High torque motor recommendation?

[fossil]

This seems to tick the boxes for power and reasonable speed?
Trackstar 540 size 4 Pole 4250KV Sensored Motor
I know that cheapness carries a risk, but it appears to be as good as some stock motors, and rc makers would not use motors like that if they did not work?? Am I wrong?

Thanks.

[Olle P]

That one should do fine!

[JatoTheRipper]

Don't let the price of the TrackStar motors fool you. They are excellent.

[fossil]

Thanks. But I did notice that in the reviews and discussion about that motor, it seems to be underrated at 55A, and that it is capable of giving a 120A(!) esc trouble with the right gearing.
So that means that its (theoretical?) max power is over 900 watts, depending on temps.
That seems like a lot for a cheap 540 BL motor.
I would probably run it at around 70A(Giving around 770 W max power)?

My main concern is that the kv rating is too high to use on a 3s battery. But that seems like a non-existent problem, given enough gear down?

Thanks.

[Olle P]

Correct gearing is crucial!
55A corresponds to a load of only 2.5% when feeding it 12V. On 2S you can increase the typical load to 4% without pulling too much current. The motor efficiency can also be expected to peak at around 3% load (I haven't done the maths to get the exact number).
Turngy's "Max current" ratings do seem a bit arbitrary. I haven't been able to figure out the limiting factor, but know that it's neither the heat generated nor how much the leads can handle.

Using this motor takes an ESC (and battery) able to handle high peaks of current, since you can expect the peaks to be around 1kA unless you run it gently.

I see also that your power interpretations are a bit off. You mention the consumed electrical power, not how much the motor deliver.

[fossil]

Originally Posted by Olle P

Correct gearing is crucial!
55A corresponds to a load of only 2.5% when feeding it 12V. On 2S you can increase the typical load to 4% without pulling too much current. The motor efficiency can also be expected to peak at around 3% load (I haven't done the maths to get the exact number).
Turngy's "Max current" ratings do seem a bit arbitrary. I haven't been able to figure out the limiting factor, but know that it's neither the heat generated nor how much the leads can handle.

Using this motor takes an ESC (and battery) able to handle high peaks of current, since you can expect the peaks to be around 1kA unless you run it gently.

I see also that your power interpretations are a bit off. You mention the consumed electrical power, not how much the motor deliver.

So, as I understand, load is how much torque the motor needs to deliver to run at a certain weight and rpm? So load goes up as either weight, or rpm go up?
And the best efficiency for the motor is at a certain load? (approx 3%?)
So if the motor(hypothetically) can provide 100 kgf-cm of torque, then max efficiency is at 3 kgf-cm load?

Also, about my interpretations, I recently learnt about wattage in, wattage out. And that efficiency is the crucial factor regarding this.
But, I also learnt that wattage out is rarely given. And that is a sticky point.
What else am I missing?

Thanks.

[Olle P]

Originally Posted by fossil

So, as I understand, load is how much torque the motor needs to deliver to run at a certain weight and rpm?

In very broad terms that's somewhat correct.

"Load" is how much torque relative to stall the motor has to deliver at any given instant.
The stall torque is proportional to the voltage fed to the motor, and that varies with the "throttle".
The torque required at a specific instant depends on factors such as; speed, vehicle weight, if you want to accelerate or decelerate, drive line friction, gearing, ground conditions, slope, ...

Originally Posted by fossil

And the best efficiency for the motor is at a certain load? (approx 3%?)

That's correct. And how much torque that is depends on the throttle used at that instant.

Gearing should be such that the load is <2% when running straight at constant speed on flat and level typical type ground.
Then you (might) need to adjust the throttle used for acceleration and going uphill.

[fossil]

Originally Posted by Olle P

In very broad terms that's somewhat correct.

"Load" is how much torque relative to stall the motor has to deliver at any given instant.
The stall torque is proportional to the voltage fed to the motor, and that varies with the "throttle".
The torque required at a specific instant depends on factors such as; speed, vehicle weight, if you want to accelerate or decelerate, drive line friction, gearing, ground conditions, slope, ...

snip

Gearing should be such that the load is <2% when running straight at constant speed on flat and level typical type ground.
Then you (might) need to adjust the throttle used for acceleration and going uphill.

