Sariel.pl

Model of the Humvee designed for Truck Trial races. Features full independent suspension, portal axles, 2-speed remote controlled gearbox, lights, opened bonnet, doors and rear hatch.  

Datasheet:

Completion date: 14/04/2011
Power: electric (Power Functions)
Dimensions: length 53 studs / width 26 studs / height 24 studs (not including antennas)
Weight: 2.319 kg
Suspension: full independent with portal axles
Propulsion: 1 x PF XL geared 8.3:1 (1st gear) / 3:1 (2nd gear)
Top speed: unknown
Motors: 2 x PF Medium, 1 x PF XL, 1 x micromotor

A little less than a year ago, I’ve built a simple Hummer H1 model designed specifically for the Truck Trial races. It turned out to perform better than expected, until the chain present in its drivetrain broke. I was relatively pleased with this model, until several months later I have come across a civilian H1 parked on a street. Having used this opportunity to inspect it as much as possible from the outside, I have come to the only proper conclusion: that my model was unsatisfactory.

While planning to build a new model – this time called Humvee to emphasize that it would be based on the military H1’s – I’ve decided to use the often-criticized black colour, because Humvees simply look excellent in plain black. I have also decided that I can’t simplify the suspension system like I did in my earlier model (which was suspended on simple pendular modules), but rather try and recreate the suspension system of the real vehicle. This is a tricky feature, as the Humvees have full independent suspension with portal axles. It’s fairly easy to build independent suspension or portal axles with Lego pieces, but it’s quite a different story to build independent suspension and portal axles. It means that Lego pieces that are designed for independent suspensions are pretty much out of the question. Instead, I had to create this kind of suspension using common pieces only.

Sophisticated suspension systems like this  are usually wide – building the new model in the same scale as the old one was impossible. When I’ve tried the old Technic wheels that have best diameter/width ratio, it turned out that I can make the suspension 23 studs wide, including the wheels (my earlier model was 20 studs wide). Since the model scaled to the size of the wheels would be 24 studs wide, that suited perfectly – except that it meant abandoning the 1/13 scale required by the Truck Trial rules, which means that the model would not be classified in the race. I decided to accept that rather than to build another small model with simplified suspension system. Additionally, the minimum weight limit for the race has been changed from 1kg to 2kg, starting this year, which meant that my earlier model would be far too small to participate.

Another decision was to use a gearbox this time (my previous model had none), and my recently developed 2-speed heavy duty linear gearbox was an obvious choice. The gearbox design was so good that I wasn’t the only one using it in the race – Maniek’s truck used a gearbox like this copied with my permission.

Mechanically, the model used a custom independent suspension system with portal axles, with a 3:1 gear reduction at the wheels, and was driven by a single XL motors using a 2-speed gearbox with ratios 8.3:1 at lower gear and 3:1 at higher gear. There were four motors total: one for drive, one for steering (housed under the bonnet), one for shifting the gearbox (housed under the gearbox) and one micromotor for rotating the steering wheel as if it was actually working (which can be considered a cheat, but is not forbidden in the Truck Trial rules). 4 IR channels were used to control the model, with the fourth one used to turn on/off the lights remotely.

Aesthetically, I have focused on improving the shortcomings of my previous model and on modelling the features which are exclusive for the military Humvees. These included intake and exhaust snorkels, which I’ve never seen used in civilian vehicles, as well as the design of the whole rear end of the model, which is quite different in civilian versions. Namely, the location of rear-lights is different, and there are recesses for two jerrycans on the sides. Inside the model, there were two well-detailed seats in front and a radio station between them. There were no rear seats, as the space was taken by the gearbox control system, and the rear seats are usually folded in military vehicles, and thus can’t be seen from the outside. The model obviously included plenty of parts that could be opened: the bonnet, all four doors and the rear hatch which was split in two pieces: one opening upwards, another opening downwards. Just like in the real Humvee, the intake snorkel stayed in place when the bonnet was opened – this was one of the features I’ve missed in my earlier model.

The model performed well at the beginning of the Truck Trial race. It drove through the first section flawlessly, with lower gear used to climb uphill and higher gear used to descend downhill. The problems started at the beginning of the second section, where it simply stopped moving. As there was no easy way to locate the damage at all, I had to give up the race. It eventually turned out that I did a big mistake by meshing three 8-tooth gears in a row (which I did to split the main driveshafts in two, as required by the suspension system). Even though the gears were protected by 3:1 gear reduction at the wheels, they have shifted on their axles (as 8-tooth gears are slightly narrower than a single stud) and started to grind each other, severely damaging their teeth. Thus a careless drivetrain design resulted in six damaged gears and in the model utterly unable to move. Additionally, the front wheels had a strong tendency to bend, for no apparent reason. I would suspect some flaw in the suspension design, but the rear axle used almost identical design and the rear wheels did not bend at all.

Despite its obvious flaws, I consider this model a step forward from the previous one. It has proven my gearbox design reliable in the racing conditions, and it used a complex suspension system, to a moderate success.  However, as I’m unable to identify the reason for the bending of the front wheels, it seems I should abandon this suspension design.

Work in progress photos:

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