Sariel.pl

Model of a Dakar race truck inspired by MAN TGA. Features full suspension, 4×4 drive, remotely locked rear differential, lights and custom stickers.

Datasheet:

Completion date: 19/10/2013
Power: electric (RC unit / 8878 battery)
Dimensions: length 60 studs / width 26 studs / height 34 studs (not including antennae)
Weight: 3.412 kg
Suspension: live axles
Propulsion: 4 x RC motor geared 4.2:1
Motors: 4 x RC motor, 1 x PF Servo motor, 1 x PF Medium motor

I have long wanted to build a large, fast Dakar-styled truck, and I hoped a propulsion system of 4 RC motors would do the trick. I was wrong.

The model was quite closely based on a MAN TGA racing truck. I have been looking for a way to fit 4 RC motors and 2 RC units into a small space with a good access to both RC units, and I got an idea around which this truck was built. The chassis was literally built around 4 RC motors lined up in a row, with the driveshaft going through all of them. By being connected to each other, the motors formed a very rigid structure which replaced a typical frame as a mounting point for the rest of the chassis. The two RC units were placed on their sides, with antennas facing inwards and bent at right angle to come out on top. Such orientation guaranteed easy access to the bottoms of the RC units from the sides of the trucks, and the antennas, being metal, were not damaged from bending. The bottom of the chassis was fully protected by a cover that was acting as a dust shield, as well as lowering the truck’s center of gravity a little.

The suspension consisted of two live axles, each connected to the chassis by a ball joint, and stabilized by long hard shock absorbers – two in the rear axle, four in the front. The front axle also included a PF Servo motor controlling the steering, which was connected to a PF switch. The switch, in turn, was sitting on an axle coming out of the steering output of one of the RC units – they both turned out to have a very similar range of movement. The switch was connected to a 8878 rechargeable battery installed over the rear axle, which was also the source of power for the lights. The rear axle was simpler, with a PF Medium motor installed in it, connected to the auxiliary output of one of the RC units, and controlling the differential lock integrated into the axle.

The building instruction for the live axle with a differential lock is available here.

The truck looked reasonably good and was very stable for its height. The propulsion system, however, struggled, because I have made the classic mistake of putting too much weight at the given gear reduction. At final weight, the truck would go quite fast for 5 to 10 minutes after inserting fresh batteries. Then it would get sluggish, and the RC units would start cutting power from the motors because of the high load on them. Another gear reduction stage could have helped the problem, but it would require a heavy modification to the whole chassis and moving the motors higher inside it. But the essential problem was the weight of the model, which reduced in a significant wear on the axles inside wheels and differentials. This could be probably remedied by greasing, which I don’t do on a principle. The steering lock was poor, too.

In the end, the model looked much better than it performed. I’m sure that using four PF XL motors and relying on the PF system instead would result in a lighter model with much better off-road capabilities. The RC motors are simply not fit for high-load applications, they become extremely power-hungry when handling heavy loads, and ignoring this was what made this model a failure.

Work in progress photos:

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