Ok. And you can use calculators like this to help with gearing?
RC Top speed calculator

Also, I heard that the rpm of a motor should not go above 45,000 rpm, and under 40,000 rpm ideally(measured at the pinion). So an 2S setup, go with a motor no higher than 6000kv. On 3S, no higher than 4000kv, etc. All depending on application of course.

So, the 4250kv 4 pole trackstar I linked above is not ideal for a 3S Truggy setup?
And is the ideal motor for 3S(depending on application!)around 2000-3000kv?

I know that the gearing should be so that, one revolution of the wheel equals one revolution of the motor. 1:1.
So there is a very particular gearing for every combination of diff, wheel, etc.

Thanks.

[Olle P]

Originally Posted by fossil

Ok. And you can use calculators like this to help with gearing?
RC Top speed calculator

That type of calculators assume there's no drag, no slopes and no friction. It's useless unless you already know what roll-out(*) you want.

Originally Posted by fossil

Also, I heard that the rpm of a motor should not go above 45,000 rpm, and under 40,000 rpm ideally(measured at the pinion).

That differs between motor designs. Some are built for very high speed whereas other can't take that much.
The manufacturer's battery recommendation is your best help.

Originally Posted by fossil

So, the 4250kv 4 pole trackstar I linked above is not ideal for a 3S Truggy setup?

That motor is rated for 3S, so it should work. Wether it's ideal or not, all factors combined, you alone can tell by testing it.

Originally Posted by fossil

I know that the gearing should be so that, one revolution of the wheel equals one revolution of the motor. 1:1.

Typically you NEVER have 1:1 motor:wheel ratio. Only with motor-in-wheel is it sometimes used.
In my crawler it's about 80:1.

Originally Posted by fossil

So there is a very particular gearing for every combination of diff, wheel, etc.

Those that run truggies os monster trucks should be able to tell you what gearing span to look for. I don't and can't tell.
This is not a truggy forum...

(*) Roll-out is the distance the vehicle moves on one motor revolution.

[new2rocks]

For what it's worth, I think you're way over-thinking this in a way that's getting you farther from your answer rather than closer. You've said you want a top speed in the high teens (presumably km/h) in a rig that weighs 3 kilos with the ability to drive on-road or off-road and carry another .5 kilo payload. The first thing you should do is pick a vehicle type and then platform for your build. If the ability to handle rugged terrain is more important than the ability to carry high speeds (i.e., higher than 20 km/h), there are any number of scaler/crawler platforms that can easily meet those specs, including the Wraith, SCX-10, Yeti, Twin Hammers, Ascender, R1 and others. If you're more concerned with high-speed stability and handling, Yeti, Wraith, Twin Hammers, a monster truck, or an open-diff 8th scale truggy platform might be a better fit at the expense of not being able to handle more rugged terrain as well.

A few minutes spent watching YouTube videos of different platforms will show you what vehicle type is going to fit your purposes best. From there, it's not going to be hard at all to find a powertrain that will work on that platform and meet your requirements. Once you've picked your platform and powertrain, you can then decide whether you want to build a custom chassis vs. modify a stock chassis to meet your needs. Good luck!

[dirtman71]

With proper gearing any 10.5t or 3000-3500kv brushless on 2s 5000mah with a 50a or greater esc will more than handle any speed or power needs
Great little motor. Used it for 2 years in a slash...till I sold it
Tacon 3650-10T Brushless Motor 3500KV for 1/10th On-Road RC Car
This is the best esc for the money. Never lets me down
https://brushlesshobbies.3dcartstore...me.asp#page-27

You could find a cheaper one but it's worth the extra $20. The owner Stan is super cool and has great CS!
But I have never ran that esc with a sensorless motor so I do not know how well it would work

[fossil]

Originally Posted by Olle P

That type of calculators assume there's no drag, no slopes and no friction. It's useless unless you already know what roll-out(*) you want.
....snip...

