The 3D-Printed Mini Semi-Truck
The Highlights
The project began as a fun idea to create a toy RC semi-truck, but evolved from there into something much more. The truck includes many scale components to illustrate how the mechanical components function, namely the differentials.
The problem with the usual RC components is that the toy no longer drives like a full size truck - it accelerates too quickly, doesn’t have shifting, and has a roll stability limit that is far too high. To work around this, I added a Raspberry Pi and virtualized some of the full scale truck driving experience.
There are many more features planned on the road map for this project including mechanical brakes, yaw rate-based steering, and a locking differential, to name a few.
The Mechanical Design
Visually the truck project is designed to aesthetically and in some cases functionally represent a full size vehicle. There is a pitman arm steering system, leaf springs, a herringbone gear reduction, and tandem rear differentials.
System Components
Steering Gear
The steering system is configured as a Pitman-arm, drag-link configuration. These have been used in vehicles of the early 20th century for their simplicity, high mechanical advantage, and robustness. They’re still the primary steering design for semi-trucks today!
Gearbox
This is a simple gearbox with a total mechanical advantage of 9.4:1. The gearing is designed to reduce the motor speed to keep the top speed of the motor close to the scaled max speed for a truck driving at highway speeds, around 65 mi/hr (~30 m/s).
Steering System
The front steering system is only actuated on the driver side via the Pitman arm from the steering servo. In order to actuate the passenger side tire, the steering spindle are connected via a ball-joint tie-rod.
The wheel end assemblies capture some skateboard bearings and use m3 bolts as functional lug nuts.
Tandem Differentials
The truck is configured as a 6x4 - that is a vehicle with 3 total axles and two of them driven. In order to improve the general drive ability, the differentials allow the truck to turn without scrubbing the tires. The differentials use a spiral bevel gear configuration with lots (10) bearings per differential to keep them running smoothly!
The ratio of the pinion-ring gear pair is 2:1, giving the powertrain a final mechanical advantage of approximately 20:1.
Software
In order to get the vehicle to drive like a full scale truck would, there are some artificial limitations placed on the response of the vehicle. For example, if hitting the gas at full throttle, the small dc motor can accelerate the truck way faster than in reality. Instead, the simulation estimates how a real truck would response, scales down the values based on size of the toy truck and uses a basic feedback controller to keep the toy matching the target values.
Future features include gear shifting, coasting, engine brakes, and other fun aspects of full-size trucks.