Tuesday, April 9, 2019

Some thoughts on Trial Truck construction & design

Having constructed a small parking lot (2-digit number) of Trial Trucks so far, and compared their live performance with their features, there are definitely some lessons to be learned―which I shall briefly cover here, hoping to help other builders possibly struggling with similar problems. And especially to help beginners avoid some tempting ideas which turn out to be cul-de-sacs.

If I had to condense all my Truck Trial experience into one single, most important lesson, it would be to avoid complexity. Features are great for bragging on the forums and in front of the less tech-savvy friends, but in practice they rarely actually help the truck's performance. Among the heaviest mistakes I have done in my early days was to cram tons of advanced mechanisms into a truck, believing they would help just they way they do in the real-life off road cars. But quite simply, they almost never did.

For example, one of my first entries (and worst failures) was an off road SUV which had no less than three independently lockable differentials, independently adjustable ride height at the front and the back axle, five-speed gearbox, four-wheel steering and, well, working headlights―and all these controlled remotely. While it was mechanically sound, in the competition it got easily beaten by the trucks half its power and quarter its features.

Not only was it too heavy, but also all these features unnecessarily raised the truck's center of mass, making it less stable. None of the advanced features actually turned out as advantages―all these differentials and ride heights only improved the performance very slightly, and altogether it was still miles behind competition

So―I can't emphasize this enough―just avoid being seduced by the fancy mechanical contraptions and go simple. A simple but responsive steering, fixed four wheel drive with plenty of power, long-traveling suspension and good ground clearance is all you will ever need. In fact, sometime later I have built a Trial Truck I got almost ashamed of due to its straightforward simplicity―yet it turned out to trample the earlier one in all the criteria important in competition.

Simplicity leads to structural strength which is another point to consider. Simple, multiply reinforced beams with several trusses in critical places will keep your truck strong and resistant to hits and vibrations, will make it easy to repair even if something goes awry, and keeps all the gears and moving parts firmly in place. This is something TLG has done finely with their official Crawler which, despite all its mechanical intricacies, is essentially simply and strongly built.

Don't be afraid to reinforce―indeed, you cannot get away without it―but simplicity makes the truck lighter and stronger, therefore requiring less reinforcements in the first place.

But what about those cases where the rules mandate the minimum truck mass? Sometimes as much as 2 kg? Does it warrant including some more advanced features if there is anyway ample capacity to build them?

Well, it seems that the general answer is no. Again, these features are hardly ever necessary, whereas they introduce issues with strength and reliability that always seem to surface exactly during trials. In such cases with minimum mass requirements, at least for me, it turned much better to devote the extra weight to very strong and reliable suspension and steering, robust transmission, and designing the chassis so that all the heaviest components are as low as possible, while retaining sufficient ground clearance. Of course, in a typical Trial Truck, it's the battery packs and the motors that have a large share in the overall mass.

Ideally, even according to the standard car construction books, the weight distribution should be approximately equal on all wheels for maximum performance. In Trial Trucks, as well as in the real world cars, it is not always possible, or at least it would require too many concessions in other areas. In such cases I've found it helpful to move the center of mass a bit forward rather than back, while it should obviously never move aside. The reason is that the forward center of mass allows for better distribution of weight over all wheels while climbing, which is the most critical operation. It makes descent slightly more unstable, but usually that drawback is largely overshadowed by better climbing performance. Of course, for this to work as intended, the truck should have all-wheel drive, but this is something one should consider as granted for a Trial Truck anyway.

And if there is some mass to be added even after all the necessary components have been completed, it helps to distribute them evenly around the chassis (or again, slightly forward).

Onward, then, to some construction details. Try always to have as few gears in the entire transmission chain as possible. If you opt for a gearbox, make it as straightforward and robust as possible, with two gears usually being enough. And try to rely on the newer, bevel-style gears as they seem to be stronger than the older, spur types. If you absolutely have to go with spurs, the new 24T and 12T seem to be all right, but avoid the small 8T, even in their newer versions. And make sure all torque-bearing gears are properly braced from both sides and mesh cleanly.

Mentioning torque: it is generally wiser to trade a bit torque for speed. What I mean to say is that, instead of letting most of the axles turn slowly but carry large forces, it makes sense to use a couple of gears to make the axles turn faster yet transmit less force (i.e. torque). Of course, one should not go into extremes as friction comes into play at very high rotation speeds. But a transmission which mostly relies on higher speed/lower torque configuration, only to reduce the speed for more torque at the final stage at the wheels (preferably portal axles!) is often a good idea. Less torque also means less reinforcement, and more reliability.

4 comments: