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379
2015.02.06, 11:21 AM
But not for a Mini-Z. I've taken a 1/35th KV-2 model kit and turned it into an RC tank using Tamiya's 70097 twin-motor gearbox. Nowhere in the documentation with that gearbox did it say the included motors were rated 3v max, so between the strain of moving that model around and the 200% overvoltage(I'm using a 5 cell NiMH receiver pack as a battery, they're getting a nominal 5.5-6.0 volts) they're cooking pretty quickly. Already burnt one out and I don't imagine my spares will last long.


I figured, since Mini-Zs use the same size of motor and that people racing those things push them pretty hard, that I could find a suitable replacement for this vehicle from the Mini-Z community. I did some digging of my own and found the PN Racing 70 turn motor, which looks pretty nice and I can get them for a song. Seems perfect! But, because getting to them requires removing the rear hull plate and re-gluing it, I'd really rather not have to do that any more than I have to. Durability is key.

I've contacted PN Racing with this, but as I'm not sure if I'll get a response(They haven't been active here since Jan and my ISP's mail filter may junk the reply they send from their email) I'm also asking you lot. That motor gonna be sturdy enough to stand up to the strain of working in a pair inside a tight, cramped engine compartment?

http://i.imgur.com/5hSBRIq.jpg?1

That's where they'll live. Two of them, nestled back to back down inside that tight little space, with some airflow but not much. Ventilation comes from two vents in the engine deck directly above the motors and a third vent between the rear armor plate and the upper engine deck. Circulation is from convection. I know some air is moving through these vents because I can smell the brushes of those 3v motors burning as I drive it, but I don't know how much is actually flowing through the compartment. It's not exactly the ideal place for them, but I can't really do much to help that.


Oh, and no need to worry about the speedo. The ESCs I'm using are rated for 280 class motors. As long as these have a stall current of roundabouts half an amp, and a chart I found puts the 70T model at ~0.6a stall current, they'll be happy as clams in there.

mleemor60
2015.02.06, 01:14 PM
Look at motors from Atomic in the 48t to 60T range. Especially the Atomic stock motor at 48T. Huge torque and low current draw uses double bearings. I would avoid the 17mm version as they seem to run a bit hotter.

379
2015.02.06, 01:41 PM
Wouldn't that also see the tank hauling a ton of ass? Applicable for a more modern tank like an Abrams, but historically the KV-2 was only barely mobile enough to be considered mobile. On paper they'd do about 20MPH, realistically they never got past 12-15, and I'd like to maintain some semblance of that ponderousness with this one. I like the speed I get out of the 3v motors that come with the 70097 gearbox, I just need motors that are more durable and are rated for the voltages I'm running so they don't burn out.


It's pretty heavy, too, and I've got a set of white metal tracks in the mail for it(The vinyl ones on it now are stretched and throw real easy) which'll add almost a pound to an already hefty RC. My understanding of brushed electric motors in RC usage is "More turns = more torque, less RPM, less current draw", and if I'm remembering things properly the 70t will be a better fit than a 60t or 48t motor. I will say I like the dual BB housing, though.

mleemor60
2015.02.06, 03:54 PM
These are torque motors not speed although they will happily pull a large pinion. You will want small pinions to do the work.

LED
2015.02.06, 05:13 PM
do not only look at the voltage, also look at the RPM of thr motor. Unless you can replace the pinion gears on the tank??
If I remember correctly the PN70T has a lower RPM then most other mini-z motors. I believe it was around 21000 RPM while even the kyosho stock does +30000.
So If you want to keep the same speed for your tank try to measure the RPM of the current motors and match that.
Also before simply replacing the motor look at the electronics of the tank. If there are FETS or something in there make sure you know how far you can go with a heavier motor so you dont burn the electronics.

Also maybe try to install a small fan for cooling. You can find fans as small as 3cm square.
Good luck

379
2015.02.07, 01:41 AM
March 2015 edit: The motors came in and slid right into the tank. They're exactly what I was looking for. They barely even get warm and they pull like a freight train. Actually broke a gear in the gearbox when I tried to drive it through some mud! An inch and a half of powdery snow was nothing to them, they powered the tank through it with enough authority that it was skiing along on its belly. Itt'l also climb pretty much anything given enough traction. The metal tracks I have on it slip pretty easily, but on a pillow with just the right texture for them to bite in, it climbed up to a 40 degree angle before weight transfer had the tracks spinning anyway.

If any of y'all happen to be looking for a super durable motor that doesn't seem to care what you do to it this is gonna do the job just fine. They ain't exactly fast, but they're tough and they're strong as an ox.

These are torque motors not speed although they will happily pull a large pinion. You will want small pinions to do the work.

do not only look at the voltage, also look at the RPM of thr motor. Unless you can replace the pinion gears on the tank??

I'm limited to just an 8t pinion for this. The gearbox I'm using has no mesh adjustment for a different sized pinion. The final drive ratio of this gearbox configured for this tank is 58:1. It has a 203:1 configuration, but I cannot use it as the output shafts are too far forward for the drive sprockets at the back of the hull to reach them. I'd have to have a chunk of the gearbox just hangin' out in the breeze to use that ratio, so I'm using the 58:1 ratio.



If I remember correctly the PN70T has a lower RPM then most other mini-z motors. I believe it was around 21000 RPM while even the kyosho stock does +30000.

Indeed it is. No load RPM of 22K according to the PN Racing datasheet. The motors that come with my gearbox are rated for 13K at 3 volts. At six I'd imagine they're fairly close to 20K or so.


Also before simply replacing the motor look at the electronics of the tank. If there are FETS or something in there make sure you know how far you can go with a heavier motor so you dont burn the electronics.

I'm using a control chip from an outfit called 35RCTank. They've rated it for 280class motors, and when purchased as part of a total conversion kit their gearbox uses 280class motors. I doubt I'll have any trouble with burnt out FETs running a pair of 70t 130s.


Straight from their website: (http://35rctank.com/html/p3.html)


- MTC-2 can work alone as a twin motor ESC, with 8A continuous current for 130 to 280 size motors.

8 amps continuous current...I could probably run some 20t super bonkers speed motors in this thing and not burn that control chip out. I doubt my chosen battery pack can even supply 8 amps continuous. It's a beast and is designed specifically for this very application. I doubt a PN 70t is gonna blow the FETs unless it somehow just fails and shorts out.

Also maybe try to install a small fan for cooling. You can find fans as small as 3cm square.
Good luck

I have pondered that. If I could find one pancake enough I could probably get it in there. The inner hull walls are ~50mm wide, so it's really only a matter of whether I have enough room vertically. The engine deck does have vents in it that'll provide an exit path, I think what I'd do is mount it such that it'd pull air in from those vents, blow it down on the motors, and it would find its way out through the rear vent between the engine deck and the hull. Same airflow path the real KV-2 used for engine cooling!