So I fixed up my Hobbyzone Super Cub today to go flying, charged up my new 3-cell lipo pack, and waited for the wind to die down so I could fly it. 7 hours later the wind was still blowing strong and I was itching to play with some R/C stuff. So I pulled down my RC18T off the shelf, which the last time I played with it I had dropped a 540-sized motor into it with a 2-cell lipo battery. Then a year later the lipo had expanded to twice its regular size and was basically dead, so it got thrown out. Otherwise the truck was in great shape and actually drivable, with the exception of needing a battery. That’s when I had an idea, why not try and fit the 3-cell lipo pack from my Super Cub into my RC18T? Sure a 540 in that truck is already overkill, but how about 11.1V to a 540 motor in this little 1/18th scale truck, that’s not overkill, is it?
So about 10 minutes later and with the help of some tape, I had strapped that 3-cell lipo battery into my 540-powered RC18T, and this thing was about to rock. I plugged in the pack and clipped on the body and ran the wheels a few times just to get a feel for how much power this thing was going to have and was quite impressed with how much the tires ballooned as they expanded to about twice their normal size under the centripetal motion of the high-velocity spinning. The Epic Binary Outlaw stock 540-motor can spin at around 33krpms at 7.2V, the nominal voltage rating for this motor, but once you bump that up to 11.1V of the 3-cell lipo, we achieve a maximum free-running RPM of just over 50krpms. That kind of speed you typically only see out of low-turn, current-hungry modified motors. I was running a 30T pinion gear and the stock 60T spur gear which amounts to a 2.0 gear ratio. Combine that with the internal gear reduction of the differentials which is 2.5:1 and we get a final gear reduction of 5.0:1. That means those wheels with this setup are spinning along at just over 10,000rpms. The total rollout (given the tires I’m running which are about 2.2″) high equates to about 7″ per revolution or 70,000 inches per minute which amounts to about 66mph. However there are many other real factors that that keep the truck from reaching that kind of speed. I usually knock off about 25% of the free running rpms due to loading, drag, and friction, which brings our real-world estimated top speed to 50mph. I was able to measure only about 47mph for a top speed (using the distance-over-time method), and that was just the one run I was able to measure and calculate. Other runs up and down the street felt faster, so in all fairness I believe this setup can reach 50mph. But WOW, was it fast. I mean 50mph is fast, it feels fast, it eats up so much ground so quickly and when it flips and rolls, it just rolls end over end endlessly. It is a ton of fun to drive too. The rear of the truck would get sucked down as I accelerated lifting the front up just like my old T-Maxx used to do. It looked super cool.
So what’s next? Make it faster! There’s a few things that could be done to make it faster. The quickest one would be to gear up and pop on a larger pinion. The downside is is harder on the motor, the ESC and the lipo to run taller gears, and as it was, both the motor and the battery were getting pretty hot with the FETs on the ESC being the hottest part (hey it’s only rated to 8.4V, that fact that it worked at all was pretty amazing). So even though gearing up might provide some short-term increase in top end, the car already felt like it needed to be geared a few teeth lower, so if anything, I should gear down. The other option is to run a faster motor, like a 19T or even a 14T or lower. The problem with the hotter winds is they are harder on the battery, and the cheap little 3S pack I have is barely rated to 18A continuous so no doubt a faster motor will burn up that pack, which means I’d have to get a new 3S pack, most likely bigger pack which can handle the higher-current demands of the faster motor, but they they get big, and they get heavy and they cost $$$. So I suppose for now it’s fast enough and hasn’t cost an arm and a leg to get up there. Making it faster from here on out would take some more money. I’m actually thinking about getting one of the Mamba MAX brushless motors/ESC combos from Tower Hobbies. They’ve got 4 different winds, the 4600kV one being just about right on par with my Epic Binary Stock motor, though having all the benefits of brushless over brushed, it would be at least a little faster than my current setup while being easier on the battery pack, lasting longer, etc. But it’s not reliability that I want, it’s speed (well, and I’ll take the reliability too). Using the same formulas I applied above, if I got the fastest Mamba MAX they make topping out at a whopping 7700kV (that’s over 85krpms unloaded at 11.1V) and even though it’s not rated to handle a 3S lipo, I’m sure it can handle the voltage fine under the right conditions, being light on the trigger, and with the right gearing. Given the same gearing as I currently have, a 30T pinion and 25% loading factor I could achieve a top speed of 84mph. Though in reality with the faster motor I should gear it much lower, so if I dropped to a 25T pinion the result would be a 6:1 final drive ratio which equates to about 70mph top speed. Sure that sounds fast, but it’s only 20mph faster than my current stock brushed motor, and that Mamba MAX costs nearly 7X what that Epic motor cost me. Though for the coolness factor, it would sweet.
After going through 3 packs, and trying it out in the dirt, I toasted the spur gear and broke another turnbuckle. In the dirt this thing was insane, but completely out of control, I couldn’t gain any traction. And then I remembered why I hate driving this thing in the dirt – rocks get in the spur gear every 10 seconds and stop the car dead. It’s definitely better in the street. After my truck was out of commission, I pulled out the battery, dropped it back into my plane, and went flying.