Sariel.pl

My second 2010 Truck Trial vehicle, and the first vehicle designed for this competition other than a truck. Features 4×4 drive, pendular suspension, lights, modular body design and an openable bonnet. Update: a complete set of instructions added.

Datasheet:

Completion date: 18/04/2010
Power: electric (Power Functions)
Dimensions: length 47 studs / width 24 studs / height 23 studs
Weight: 1.224 kg
Suspension: pendular, stabilized with 2 shock absorbers per axle
Motors: 1 x PF Medium, 1 x PF XL

After the disappointing failure of my Tatra T815 truck, I was left with less than two weeks time until the next race. Re-using Tatra was out of the question, so my only option was to build some simple and small 4×4 vehicle. Since our current rules allow this type of vehicle into competition, I decided to model the iconic Jeep Wrangler in the acclaimed Rubicon version. It was built in roughly 4 days.

The basic rule of this construction was to keep everything as simple as possible and to reduce the weight to minimum. This is why the body was built with the new Technic panels, which provide the best combination of stiffness and lightness. Both front and rear axle are built entirely with liftarms, but the frame that connects them is built with Technic bricks. The reason to use bricks was that I wanted to build a roofless version of the Jeep in order to obtain low center of gravity, and since the sides of the cabin are made of single panels, the whole model had to be kept together by the cabin’s floor. Therefore there are bricks in the floor, that provide excellent stiffness and robustness while keeping the floor thin.

The chassis, which weights just 0.75 kg, consists basically of the studfull frame connected with studless axles. Since there is very little space in front of the front axle, both drive and steering had to be transferred to the front axle from behind. This was achieved by placing a 16-teeth gear with a clutch on the driveshaft – this gear works as an idler gear between two other gears, thus transferring the steering independently to the drive. Front axle is compact but massive, strong enough to handle much heavier vehicle, and it’s bottom is entirely covered with liftarms to prevent it from getting stuck on an obstacle. It should be noted that both axles use knob wheels instead of differentials, and both are portal axles with an integrated 3:1 gear reduction. Each axle is stabilized by a pair of short shock absorbers located between the axle and the frame for maximum structural integrity. In order to minimize the effect of central driveshaft’s torque tilting the body, which occurs when only pendular axles are used, the axles are stabilized with the hardest shock absorbers available. The rear axle is built around the 7×5 liftarm frame and is not covered from below.

The model is driven by a single PF XL motor with a total 5:1 gear reduction, and the motor’s location is strictly related to our current rules. Our rules state that every model has to be equipped with a piston engine whose set-up and location are consistent with the original engine. This rule, however, can be omitted if the drive motor is located exactly where the original engine is. This is Jeep’s case – the PF XL motor occupies exactly the same place that is taken by the original engine in a real Jeep, hence there is no need for the piston engine and the model can be simpler and lighter. You can see the top ends of the front axle’s shock absorbers next to the motor. It should be noted that the PF XL motor actually touches the front axle, but because it’s located in the center of the chassis and because it has a round shape, the axle can still oscillate under it. Such a trick was necessary to fit the XL motor under the bonnet.

The steering is controlled by a PF Medium motor with a 9:1 gear reduction. The motor is located between the seats in the cabin, and the steering wheel is connected to it. The cabin has a safety cage built on top of it, which is strong enough to survive a turnover without any damage. Finally, there is the 8878 rechargeable battery box and the IR receiver located over the rear axle, as a counterweight against the PF XL over the front axle.

One special feature of the bodywork is a droppable modules design. The obstacles used in our races are often larger than the ones the real vehicles are designed for, so the usual practice to handle this difference is to increase the model’s ground clearance. I wanted to avoid it, however, to keep the Jeep’s center of gravity as low as possible. It meant that some parts of the body are very likely to touch the ground e.g.  when the approach angle proves to be insufficient. Therefore I designed a number of body elements as independent modules that would fall off when stressed, on assumption that it’s better to drop something than let it get the whole vehicle stuck. The list of droppable elements includes front and rear bumper, rear spare wheel and front mudguards. The sides of the cabin are somewhat elastic too. This design is also helpful when the front wheels’ steering lock makes them collide into some parts of the body.

The model turned out to be pretty stable, with a well performing suspension and an excellent amount of torque available. It was my goal to prefer the torque over speed, since I did not want the model to get stalled under any circumstances while its speed didn’t seem crucial. This is second model of a Jeep used in our races, and it’s much different from its predecessor, the Jeep Willys built by Atr. Among other things it is heavier, larger and slower. Observing the two models compete against each other and comparing their performance will be certainly a great experience.

The main disadvantage of this model is its size. The standard scale used for vehicles in our races is 1/13. I was in hurry while building the Jeep, so I picked up the wheels I found most suitable and scaled the whole model accordingly to their size. Eventually it turned out to be in 9/1 scale which is much different from the required one, and will probably result in letting this model race outside the official ranking (in a so-called ‘open class’). Still, building and testing this model was a valuable experience, and driving it in a race will be so too. It was also acclaimed by other builders and race contestants for its look, its authenticity and its performance. If it turns out to be successful in the race (meaning that it suffers no fatal malfunction and doesn’t end up last), I will prepare and publish a complete step-by-step instruction for it, as well as schemes of front and rear axle along with their individual instructions.

Update: since the Jeep performed well at the race, the instructions are under the photos.

Photos:

Full instruction with parts list:

Photo-instruction for the whole model:

Videos:

Media reference:

AutoMotto, GadgetSin