The cars may be small but their performance is remarkable. We investigate the sport of radio-controlled racing...
I am standing with a controller in my hand connected to a 1:10-scale racer on the track below. This can only end badly…
To preserve the expensive toy from a rookie mistake, Francois Laubscher, the car’s owner and member of Parow Radio Control Car Club (PRCCC), has dialled down the responses to about 25 percent. Even so, the little racecar responds instantly as if there is a miniature version of myself behind the wheel. I lap the circuit at a snail’s pace compared to the professionals, but it is clear: this is real racing.
There are many recognised formulas in the industry but we will focus on the 1:10-scale modified class. It all starts with a chassis. This carbon-fibre backbone of the vehicle forms the skid-plate underneath and provides the mounting points for the drivetrain, suspension and control electronics. The ground clearance varies between 5 and 8 mm depending on the smoothness of the surface.
This specific RC class determines the motor specification (in this case, a 4,5T referring to 4,5 turns) where the number of turns (or windings) around a stack on the armature (rotor) of a brushed motor or stator on a brushless motor define the speed and torque characteristics of the unit. In general, the fewer the turns, the less electrical resistance and faster the motor. The brushless motors can spin up to 40 000 r/min and have a power rating of up to 800 W. This gives the RC car featured here a power-to-weight ratio of 593 W/kg (versus the “dainty” 362 W/kg of a McLaren 720S). These motors are air-cooled, although temperatures can exceed 80 °C. As the brushless motors run on three-phase alternating current, a speed controller is used to convert the direct-current supply from the battery. A brushless motor with a speed sensor allows for phase-timing adjustment to change the torque-speed characteristics of the unit.
Battery technology has improved dramatically; nickel-cadmium has made way for lithium-ion. In the case of RC cars, lithium-polymer is now the preferred choice with a capacity of 6 200 mAh in the case of the Xray T4’17 (see previous page). The capacity-versus-mass ratio (energy density) is impressive but even more so the current-draw capability of more than 100 A. At 8,4 V, this means 0,84 kW to power the electric motor, cooling fans and electronic control systems under race conditions.
The modified class is all-wheel drive with a drive belt connecting the front and rear axles. The front axle is solid but aft employs a differential; its characteristics can be tuned by using different-viscosity oils inside.
The Xray T4’17 employs double wishbone suspension front and rear, coupled with a spring and damper setup. Spring rates and lengths can be altered and the damper oil’s viscosity tweaked to adjust the suspension behaviour. Wheel alignment is critical and a special tool is used for checking and correcting.
Tyres are rubber or foam. In general, the former results in higher performance but is difficult to control at the limit where the grip drops off. Foam tyres are more predictable but the ultimate pace is slower. For competitions, a set of control rubber tyres are mandated with certified specifications because this takes compounds and the different sponge insert (no air pressure) out of the equation. A set of tyres will last about three sessions of five minutes and “muti” (a special liquid) is applied to the surface of the tyres to increase grip before a race.
Forget the radio-controlled (RC) cars you can buy at toy stores. The ones these enthusiasts use are bespoke road racers built from the ground up and not bought as complete units.
Whereas internal-combustion-engined cars used to be top class a few years ago, the electric revolution has reached RC racing, too. According to PRCCC member Anthony Smith, electric cars are now cheaper and, more importantly, faster than petrol cars. Plus, there is no pollution (noise and exhaust emissions), so electric RC racetracks can be located in easy-to-reach neighbourhoods. Is this a glimpse of the future of conventional motorsport? Certainly.
Performance measurement challenge
CAR magazine regularly measures and analyses the performance of production cars around a racetrack, usually with ace racing driver Deon Joubert behind the wheel. We time with a Racelogic Vbox GPS-based system. Unfortunately, this unit is too bulky and heavy to employ on an RC car. The other challenge is the sheer speed and manoeuvrability of the scale model which taxes the GPS’ accuracy on the relatively tiny circuit.
We contacted an engineering-consultancy firm specialising in such projects but even they were unable to provide a feasible solution. Plan B was to employ a cellphone weighing 165 g equipped with a track analysis app called RaceChrono Pro. The cellphone was carefully strapped inside the body of the Xray T4’17 piloted by Brandon Solomon. The added mass dulled the performance but allowed us to record some of the data. Below is a summary of the results.
We’d like to challenge the local engineering fraternity to come forward with a measurement solution (to record position, speed and accelerations) weighing less than 50 g with a data acquisition rate of at least 10 Hz. Do that and we’ll retest an RC car.
After attending the race event which prompted this article, I can confirm it is as fierce and competitive as full-scale motorsport. Transponders are mounted on cars to record lap times, vehicles are randomly inspected to make sure they abide by the category rules and racing is wheel to wheel. The quick reactions of the “drivers” on the balcony overlooking the track are cat-like as they pilot their four-wheel missiles around the circuit.
The pit area consists of benches where batteries are charged, tyres are warmed to 50 °C (yes, tyre warmers are used) and the vehicle setup is completed. The professionals have toolboxes that would not look out of place in the pits at Killarney Raceway. It is such a focused sport that it attracts actual racing drivers such as Johan Fourie, multiple SA champion, who is a regular at PRCCC to hone his reactions and practise his racecar setup skills. At least when things go wrong on the track, it is only your car and ego that get hurt. Now, if only they could shrink me so I would fit into one of those racers...