A tiny monster truck, features all wheel drive and steering, torsion bar suspensions, changeable gear reductions and power source, and openable hood, with building instructions.
I wanted for a while to build a tiny monster truck with the big wheels of the Lego Xerion with a special color scheme. The first idea was to use lime and blue color from the lego #42037 but I had not enough parts in correct color to have a good result, that is why I changed the idea and try a different approach with a red chassis, white body and some touch of lime, but without neglecting the bodywork.
I have chosen to design the chassis in red color, like the iconic lego crawler #9398, using one L-motor on each axle with differential, with a simple but effective drive train. The use of portal hub enables the easy change of the gear reduction, to have a maximum torque (8/24t) or maximum speed (16/16t). It was an important thing for me for the drive train to be easy changed.
The steering is operated by a servo motor, placed centrally in the chassis, and uses the ball-join frame to passes the axle of the steering. As the servo motor is not directly on the steering axle and uses 16t gearing, I have to use a small trick to avoid miss-alignment of the steering, by shifting some tooth of the 16t gear to have the correct tooth of the 12t gear on the rack. (you can see it on the building instructions).
The suspensions are based on the same design like my Mongo Truck, the CVT truck, or orange trial crawler since lego have released the ball-join hub with frame. Therefore the axle are composed of a ball-join hub frame, with two links on the side but the shock absorbers are not made by using springs, but torsion bar. A link connects the axle with a lever, which is mounted on an axle which can not rotate. The suspensions are therefore create by the torsion of the axle. It enables to have adjustable soft suspension (by moving the fixed position of the axle) and more compact than a spring for vehicle in small scale. But this suspension can not handle a drop of the vehicle without end stop to prevent breaking the axle.
Regarding the design of the bodywork, as I mentioned, I wanted to create a tiny truck with good mechanical and simple design but also a nice bodywork. I have chosen the white color for the bodywork which fits nicely with the red chassis, and some touch of green using the lime soft axle, but you can of course consider every color you want as the design is pretty simple in the parts used. I also wanted to put some nice details in it to reinforce the playability, like the possibility to choose the power source. My model uses standard battery box with IR received V2 but a buwizz of a lipo with sbrick can also be installed. The last detail is about the openable hood which reveals the big V10 block (not working sorry) and some others details like a dashboard and exhaust.
To finish with the trial truck abilities, there are good, despite the fact there are differentials on each axle and the wheel base is short, but the truck has a low center of gravity, a good torque on wheel (with the 8/24 portal hub), good traction with the tires and the responsive suspension and small turning radius. Of course it can not compete with heavy slower truck with many motors but it is very playable and fun, especially if you choose the buwizz of sbrick solution to avoid the IR detection problem under the sun. To sum up, this is a fun and powerful tiny monster truck, which can be modified by the user (changing power supply, gear ratio, color scheme), that is why I have created a free building instructions based on reverse photo sequence !
You can see the bill of material here :
or watch it on rebrickable :
You can download the PDF of the building instructions here : LegoMonsterTruckInstructionsByNico71.pdf (25mo) or browsing here :