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1/8 scale monster truck brushless conversion

Sandy Johnson

Sandy Johnson

Harry Member
Joined
May 19, 2020
Member Number
247
Messages
2,809
Loc
Spreckels, Ca
So I had this old Associated MTG 4.6 sitting around and the kids kept pestering me to get it running. Decided I was over nitro when the motor blew up trying to start it, soooo I figured I'd go brushless instead. Everything I really know about RC cars is from racing the original RC10T in its heyday back when electronic speed controls were still a fairly new idea, so basically I am bumbling through this.

Here it is getting used as a giant toy car

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Here it is just before I blew the motor up trying to get it to run after sitting for a few years.
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There's a complicated 2 speed transmission with a bunch of extra shit to make the drivetrain work. I figured it would be more simple to use a center diff out of another 1/8 scale rc rather than hook up to all the existing mumbo jumbo. Turns out the hardest part is finding a bracket that will hold everything in place. I did some google and found GRCWorks in Greece still makes what I need. THUNDER TIGER MTA4-TEAM ASSOCIATED MGT BRUSHLESS CONVERSION
I rolled the dice and ordered. It showed up about a week later, score!
PXL_20221025_185438792.MP.jpg


After that I bought some chineseum motor and speed controller off of amazon(we'll see if the speed controller holds up). The plan is to run 4s and 6s batteries. Probably 4s to start since my 5 year old will be crashing...er driving it more than I.
So this is where I'm at while a wait for a pinion gear for my electric motor and try to decide what radio and receiver I'm gonna run: 2200kv brushless 1/8 scale motor, a 120a brushless esc, and a center diff and spur gear out of a associated RC8b3e.
PXL_20221104_041305069.MP.jpg
 
Still waiting on some parts, but I slapped in a spektrum receiver I had and a 2s battery and drove it around the yard.

Things that bothered me:
There appears to be a throttle delay. I have to floor it and wait a second for the motor to spool up.. so either there is something I don't know about 2s batteries with an over-powered motor, or I need to mess with the program in the esc.

Also it atw a 5000 mah 2s battery in about 10 minutes... I had planned on either a 6000 ot 8000 mah 4s. I guess that means I want to go with the 8000?
 
So, I got some 4s batteries and it hauls ass. Played with the config on the ESC and got it pretty close to where I wanted it.. until kerblooie- too much wheel speed:
PXL_20221119_182112412.jpg


Also noticed this thing has no down low power- sometimes it would stall in the grass. 2000kv motor on 4s battery, so I figure it's not geared low enough. Currently has a 15t pinion gear and a 46t spur gear. I can't go bigger with the spur gear because it'll hit the chassis, and it's really hard to find anything lower than 15t in a 5mm bore, 1p spaced gear. I hunted around and found an 11t online, so we'll see how that fits when it shows up. Is there any other way to get lower gears in these things? I don't see any kits to gear the diffs lower. I guess if the smaller pinion gear doesn't cut it I'd have to go with a lower KV motor?
 
Harry Johnson said:
So, I got some 4s batteries and it hauls ass. Played with the config on the ESC and got it pretty close to where I wanted it.. until kerblooie- too much wheel speed:
PXL_20221119_182112412.jpg


Also noticed this thing has no down low power- sometimes it would stall in the grass. 2000kv motor on 4s battery, so I figure it's not geared low enough. Currently has a 15t pinion gear and a 46t spur gear. I can't go bigger with the spur gear because it'll hit the chassis, and it's really hard to find anything lower than 15t in a 5mm bore, 1p spaced gear. I hunted around and found an 11t online, so we'll see how that fits when it shows up. Is there any other way to get lower gears in these things? I don't see any kits to gear the diffs lower. I guess if the smaller pinion gear doesn't cut it I'd have to go with a lower KV motor?
Click to expand...
Going 4 teeth down on the pinion will be a huge difference.. basically from 3-1. To 4-1
 
2big bronco said:
Going 4 teeth down on the pinion will be a huge difference.. basically from 3-1. To 4-1
Click to expand...
Someone on an rc nerd forum told me my 120A speed control wasn't enough for the weight of an old monster truck and I should bump up to 150A.

Gonna see what the pinion gear does, but may have also ordered a bigger esc. I pretty much just want it to drive just like a regular monster truck (wheelies and stuff but not a lot of top end), and then I'll be happy.
 
My guess is your cheap brushless isn't sensored, and the delay/hesitation is cogging.

I converted my nitro Savage to electric with a Hobbywing brushless. Waaaaay too much power. It'll do backflips at 35mph.
 
Brecklin said:
My guess is your cheap brushless isn't sensored, and the delay/hesitation is cogging.

I converted my nitro Savage to electric with a Hobbywing brushless. Waaaaay too much power. It'll do backflips at 35mph.
Click to expand...
Yes, not sensored. EDIT- I just looked up what cogging is, that's definitely what's going on. We'll see what the beefier esc does to help. This is a project that was mostly parts bin on the cheap stuff, so I'm not quite ready to thrown a whole sensored setup on it until I see my kids actually keep an interest first(their first love is rc crawling).
 
