Sariel.pl

1/18 model of a German Main Battle Tank, features torsion bars suspension, rotating turret, rotating commander’s cupola, self-levelling main gun, gunner’s gun counter-rotation system, lights and custom stickers.

Datasheet:

Completion date: 28/10/2010
Power: electric (Power Functions)
Dimensions: length 70 studs (including main gun’s barrel) / width 26 studs / height 23 studs (not including antennas)
Weight: 3.42 kg
Suspension: torsion bars
Propulsion: 4 x PF XL motor geared 1:1
Top speed: 1.5 kmph
Motors: 4 x PF XL, 2 x PF Medium, 1 x Micromotor

My first dark grey tank with a conventional construction (i.e. with a turret). I broke my ‘no German tanks’ rule, but at least the 2A4 version serves also with Polish army. It’s also the last version of Leopard with it’s original turret, whose distinctive shape (resulting from limitations of then-early modular armour technology) reminds me somewhat of German WW2 tanks. Leopard 2 is the world’s first 3rd generation tank (a brief explanation on tanks generations is available here), resulting from a failed joint German-US Main Battle Tank project (one that gave birth to Abrams M1 too) and introduced in late seventies. It’s a very universal construction with strong focus on mobility, and it has become quite popular, especially within EU. Widely considered as European Abrams’ counterpart, it has seen action e.g. in Kosovo and Afghanistan. The newest version currently in service is 2A6, while a 2A7+ version is under development.

I have started to work on this model in an unprecedented way, that is by buying a 1/35 scale plastic model kit of the Leopard 2A4 (which alone took a couple of months, as models of this versions of this tank at this scale are exceptionally rare, I only found them in Hobby Boss’ catalogue and already out of production) and putting it together for reference. I have obviously used a blueprint for calculating the dimensions too, but the plastic model gave me incomparably better reference, such as details of the portion of the hull that is covered by the turret or the decals compliant with German army’s marking patterns. It also helped me to get the idea of various subtle angles and shapes that were difficult to notice on a blueprint, and provided a clear view of some details that appeared vague on it. I am generally happy with the model’s look which suffered few compromises, except for a single serious one: the two grills on top of the rear hull surface, which should be round. I was forced to place three IR receivers beneath this surface, which made it impossible to accurately model the round shape of these grills – I wish I could do that e.g. like here.

Technically, the model was primarily meant to test a new suspension concept, based on torsion bars – a solution taken straight from the real tanks. Torsion bars suspension uses road wheel mounted on end of short rods, whose other end is set on a flexible element which twists under load, thus allowing the road wheel travel up and down. The simplest application of this solution with Lego pieces was a bit unorthodox: it required regular axles, which have a degree of elasticity, to be twisted.

Just like  in my previous tank model, the 6595 wheels have been used as the road wheels. They were mounted on bent 2×4 liftarms, so that they could rotate and maintain good ground clearance. The other ends of the liftarms were set on 8 studs long axles inserted transversely into the hull’s floor. Both ends of the axles were supported, but the inner ends, in the middle of the hull, were locked so they couldn’t rotate. Thus the axles were firmly braced in the chassis, with one end still free to rotate – the end with the liftarm and road wheel on it. It resulted in axle twisting slightly under load and then twisting back when the load was gone. Contrary to what I was afraid of, no apparent damage to the axles occurred, and this kind of suspension turned out to work excellently and to be extremely space-efficient (all it took was basically a single stud of hull’s height). Moreover, the hardness of this suspension can be easily adjusted by using shorter axles or by changing the locking point on them. In this model, each road wheel was subject to an average load of almost 250 grams and the suspension worked perfectly well with an 8 studs long axle, given 4 studs of space between the liftarm and the axle’s locking point.

