Kettenkrad
Model of a German WW2 half-track motorcycle. Features drive and steering with an automated braking system.
Datasheet:
Completion date: 20/10/2024
Power: electric (Circuit Cubes)
Remote control: Circuit Cubes
Dimensions: length 22 studs / width 8 studs / height 12 studs
Weight: 0.265 kg
Suspension: none
Propulsion: 2 x Circuit Cubes regular motor
Motors: 3 x Circuit Cubes regular motor
The Kettenkrad, or Sonderkraftfahrzeug 2, as it was formally called, was a legendary WW2 German vehicle because of its uniqueness. Designed as a light tractor for airborne units, it combined front wheel of a motorbike with tracks of a small tank. It was known for its reliability and cross-terrain capability, but also for its complex steering system which worked like a regular front wheel of a motorcycle to a certain point, but when the handlebars were rotated beyond that point, the track on the inner side of the intended turn would get brakes applied to it automatically. I was interested in re-creating this steering system in a LEGO version, and after noticing that the Kettengrad’s road wheels look very much like LEGO wedge belt wheels, I set out to build a model scaled to these wheels.
The wheels dictated a very small scale, with a 7 studs wide chassis and with tracks being just 1 stud wide each. I have used LEGO chains for tracks and – since the LEGO power supplies and motors were far too large here – turned to Circuit Cubes in order to power the model and control it remotely.
I have succeeded in building an automated steering system that produced similar result to the real one, although without using brakes. In my model, the tracks were driven via two separate LEGO transmission driving rings, with each ring being kept engaged by a rubber band. Once the handlebars would get rotated sufficiently far, the bumps on them would press on a slider on the inner side of the turn, disengaging the corresponding driving ring and thus disconnecting the track on this side from the drive motor. Once the handlebars returned closer to the central position, the rubber band pushed the slider back and the ring was engaged again, driving the track just like before.
On the aesthetic side, the model looked pretty, and I mean pretty terrible because of being entirely filled with mechanisms (16 gear wheels total). I was able to keep the chassis 7 studs wide, but in order to fit everything in, I had to make the model about 1 stud taller than it should have been. The bodywork was also extremely fragile and the drivetrain wasn’t much stronger. To make things worse, the front section of the tracks didn’t touch the ground because I have built the chassis first with a smaller front wheel, which I then replaced with a more accurate, larger one, and adjusting the chassis for the difference in their sizes would make the front of the model even taller. The final front wheel used the LEGO 30.4 x 14 VR tire, which I found to be exceptionally soft, pulled over a LEGO large steering pulley. This worked quite well and the tire wasn’t damaged or misshapen as a result.
As for the performance, the model was very slow because there was only one gear ratio that fit inside the chassis, and it suffered from low traction – so low, that when I attached a trailer with a GoPro camera on it to the model, it was unable to pull it. But the steering system worked flawlessly and efficiently – in fact, it was very responsive to even slight motions of the handlebars, even before one track or the other would get disengaged.
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