2. This was one of our prototypes pulling the car, that's why it was 3D printed. The final production units use injection molded plastic parts that are more durable and easier for us to produce in high quantities. We are selling the Rover Mini as both a kit (injection molded parts included) and as an assembled unit. We have still chosen to publish the files needed to 3D print a full chassis on our GrabCAD page so people can print their own, but since they come with the kit, and we sell spares, and they are kind of hard to 3D print, we really only expect people to do this for the novelty of it.
Part 1: Printing the Robots
During the prototyping phase of designing the Rover Mini we heavily utilized our offices 2 Prusa i3 3D printers. Huge thanks to Prusa Research for making such a great product.
It takes about 20 hours of machine time with a 0.6mm nozzle to 3D print one half of the Rover Mini's chassis. We decided to make both halves of the chassis identical to lower the cost of injection molding tooling. So to 3D print a robot you need to print the same file twice for a total of 40 hours of machine time.
After printing the two sides of the chassis you then assemble the robot together. Each Rover Mini gets 4 hubs motors, 4 ESCs (shown in green below), a main logic board (shown in blue), and a battery pack. The robot is controlled by the main logic board which has Bluetooth, Ethernet, and USB as communication options.
The two halves of the chassis are then secured together with some long bolts. The stems of the wheel hubs are sandwiched between the 3D printed pieces. To keep the stems from spinning they each have a flat side that matches with a flat on the 3D printed part. The end result of all this is a 3D printed robot that can support 150lbs before flexing and about 300lbs before breaking.
Part 2: The Physics of Towing a Car
Lots of people ask us if this video is real or not, and our answer is it 100% is, no trickery involved. So to dispel any doubts, here is a deep dive into the physical characteristic of the Rover Mini that make it capable of towing a car.
It takes 200N-250N to pull a car
Towing a car is all about the initial force to get the car rolling. It takes around 200N-250N to get a normal sized sedan rolling on flat ground and around 150N to keep it rolling. We researched these number before the test and verified them to be accurate using a pull gauge. Our test car had pretty flat tires that we didn't bother to inflate and so it was on the upper bound of this range, measuring in at 240N.
The Rover Mini is equipped with x4 hub motors that can each output 250W of power for a combined 1000W or 1.33 horsepower. Each hub motor has a peak stall torque of 11Nm and a diameter of 0.08 meters. Using the below equation we can work out its peak towing force when all 4 motors are working together.
Towing Force = [# of motors] * [stall torque] / [wheel radius]
 * [11Nm] / [0.08] = 550N
550N > 250N
As you can see the peak towing force of the Rover Mini's hub motors is much greater than the force needed to tow a car, but there are other limiting factors discussed below.
So you may be asking yourself "if the peak towing force is over double the amount needed, why did the robot struggle so much in the video". The answer to that question has to do with the electronics of the Rover Mini. In order to output 11Nm of torque, you need to send 35A (at ~40V) to each motor. That's a lot of amps and the Rover Mini runs on a battery which is asking a lot of those 18650 cells. By default the Rover Mini ships with a 10S1P battery pack capable of delivering 100A total. When we performed this test with the stock battery the 25A per motor was not enough to tow the car. Luckily we built in a feature that lets you to add an external battery pack to the Rover Mini. This allowed us to hook up a second 10S1P pack to double the current output to 200A. This allowed each motor to receive its full 35A which brings us to our final problem.
Remember when we mentioned our motors are rated for 250W. That's a max continuous power rating set by the manufacturer because if run continuously above that power level, the motor will heat up above the melting temp of the glue that holds the magnets into place. We know because we tried it. The 35A at 40V needed to produce 11Nm of stall torque requires a massive 1400W, well above the rated 250W. Luckily to tow a car you only need that peak torque for a short period of time.
You can tow a car with a 3D printed robot and we do encourage people to try this yourself, its a very fun and rewarding project. However if you do try this at home please be very careful. Make sure to constantly be checking the temperature of the robots battery and have a fire extinguisher handy just in case something goes wrong.
Good luck and Happy Roving!!!