August 9, 2013 Leave a comment
I have said it before, but my favorite things to build are Trial Trucks. The combination of the driveline construction, forces on the truck, diversity of body style, and various propulsion systems offered by LEGO combine for a great building experience. Because of this, I usually am building a Trial Truck, or have one built at all times. But for some reason, this truck seemed to sit for a long time unfinished. I struggle with deciding if a truck will be a model of something, or something fictional. This decision is often made too late in the construction process. After toying with a Daimler Scout body, I decided I needed to finish this project and the Bedford MWD body was chosen.
The full gallery may be seen here.
After some some experience with various designs, I decided to construct a truck around a simple locking differential idea I had recently designed. Because I would need an extra IR Receiver for the locking function, I decided a simple two speed gearbox (1:6 and 1:10) could use the other IR channel. I placed all the controls in the middle of the chassis. The driveline and the steering axle would run through the middle. On the left side was the Battery Box and the motor for the gear change, and on the right side was the XL drive motor, the gear box, and the motor for the locking differential mechanism. The steering motor would hang out the back of the chassis over the rear pendular suspension unit. Both axle were connected by my favorite linked suspension system.
Each axle took a little bit of work. I selected a simple design for the locking differential. Basically, it is a 24 tooth differential placed directly next to a 24 tooth gear. A pair of sliding 12 tooth double bevel gears slide back and forth one stud to connect only with the differential, or with both the differential and the neighboring gear locking out the differential. After toying with some old flex cable, and some pneumatics, I figured I was making it too complicated. I added a small pivot with a Small Technic Steering Arm, and connected it to a 9L link. This way both axles could be connected, the suspension and lock could keep operating unaffected by each other, and it all could be controlled by a mini Linear Actuator.
Initial tests were positive, so I then decided to figure out a body for the design. I worked for too long on a Daimler Scout body. I had the structure made, but the paneling was just not happening. After sitting on the project for 5 months, I decided it was time to make something new. The Bedford design worked well, and helped my get excited again in the project.
Now, once I got outside to drive the truck a glaring problem occurred. The bevel connection in each axle that transmits the longitudinal drive forces to transversal drive forces kept slipping. You can hear it in the video. Because of this, it did not matter if it had locking differentials, or if it had a two speed gearbox, or if it had working suspension. Anything could stop it. I though about reworking the axles, but then, I have been working on this for 11 months, it was time to be done. I’ll use the locking mechanism again. That worked great.
Thanks for reading.