PatchKart - The Ultimate Sub-Miniature Go-Kart

"Wow! What a cool and original idea!" - Some guy

We designed this sub-miniature gokart some time ago after seeing countless power wheels races, drift trikes, chibi karts, and other DIY micro electric vehicles. We have been testing, abusing, and iterating on this design with friends and unfortunate riders ever since. The goal was to have minimal part count and cost while maximizing fun. You likely already have most of the parts you need to build it too, which means you can probably build this for UNDER $100 in hardware, and 400g of filament!!

PatchKarts have proper Ackermann geometry and can run for hours on simple tool batteries. Top speed is typically 16-20mph (26-30kph),

Slip on some 6" PVC end caps to the wheels, and you have an amazing drift kart! 

Print the 4WD version and you have a 2kW+ Gymkhana that can fit in a suitcase. (Yes, these have been flown in checked baggage (tool batteries in carry-on).

BUILD IT! 

2020/2040 Series T-slot options:

MAKE SURE the t-slot you order has a 4.2mm hole in the middle. This lets you tap it to M5 where needed. A 5mm slot width (as opposed to 6mm slot) will also improve stiffness.

You can make the stuff with a larger hole work, but you will have to tap to M6 and source 35mm long M6 bolts as well.

Print:

All components should be printed in PETG (or other high-performance plastic) with 10 perimeters, 5 top/bottom layers, and 60% infill. Select your "STRUCTURAL" preset for better layer adhesion. No support. The steering wheel can be half as much infill and perimeters.

The file names are self-defining.

• 2 wheel-drive version (2WD- parts) is lighter and more efficient. (recommended)

• 4 wheel-drive version (4WD- parts) is heavy but has ludicrous acceleration, and more ground clearance.

Tools:

• 3mm hex driver bit

• Saw that cuts right angles in aluminum

• 5mm (3/16") and 9mm (11/32" or 23/64") drill bit

• M5 thread tap

Parts List:

My personal Battery Adapter:

A note about 6.5" hoverboard motors:
Of the dozens of hoverboards we have tested, we found that the Kv (RPM/Volt) rating is often anywhere from 15 to 25Kv. Lower Kv motors will have much higher torque.  Higher Kv motors seem to be found in the cheaper "24v" hoverboards. 

Cut List:

(!) - These lengths are important for proper Ackermann steering geometry. Make them as precise as possible. Changing them will make steering harder. You can experiment with other beam lengths without worry.

Assemble!

Cut and drill aluminum extrusions:

1. Cut, then tap M5 threads into the ends of the three bars mentioned in the list above.

2. Drill 5mm through-holes according to the cut-list mentioned above. 

3. The 90mm segment of 2040 is used for the bike seat. You may need to cut some corners off to ensure it fits between the mounting bars of most standard bike seats. Most seats let you remove the metal bars from the pad for easier assembly.

Assemble chassis 

Pro tip: You can pre-mount your t-nuts to the exposed bolts, THEN slide the bracket/part onto the end of your extrusion. No need to try to line up nuts after the fact. Take careful note of brackets used, their orientation, and where the tapped holes for screws are.

1. Referring to the photos below, attach the front and rear beams to the central beam using the 5-hole plates, and 2-hole inside-angle brackets.

2. Use 35mm M5 screws to secure your front and rear beams to the center beam's threaded holes. 

3. Use two 25mm M5 screws into the remaining two holes of the rear beam.

4. Drop in the 170mm "seat post" along with four 2-hole inside-angle brackets on the back.

5. Mount the 90mm seat adapter on top using at two more 35mm M5 screws.

6. Slide on your 300mm steering support beam into the top of the front beam using two 5-hole "T" plates.

Assemble the front spindles (2WD only)

1. De-bur and chamfer the 75x8mm smooth rods. 

2. Using a vice or hammer, and the "Strike plate L/R.stl" prints, force the smooth rods into their respective holes on the front spindles. Check progress every few millimeters. BE EXTRA CAREFUL to not drive these rods too far or at an angle!

   

3. Insert a normal M5 hex nut into the rectangular hole and a 25mm M5 screw perpendicular to that to lock in the shaft.

4. You can now install your scooter wheels using the "Tapered Washer" prints and your split-shaft locking collar.

Note: There is an intentionally unused screw hole in these spindles.

Assemble the front spindles (4WD only)

1. Insert your hub motors into the spindles. The shaft's flat should interface with the two intersecting screw holes. Insure a small gap is between the motor body and your print.

2. Lock the motor shaft in place using all three 35mm bolts and nylock nuts. The pivot hole should not be obstructed. 

Assemble Steering (front end)

1. With a 9mm drill bit, drill out the outer most hole on two 5-hole angle plates, and two 3-hole straight plates. Insert your brass sleeves from the outside. The ends should be flush with the inside of the brackets.
   

   

2. On the front beam, slide the 5-hole angle plates (plus 2 unmodified ones) onto top ends of the front beam as pictured below. Do the same for the 3-hole plates on the underside. The sleeved ends should stick out exactly 20mm from the end of the bar. 

3. add grease to the inside faces of the exposed brackets, then slide in your steering spindles.

4. Insert a 60mm M5 bolt through the assembly, and tighten with a nylock nut; ensuring pivot can still move with some freedom. 

4WD SPINDLES ONLY: You will need to use a file or coping saw to gouge out a small section from the rearward corners of your slightly smaller (260mm) front beam to allow the motor axle and wires enough clearance to pivot inwards. Keep as much of the original profile in tact as you can. Also remove sharp corners that could damage the cable.

