I now have my low buck 1/8 conversion running, and I felt I should start a new thread to talk about it rather than hijacking the racing thread.
SO far I have about 15 minutes of run time on the car, for those who have not seen my posts, it is a beat up old Duratrax AXIS chassis that needs some work, I am just using it as a test bed for the drive train to see if this minimalist setup will have what it takes to run with a current 1/8 nitro. I don't expect this thing to corner or jump with a modern race bred chassis, but if I can accellerate and brake as well at any speed, I know I am in the ball park and I will likely look for a better chassis. For now I am looking only at motor performance.
My motor choice bucks a few trends. Even in my own discussions this was an oddball shot, but I based it on a lot of math and data from my 1/10 4WD setup. I bought a Medusa 36x50 mm motor with a 3300 KV rating. This is much higher rpm that I was originally going to use, but I also decided on limiting to just 4S LiPo for the batteries, so the RPM is not stupid high with an unloaded 48,840. This motor is a bargain at just 5 now straight from Medusa. It is rated for 90 amps peak, 75 amps sustained with good air flow, and a 700 watt input rating. For my initial testing I have it geared very conservatively. If it hit the KV x volts rpm, it would be going 34.8 mph. I am running a Mamba Max ESC with a Common Sence RC BEC to power the radio gear. For testing I wanted to limit the current a bit, so I am using my two well worn (ABUSED) Max Amps 2S 3000 20C packs wired in series. These packs have shown to limit current to under 40 amps with the LVC set at 3.0 volts per cell, so I knew ESC and motor heat would not be an issue. SO onto some data log results.
The first run, I had not re-cal'd the logger for 4S and the numbers were just plain wrong. I also realized the ESC LVC was still set for 2S Oops!! The un cal'd log showed a solid 9.0 volts, no variation ever, and the current seems very low as well, so the run was useless, except that I found it to drive quite well with much better acceleration than I expected from these worn packs and the motor and ESC stayed cool. After 5 minutes of hard running the motor was barely warm and the ESC was just noticeably warmer than the chassis around it. I am guessing the voltage was being pulled down too far as the batteries were fairly warm.
I went through the calibration and sure enough the voltage was cal'd wrong and clipped at 9.0 and the current was only reading 2/3's. I now have it agreeing with my Fluke 76 meter within 0.05 amps and volts on various test loads with the full 16 volts going in, and I was now able to see the voltage dip when I pulled current. I also set the LVC to 12 volts for the 4S setup. I drove it as hard as I could and with the higher LVC it did take a bit off the low speed acceleraion, but it is still no slouch. This time the data log was real. I peaked at just 522 watts at 57.65 amps. This is scary as it means the pack voltage was still dipping to just 9.05 volts before the LVC could back the current down. With the battery dead cold, it would only hold 25 amps at 12 volts, and once warmed up, it was still only hitting 35 amps for a sample or 2 and then falling to the 30ish range for the first full second with the voltage staying at 12 volts for a motor input power of just 360 watts. I never expected the acceleration rate I was seeing with just 360 watts going to the motor. On a fairly grippy asphault with Bow Tie tires biting it, it got just a tick of wheelspin and the chassis would rock back so the front tires were just shy of lifting off the ground. I was able to hold on for just over 3 seconds to wind it out. It was not quite able to truely top out because as the current would unload the battery voltage recovered and kept climbing so it was still pulling 15 amps with the voltage up to 13 when I had to get out of it as I ran out of room. This works out to reaching a true speed of just 29.5 mph. 13 - 0.45 volts of losses etc. It looks fast, but I know it will ned a bit more on the track, and I already figured I need 2 more tteth on the pinion, but these batteries are certainly not up to race pace running. I know for a fact my 2 5000 20C packs will hold over 3 volts per cell at over 60 amps meaning I should get over double the watts into the ESC. I need to finish the battery trays for them, I have the tiny Max Amps 3000's just stuck on with double sided tape for now.
All in all, I am very pleased with the result so far. I have 0 in the ESC, in the BEC, 5 in the motor, a few hours in my shop building the mounts and spool, and probably another 70 to 100 in various hardware and connectors etc. Certainly on the very low end for a 1/8 electric conversion. I hope to have the 5000 packs on it tomorow some time and get the suspension working a bit better, the shocks have quite a bit of drag making it a bit jerky in the handling department. I did get brave enough to jump it over driveway aprons and whip it through some very fast turns, but on a smooth surface. I like the mass of it over the 1/10, it certainly has a big car feel where my 1/10 XXX-4 is so nervous.
The Datalog also showed something I never saw before. With all that mas and the spool maing the braked work very hard, I actually see 5 to 7 amps go back into the battery raising the pack voltage almost 0.5 volts for over 1/2 second when I lay on the brakes from full speed. I don't think it will ever overcharge a battery, but this was a peak of nearly 2C charge current into these weak packs. On my 1/10 cars, the data log never showed enough braking power back into the batteries to be any concern at all, this just might be. And I even have the braking dialed back to just 30% now. That 4 pole motor and this high revving gearing sure makes for a ton of braking. It stops harder than it ever did on the friction brakes. The lack of bias adjust may be a small issue. It does understeer under braking right now, but it is also front heavy with the light 3000 packs and no battery trays. I will also need to dial it in on the actual dirt track to know for sure what it can do.
Look here for more data as I get some test miles on this setup. With the performance I am seeing at just 30 amps, I can't wait to see it fly at 60 amps, still well under the motor rating. The Mamba Max Medusa combo really looks good so far. Especially when you consider it is nearly 0 less than a Monster Max + motor combo will run. The earlier non-calibrated run did show wheel spin with the LVC set wrong. If we can believe the current was off by 50%, it looks like the wheel spin was happening at about 52 amps which is a little higher than my prediction, but still in range of the electronics. I know it was reading low, as it only reported 35 amps.
My 2 minute run also took just 350 mah out of the battery. I know this is not realistic. The current limiting to just 30 amps kept this low. My goal of 15 minute races on the 5000's looks possible though.
I will try to ge a few pics of it to post.
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