My Losi Mini 8ight on a 2s Lipo

[Psychron]

So I bought the Mini 8ight and decided I'd like to know what the stock top speed of this thing is on a 2s lipo once you change the gearing to match the recommendations in the manual. Here are my results.

Even running the stock motor with the correct gearing this motor gets hot.. and I mean fry an egg hot. Scorched fingertips hot. I'm on the third motor in this thing and I'm convinced the 4500KV motor installed by default just doesn't have the torque required to drive this buggy without overheating.

 

[Certified Mike]

30+ mph out of a stock motor like that is pretty crazy! NICE!!! It might be worth playing with the gearing a little more or just double checking for an issue. 2s or no, 'fry an egg hot' just isn't right!

 

[Psychron]

30+ mph out of a stock motor like that is pretty crazy! NICE!!! It might be worth playing with the gearing a little more or just double checking for an issue. 2s or no, 'fry an egg hot' just isn't right!

I've double checked the set screw on the motor and used the piece of paper technique between the gears to make sure there was no binding. I'm using the manufacturer's spec.. I could put a shorter pinion gear on, but that just takes away from top end and the motor still gets hot. I'm sure the issue is torque. I bought a second for my son and his has the same problem. the final drive ratio is 9.??:1 on this thing and if you're running full bore for any amount of time things overheat and the motor starts to stutter before shutting down.. The Mini 8ight DB I have has a final drive ratio of 12.??:1 and I bought a pair of those after the disappointment these gave me and I can run pack after pack after pack through them with no break and that motor is easily touched for 7 seconds or more before you have to remove your figer due to discomfort. I think the 3800KV motor from the DB could be mounted in the mini 8ight proper and then I could have four vehicles I could abuse all day long without issue and smile the whole time.

For us, it's not really about the speed, the DB is slower, but more fun, and when you crash it doesn't break because if it. I had the mini 8ight geared for and running on a 2200 3s, and it was too fast to enjoy. Ok, let me take that back, it was excitingly fast, but I spent more time running after it after it cartwheeled and launched itself 7 feet in the air and came down breaking one of those ball and cup suspension parts.
If I could figure out a way to get the final drive ratio changed I might be in business, but I think some of that would have to be done at the diff's right?

 

[Certified Mike]

There is only so much that a pinion and sour swap are going to do for you. That is true. I am not sure if there are different trans/rear end gears that might help. (I am not very familiar with this particular kit) With that kind of speed out of something so small, it seems you're Def pushing its limitations but the super heat in general leads me to think that something may be off. Of course, ambient temps will have some effect on it. There may be a way to gear it up a little and change tire height to net the same effects without having to get into the rear end gearing tho. Just as a thought...

 

[Psychron]

There is only so much that a pinion and sour swap are going to do for you. That is true. I am not sure if there are different trans/rear end gears that might help. (I am not very familiar with this particular kit) With that kind of speed out of something so small, it seems you're Def pushing its limitations but the super heat in general leads me to think that something may be off. Of course, ambient temps will have some effect on it. There may be a way to gear it up a little and change tire height to net the same effects without having to get into the rear end gearing tho. Just as a thought...

Agreed. My only thought is maybe I drop the 58 t spur for a 60t spur and then the 12 tooth gear will be 5:1 drive ratio, but final drive is a mystery.. I suppose I could figure out what makes 9:1 .. that math is not easy though.. I'd have to start with the stock gearing to find out what the transaxle drive ratio is.. like 60/12 is 5:1.. final drive ratio is 9:1 so I'd guess that the diffs are running at almost 2:1 to get the 9:1 final drive ratio.... given that I should be able to find a gearing that keeps it from overheating. I have new pinions and spurs laying around.. I've just had the thing apart so many times that I'd like to avoid screwing it up anymore lol. I'll do the math, check it out and come back to let you know.. though most people I've talked to have said get a lower rpm motor with higher torque like that in the Mini 8ight DB which is supposed to (with the 12:1 FDR) be able to use front wheels off any 10'th scale 4wd offroad buggy. Those are pretty tall tires. I have my doubts. Also, I've run it with tires that were a full inch shorter than the stock wheels and she still overheats. same with two I bought for my buddy (payment for general contract work) and my son. They've really been a disappointment.

 

[Certified Mike]

I do agree, the motor does seem to be the culprit by what you've experienced. I guess the question really is what you can truly expect to get out of this rig and what is worth doing vs what you would get out of just going with a different rig. You've Def got more experience with that than I do. I am Def curious to know what you come up with tho. It's difficult to walk away from a kit you already own. The thing is, if you could just get the temp down a bit, 30+ mph out of these cars is Def a respectable speed!!!

 

[Psychron]

I do agree, the motor does seem to be the culprit by what you've experienced. I guess the question really is what you can truly expect to get out of this rig and what is worth doing vs what you would get out of just going with a different rig. You've Def got more experience with that than I do. I am Def curious to know what you come up with tho. It's difficult to walk away from a kit you already own. The thing is, if you could just get the temp down a bit, 30+ mph out of these cars is Def a respectable speed!!!

I agree, It is a respectable speed, to be sure. so here's what I figured out. for about 60 total for each buggy, I can put the 3800KV motor they use in the Mini 8ight DB, move the 14 tooth pinion over, to it, and put a 62 tooth spur gear. I'll upload my excel file.. it has some calculations in it. Essentially I calculated the final axle speed at full throttle for all three of the losi mini's. Showing my first config scorching hot, followed by gearing for both the 4500 and the 3800 that would result in the same top speed as the Mini 8ight DB because the axle speeds would be close to identical. Because the 4500 wants to get to 4500 really fast I believe I'll still cook it and it'll accellerate too fast putting it out of the league of the Mini DB.

