Tow Truck: 12 Pounds of Lego Technic

Cyrus Tabrizi, 7/10/12
In the Works (25)

      The base of the truck has four axles, each consisting of its own steering and suspension assemblies. In this, nothing really varied between the axles. One difference is that the rear two axles were flipped around, putting each of their steering racks behind their respective axles whereas the front two axles have their steering racks in front of their suspension assemblies. All eight wheels were steered via one axle that ran along the very bottom of the undercarriage up until the front of the cabin so the rear two axles were flipped to have them steer in the opposite direction as the front axles, improving the truck’s steering radius. Originally, I was planning to use the only four XL motors that I have for the sole purpose of powering the truck’s drivetrain – this would mean giving each axle its own motor. I knew that I would have to power the steering with something that had a lot of torque so I tried gearing down an RC motor with a worm gear and some 24t gears but I found that the setup would not be enough to steer the entire truck through that one axle. Instead, I decided to drive the rear two axles with a single XL motor and use the fourth XL motor as the steering motor. This remained at the front of the truck below the opening cabin and fake V8 engine (behind the V8 engine, there is a small V2 that could almost be thought of as a fanciful turbocharger of some sort).

Tow Truck (18)

      In addition to having motorized drive and steering for all eight wheels, all eight wheels have adjustable suspension. This is done in nearly the same fashion as is done in the 8297 Offroader where two 2L beams (hole and axlehole) are attached to each shock absorber. At the hole-end, they are connected to the shock absorber by a 3L pin, and, at the axlehole-end, to a motorized axle. There are four such axles, two in the front and two in the rear. Each axle handles two wheels and each pair of axles is rotated via knob gears and a worm gear. The worm gear in the front and the worm gear in the rear are, in turn, driven by a single axle, connecting the front suspension to the rear.  This axle is driven by one of the truck’s three gearboxes. When this function is selected, the output rotates, slowly adjusting the angle at which the shock absorbers are mounted to their respective steering arms and raising or lowering the steering arms as well. This actually does two things: it adjusts the riding height of the truck and it adjusts the stiffness and travel of the vehicle’s suspension – this is all the result of changing the geometry of the shock’s mount. Although each of these characteristics cannot be adjusted independently of each other, the vehicle’s ride height can be adjusted despite the weight that the suspension is supporting. The only condition here is that the ride height can only be raised when the vehicle is off the ground – the mechanism responsible for adjusting the suspension does not have the power by itself to lift twelve pounds of truck. While it cannot accomplish this feat alone, it can, however, do so when done in conjunction with the extension of the truck’s four pneumatic stabilizers. When these two operations are run simultaneously, it is possible to remove the wheels from the ground and rest the entire weight of the truck on these four stabilizers. Doing this is no easy task and it requires a significant amount of time for the truck to lift, but, if conditions are right, it normally manages to do so.

Tow Truck (15)

      The pneumatic stabilizers consist of four pneumatic cylinders connected to a pneumatic switch in the compartment behind the cabin. This switch is controlled remotely via one of the outputs from the main gearbox (will be discussed again later). The gearbox outputs a rotational force which is transferred to a worm gear which is connected to the switch via some lift arms. To power the four cylinders, an RC buggy motor drives a worm gear which then spins a belt wheel connected by a pin to a pneumatic pump. This arrangement is not ideal and barely provides enough pressure to lift the truck but was picked because I had no pneumatic T piece to use a second pump (all of the ones I had were used to connect the cylinders in the first place). If I had had another one, I would have used two RC motors and two pumps – I actually tried this setup and it ran much faster and much smoother – the pumps were connected oppositely to lower the tension caused by the pumps’ coils – with only one pump, such an arrangement is not possible. To turn the RC motor and pump the cylinders, yet another output from the main gearbox was used. This output was connected to a PF switch which was connected to the RC motor via an extension cord – this cord was not needed as an extension but simply as an adapter for the old connection type present on the RC motor. All of this was housed in the compartment behind the cabin.