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This model was built for essentially one purpose: To build a model with a functioning four-speed automatic gearbox, using PU programming.



4-Speed gearbox with PU L-motor

Steering with PU L-motor

4WD with 2x PU XL-motor

Dual live-axle suspension

Opening hood


More details:


The real vehicle is quite boxy, making it easy prey for reluctant bodywork-builders such as myself. I think my version was easily recognizable, which was good enough for me, but not exactly excellent.







Steering is a basic rack-and pinion setup with a PU L-motor. The app provides return-to-center steering.





Two PU XL-motors drive all four wheels. The front axle uses a new style differential, while the rear one uses a perpendicular 12T to 28T gear meshing, which is a good compromise between off-road capabilities and handling.







There is full live-axle suspension, using ball joints and shock absorbers. The system worked fairly well.





Opening hood:





The gearbox was a fairly basic four-speed design using a rotary catch. The only interesting thing about the gearbox itself is that I designed it such that the four speeds were not actually in sequential order, which made for a significantly smaller, more efficient gearbox. This lack of sequentiality was not an issue for me, because I knew it would be electronically controlled, which could mask this mechanical shortcoming. The programming was definitely the most complicated part of the gearbox, and allowed for both manual and automatic modes. In manual mode, one could toggle up and down gears with a pair of virtual buttons, which worked quite seamlessly! The automatic mode would essentially keep tabs on the speed of the drive motors (actually an average over time), and if their speed went above or below specific limits (relative to the commanded speed of the motor), the gearbox would then shift to offset this. In practice, the shifting was rather sluggish, and is certainly not a good choice from a performance point of view, but it would eventually get around to the right gear for crawling or cruising, if one were patient. Shifting was mechanically controlled with a PU L-motor, which drove the wave selector through a 20:12 gear ratio (chosen because of space considerations, rather than torque/speed ones). The wave selector also had a stepper (?) mechanism attached to help it snap into position.



The program was quite complicated, so I won't go into it here, but there is some explanation in my video, and I would be glad to answer questions or provide screenshots of the code if requested.


Overall, I consider this model a success. It combined decent looks with considerable speed, tolerable off-road abilities (Ground clearance seriously hurt this), and a reasonably well-sorted automatic gearbox!

Images at:


Edited by 2GodBDGlory

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5 minutes ago, msk6003 said:

Non-sequential gearbox idea is great!

Thanks! It saved adding three gears, which is definitely a win.

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Nice model! The bodywork isn't super amazing, and the wheels look a little small, but it's still quite recognizable. The suspension looks a little flimsy, but it's also compact. It's great to know that non-sequential gearboxes can be programmed into PU, congratulations on putting in the effort there. Any idea what the approximate ratios are in the gearbox? It's kind of hard to see what the gear meshes are in your pictures.

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The gear ratios would be (provided that the inputs and outputs are set up to provide no overdrive) 1:1, 1:1.67, 1:2, and 1:3.33

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