Wednesday, July 18, 2007

How-To: Nitro-to-Brushless Conversion: Part 5

Part 4 can be found here.

We have finally arrived at perhaps the most interesting part of this how-to on the conversion of a Losi 8IGHT 1/8th-scale buggy from nitro to brushless/LiPo power: the installation of the motor, battery, ESC, and supporting components.

Here are a pair photos of the completed car without the body or battery:

This photo shows the 5000-mA-hr battery in place:

You can see a few important aspect of the car's construction from these pics:
  • I've put everything except the battery on the right side of the car or along the centerline. Id did so because, as I said, the battery is quite heavy. With the 5-A-hr battery shown, the weight is distributed slightly to the right, but with the larger 8-A-hr battery, the left tires carry more weight so I think my arrangement is a pretty fair compromise.
  • You may also be able to make out the custom speed-control mount/heatsink that I bent and cut out of 5052 (bendable) aluminum. I'm quite pleased with it; it places the ESC near the centerline of the vehicle and keeps the ESC running cool.
  • The little circuit board thermal-epoxied to the top of the ESC is a temperature sensor. I plan to put another on the motor. Since the transceiver and servo run off a 5-V voltage regulator, I added a voltage sensor---really just a pair of wires with a Rx/servo connector on the end to the setup. You can see the leads disapearing into the ESC's input connectors in the photos.
  • I've dispensed with the front and read disc brakes, which normally sit just ahead of and just behind the center differential. Instead, I'm relying on motor braking. It seems to me that disk brakes are more efficient, from a current-draw standpoint, but I just didn't have room for the servo that would actuate them.
  • As I mentioned last time, I had to grind the motor/diff mount down to fit under the body. (The part was actually made to fit the Losi 8IGHT-T truggy.)
  • Those of you who know me are aware of the significance of the number I've chosen to place on the wing.
Here're a few photographs of the car with the body on. I'm very pleased with this particular color scheme. It addition to being attractive---at least, in my opinion, which is the only one that matters---it has proven itself to be quite visible from distance. Incidentally, I wonder if Lokai and Bele could find common ground by agreeing on the virtues of this paint job.

Next: some shots of the car with road tires---Pro-Line Road Rages---mounted in place of the dirt-track tires---Pro-Line Crime Fighters1 ---shown above. I also purchased some extra-knobby all-terrain tires---Pro-Line Badlands---for more rugged surfaces.

This last photograph shows how I measured the weight distribution. Yes, those are season box sets of Star Trek: Deep Space Nine. And yes, that is the second Trek reference in this post, but I'm not some kind of raging Trekkie; these boxes were the first objects I could find that are all the same thickness, and very close to the thickness of the scale.

At this point, the conversion is done, and I'm quite please with the way it turned out. The car accelerates like nobody's business. The acceleration is far better than a nitro-powered buggy can produce, since nitro engines' torque peaks at many thousands of RPM, while the motor in my buggy produces full torque at zero speed. The vehicle also hits remarkably high speeds. I used Alisons GPS receiver2 to measure the top speed to be 41.8 mi/hr with a 14/48 pinon/spur and the road tires. (This test was done on a straight, nearly flat strip of asphalt with nearly no wind.) Additionally, the buggy remains flat---doesn't roll---in corners, and it can be slid very dramatically on smooth pavement.

I only plan minor tweaks from here on out. Here are some of the changes I'd like to make, in approximate order of precedence:
  • Adjust the motor braking. It's way too aggressive at the moment.
  • Add a second temperature sensor to monitor the motor itself.
  • Add another strap (or some other component) to the battery tray to hold the larger, 8000-mA-hr batteries in place securely and cleanly.
  • Replace the Deans Ultra Plugs I have on the batteries and the input to the ESC with 5.5-mm bullet connectors, like I have on the motor and the ouput of the ESC. Deans aren't really meant to be used over 50 amperes, while the 5.5-mm bullets can be run up to 200 amps.
  • Experiment with gearing. A simple pinion change to 15/48 should put the vehicle just under 45 mi/hr. If that doesn't hurt the low-end acceleration, I might go to 15/47. I suspect 15/48 will be close to the ideal gearing, though.
  • Experiment with the suspension parameters. The setup suggested by the manual is pretty good, but I think I should stiffen it up a bit, since the car is a bit heavier than the nitro version.
  • Possibly solder the ESC directly to the motor to reduce loss and improve reliability. I'm not sure if that step will be necessary.
I would be remiss if I failed to point out that I got a lot inspiration and information from the forum at RC-Monster. com. Users BrianG and glassdoctor, as well as Mike Cronin, who runs RC Monster, were especially helpful.

Your comments and suggestions are welcome.

1 Pro-Line's tire names, especially those designed for off-road racing, have a criminal theme to them. I addition to the aforementioned Road Rage and Crime Fighter models, there are also the Cell Block, Mugshot, Inside Job, and Dirty Harry. It not clear to me whether the Knuckles model counts.
2 We have named the device Garmin Tanzarian.

Part 6 can be found here.

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