Sariel.pl

An experimental halftrack vehicle. Features three types of suspension, a 4-speed manual gearbox and openable doors.

Datasheet:

Completion date: 13/04/2009
Power: electric (Power Functions)
Dimensions:  length 54 studs / width 17 studs / height 16 studs
Weight: 1.41 kg
Suspension:  front – independent / rear –  torsion bars + pendular bogie
Motors: 1 x PF Medium, 1 x PF XL

Having built a dozen or more tracked vehicles, I eventually felt compelled to try a halftrack. Halftracks are an interesting blend of tracked and wheeled vehicles, combining the advantages of both. Namely, they perform better off-road with tracks than typical wheeled vehicles, and they can steer much easier than typical tracked vehicles thanks to the wheels (which results in a simplier construction rather than in a better performance).

There is a number of problems involved in building a halftrack, and I was curious as to how they would affect a small, lightweight model. Basically, the real halftracks often have the front wheels driven for better traction and better steering. There is usually a differential between the tracks, which serves the same purpose, and some of the heavy German halftracks were fitted with automatic brakes that would slow down one track while turning.

There was a single technical solution that greatly affected the look of the vehicle: the torsion bars. I was earlier suggested to try this kind of suspension, popular even in heavy tanks of the WW2, but it did not occur to me at that time how to make such a bar bend without sustaining any damage. Only sometime later it hit me that the elastic 16-studs long axles from the Jumping Giant would be an ideal and natural choice. It’s just their limited length that was not ideal, and hence the width of the entire vehicle was limited by the transverse 16-studs long axles used as torsion bars.

With the vehicle’s body only 16 studs wide I’ve given up the intent to make the front wheels driven. Instead, they were just fitted with a simple independent suspension, and the suspension of the tracks was complemented by a central pendular bogies because I only had two axles that could serve as torsion bars. The drive was transferred through the usual 4-speed manual gearbox I’ve used many times before to the tracks’ front return rollers without the use of a differential. I have assumed that it would be catastrophic to drive a lightweight vehicle with a slippery tracks via a differential – the slip situation would be more than likely to happen.

Eventually, the vehicle had a surprisingly efficient steering system – which was due to the tracks’ poor traction rather that to its construction. The turning radius was even slightly better than it would be with two regular rear axles instead of tracks. I suppose that given a sufficient speed it could be a great drifting truck.

As for the look of the truck – I had no clear idea what should it look like, but I definitely didn’t want it to look like German military halftracks of the WW2, which are often considered iconic for this type of vehicle. That is why I pursued the look of a vintage truck, which eventually proved to have a lot in common with the US Army’s halftracks (M3 and its successors in particular). Still, I was unhappy about the bonnet which had to be tall and wide to cover the shock absorbers of the front suspension. Actually, the front of the truck was so massive that its center of gravity ended up below the cab – despite the fact that all motors and the battery box were located between the tracks. I felt that the back of the truck should be openable, but that was impossible because of the battery box being fixed there. The battery box and the motors were left uncovered deliberately: this way they could appear to be the truck’s load, and the cargo space seemed much larger than it would with these parts covered up.

The truck was technically very compact, which resulted in some limitations. There was no place for working lights nor for the reinforced drivetrain, the latter resulting in an inconsiderable vehicle’s top speed.  The suspension system, however, worked pretty well. The torsion bars functioned in a proper and realistic way, even though they were probably too weak to support the entire truck’s weight. Still, there was a margin of calibration available, as I created a 2 studs long ‘torsion space’ – that is a space between the actual suspension and the point where the torsion bars were fixed to the chassis. If this space was reduced to a single stud, the suspension could probably handle a significant weight. Tests with the gearbox proved the torsion bars to be a bit unreliable; they tended to produce vibrations at lower speeds and worked smoothly only at the higher ones. I suppose that this effect is proportional to the size of tracks’ links, which means that the newer tracks system would require even higher speed to run smoothly with this suspension system.

Although I have initially considered it a minor, highly experimental project, the Crusader performed almost flawlessly and was more fun to drive than many more advanced vehicles. Moreover, to my sincere surprise its aesthetic aspect was much acclaimed at the LUGPol and beyond.

Photos:

Video:

Media reference:

Klocki, The Brothers Brick