I wanted to make a new downhill billycart for Space Pilots camp that I didn’t have to push up the hill. Unfortunately that didn’t happen this year, but I did make an awesome attempt that will need to finished before next year.

William driving me down the hill at high speed-gravity power only

It is powered by (or will have) 2x 2kW motors, 6x 9Ah 18V Makita cordless drill batteries.

Billy cart as driven with 1 motor

I got 1 motor driving 1 wheel, which has enough power to drive it up the hill, but unfortunately a single driving wheel doesn’t have enough traction up the hill and just spins. I now have the next 12 months to get the second motor installed.


I started off looking for 2 bikes of the same model with dual suspension. The second bike was bit of an issue after the one I found the guy had given it away to someone else so I had search for 6 weeks for another to match the first one I had.

The donor Terrain 2x dual suspension DX7800 18 speed

After pulling apart the frame, I made the lower extension arm. My fish mouthing of the end of the RHS was dreadful, but nothing a lot of filler weld couldn’t fix.

William machining up the bushes for the lower frame extension
Back part of the bike frame with the lower frame extension
Centralizing washers for the lower frame extension
Beginning of lower frame pivot point mounts.
Motors with gearboxes attached with 3D printed flange and coupling
Lower frame pivot point assembly
Bottom frame extension installed
Trial fitting and measurements to work out top frame placement
Both extension drams installed into the bike frames
Donor frame I was going to cut the headstock tube out of but decided it was easier to machine one up. Bearing cups and other hardware was reused.
Seat frame cross members cut and mounting holes added
Elongating mounting holes on motor enabling chain adjustment
Elongating mounting holes on motor to make chain adjustment

Motor with laser cut mounting plate

Lasercut gears being drilled, tapped, linished and filed to Mount into motor shaft.
Lasercut gears being drilled, tapped, linished and filed to Mount into motor shaft.
Lasercut gears being drilled, tapped, linished and filed to Mount into motor shaft.
Lasercut gears being drilled, tapped, linished and filed to Mount into motor shaft.
Rear frame with motor mounting plate.
Rear frame with motor attached.
Inner seat frame being designed with motor clearance being account for
3D printed battery holder.
3D printed battery holder.
Seat frame assembly
Seat frame assembly
Seat frame assembly
Seat frame assembly
Seat frame assembly mounted to bike frames
Steering tube head machined
Forks cut off and machined.
Current state of frame build
Drilling holes into repurposed seat pillars with steel slug for strength. These are the steering arms that mount where the steering was in the head.
Gas struts purchased from bunnings for the ball ends for steering. Also seat pillars repurposed as steering arms.
Gazebo taken from works bin to salvage aluminum for steering arms and linkages
Working out length of pivoting steering arm holder
Working out length of pivoting steering arm holder
Steering arm pivot about to be tacked into steering arm holder.
Adjustable steering arm installed
Current state of build
Steering linkages
Steering linkages
Chain tensioner for steering coupling. This chain was replaced with a slightly less seized chain later
Both ends attached to the steering chain tensioner.
Slightly less dodgy chain on steering link coupler.
Cutting aluminum on the saw for the brake cable joiner
The beginnings of a 1 to 2 brake cable joiner. The pieces of steel plate (if you can call 2mm thick steel plate) are from brakes to use the oblong holes.
Welding them together on brass so I could build material between them
Cutting a pedal off the crankset
1 to 2 brake cable adapter
Temporary seat
Chain attached to motor and crankset
Milling brake cable joiner

I tried ordering longer brake cables from a local bike shop but unfortunately they didn’t make it in time. So that left me with joining and extending shorter brake cables.

Brake cable joiner block before being cut
Brake cable joiner joint brake cables.
Brake cable joiner drawing

I did do I quick power up test and calculations that running the motor off a slightly higher voltage the inverters would survive. A 12v lead acid battery has a nominal voltage of 12v, and is considered flat at about 10.5v. What we are interested in is the charging voltage. The charging voltage of a lead acid battery is about 14.5V, so the max system voltage would be 4×14.5V=58V. The full charge voltage of a 18V lithium battery is 20.5V, making the system voltage of 3x18V batteries 3×20.5V=61.5V.

This means I am applying about 3.5V more than what it is designed. I did pull the end cover off to see if I could see what the components were rated for but the MOSFETs/transistors are mounted to the case and I couldn’t be bothered pulling them off to get it apart. I did see a SKD502T which is rated for 85V 120A. So hopefully the internal components have had a bit of headroom designed in to cope for this increase in supply voltage. Unfortunately 2 batteries don’t quite meet the low end cutoff voltage of 42V to run it so 3 batteries it is.

The Makita 18V battery low voltage cutoff seems to be 14V from measuring the voltage of the battery when my Makita DUB184 blower cuts out with a genuine battery, which would mean 2.8V per cell inside the battery, which seems a bit low to me. But working off this it would mean the low voltage cut off of 42V would be appropriate. I would personally go as low as 3V/cell which is 15V on the battery to try to give it some more life.

My main concern is the amount of current being drawn from the battery which might be too much. 2000W/42V=47.6A, which may not be a problem if the motor kits are lower power and not as advertised, which may get their own post once I investigate. The website states rated current and voltage as 34Ax48V=1632W, so something doesn’t add up. Maximum current is stated as 42A, so 42Ax48V=2016W input, and assuming there is efficiency losses in the inverter and motor, this will be less.

Batteries mounted on ply board with 40A breakers

The batteries are set up in 2 groups of 3 in series, with the intent of powering each inverter with 1 set. I have the 0V commoned up between the 2 sets and am only breaking the positive. I was going to put the circuit breaker on positive and negative, but because I am joining signal cables to control both units together I can’t have the 0V on one go open and somehow ground out through another sensor wire. I also have another breaker allowing both battery banks to be paralleled.

Motor drives mounted on other side.
Completed buggy after some quick modifications to steering, g-clamp added to motor to hold gearbox on.
Prototype motor gearbox coupling
Coupling sitting in clutch housing of 5:1 gearbox
Carbon fiber filled nylon 3D printed coupling

The motor I purchased. I went through eBay but this is the same

Wish list of improvements/additions

Modify voltmeter on ignition key to display 46V as flat

Make a voltage/current sense circuit to measure voltages of batteries so keeping them as a set is not so important. If batteries of different capacities are connected together in series, the lowest capacity battery voltage goes down fist and can easily be over discharged by the other batteries.

Add wider tyres on the rear for more traction

Better brakes?

Improve steering

Make the cart tilt/pivot the direction turned

Remote kill switch?

Chain tensioner on motor?

Enable changing of gears?

Brass bushes for rear pivot point to replace the plastic ones

By David Dobson

I try not to let my age dictate how old I act