Friday, July 15, 2016

Competition Crawler Mk4

In the wake of the previous GT Crawler featuring an unconventional yet a slightly lacking chassis concept, for my next one I decided to get back to the roots a bit. Therefore this CC4 (Competetion Crawler, Mark 4) got a standard dual floating axle suspension, somewhat similar to the one seen in the official 9398 Crawler, though with a few modifications I wanted to try out.

For starters, although it is not easily visible on the photographs, this chassis is slightly (2-3 studs) shorter and narrower than the 9398's. The intention was to make this crawler more maneuverable and easier to sneak through the tight passages with. This, of course, in turn reduces the stability, which I attempted to compensate by mounting the heaviest parts (the batteries and the motors) as low as comofortably possible.

The other difference regards the suspension angles and tuning. A thing that bugs me for a while already, not only on 9398 but on majority of official Technic sets, are their suspension springs which are far, far too stiff. Typically they are hard enough to keep the entire chassis in the most expanded position at rest, and even requiring some force to start compressing them at all. Not only is it unrealistic, but significantly reduces the off-road performance as well. Having the suspension compressed about halfway at stillstand is the correct approach. True, the official LEGO approach with fully extended springs is perhaps useful in the specific case of the chassis violently landing on a surface, but this is not its purpose, despite it being very popular thing to film on YouTube.

That is why I chose to, apart from the hard yellow springs, use the gray medium ones, with the result being the chassis floating halfway up, just as intended. The springs' mounting points are narrower and further back, to allow the longer wheel travel and reduce the resistance of the bodywork to rolling. The chassis itself is raised by using the portal axle parts. Although I don't like those parts too much, I've got to admit that the other tries to make the same using the standard Technic parts turned out too fragile.

As typical for any crawler, CC features a four wheel drive. Three PF XL motors take care of that: one powers the front wheels without a differential, while each of the rear wheels has its own PF XL. They are attached to the axles and move along with them. As for the steering, it's only on the front wheels, attached to a standard rack and pinion rotated by the PF Servo, mounted as well on the axle. Actually, no mechanical work is done in the central chassis section ― it only carries two battery packs and their IR receivers.

Reducing the steering to the front wheels only somewhat hampers the maneuvrability, but I planned to counter that by introducing another, slightly "naughty" trick - controlling each drive motor independently. For exampole, if the front wheels steer right and drive fotware, while the rear right pulls back, the crawler is supposed to steer (with some slippage e.g. on the soil or sand) harder than if it had a four wheel steering.

Since, apart from the steering, there are three drive motors that need to be controlled independently, I was forced to build a sligthly adapted controller, with a lever that allows to let all three drive motors drive forward simultaneously, yet can be retracted for those rare maneuvres when each wheel needs to be controlled manually.

The bodywork serves no other purpose apart from the decorative one; as per old Technic off-road tradition, it is entirely held in place at only a couple of strongpoints and can be detached or mounted literally in five seconds. Since the wheels have plenty of vertical and longitudinal range of movement, the bodywork needed to be rather tall and retracted. Contrary to the GT Crawler and its shapely, rounded surface, this time I wanted to build something more utilitarian (not to say "Russian"). I don't mean to say this couldn't have been done better, and I've got to admit I was never very successful at form design. A lever inside the cabin allows turning on the headlights, but this is hardly a feature.

Well then, after all these technical descriptions, the logical question is ― how does CC4 perform in practice? I've got to admit that, even up to the point when I loaded the chassis for the first time with the batteries and started the first test drive, I did not know what to expect.

In the role of a climber-crawler, this experiment was pretty successful. Chassis at first, and later the entire crawler, had no problem handling the inclines, stairs, pits, downhills and other obstacles, and maneuvered well on the dirt, grass, sand, etc. The low center of gravity, high chassis ride, rather powerful four wheel drive and the long suspension travel helped a lot ― at almost all times were all four wheels on the surface, regardless of the obstacles in the path. The chassis also proved to be pretty strong. The first version was prone to detaching the rear springs while falling off the platforms, but that was easy to reinforce additionally with about ten parts.

What I'm not particularly satisfied with is the staright line drive. Because of a long suspension travel and low forces acting on the springs, the axles allow small side movements that, over longer distances, make the crawler turn slightly instead of making it drive straight. In comparison to the laser-like precision of 9398, CC4 almost looks drunk.

Fortunately, the idea about the independent drive control turned out to work ― for example, turning the forward wheel forward and steering to the right, with the rear left static and rear right turning backward, indeed made the CC4 to turn noticeably sharped than it could using even four wheel steering. Other combinations are possible as well. However, this works reliably only in the suitable conditions, when the grip of all the tyres is about equal, such as the wooden floor, a carpet or sand. But as soon as the surface becomes bumpy or unhomogenous, one or two wheels get more grip than the others and simply overpower them, returning the crawler to the "standard", no-tricks chassis.

The test drives have shown that we still have not got standard off-road wheels that could compete with the custom ones from the non-LEGO sources. The 54120 wheels, used in the official Crawler among other sets, are not a bad compromise of all roles ― they do not have unnecessary friction on the smoothe surface and are more or less useable for the off-roading. However, in the serious off-road conditions, regardless of how clever the chassis has been built, they are still the weak spot, especially when they pick up the layer of dust. It is not surprising that many are trying to compensate for it with the 8x8 configurations. But one needs to understand LEGO as well; it is not feasible to develop the extreme tyres only for a narrow community of Technic AFOL's dealing with extreme off-road vehicles. And even then, the most would not buy more than four.

Altogether, building the CC4 was an interesting experience, as well as driving it on various grounds. The concept itself is interesting, though I think that the option of sharp turning would not be that useful in the Truck Trials. A bit like crab steering ― fascinating from a mechanical point of view and a seemingly an ace in the sleeve, but unnecessarily complex and not that useful in real driving. But even without that, this would turn out to be a quite acceptable off-roader.



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