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I would like to present my Heavy trial truck. It is indeed the result of many attempts that began with a 6x6 heavy truck to this version, with one less axle. But in the middle, there was two additional versions that, maybe, I would refine and finish for a functional MOC. For the truck, I wanted a functional “Dynamic automatic gearbox”-driven heavy vehicle, featuring a significant body resistance, differential locks (at least, front and rear axles) and a good geometry for climbing obstacles (including front and rear attacking angles, big ground clearance, and low gravity center). And this is the result. More images at Bricksafe: https://bricksafe.com/pages/HectorMB/heavy-trial-truck-public Features (motorized and powered by a BuWizz 3.0 unit): Dynamic automatic gearbox (powered by 2 L PU motors) Differential locks (front and rear axle) (1 L PU Motor) Steering (1 L PU motor) and moving steering wheel Features (manual): Linked suspension with centering system Opening doors, openable roof window (which also serves for an easy access to ON/OFF switch), and 3 openable engine doors Effective ground clearance of 7 studs Detailed cabin Gear ratio from (continuous) 10:1 (177 Ncm) to 90:1 (1600 Ncm) (Calculated by Sariel's app) Video 01 Video 02 Features (video) Features description: Dynamic automatic gearbox: It was the starting point of this MOC. Since I found them, I was very interested in the potential of the differential-based automatic gearboxes. I found that for heavy models, it was not very easy to use. In this case, I opted for doubling the gearbox to avoid any cracking when stepping up obstacles as well as increasing the resistance of the “high torque/low speed” pathway. After the two gearboxes, all the power is driven through a common central axle. After several testing, I found that the resistance should be proportional to the difference between the max and the low gearing; the bigger the difference, the bigger the resistance. The problem is that increasing the resistance, obviously, decreases the efficiency of the system… but, by now, I did not find any other solution. With this gearing, and the vehicle geometry, the truck can climb up to 42º steps. Suspension: Although not motor-driven, this was an aspect that gave me several headaches. The starting idea was to have a suspension on which when a wheel is elevated because and obstacle, it benefits from the suspension of the opposite wheel, increasing the ground adherence. In practical terms, this means that you can rise one wheel up to 10 studs keeping all other three wheels still touching the ground. An issue with this system (and having only 2 axles) is that the body of the vehicle tends to stay tilted after overcoming obstacles. To solve that, I included an extra pair of shock absorbers that, instead of being supported in the opposite wheel, they are attached to the body of the truck. Differential lock: Because of the geometry of the vehicle, front and rear differential locks were not indeed and option but obligatory. Indeed, the turning radius turns from 90 cm (diff lock closed) to 47cm (diff lock open). Although it is driven by a single motor, thanks to two clunch 24 gears, it is ensured that, despite a significant different backlash, both locks perform perfectly fine. Steering: About this feature, the major challenge was to minimize as much as possible the backlash of the steering rack. I even considered including an actuator-based steering system. Unfortunately, for this model, I could not manage to make it happen. In exchange, the steering rack is designed to reduce the backlash in any direction. Openable doors: In total, the model has 6 openable doors. 2 that access the pilot and co-pilot positions, 2 laterals that allow to see part of the gearboxes (and how they work in flat and step terrain), 1 rear door that access the rear differential lock and 1 last roof door that, besides the appearance, it allows an easy access to the On/Off switch. The instructions are available in Rebrickable: https://rebrickable.com/mocs/MOC-100386/HectorMB/heavy-trial-truck/#parts Many thanks for the reading and please, leave your comments and feedbacks! :)