Posted by Emese Nemeth

Here we have 2 very different LEGO RC cars -- a very light "R-kart" (yellow), and the much heavier "L24", a motorized MOD of the "24 Hours Race Car" set (42039, green). Both have the same 0.37 N m of rear-wheel torque, but the R-kart's power-to-weight ratio (PWR) is almost 9 times greater, and that makes all the difference. Each car is powered by a single 7.4V PF LiPo battery, steered by a PF servo motor, and controlled by a Bluetooth-based SBrick RC receiver and Android phone. The wheels and tires are also identical except for color. The bare-bones 0.59 kg R-kart opening the video is driven by a pair of "R motors" (RC Race Buggy, 5292) delivering 11 W of total peak mechanical power. The resulting PWR is a whopping 18.8 W/kg, and it shows in the R-kart's top speed of ~3.2 m/s (~11.5 kph) -- pretty darned fast for any LEGO RC car. The R-kart ran balloon tires in its YouTube debut on a smoothly paved tennis court at https://www.youtube.com/watch?v=Jd91-TCza8c. Here it's running smoother low-profile ZR tires to reduce rolling resistance on rough asphault. No other changes were made, and the rear wheel radii are the same. The R-kart went out of Bluetooth range once at 0:35, but neither car shut down from thermal overload despite the hot day and warm pavement. The L24 enters at 1:36. This much more complex model weighs 1.40 kg and is driven by a single L motor delivering only 2.2 W of peak mechanical power. The much lower PWR (1.5 W/kg) is reflected in its top speed, which I estimate at only 0.8 m/s (2.9 kph) or so. Adding a single L drive motor and PF servo steering motor to the L24 turned out to be surprisingly easy. Other than cleaning up the color scheme, I left the original set's excellent styling alone. I later went on to add a 2nd L drive motor to the L24. The resulting "L24t" MOD (not shown) is ~30% faster than the L24 and much stronger on hills, on rough or soft surfaces, and in tight turns. In fact, it's much more capable than the R-kart in these settings. L24 and L24t write-up and photos @ http://www.moc-pages.com/moc.php/429574 L24t video @ https://www.youtube.com/watch?v=iYHLZrI_HJU R-kart video with photos @ https://www.youtube.com/watch?v=Jd91-TCza8c R-kart write-up and more photos coming soon @ http://www.moc-pages.com/home.php/99234