would be a through screw.

Wiring the potentiometer is simple, 3V3 (red), analog out (yellow, mid-pin), and ground (black). This connects to the break-out board located inside the steering wheel column.

Electronics

Break-out board

This is board makes it easier to finish the build. It has easy to connect plugs for the throttle and brake, analog signal filtering, master and slave throttle isolation - where one driver board can be turned off without interfering with the other. It tucks away inside the steering wheel under the red beeper cap.

Inspired by the Jan Henrik break-out boards.

• Schematic of the throttle breakout board
• Break-out PCB Gerber-files

Motor wiring

Connecting the front motors to controller is straight plug and play, but the rear main motor wires needed to be extended 10 cm to reach the controller. Simple cut-and-solder in 16 AWG extension wires, keeping the existing bullet connectors.

The motor hall-effect wires were easily extended using custom made 15cm extension wires with connectors on each end.

File:Electric bobby car build motor wiring3.jpg

Mounting controllers

The controllers are essentially identical, except for slightly different caps, power transistors, and wiring method. Both are from Hoverboard productions in 2015. Both rock the same STM32F103 ARM Cortext-M3 MCU.

Both controllers are mounted back-to-back using four M4 x 30mm screws and nuts with a stand-off spacer in-between, and installed vertically using a custom 3D-printed bracket and two slotted M5 x 16mm mushroom screws and nuts.

Battery

This build is using the stock 18850 10S2P 36V Lithium-ion battery with BMS built-in, from the Hoverboard. Although a 12S2P 44.4V battery is supported by the controllers, for future upgrades.

The battery is kept secure by a hook-and-look strap around the battery and through two slots in a Lexan cover plate.

File:Electric bobby car build battery2.jpg

Power buttons

As this is a four motor, two controller setup, I wanted to have the possibility to power-on each controller separately. This would allow for FWD or RWD, depending on the occasion. This requires the custom design break-out board in order to isolate the two controllers so they would not power each other or block/pull-down the throttle/brake signal when one of the controllers were powered down.

Two momentary LED push latching buttons were prepared with a four-pin connector/socket. The corresponding socket were connected to the 15V regulator on the controller and power-on pins. A 2k resistor were put on the button LED power pin to lower the illumination. A Ø12mm hole was prepared and the buttons installed next to the steering rod.

Power plug and charging

A 3D-printed bracket holds the power plug (cuts all power) and charging port in place just rear of the equipment bay slot. Fastened using three screws.

Flash firmware

The Hoverboard controller board expose the programming port, requiring only three wires or a header to be soldered.

Make sure the power-button is connected as this is required to put the MCU into DFU flashing mode. Connect the ST-Link V2 Mini programmer to the mainboard.

GND -> GND
SWDIO -> DIO
SWDCLK -> CLK
3V3 unconnected, powered by board

Power off, press and hold the power-button while executing the following commands.

Unlock MCU:

openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c init -c "reset halt" -c "stm32f1x unlock 0"

Flash MCU:

openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c "reset halt" -c "stm32f1x mass_erase 0" -c "flash write_bank 0 firmware.bin 0" -c "reset run"

BOM

General parts

Quantity	Description	Destination
1x	Big New Bobby Car	Main base
1x	Xbox 360 Game controller	Throttle and brake potentiometer
2x	Hoverboard with STM32 MCU	Wheels, controllers, batteries and misc.
1x	ST Link V2 compatible programmer	Flash new firmware
1x	Aluminium 6061 200x70x12mm plate	Rear wheel plate
1x	Round Stainless Steel Rod Ø10mm x 100cm	Steering wheel rod
1x	Plastic PVC Ø32mm x 50cm tube	Steering wheel height and wire protection
1x	XT60 60A connectors	Power plug and power cable
1x	Silicone Wire 14AWG red	Power cable
1x	Silicone Wire 14AWG black	Power cable
2x	Deep Groove Ball Bearing 608ZZ ID Ø8mm	Front upper wheel join load distribution
1x	NMB Steel Bearing Motor Grade Bearing ID Ø10mm	Steering rod lower load distribution

Screws

Quantity	Description	Destination
16x	4 x 30mm Screws Chipboard Pozi Pan in A2 Stainless	Rear plastic body to wood structure
16x	M4 Washer Form A in A2 Stainless - ISO 7089 DIN 125A	Rear plastic body to wood structure
12x	6 x 45mm Torx Countersunk Chipboard Screws in A2 Stainless Steel	Rear alu-plate to wood structure
8x	M8 x 30mm screws for motor axle bracket - use existing Hoverboard screws, 1.25mm pitch	Rear motor bracket to alu-plate screws
8x	M8 x 40mm Socket Low Head Cap Screws 3mm profile A2 DIN 7984	Steering assembly motor clamp
2x	M8 x 25mm Low Socket Head Cap Screw 5mm low head in A2 Stainless - DIN 7984	Steering wheel inner stability top plate core
2x	M8 x 80mm Socket Head Cap Screw Full Thread cut to 74mm in A2 Stainless - ISO 4762 DIN 912	Steering wheel inner stability body and bearing
2x	M8 x 40mm Cap Socket Head Cap Screw 8mm head depth in A2 Stainless - ISO 4762 DIN 912	Steering wheel inner stability arm core
14x	M8 Nyloc Nut Type T Thin 8mm in A2 Stainless - DIN 985	Nuts for steering wheel assembly
8x	M4 x 30mm Socket Head Cap Screws ISO 4762 DIN 912 in A2 Stainless Steel	Reinforcement screws for cross bracket
4x	M4 Nyloc Nut Type T Thin in A2 Stainless - DIN 985	Reinforcement screws for cross bracket
4x	M4 Washer Form C in A2 Stainless - BS4320	Reinforcement screws for cross bracket
1x	M5 304 Compression spring rod	Steering wheel to steering rod lock screw
4x	M4 x 25mm Slotted Mushroom Screw in A2 Stainless	Steering rod support brackets top and bottom
4x	M4 Locking Nuts Nyloc Type T Thin in A2 Stainless - DIN 985	Steering rod support brackets top and bottom
4x	M4 Washer Form C in A2 Stainless - BS4320	Steering rod support brackets top and bottom
4x	M4 x 30mm Socket Head Cap Screws ISO 4762 DIN 912 in A2 Stainless Steel	Motor controller board assembly
4x	M4 Nyloc Nut Type T Thin in A2 Stainless - DIN 985	Motor controller board assembly
2x	M5 x 16mm Slotted Mushroom Screw in A2 Stainless	Motor controller board assembly mounting to plastic body
2x	M5 Locking Nuts Nyloc Type T Thin in A2 Stainless - DIN 985	Motor controller board assembly mounting to plastic body
2x	M5 x 16mm Slotted Mushroom Screw in A2 Stainless	Bottom cover plate
2x	M5 Locking Nuts Nyloc Type T Thin in A2 Stainless - DIN 985	Bottom cover plate

References

• Larsm Allrad e-Bobby Car - great write up and custom firmware fork
• Niklas Fauth Hoverboard firmware hack - original firmware
  • CCC GPN18 HowTo: Moving Objects - great talk
  • Build Instruction: TranspOtter - Another build, lots of helpful pictures
• Fisch's detailed E-Bobby Car build guide - great documentation of the assembly
  • Fisch's trailer build - trailer for the Bobby Car
• Jan Henrik breakout boards - PCBs for easy connection to serial and Nunchuck
• Peter Pötzi Electric Bobby-Car - Blue Bobby Car with LCD steering display