My first racing truck. Features full suspension, dual motor drive, and lifted cabin.

Datasheet:

Completion date: 16/08/2008
Power: electric (Power Functions)
Dimensions:  length 47 studs / width 18 studs / height 26 studs
Weight: 1.92 kg
Suspension: front independent / rear pendular with shock absorbers
Motors: 1 x PF Medium, 2 x PF XL

In August 2008, I’ve come across the European truck racing, and this sport appeared to me as very potent in the terms of a LEGO Technic competition. To try out some assumptions of such a competition, and demonstrate my idea, I have built an exemplary truck.

The assumptions of the LEGO Truck Trial competition were as follows:
- trucks scaled to the 62.4 x 20 S wheels, or different wheels of similar size
- maximum width of a vehicle: 19 studs, exluding side mirrors
- full suspension on all axles
- at least one axle driven, at least one axle steered
- must refer to an existing racing truck; therefore I’ve assumed that truck with more than 3 axles won’t be used, as there are no such trucks in the real sport
- no weight limits, and unlimited number of motors used to drive

The idea, presented at LugPOL, prompted several builders to build their own racing trucks, most of which did not comply with the assumed rules (a lot of people complained about the suspension system requirement, which I have viewed as crucial for realism). As of the end of 2008, not a single contest took place.

The truck was driven by two PF XL motors connected by an adder. The rear axle was driven via the new type of differential from the 8297 set, and had pendular suspension stabilized by shock absorbers. The front axle was not driven, and had an independent suspension based on the components from the 8435 set, with very hard shock absorbers (to support the weight of the cabin and to improve stability of the truck, which could rely on the rear axle only). The dimensions and the look of the truck have been to a large extent based on layout of Scania R 340. The battery box was located above the front axle, and the cabin could be lifted to access the box. The truck was equipped with a large array of lights, both front and rear. The IR receiver was located in the chassis and well exposed.

For the lack of any official contest, the truck went through a lot of test-driving. It performed well, with a drivetrain which provided speed below my expectation, but proved entirely capable of handling all the torque involved in driving such a heavy vehicle. Because of the large accelerating gear ratio in the drivetrain, the truck suffered from a large inertia while stopping. I have assumed that a braking manoeuvre must be used to eliminate that, namely the motors were switched to reverse to stop the truck. Again, the drivetrain endured a huge torque generated while stopping truck this way, and since that time, I required all my racing trucks to be able to endure such a manoeuvre.

The disadvantages of the truck was a significant weight, and a tall center of gravity, which did not cause any trouble here, but I have anticipated that it would, if the truck was faster. Moreover, there was a noticeable part wear in the front suspension, which resulted probably from the large weight of the vehicle, located mainly on the front axle.

Photos:

Video:

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Model of an 8×8 special truck designed to tow tanks. Features 8×8 drive, 4 wheels steering, full suspension, drivetrain with an adder, pneumatic carriage outriggers and remotely raised carriage ramp.

Datasheet:

Completion date: 24/03/2008
Power: electric (Power Functions) / pneumatic (fed from internal manual pump)
Dimensions:  tractor – length 59 studs / width 20 studs / height 23 studs, carriage – length 109 studs / width 24 studs / height 23 studs
Weight: tractor – 2.1 kg, carriage – 1.2 kg
Suspension: full pendular without shock absorbers
Motors: 2 x PF XL, 2 x PF Medium
Pneumatics: one circuit with manual valve and internal manual pump in the carriage

There are two versions of the Faun SLT tank transporter: Elefant and Franziska. While externally almost identic, they are very different internally, with Franziska being a simplified, cheaper version with a limited off-road performance, and built from less specialized, more common parts. Both versions use different carriages, with different number of axles. What I’ve built is a 1:19 model of the Elefant version.

The look of the truck is fairly accurate, except for the cab, which is too tall by at  least 2 studs. I have also sacrificed the details of gear located behing the cab, in order to gain space for the drivetrain.

The drivetrain is quite space-consuming, as it consists of two PF XL motors connected by an adder. Normally, an adder sums up the speed and torque of two motors into a single output. Here, however, I have used a different gear ratio between the adder and each of the motors. This results in a different speed and torque the adder receives from every motor, which means that they can be combined to work like gears of a gearbox. All it takes is an independent control of each motor, and thus 4 combinations are possible: motor A alone, motor B alone, motor A minus motor B, and motor A plus motor B. Unfortunately, this theoretically great solution has one real disadvantage: if the adder’s output meets sufficient resistance, it stops and the motors start to drive each other instead. More precisely: the motor which is geared down more drives the other motor. It doesn’t cause any damage to the PF system, but it stops the vehicle entirely. Since the adder uses a differential, this situation can be supposedly prevented by using a differential lock. The locked adder would then work like a regular rigid axle, and transfer the drive to the output like it should, provided on of the motors is off & driven by the other (because of the gear ratio difference). I have not yet tried locking the adder under such circumstances.

The steering system includes axle 1 & 2, which have the same turning radius, just like in the real truck. Both are controlled by a PF Medium motor. I have used my compact pendular driven & steered suspension module again, with some additional reinforcement. It works well, but under extremely heavy low a single weak point fails: the 8-teeth gears. These can’t be replaced in this construction, and they are the weakest gears possible, even though preceded by a worm gear here. Both front axles have no shock absorbers, to save space. The vehicle is stable thanks to the axles 3 & 4, which are driven through a single differential, and suspended on longitudinal rocking levers. This is a very simple solution, which needs no shock absorbers neither.

The last motor, a second PF Medium, is located at the back of the truck and has an output that exits the hull in form of an universal joint in the middle of the carriage latch. Upon connecting to the carriage, the output drives a ramp raising mechanism in the carriage, through a long axle that reaches the carriage’s rear end, and a worm gear.

The carriage is pretty simple, fitted with 4 axles that use the same pendular solution as axles 3 & 4 in the truck: the longitudinal rocking levers. There is a very simple pneumatic system in the front part of the carriage, that deploys a pair of outriggers to support the carriage when disconnected from the tractor. Some of the photos show the carriage with the hull of the S Tank, which has been at the disassembly stage when the Faun was finished.

The model met with a very positive response, both local & worldwide. The use of worm gears, however, caused the vehicle to be very slow, which prompted me to cut and shorten the video, and some people believe it to be an attempt to put them on. I’ve been also disappointed by the problems with 8-teeth gears in the drivetrain, which rendered the truck unable to tow loads heavier than approx. 6 kilograms despite all the torque it had.

Photos:

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Video:

Click here to view the embedded video.

Media reference:

YFOLe (Polish only), Young Spacers Association Blog

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