[MOC] Classic Dump Truck XL

JLiu15 · April 6

A sequel to my Classic Dump Truck from 2022, built at a larger scale. Features AWD drivetrain, a flat-six piston engine, oscillating rear axle and a pneumatic dumping mechanism.

Functions:

Drive (AWD)
Steering
Flat-six piston engine
Oscillating rear axle
Opening hood and doors
Pneumatic dumping

Ever since building my Classic Dump Truck MOC in 2022, I always felt like it still had more potential. I managed to pack plenty of functions/features into it, but felt like a larger scale would allow me to do more. I wanted to scale it around either the 52985 or 2491 tires. This would at the very least allow me to use wheel hubs, making an AWD drivetrain easier to incorporate as it was something missing from my previous, smaller version. I went with the latter option for the tires since I found their tread to be a good fit for trucks.

The building process started as a very compact build around the front axle. I used the Control+ large angular motor for propulsion, as it has a front and rear output which makes a longitudinal driveshaft possible while taking up minimal longitudinal space. I initially wanted to use planetary hubs, but found that the drive speed would be too slow. The final drivetrain design used normal hubs instead (the front hubs are the ones from the 42160 set so I could still drive them with heavy duty CV joints). The only gear reduction in the drivetrain is the 12:28 reduction at the differentials, and while it's able to handle some light off-road driving, it still felt underpowered especially when driving up inclines or over larger obstacles. Another stage of gear reduction could've helped, but that would most likely require moving the drive motor at least two studs up. A flat-six piston engine is placed just above the steering motor, connected to the drivetrain via a set of gears in front of the front axle. Directly behind the drive motor is the Control+ hub. Because the driveshaft to the rear axle passes under it, I used a long heavy duty CV joint to allow the driveshaft to be pushed into the extendable section of the CV joint and removed from the large angular motor's output. Removing the driveshaft here allows access to the battery compartment for replacing the batteries. The hub is reinforced using two 3x19 frames, and just behind the hub are two Control+ large motors placed side by side for the compressor and valve of the pneumatic dumping mechanism. All in all, I feel that the design made good use of every inch of space available. The large number of pin holes on the Control+ motors and hub also allowed them to be used as reinforcement points, which also facilitated the compact build.

It's been a few years since I used pneumatics on a MOC, and to give it a little something extra over my previous version, I decided to use pneumatics instead of a linear actuator for the dumping mechanism. I was able to fit three 6L compressors into the limited space just before the oscillating rear axle. In the Powered Up app control profile, I set up one slider to drive the compressors' motor with proportional speed and also the valve motor like a steering servo motor, returning to the center position when the slider is released. This closes the valve when the compressors are not running, preventing air from leaking. In fact, I was able to display the model with the bed fully elevated for several days without it collapsing back down. Because of the size of the dump bed, I used two 2x11 pneumatic cylinders instead of just one, as it offers more capacity. In the end, the pneumatic mechanism was able to handle a decent amount of load in the dump bed without the compressors struggling. I wanted the dump bed to be more detailed, but was worried that it may cause the pneumatic system to struggle with heavier loads so I kept it simple. Overall, the pneumatic dumping mechanism has been my favorite part about this MOC.

As for the bodywork, my favorite part is how the front grille and hood turned out. The grille and front bumper are built with System pieces as a single module, which allowed it to be more detailed. I also incorporated metallic silver elements for the grille, bumper, side steps and mirrors (although I had to settle for light bluish gray at certain spots where a metallic silver piece isn't available). The sides of the hood are also finished with System pieces. The hood and the mudguards from the 42114 set allowed for a good blend of System and Technic in the front. Because the grille/front bumper are built using System pieces, it is built at an even stud width, which made things trickier with the odd width of the Technic chassis. The doors maintain the even width, but the rear and roof of the cab are built at an odd width connected to the chassis, with tiles adding a little more width and also covering up pin holes. I feel the cab section could've turned out better, but still I've managed to put plenty of details in the cab including a steering wheel and bench seat. This is an improvement over the previous version, as that one lacked an interior. To access the power button on the Control+ hub, a lever is placed behind the seat to press the power button.

Overall, the model was an improvement over my previous Classic Dump Truck. It offered better functionality and a better designed bodywork, although there was definitely still room for improvement. The pneumatic system worked well and the oscillating rear axle allowed it to do some simple off-road driving. However, the model could've benefitted by having a little more torque in the drivetrain, as well as a more reinforced chassis as I found the chassis to still bend a little when the model is placed on the ground. This is especially noticeable with the cab and dump bed in close proximity to each other when the dump bed is lowered. Still, I'm pretty satisfied with how the model turned out, especially being able to fit all the electronics and mechanisms in such a compact way in the model.

Instructions will be published for this MOC on Rebrickable once they are finished.

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