Typically you NEVER have 1:1 motor:wheel ratio. Only with motor-in-wheel is it sometimes used.
In my crawler it's about 80:1.
...snip...
(*) Roll-out is the distance the vehicle moves on one motor revolution.

I'm sorry, I got mixed up. I meant back there, when I was talking about motor-wheel revolution ratios, I actually meant roll-out.
So I was asking if a 1:1 roll-out ratio is ideal...
Sorry.
And I'll look on a truggy related form for the gear ratio, thanks.

Originally Posted by new2rocks

For what it's worth, I think you're way over-thinking this in a way that's getting you farther from your answer rather than closer. ...snip... The first thing you should do is pick a vehicle type and then platform for your build. ...snip... If you're more concerned with high-speed stability and handling, Yeti, Wraith, Twin Hammers, a monster truck, or an open-diff 8th scale truggy platform might be a better fit at the expense of not being able to handle more rugged terrain as well.

snip...
Once you've picked your platform and powertrain, you can then decide whether you want to build a custom chassis vs. modify a stock chassis to meet your needs. Good luck!

Ok, thanks. High speed and handling are more important to me than rugged terrain capability. The is not much rugged terrain in this part of Ireland, anyway.
And I have been looking at the yeti closely. It seems to be a close to ideal platform, with a bit of modification.
I'll focus on getting an appropriate platform first.

Originally Posted by dirtman71

...snip...
Great little motor. Used it for 2 years in a slash...till I sold it
Tacon 3650-10T Brushless Motor 3500KV for 1/10th On-Road RC Car
This is the best esc for the money. Never lets me down
https://brushlesshobbies.3dcartstore...me.asp#page-27

You could find a cheaper one but it's worth the extra $20. The owner Stan is super cool and has great CS!
...snip...

Thanks, I've heard of some people on RCGroups using that motor, and they say its good too.
I'll consider that ESC and motor, I'm looking for something in the 3000-3500kv range now, anyway.

Thanks everyone!

[Olle P]

Originally Posted by fossil

... when I was talking about motor-wheel revolution ratios, I actually meant roll-out.
So I was asking if a 1:1 roll-out ratio is ideal...

That's even more confusing to me.
The roll-out being 1:1 relative to what?

[fossil]

Originally Posted by Olle P

That's even more confusing to me.
The roll-out being 1:1 relative to what?

The "1:1 roll-out rule". One motor revolution moves the wheels forward one inch.
To achieve this, to total gear reduction has to equal the tire circumference.
Apparently its a good starting point?

Thanks

[dirtman71]

I think it's been said before but your way overthinking this lol

Yes you can take the trans ratio devided by the spur and pinion ..... But in the end it really doesn't really matter. The motors can run on a wide range. I don't know what trans you are running or wheels and tires. But I would start with an 87 spur and 18 pinion and run it and keep checking the temps

I would use a temp gun and not let it get to 130 degrees and would shoot for 120

If your temp is good after 7-8 min of hard running and it seem to top out your good. If you want more top end add a tooth or 2 and check again

[Olle P]

Originally Posted by fossil

The "1:1 roll-out rule". One motor revolution moves the wheels forward one inch. ... Apparently its a good starting point?

Unless I had some firm indication that 1"/rev should be correct for my use I'd start with less roll-out, about ½"/rev, just to be safe.
On firm and open ground 1" might be about correct, but the rule of thumb is to start low and then gear up incrementally if there's a need and the motor temp allows it.

Originally Posted by dirtman71

... use a temp gun and not let it get to 130 degrees and would shoot for 120...

Just to be clear: Those are temperatures in fahrenheit, not celsius.

[dirtman71]

Yes Fahrenheit

Some guys run there stuff way hotter and then wonder why they smoked a motor....heat kills

Right now I run a Novak 25.5t and even in the heat of the summer I never broke 100 degrees

Don't forget about ambient temp...if your testing your motor with an outside temp of 70 degrees and your getting 130 remember the if the outside temp goes up to 100 your motor may go to 160