Sariel.pl

My third 2010 Truck Trial vehicle. Uses modified Jeep Wrangler’s chassis, features 4×4 drive, full suspension, opened doors, bonnet and trunk, headlights and rear lights with LEDs.

Datasheet:

Completion date: 21/05/2010
Power: electric (Power Functions)
Dimensions: length 41 studs / width 20 studs / height 19 studs (not including aerial)
Weight: 1.518 kg
Suspension: pendular, stabilized with 2 shock absorbers per axle
Motors: 1 x PF Medium, 1 x PF XL

My third 2010 Truck Trial vehicle was based largely on the previous Jeep Wrangler Rubicon. It took me over a year to come up with this model since the first plans for it appeared in early 2009 and were postponed since then. It turned out to be good for the model, as I was able to build it entirely in dark gray and using race-proven chassis components. The basic chassis concept remains the same as it was for the Jeep: two pendular studless axles held together by a studfull frame embedded into the cabin’s floor. All the components taken from the Jeep have been modified though: the front axle has a modified steering system that makes the wheels protrude forwards, the rear axle is narrower, and the frame is wider and fully covered with plates from below.

My goal from the beginning was to make this model accurate, even at the cost of its off-road performance. Unlike the Jeep, this model is scaled down accordingly to our Truck Trial regulations, which makes it much smaller than usual TrTr vehicle. Therefore it was clear that it share some disadvantages with the real Hummer, such as relatively small wheels and large wheelbase with low ground clearance. The latter resulted in numerous ‘getting stuck on belly’ accidents during tests.

The Hummer is much heavier that the Jeep was, which was exactly my intent. Based on the performance of the Jeep, which had enough torque to be practically unstoppable, I have estimated that its drivetrain and chassis components should handle weight of some 1.5 kg without problems. Since our rules take power-to-weight ratio into account, heavier vehicles get more points. Therefore I was trying to achieve greater weight while maintaining Jeep’s speed and performance. The speed is almost identical, despite smaller wheels (8 studs in diameter compared to Jeep’s 10 studs), as the gearing in the drivetrain was changed from 5:1 to 4.5:1. In order to obtain this exact gear ratio I have used a double chain between the drive motor and the driveshaft. A single chain would be broken instantly, while a double chain survived all tests I’ve ran. The use of a chain had one extra advantage: it required much less reinforcements in the surrounding structure that the gears alone would, since the gears were, well, chained together. Eventually, there were some minor inaccuracies in the model as compared to the real H1. The bonnet had a cut-out section in the middle of it, which was necessary to accommodate for the upper ends of the front shock absorbers. The air intake, which in reality is fixed to the chassis and goes through an opening in the bonnet, was attached to it in the model. I have also reduced the size of the front bumper and gave up some extra elements such as the famous “cow catcher” in order to achieve a better approach angle. Finally, there were openings in the sides of the cabin that allowed the doors to open up easily, as there was not enough internal space to use opposing hinges and I didn’t want the doors to require force to open.

The model participated in a Truck Trial race on May 22nd, and it generally performed above my expectations. It had sufficient torque and displayed some excellent manoeuvrability. The stability was satisfactory, except for one incident when I managed to flip the model over somehow. There were some traction problems, but not larger than those experienced by other vehicles (we keep having problems when driving on a dry forest bed). After the first section of the race Hummer placed 4th out of 11 vehicles, which was partially thanks to the power-to-weight ratio more favorable than it was for the Jeep. During the second section it easily climbed over a large branch that seemed too big for its ground clearance. Then during the third section both chains in the drivetrain broke at once. Since it was impossible to fix it on the field (large portion of the cabin had to be taken apart to access the chains, and I needed tweezers), I was forced to give up the rest of the race and thus the model ended up on the last place. It seems that the model had a considerable potential and could effectively compete against larger vehicles, but the idea of using chains in the drivetrain was clearly wrong. The thing I’m happiest about is that I’ve finally managed to put the damped shock absorbers to some practical use, even if it didn’t look as good as simple plates on hinges would.

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