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2big bronco said:
Hows the new pinion gear?
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Well, it turns out I'm learning lots about RC cars with this thing. It had bigger hex hubs than my 1:10 crawler and short course truck so when I bought new wheels and tires and saw that some were 12mm and some were 17mm, I assumed it was that. Turns out no. So then I bought some 12mm to 14mm adaptors and when they showed up and were too small (12mm is the 1:10 scale size apparently), I finally got off my ass and measured. 14mm.. wtf. It looks like 17 and 14 mm might use the same axle size, so I ordered some 17mm hex hubs and they're gonna get here tomorrow.

Meanwhile, my kids have literally been driving the wheels off the current setup(RIP any and all tire glue), so hopefully this next attempt solves the issue.

On a positive note, it turns out my 2.5 year old can actually drive this thing reasonably well as long as he's in a big grassy field with nothing to distract him.
 
Also, funny story from this weekend:

I'm letting my 2.5 year old drive this thing in the grass at our local park. He had proven himself the day before with no one around, so today I let him do it while other kids were playing there as well. My older kid is running around kicking a soccer ball with some of his buddies, one of our neighbor's 4 year olds has borrowed our little 1:18 monster truck and is ripping it around the grass, some random kids are playing kick ball in the corner, even a few dogs out running around.

I have a talk with my little guy and tell him he can drive the thing fast, but not near anyone else, and if another kid comes near, he has to slow down. He's doing a great job at it when suddenly... The hyper active 8 year old who lives across the street from us shows up. Somewhere in the mix while his mom is talking with my wife, he gets a hold of the soccer ball and starts playing try-to-smash-the-little-rc-car-with-the-ball. The 4 year old is good enough and his soccer ball throwing aim is bad enough that he hasn't hit that little truck yet. I see it go down out of the corner of my eye but figure I better hang out with my youngest in case he sends a 1/8 scale missile toward any unsuspecting children. Suddenly this kid appears in front of us. My son dutifully stops the truck and it's a face off of about 20 feet.
"TRY TO DOGE IT!" the neighbor kid shrieks with wild eyes, and starts running toward the larger RC with the soccer ball over his head.
"Hold on, that's not a good idea!" I try to command in my best booming dad voice, but it's too late. 2.5 year old makes some sort of an excited/panicked squeal and drops the hammer. The RC shoots forward amid a cloud of flying grass blades and catches Mr. Hyperactivity by surprise with how fast it moved compared to the other RC he had been hunting. At the last minute before they came together, the kid tried to jump over the RC but his timing was waaaay late and the truck caught him in the shin at mach chicken just as he left the ground.

The result was spectacular, and made even more amazing by the fact that this kid runs everywhere in crocs with the safety strap disengaged. He got cartwheeled in the air, the soccer ball went flying, but the most impressive part was his left croc getting sent into orbit like it was shot out of a cannon.

In the aftermath, I got a 2 for 1 dad yelling at both of them. My youngest for "driving fast" around other kids when I told him not to, with a great example why he shouldn't(taking out the older kid made him cry), and at our neighbor's kid for charging a monstrously fast RC and trying to break it with soccer ball. The second conversation came after I made sure he hadn't broken a leg or foot or anything like that. Meanwhile, that kid is on the ground holding his shin and his mom up on the hill missed the whole thing told him to get up and stop being dramatic. She clearly missed the croc about 100 yards away, and when I pointed out she dismissed it with "oh he's always losing those".

Anyway, same kid was back over at our house yesterday and seems to be good to go other than an impressive shin bruise and a healthy respect for RC cars.
 
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Wheel adapters came in the mail today and I finally got the new tires on. It's looking pretty good.
Newtires.jpg



per some RC nerd forums, I kept my gearing where it was and tested with just a 150a ESC rather than the 120a I had before. It's a little better, but I still can't drive up a curb. Going to try the pinion gear next and see if that helps.

My next problem is wheelies. I've got 50,000 cst oil in my center diff and that seems too light because it'll hop the front end down the road, but not pull a full wheelie. I guess I have to look around and see what other people with big heavy awd trucks are running. Wheelies aren't mandatory, but they would be nice.
 
2big bronco said:
Your boys would be disappointed hearing you talk like that :(
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It's like swear words. I try not to say stuff like that in front of them.

There's only so much time I want to sink into this. Gotta get going on a Kx85 fork swap on the beater little bike.
 
Sandy Johnson said:
Pinion gear made a world of difference. It can actually drive up a curb now. I went down from a 16t pinion to a 12t.. I've got a 11t sitting in the garage if I want to go a little bit lower, but so far it seems pretty solid.
Click to expand...

But does it do wheelies?.....

What are you KX85 fork swapping?
 
SLOWPOKE693 said:
But does it do wheelies?.....

What are you KX85 fork swapping?
Click to expand...
Tested today. It'll do wheelies, but something is weird about full throttle on it- I get a stutter if I slam it wide open from a stop. I have to roll in a bit and then hammer it. I'm starting to think it's not the center diff, but something in the cheapo ESC. Also the motor gets effin hot on this thing. I guess I need to pick up a heat sink/fan combo for it. After having a good time with it for more than 10 minutes, the motor is too hot to touch.

Got a beater klx125 I bought to chase my oldest around on his 50 with. Figured I might as well make it enjoyable to ride, so plan is to find a kx 85 fork and fork tree for the front and do a cr85 rear shock, then get some springs that actually tolerate someone 200lbs sitting on it. Got a engine cradle coming in the mail on monday, and a new carb to slap in it this weekend.
 
We've been having a pretty good time with this thing. My oldest wanted to take it to the local county park this weekend:

 
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