Tatra T-813 8×8 Kolos
Model of a legendary Czech heavy-duty truck. Features realistic suspension, drivetrain and steering system, V12 engine with moving pistons, working steering wheel, lights and openable tailgate.
Datasheet:
Completion date: 29/06/2018
Power: electric (Power Functions)
Dimensions: length 56 studs / width 17 studs / height 22 studs
Weight: 1.72 kg
Suspension: Tatra suspension
Propulsion: 2 x PF XL geared 3:1
Motors: 2 x PF XL, 1 x PF M, 1 x PF Servo
Tatra T-813 Kolos is a Czech heavy-duty truck famous for its off-road performance and popular with armed forces around the world. It’s also extremely popular with LEGO builders, resulting in hundreds of LEGO T-813 models, most of which fall in one of two categories: very big and complex, or small but ignoring the Tatra suspension and drivetrain design. I wanted to create a model that fit between these two categories, and the whole idea was started by playing with 3×3 braces and new wheel hub pieces. I wanted to see the narrowest axle possible with the unique Tatra drivetrain, suspension and steering that I could make using these pieces, and the model was the result. To put it simply, my goal was to make the smallest possible model with realistic Tatra drivetrain, suspension and steering.
The resulting model was close to 1:18 scale (significantly smaller than the 1:13 scale typically used in LEGO trial trucks) and was fitted with drivetrain and suspension that worked just like in the real Tatra truck. The steering system worked a little differently, but produced very similar effects as the original Tatra steering. The axles were just 15 studs wide with wheels and tires included. Speaking of tires, the model was a test bed for the new kind of third-party tires: the RC4WD Rock Crusher tires reduced to 1.2″ size to fit the LEGO 43.2mm D. x 18mm wheels.
The model was all studless except for the outer shell of the cabin. The two front axles were steered using a PF Servo motor located above the third axle, and the steering was intentionally limited to reduced the stress in the drivetrain. I was afraid that such a stress would be critical, since the model is long but doesn’t have a single differential, and each wheel is driven using a small, thin 12-teeth single bevel gear which is prone to breaking (plus the very grippy custom tires added extra stress since they were less likely to slip than LEGO tires). I wanted to avoid steering sharply, as it would most likely destroy the gears – by the way, I did destroy a single knob gear in the truck’s drivetrain while testing it. The propulsion was provided by two hard-coupled PF XL motors located vertically between axles #2 and #3. The model was so small that there was really no way to put the motors horizontally – instead, I’ve made use of the cargo space. The suspension was using the long 9.5 shock absorbers: soft ones on the two rear axles and medium ones on the two first axles. This resulting suspension proved very responsive, with long travel and great softness.
The rear part of the truck was mostly taken up by motors (all four of them), and covered up using a piece of canvas mounted on a complex frame of angle connectors forming “ribs”, just like in the real truck. There was also the LEGO 8878 rechargeable battery located between axles #3 and #4 and turned on or off using an unobtrusive switch on truck’s side. Another switch on the other side of the truck activated a PF M motor which was driving a miniature V12 piston engine located in front part of the cabin. The V12 was a modified, shorter version of the piston engine I’ve used in my T-34/85 tank model. Next to the V12, there was a working steering wheel, and the cabin was also fitted with LEDs in the headlights and blinking LEGO 9V lights on the roof. In the back, there was a pair of tail lights with LEDs in them, as well as a tail gate that could be opened manually. For a time I was toying with the idea of adding a motorized winch, but in the end I’ve decided that there is simply not enough room for it, with the entire front of the cabin being taken by the V12 and the steering wheel mechanism.
In the end, the model turned out heavier than I anticipated, and its relatively small scale came at a cost: it was simply too small to handle the same obstacles that a full-sized LEGO trial truck can handle. I was also afraid of destroying the single bevel gears in the drivetrain, so I didn’t put too much torque in it, which is why the PF XL motors were geared down only 3 times. It was sufficient for some moderately-sized obstacles, but not enough to e.g. climb up a steep ramp. The truck could really use portal axles, they would be great for reducing stress, but it wasn’t really doable at that scale. I really liked look of the truck, but it was largely helped by non-LEGO tires, and the truck generally appeared too tall because of the increased suspension travel (as compared to the real truck). On the bright side, the suspension was a delight to watch and it operated against the obstacles beautifully.
