Picking the right motor for the right application

[VintageMA]

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OK - this is a very confusing topic for me because there seems to be little out there for reference short of reading through the experiences of what motor people have selected for there car and trying to approximate that motor based on the weight/style of your application.

My question is this - how do you pick the right motor for your car? It seems much easier to pick and ESC. But for the motors there are so many different variations out there. You have only a few primary variables that need to be taken into consideration in my mind:
1) weight of the car
2) top end speed versus acceleration
3) # of cells being used / current draw
4) heat dissipation

With that in mind, none of the motor specs really translates into the variables shown above directly.

In my first noob conversion I have converted an Kyosho MP 7.5 to brushless and used a Nemesis 7XL motor. The thing doesn't cog at all, the acceleration is mind blowing, and I get about 10 - 15 minutes off of a 4S 3700 mAh lipo pack. Top speed with 14/46 gearing is easily in the area of 40mph. Using a Warrior 9920 ESC. Temp wise, at the end of the run the motor is around 130 on the can and 150-160 at the endbell, the ESC never goes over about 100.

My next conversion is going to be my LST2 because nitro has just begun to piss me off (granted I still like the sounds and smoke). I have guestimated that I will do well with a LMT 1950/7 motor (stock gearing) with 6S 8000 mAh and the MGM Compro 160 amp, 24 cell ESC. Based on what I have read others do, this seems to be about right.

But I'm still uncertain, how do I know a 1930 or a 1940 with higher winds wouldn't be enough? Or is LMT 2200 series really needed for that much weight?:026: :026: :020:

It's very confusing - they tell you what the motor winds are, the max current is, the RPM/v, the max RPMS, and the max power in Watts. But if you compare a 1930 with high winds to a 1950 with low winds the RPM/v and max rpms are the same, so that would leave me to believe that I could use the same # of cells to get the same speed, but how will I know if I am going to draw to many Amps with the 1930 and kill it versus going with the 1950 which will weigh more?

Aaaarrgh..... Thoughts? Please let me know if there is something I am missing here.

Thx

[captain harlock]

Well, its as simple as that. Choose the small and fast for your lightweight applications and choose the big and torqey for your heavyweight applications.
The 1950/7 is quite a good motor to go with the LST2, but I doubt it'll run as effecient as..say..a 1950/9 or 10 turns. The higher winding gives you more effeciency, but less speed. The longer the rotor and the bigger the motor's diameter, the higher the torque a motor has to offer.
So, the bottomline:
Go with a 1950/9 with 6s and MGM16024.
This is one nice setup for your truck.

[VintageMA]

So basically is your thinking that the 1950/7 would be little hot because the RPM/v is higher than that of the 1950/9 or 10? In that the 7 may try to draw more Amps in order to make it up to the spec speed at any given input voltage - thus causing the motor to heat up more quickly than the 9 or 10 turn motor?

Thanks for the input - but I think my point was mainly that there is some guesstimation that comes into play here and the accuracy level in picking the best motor only comes with experience

[captain harlock]

Exactly, picking the right motor comes with experience.
If I were a fresh hobbyest in MT brushless setups, I would've gone for even a 1950/4 just because its insanely fast. However, soon you'll realize that you are running both the motor and the controller at their hot steak.
The LST2 is impressively heavy and you need to keep the motor and controller at their best effeciency.
I dont think that its impossible to use the 1950/7 with the LST, though you can use it and get some serious speed with the right gearing and number of cells, but your truck will run with more power if you desire more speed and torque from your motor and controller without making them beg for mercy if you choose a motor with less rpm/v.
The 1950/9 is a centerline between speed and torque. You can run it in the LST2 with 5s for nice speed and torque with moderate temps or you can go for 6s for some 45-50mph speed runs. You can easily use even a 7s pack, but you can no longer handle the truck with ease like you would with a lower cell counts.

[VintageMA]

According to BK's website the specs are:
1950/7 Motor LMT RPM per Volt 1867 (@6S, 1867x22.2 = 41447)
1950/9 Motor LMT RPM per Volt 1452 (@6S, 1452x22.2 = 32234)

Given those specs and comparing it to the stock nitro engine which I believe is rated to ~33Krpm I would've thought 1950/9 (with stock gearing) motor would only get it going top speed as stock (although getting there much quicker). I also have the high/low gear settings to play with in addition to tweaking the shift point on the 2 speed - so hopefully that'll keep heat down on accelarations from stop.

Either way this motor is getting a heat sink and a fan!!

Thanks for your input.

[crazyjr]

Honestly the 9 turn would be better, less stress on the esc to run the same rpms and it runs in the 30,000 to 35,00 that others here call the "sweet spot", Anything lost in RPM can be got back in gearing and temps would be better for everything.

[Serum]

Yeah, keep that sweet spot on mind.

A 1940 would be enough allready bytheway, to push the truck around.

Why are the Neu motors missing on your list? the 1515 with a KV of 1700 would be very nice on 6S.

You don't need to copare the RPM's of nitro with the electrics. It's a whole different bag, and gearing is the key to speed, not RPM's.

[mickeymoose]

Hi everyone, I like to jump in with some questions while this subbject is being discussed if it's OK? I have built an LST2 using a Feigao 8XL, Quark 125B Monster Pro esc with a Quark external BEC, E-Maxx Tranny, custom top and botttom aluminium chassis plate. the rest of the truck is stock LST2. I've just been running 2 GP3300 stick packs so far for testing and tweaking the setup. I'm using the stock 66T E-Maxx spur gear with a 16T and 18T pinions so far. I have also locked the Tranny in 2nd gear. Even with this setup there is plenty of torque, the truck will pull wheelies from a dead stop and flip if you don't get out of the throttle. The motor temps are low to mid 130's and the esc is barely over 100. top speed as a guesstimate is probably around 30mph ( it's a hair faster than my buddy's E-Zilla ). Do you guy's have any suggestions on better overall gearing. Oh yeah the truck weighs about 10.5lbs without the batteries, and I'm running proline 40 series dirt hawgs. Thanks for any help.

[BrianG]

IMO, the size of the motor depends on the weight of the truck. If the motor is too big/powerfull, it's a waste of money. Kinda like putting a 1,000HP motor in a mid-size car; it's nice and all, but not very practical unless you can actually use it. Likewise, putting a 50HP motor in a mid-size car is not practical either because it is underpowered.

Plus, using a motor too big doesn't load it enough. Using a motor that's too small loads it too much. Some of this can be compensated with via gearing to a point.

The wind of the motor depends on the voltage used. Obviously, for a very large motor, you want to use higher voltage to produce the current needed for the HP.

From what experience I've had (and that's not really all that much). Using a high wind (low KV) motor geared really high is not ideal. Nor is using an unreasonably low wind (high KV) geared really low. In either case, some or all parts gets too hot and efficiency falls. It seems moderation works best.

[squeeforever]

You could gear up slightly I would imagine. Just keep the temps below 160 on the motor and ESC.

[Serum]

How many cells are you using?

[mickeymoose]

I am running 12 GP3300 cells in series for now, but I plan on 4s Lipo eventually.