The model is driven by an unusual number of four motors, all XL. Each of its four sprocket wheels is driven by a separate motor, and the whole set is powered from two separate battery boxes through two separate IR receivers (set on the same channel), with each battery box powering one left and one right motor. I hoped that the massive torque provided by all these motors together would make the model fast despite its weight, and early version with almost 3:1 gear ratio proved to be very fast. Unfortunately, the performance degraded rapidly with the growing weight, and at 3 kg the motors were unable to move the model at all. I’ve spent almost 2 hours changing the gear ratio to 1:1, viewing it as the most efficient ratio possible and unwilling to experiment with something between 3:1 and 1:1 lest I spent another 2 hours changing it again. It appears that XL motors, despite all their power, become increasingly ineffective when used with accelerating gear ratio. It is also possible that it takes more torque to move heavy tracked vehicle than it does for a wheeled one of the same weight.

Except for the four XL motors and two battery boxes, the hull houses three IR receivers, a single PF Medium motor that rotates the turret and the entire turret rotation mechanism along with a turntable. Placing the turret’s turntable inside the hull had a number of advantages: most importantly, it made the rotation mechanism simpler and more robust, and made the construction of turret’s base much simpler while leaving plenty of free room inside the turret. There are two wires going through the turntable, so it can’t rotate infinitely, and there is a simple liftarm frame the turret is built around. The frame includes a single roller behind the turntable, which keeps the turret’s rear end from touching the hull.

Inside the turret, I wanted to have a self-levelling main gun with remote elevation control. To put these two features together, I have built a module suspended on a transverse axle, which connected the main gun with its counterweight and had a micromotor in between, so that the angle between the main gun and its counterweight could be changed remotely. It worked as expected, joining the two features: changing the angle between the main gun and the counterweight by e.g. 30 degrees would result in the main gun going up (or down) by 15 degrees and the counterweight doing exactly the same, while the whole module kept self-levelling all the time. Two problems prevented me from using this mechanism: firstly, raising and lowering the counterweight required more space inside the turret than was available, and secondly, the micromotor’s wire kept affecting the module’s position, no matter how I arranged the wire. Eventually, I decided to keep the self-levelling feature and to remove the elevation control at all. Tank model with no main gun’s elevation control is certainly at disadvantage, but I thought that since all my tank models had this feature, I can do something different just this once. So, I removed the micromotor from the module, positioned the counterweight as close to the main gun as possible, and added another counterweight below the pivot point to lower the module’s centre of gravity – as it was apparent that the accuracy of self-levelling increases with the centre of gravity being further from the pivot point.

The micromotor which was removed from the self-levelling module served as a propulsion for commander’s cupola. It proved perfectly suited for this purpose, because no gear reduction was needed and the cupola is mounted directly on the motor.

Finally, there is a PF Medium motor in the rear part of the turret. I had a lot of space available in the turret, but all the IR channels were already in use, so I was looking for some additional use of the channel controlling turret’s rotation. Eventually, I have built a copy of turret’s rotation mechanism inside the turret, driven by the aforementioned Medium motor and working in opposite direction. The motor was connected to a switch inside the hull, and the switch was connected to the same IR receiver’s output as the motor rotating the turret. It resulted in a possibility to turn on or off a mechanism that would rotate something on top of the turret against the turret itself, thus keeping it aligned with the hull. The obvious choice was to select gunner’s machinegun as the counter-rotated element. The mechanism worked well, but the difference in resistance met by the motor rotating the whole turret and the motor rotating the machinegun alone resulted in different speeds of rotation, with the machinegun rotating slightly faster than the turret. Thus the machinegun does not really stay aligned with the hull, but slowly rotates in opposite direction that the turret does. It should be noted that since two copies of turret’s rotation mechanism were built, there are actually two turntables in this model: one below the turret and another one inside it.

Despite its few shortcomings, the model met a very positive response, being occasionally called my best tank model yet. I was slightly disappointed by its speed, but delighted at its suspension which worked way beyond my expectations, providing excellent flotation without using a single shock absorber. Personally, I consider this model as a preliminary study for my next tank, the South Korean K2 Black Panther, which will develop some of Leopard’s solutions further and hopefully improve in general wherever possible.

Photos:

Work In Progress photos:

Video:

Media Reference:

Rock’n Tech (Portuguese only), The Brothers Brick