Sariel.pl

Model of an experimental US Army truck. Features 8×8 drive, steering, 2-speed transmission and 2-stage suspension.

Datasheet:

Completion date: 23/09/2024
Power: electric (Power Functions)
Remote control: SBrick
Dimensions: length 69 studs / width 28 studs / height 29 studs
Weight: 2.873 kg
Suspension: 2-stage double wishbone dependent
Propulsion: 4 x PF L motor via a 2-speed transmission (1st speed gear ratio: 16.2:1, 2nd speed gear ratio: 5.4:1) on all wheels
Motors: 4 x PF L, 1 x PF M, 1 x PF Servo

This model was a follow-up to my MAZ-535 model from 2019. I was unhappy with MAZ’s performance, especially with its overengineered pendular axle suspension, and I wanted to use new types of LEGO pieces to improve on it. To that end, I kept looking for a short 8×8 off-road truck with good approach and departure angles, but most of the trucks of this kind are Soviet-made and for the time being I wanted nothing to do with Russia. Eventually I stumbled upon this rejected project for US Army, which was interesting but extremely limited when it comes to available documentation.

The XM376 and XM377 were two parts of a single transport solution submitted to US Army in 1957, with the goal of facilitating transportation of tanks over rough terrain. The idea was that these two 8×8 heavy-duty off-road trucks would be connected by a suspended trailer with a tank resting on it. To make this work, the truck at the rear – the XM377 – had its cabin rotated front to back, facing towards the trailer rather than away from it. The whole setup was operated by two drivers, one in each truck, coordinating over radio. The project was rejected after field trials which proved that the trucks with the trailer weigh almost 3 times as much as the tank itself, need more fuel than the tank does, and require enormous amounts of free space to maneouver. It appears that there was only one of each trucks made and that they were both scrapped some time later, with very little documentation left. In fact, all I was able to find were three photos.

Unlike my MAZ-535, the model was built with double wishbone suspension, utilizing the wheel rims introduced in the 42110 Land Rover Defender set to reduce its width. In order to increase the suspension travel and make the suspension more responsive, I didn’t use the regular independent wishbone suspension setup where each wishbone is supported by a shock absorber connected to the chassis. Instead, my model had each wishbone supported by a shock absorber connected to the shock absorber of the corresponding wishbone on the opposite side (left/right). This allowed the suspension to work in two stages: one stage where wheels moved by raising on one side and lowering on the opposite side without compressing the shock absorbers, and second stage where the shock absorbers would get compressed. This resulted in a significantly increased suspension travel, because it wasn’t limited by the shock absorber’s range of compression, and in increased responsiveness, because the wheels could react to loads that were too small to compress the shock absorbers. The downside was inferior sideways stability – with a center of gravity sufficiently high above the ground, it was possible for the chassis to tilt left or right due to the lack of rigid connection between the wishbones and the chassis. Because of that, it was my goal to keep the center of gravity as low as possible, and I ended up squeezing almost all mechanical and electrical stuff between the axles. The only things that were located above the axles were the motor controlling the steering, the motor controlling the transmission and a single SBrick.

As shown below, the steering system was located between axles #1 and #2, the transmission – between axles #2 and #3, the propulsion motors – between axles #3 and #4, and the single battery box was installed behind axle #4 to counteract the weight of the cabin. In fact, thanks to the new pieces such as the 3×19 liftarm frames, the whole suspension was impressively compact and almost everything above the wheels was empty body shell, which was a huge improvement over the MAZ-535 model.

The propulsion consisted of 4 hard-coupled PF L motors powered from a single battery box, but using the high-powered Ni-Zn batteries. These drove a 2-speed transmission operated by a PF M motor, producing 3:1 and 1:1 gear ratios internally, which – combined with the planetary wheel hubs – resulted in final gear ratios of 16.2:1 at the lower gear and 5.4:1 at the higher one. All four axles were driven using knob gears instead of differentials.

As for the look of the model, I could only see two sides of the truck on the photos I found, so plenty of details had to be made up. There were no blueprints, so I had to use my best judgement for the proportions, but I think the end result looked good enough, except for being clearly to tall to account for the increased suspension travel.

In terms of performance, the model turned out to be a disappointment. It turned out that after about 1 hour of test-driving in rough terrain the driving ring in the transmission got so worn that it would disengage under stress, even though it was firmly kept in place by the rotary wave selector. It’s possible that the 24t clutch gear which was engaged by this ring at lower gear was also at fault. All in all, it was obvious that the transmission couldn’t handle the torque and there was no easy solution to this other than adding extra gear reduction between the transmission and the wheels, or removing the transmission entirely. There was also one incident of the 16t gears by the PF L motors collecting so much grass and dirt that the motors were stalled. Luckily, it was easy to detach axle #4 and clean the gears, which resolved the problem. While the PF L motors never stalled except for this one case, it was my impression that if I used PF XL motors instead, they would simply grind through any grass without stopping. And while the single battery box with six Ni-Zn batteries kept powering the motors at all times without stopping, it was my impression that it was getting hot and that the overall performance degraded over time when operated outdoors in hot sunny weather.

The model proved a valuable lesson. It was a marked improvement over the MAZ-535 model in several areas, such as compactness, chassis rigidity and suspension responsiveness, but just like the MAZ, it suffered from transmission problems.

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