Short answer is, I want boith my Mini 8ight DB buggies which run warm and fine through 6-8 full packs without needing a break to be mirrored in the Mini 8ight. We're not a racing family and if we get there I think we'd go larger scale anyway, but I just want four 25 mph buggies that all accelerate about the same and can be played with together and can run 6-8 full packs each without needing a cool down period. You'll see how in the chart the scorching hot config for the mini 8ight puts a lot of strain on that little motor. We go camping as a foursome and it's fun to pull these all out and run them around but the poor friends that get stuck with the mini 8ight get left out every time because of the heat problem.

You can see what I tried to do when you play with the pinion and spur gear sizes. 8.4 is used as a standard power pack for these things. I really think the 3800 and the gearing for that is the best option. lower rpm = higher torque.

 

[Certified Mike]

I'll have to open all this when I get home. Poor signal on the phone here while I wait for the kiddo to get out of school. It sounds like you've pretty much for it sorted if your calculations are correct. I hope it all works out. It sounds like you all enjoy these cars and actually use them as they are intended!

 

[Psychron]

I'll have to open all this when I get home. Poor signal on the phone here while I wait for the kiddo to get out of school. It sounds like you've pretty much for it sorted if your calculations are correct. I hope it all works out. It sounds like you all enjoy these cars and actually use them as they are intended!

Oh then you'll enjoy this reading material.

so you put in the pinion and spur and it does all the rest basically.. Motor KV is known, as is internal drive ratio (differential ratio as listed on the specs) Voltage is a constant as well. After that the chart essentially divides the spur by the pinion to give you the transmission or motor drive ratio, then it multiplies the internal (differential) ratio by the motor drive ratio to give you the final drive ratio. Then your battery voltage is multiplied by the kv to give you the max rpm of the motor and then is divided by the final drive ratio to give you the rpm at the axle. It's fun to play with. There are some guys on another forum just not looking at any of this and saying "just buy some gears and play around and see what works best" I gave that crap up 20 years ago when I realized math really is good and works lol. Anyway, if you look at the two below, it's pretty clear I get very close with the 3800kv motor. The differences are negligible.

Consideration Mini 8ight DB
Motor KV 3800
Spur (t) 60
Pinion (t) 14
Motor Drive ratio 4.29
Internal Drive Ratio 2.6
Final Drive Ratio 11.14
Full 2s pack voltage 8.4
Full Throttle motor RPM 31920
Axle RPM 2864.62


Consideration Mini 8ight w/3800
Motor KV 3800
Spur (t) 62
Pinion (t) 14
Motor Drive ratio 4.43
Internal Drive Ratio 2.47
Final Drive Ratio 10.94
Full 2s pack voltage 8.4
Full Throttle motor RPM 31920
Axle RPM 2918.11

 

[Psychron]

So because the Mini 8ight has been a huge pain with all the excess heat even when using the manufacturer's recommended gearing for a 2s lipo, I've come up with this to solve it and make it run more like the Mini 8ight DB which has the higher torque motor and doesn't overheat even after 8 packs have been run through it. And I show you how I did it.

 

[Certified Mike]

Wow! I've finally got thru a good amount of your vids. A lot of good info! Curious if you've got a chance to really run these kits side by side and if you're results are as positive as they look on paper etc... Also got to see the 'wall of flight'... Looks like a lot of fun hanging up there!

 

[Psychron]

Wow! I've finally got thru a good amount of your vids. A lot of good info! Curious if you've got a chance to really run these kits side by side and if you're results are as positive as they look on paper etc... Also got to see the 'wall of flight'... Looks like a lot of fun hanging up there!

Hey Mike,
Thanks for the compliments, (bows head modestly) I'm trying. The wall of flight needs more! lol I think a scratch built motor glider would round it out quite nicely. And I used e-calc the other day to try to put together a 90mph plane powerplant for a second aircraft. I'm not sure I'll be successful with it but I'm going to give it a shot anyway. My son needed something to do that day so I let him run the camera. Not bad for his first time, but my mini tripod came in today so I might start letting him help me out more with 'other" lol.

As for running the cars side by side, I don't have a partner to do it with today, but the weather is going to cooperate so I'm going to take it out to the community college today and give it a "post mod speed run" and hook it up like the last time, with the gps and all, then throw it up on the channel sometime this week. All I can say for sure at this point is that I ran a full pack that night in the video and the temps were perfect. and the bumps in the street kept sending her airborne and upside down (hence the run to the skid pad today)

I called horizon hobby and told them what I did and had to coax the answer out of the ground team that I was expecting which is that the 3800 does have more torque than the 4500, so already it'll be more efficient and probably run longer. I couldn't get a definitive answer on running a 3.25" vs a 2.75" diameter tire aside from what essentally amounted to "you might have to put on a 12t or 13t pinion if you want to keep the heat low." And I couldn't find any simple math regarding tire size / torque comparisons that didn't apply to full sized cars or industrial machinery. Rolling resistance doesn't interest me etc. At this scale, and weight, I just need to ballpark it. I suppose I can just go back to the run it for 5 minutes, take the motor temp, another 5, take the motor temp, etc, and if the taller tires make that big a deal, I'll have to drop to a 12t pinion gear to get my numbers back to what was in the chart. I added a section that tells my mph based on the circumference of the tire. The Equation is (Tire circumference x axle rpm x 60 minutes in an hour) / 12 inches in a foot / 5280 feet in a mile. It comes very close to actual given a speed on the chart of 23.4 mph, and the gps results of 25mph, and then factor in the gps fudge factor and and we're right in the ballpark. Regardless, 2.2" rims with pre mounted 2.75" diameter tires are plentiful and cheap on amazon so I might just stick with those.