Posted by Emese Nemeth

This sturdy bare-bones 0.58 kg, 14.4 W/kg racer is my fastest, best-handling, and most play-worthy LEGO RC car to date -- but only when the twin R motors (RC Race Buggy, 5292) are willing to play along. Top speed is ~3.0 m/s. R motors put out more peak mechanical power than any other LEGO motor by a wide margin, but that power is very difficult to harness in practice. Huge starting currents and relatively low torque output make it all too easy to trip an R motor's built-in thermal protection (a single thermistor) -- especially when lugged or stalled at full throttle. Once tripped, the R motor may be down for several minutes to over an hour depending on how quickly prevailing air and surface temperatures allow the thermistor to cool. Thermal shutdowns can be avoided entirely if the R-kart is restricted to surfaces that are cool, hard, smooth, and nearly flat. (An air-conditioned gym floor would be ideal!) On a steep enough slope or soft enough surface, however, the R-kart's motors can shut down before it even moves! The R motors never went into thermal shutdown during the clips shown here, but they did afterward. And once the shutdowns started, they kept getting more frequent, and the recovery times longer. The reason: Both the air and the tennis court surface were heating up rapidly in the noonday sun, and it was becoming increasingly difficult for the thermistors to shed heat. Mass: 585 g Drive motors: An R motor on each rear wheel (slower power take-offs only) Final drive ratios: 1:1 (direct drive) Steering motor: Power Functions servo RC receiver: Bluetooth-based SBrick (www.sbrick.com) Battery: 7.4V Power Functions rechargeable LiPo Photos and write-up coming soon at http://www.moc-pages.com/home.php/99234.