4WD SPINDLES ONLY: Mounting 4WD Spindles:

Note: if you are using non-standard front beam, or center chassis beam lengths, you will need different length control arms. Find some "M5 Rod End Bearings" and associated threaded rod to get infinite alignment adjustment. 

Rear Wheel Motor Mounting

1. Slide your chosen rear motor mounts onto the shafts of your wheel motors. The shaft's flat should interface with the THREE intersecting screw holes. Keep a small gap between the spinny bit and mount.

2. Fully insert your M5 screws and loosely screw wide t-nuts on each.

   1. (If the screw just wont spin when fully inserted, a few passes with a round needle file on the motor shaft near where the screw intersects will add just enough clearance.) 

3. Slide each motor mount on to the rear faces of your rear beam. (For the 4WD version, the shaft's flat points up.) The print should be flush with the end of the 2040 extrusion.

4. Tighten every screw firmly.
   

2WD Motor Mounts (3pc M5x35mm, 2pc M5x10mm):

4WD Motor Mounts (6pc M5 x 30mm):

Assemble Steering (steering shaft)

1. Firmly bolt one half of each pivot to the rigid 300mm upright; one at the bottom, touching the "T" bracket you installed earlier, and one at the very top.

   1. (Try to keep the pivot's screws facing upward so they are less likely to be lost if they come loose.)

   2. (If two small tabs are in the way, you can file them down or break them off with pliers.)

2. Loosely attach the other sides of the pivots to the 400mm beam. 

3. The bottom pivot can be locked into place with its bottom edge placed exactly 40mm from the end of this 400mm beam.

   

4. Bring these two beams together to align and lock down the final pivot piece and set it aside.
   

5. Grab your "control arms L/R" and slide a single 30mm M5 bolt up through the overlapping holes on the larger ends. Loosely screw on an M5 hex nut.

6. Screw that all into the bottom tapped hole of the 400mm beam and tighten the jam-nut against the extrusion, ensuring the steering can still pivot easily and that there is a ~5mm gap between the control arm and extrusion.

7. Reassemble the pivots with their included countersink screw; mating the steering shaft with its support.

8. Use 30mm M5 bolts and nylock nuts to screw the control arms to the spindle's arms. Apply a bit of grease where needed so they pivot freely.

Steering Wheel (Electronics)

A blank steering wheel model is provided to suit your own switch/pot/indicator component inventory. However the "18mm 1k Ohm Linear Potentiometer, 10mm stroke." and " 4.5mm by 20mm compression spring" from a ball point pen are required. A tiny bit of hot glue is also required. 

1. Solder a wire to each of the three pins of the 18mm slide pot for later use.

2. Spread the four mounting pins of the slide pot slightly outward so it has friction in the next step.

3. Fully and evenly insert the slide pot into the cavity on the rear of the steering wheel print.

4. On the front of the steering wheel, press your "thumb throttle nib" onto the nib of the slide pot.

5. Insert your pen spring into the gap between the nib and spring-stop hole.

6. Ensure the slide pot is still fully seated then verify the nib slides easily and returns to the zero position.

7. Once satisfied, fill the back of the slide pot cavity with hot glue. 

8. You can also secure the printed nib to the slide pot's stem with CA glue for added security. 

9. Test again to ensure reliable operation of the throttle.

10. Attach brake button, direction switch, speed governor pot, and any other switches/indicators you need. A simple schematic is shown in the next part.

11. Finally, attach the printed steering wheel to the steering shaft with a 25mm M5 from the top.

Traction Electronics

This will be specific to the basic 6-60V motor controllers we used. You are encouraged to study how these or your own motor controllers should be wired to these hoverboard wheels. Note that the phase order of the hall sensors and motor windings is always going to be critical for proper operation and rotation direction.

WARNING: Prop up your entire kart and keep clear of the wheels. They tend to get excited at this stage. 

Tip: Reuse the inline XT60 power leads, inline motor bullet connectors, and motor hall sense connectors that likely came with your donor hoverboard. 

1. Mount a pair of motor controllers (ESCs) to the seat post under the seat using the included "ESC mount" printed bracket (remove the seat for better access).  

2. Mount your 40v tool battery pack to the center beam.

3. (Recommended) Solder in an inline 20-30A fuse/circuit breaker and power switch. (40-50A for 4WD)

4. Terminate the power wires to the ESC screw terminals

5. Terminate the A/B/C phase wires to the phase outputs of the ESC

6. Terminate the A/B/C/5v/Gnd sense wires to the sense inputs of the ESC's

7. Route a long wire from your steering wheel controls to the control pins of the ESC. Terminate them according to the schematic below. All ESCs will be controlled in parallel however 5v only needs to come from one. 

Tighten every screw

I know it was easy to build when every screw was loose, but I guarantee you forgot to tighten some of them. It's time to tighten every screw again to a "firm grunt" level of tightness. Now you can go ride!

Upgrades

PVC Drift Slicks: 6" SCH40 PVC end caps can press-fit on the 6.5" tires with a bit of force and a spritz of rubbing alcohol or soapy water. If the fit is too loose, add a few layers of electrical tape. Some types of PVC end cap may require a bit of rubber to be worn down first. 

Add a couple 2" long drywall screws through the edge and into the rubber tire for added security.  

Tip: drill a 3mm hole in the middle so you can use compressed air to remove the end cap easily. 

Carry Handle: off-the shelf 2020 series handle works great on the front bumper.

PatchKart = Phys (me) + Mismatch (friend) Kart; i.e. the guys who designed and built this thing.
It's also a patchwork of parts you may already have in your workshop.
It's also small, like like a patch or patch cables.