Sariel.pl

Simple inverted trike built for speed record attempts. Features front suspension and single driven rear wheel.

Datasheet:

Completion date: 12/04/2014
Power: electric (RC unit)
Dimensions: length 41 studs / width 20 studs / height 12 studs (not including antennae)
Weight: 0.684 kg
Suspension: front – full independent / rear – none
Propulsion: 2 x RC motor geared 1:1
Motors: 2 x RC motor
Top speed: 20.78 kph

I have been thinking about possible ways of topping my personal speed record of a LEGO creation, which until now was held by my Pagani Zonda at 15.41 kph. Having been somewhat disappointed with performance of four RC motors, I wanted to try to use just two, with a single RC unit, in some very lightweight vehicle. One of the issues was the choice of wheels – the Zonda’s wheels, for example, were exceptionally heavy, although they did provide excellent traction. Eventually, I decided to go for some lightweight, large wheels with slick tires made of solid rubber, without air inside. This is because I wanted lowest rolling resistance possible, and this kind of resistance comes partially from the tires being deformed by vehicle’s weight, which is minimal with solid rubber. At the same time, slick tires would provide much better traction than no tires at all.

Another question was the drivetrain – using just two RC motors called for RWD vehicle, because AWD system would add much weight and many moving parts with extra friction. In my experience, very fast RWD vehicles have great need for differentials, because they easily spin out at high speeds. The differentials increase level of complexity and are fragile – the rear differential in my Zonda got it’s inner gears snapped while braking with motors. It took me a while to arrive at a conclusion that the best option is to simply use a single rear wheel. This removed any need for a differential while making the vehicle perfectly steerable and free from tendencies to spin, and with the wheel installed between RC motors, the drivetrain was extremely simple and robust, allowing to brake with motors without any damage.

The resulting vehicle was an inverted trike – with two front and single rear wheel. All wheels were Lego motorbike wheels with slick tires – narrow in the front and wide in the back. The wide narrow tire proved to ensure great traction, partially thanks to the fact that with just one rear wheel more than half of vehicle’s weight rested on it. The rear wheel was mounted between the motors so that it could be easily swapped between their faster and slower outputs. It was obvious, though, that the vehicle was much faster with the faster outputs driving the wheel, and this is the configuration I stayed with. The front wheels were attached to the RC unit’s steering attachment, which was robust, lightweight, and provided proportional steering and positive caster angle. This allowed me to use independent front suspension, thus minimizing amount of shocks transferred to the rear wheel, and to make minimal directional corrections while driving, using the proportional steering.

The final top speed was decent, but I’m sure it can be topped, perhaps on a different kind of surface, with a different kind of wheels. Seeing as the trike took quite a while to fully accelerate, I would say that lowering weight may be an option, because torque seemed like an issue here. There may also be potential in a gearbox, with two or more speeds shifted remotely during acceleration, but with LEGO gearboxes adding a lot of weight and friction, the advantages may not be worth the effort. In any case, I don’t think that “legal” LEGO solutions allow significantly higher speeds – this model probably cuts it pretty close.

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