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Ural 375D

April 21st, 2021 Leave a comment Go to comments

Model of a Soviet heavy-duty military truck. Features 6×6 drive, steering, suspension, remotely controlled transmission and remotely locked differentials, rear winch, a V8 piston engine, lights and LED beacons.


Completion date: 21/04/2021
Power: electric (Power Functions) / pneumatic
Remote control: SBrick
Dimensions: length 86 studs / width 30 studs / height 32 studs
Weight: 3.94 kg
Suspension: axles #1 and #3- live axles, axle #2 – pendular
Propulsion: 4 x PF L motor via a 3-speed remotely controlled sequential transmission
Motors: 4 x PF L, 2 x PF M, 1 x PF XL, 1 x PF Servo

The Ural 375D, which I had the pleasure of driving last year, is a heavy-duty military truck that started serving with Soviet Army in early 60’s. It has undergone a series of upgrades and was modified into dozens of variants, and it managed to become legendary for its toughness and its cross-country capabilities which rival many modern vehicles. It’s quite possibly one of the most capable regular wheeled vehicles in existence, and its simplicity and crudeness translate perfectly into reliability.

My goal when building a model was to perform a serious stress-testing of the LEGO planetary wheel hubs, the two-piece differentials, and of my own 4-speed sequential transmission. The completed model has fulfilled all these goals, but at the cost of compromised look and somewhat convoluted construction.

A sum of design choices has made this one quirky model. Most importantly, it was truly slow because of the axles which combined the planetary wheel hubs (5.4:1 gear reduction) with the two-piece differentials (2.33:1 gear reduction), resulting in a massive 12.6:1 gear ratio just inside the axle. That meant a lot of torque but not much speed. For that reason I’ve abandoned my original plan of driving this model with PF XL motors – they were simply too slow – and used four PF L motors instead. Still, the truck was fairly sluggish even at the highest gear. The transmission technically featured 4 gears, but the lowest gear was unused. This was so that I could use a PF Servo motor to select gears (this motor only has 3 positions, not 4) – I could have used a stepper mechanism, like the one I’ve developed recently, to shift through all 4 gears, but this would require checking visually which gear I’m on and how far the stepper has revolved, and the lowest gear was ridiculously slow anyway (with a gear ratio of 63:1).

Another design choice was making sure the ground clearance between axles #1 and #2 was maxed out. To this end, I have built the truck around a massive frame that was located fairly high and had the entire suspension located below it. The suspension consisted of live axles in front and rear, and of a pendular second axle. It was an unusual solution, but I was worried about the rigidity and stability of the second axle when turning under such a large, heavy model, and it was certainly not as good for off-road performance as another live axle, but it kept the truck stable, enabled it to steer surprisingly well, and maintained a constant ground clearance.

Finally, I have chosen to place the drive motors on the sides of the lower part of the cabin, covering them in black tiles pretending (poorly) to be toolboxes. I could have installed them at the back of the cargo hold and keep the cabin looking good, but I really wanted to make the V8 piston engine run at constant speed regardless of transmission, like a real engine does, and this would make it fairly complicated. So I went with the following drivetrain arrangement:

All three axles included pneumatic differential locks that could be operated remotely using a compressor/valve unit inside the cargo hold. Locking all three differentials at once proved very efficient at overcoming slip situations, as demonstrated in the video below. The front axle was steered using a PF Medium motor in the chassis, connected via an axle sliding into a CV joint, allowing a certain range of sliding movement as the suspension was compressed and uncompressed. In order to prevent the axle from slipping out of the CV joint, I have installed two short LEGO strings connecting the front axle with the chassis and limiting how far the suspension could uncompress. The single steering shaft was split in two inside the front axle, so that there were actually two adjacent pinions on a single rack. This allowed to reduce the amount of gearing in the chassis by moving it inside the axle.

There was one last aesthetic compromise: because the V8 piston engine was longer than the real trucks engine, and because I’ve installed complex thick grille in front of it (I really wanted to make the grille see-through, with engine fan being visible behind it), I found it necessary to move the cabin roughly 1.5 stud forward. As a result, the front axle looked like it was moved too far back, but actually its position was correct, it was the cabin that was moved too far to the front to accommodate the engine.

Building in dark green was traditionally difficult, with e.g. 1×2 tiles being sold for as much as $5 each, and with e.g. 1×8 and 1×10 plates only released in a single LEGO train set from 2004 (so, basically was too expensive for my budget). Then there was the very limited selection of hinges available, and a complete lack of any dark green bar pieces.

The model was completed by 1.9″ Rock Crusher tires from RC4WD on LEGO rims, providing more realistic proportions and much better grip than LEGO tires would, and by Brickstuff LED beacons on the roof, activated by turning on a separate LEGO 8878 battery. Finally, there was a single-piece canvas cover, which was admittedly fairly crude.

To sum it up, the model performed quite well given the complex drivetrain – it proved capable of driving over a 6 cm thick book or climbing up a nearly 20° incline, and all of it with nearly 1 kg weight per a single PF L motor. This performance should be mostly attributed to the use of planetary wheel hubs, but I was happy that my transmission endured. The look of the model was largely compromised though, and some design choices turned out to be questionable.

Work in progress photos:

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Media coverage:

The LEGO Car Blog

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