Mojave Motor kv discussion.

[UPPERDECKER]

After some lengthy reading across the corners of different rc forums, I'm having difficulties trying to wrap my head around the concept of kv in regards to heat and gearing.
I know higher kv/more rpms per volt, higher volts/less amps per same load, and less amps/less heat. So what if volts and load is the same? Does going to lower kv, and gearing up to regain some wheel speed increase load/generate more heat then less load (smaller pinion) on a higher kv? If I lower my kv but keep the volts and load the same I should run cooler, even at maximum throttle, correct?
My concern is more about heat than speed, I have more than enough wheel speed with the higher kv and find myself turning max output down but temps are high. Not dangerous, but not comfortable. I imagine I would be able to use more of the available power with the lower kv and the same load, but does that generate less heat?
I read that motors of the same size can will generate the same power/torque just at different volts/amps, but my volts and load are staying the same, so amps is the varying factor. Following the formula I should be pulling less amps (heat) than the higher kv, but will that be negated by increasing my max power output to regain wheel speed instead of a bigger pinion (more load).?
Lot of questions going on here, sorry. I'm not great with words either but I hope I was able to convey my thoughts well enough.

 

[GraniteFox]

I generally go by the rule of larger motor size = less heat. I run a 6s motor in my BRCC because I run in cut grass and the standard 3660 would fry itself. You also need to understand that lower KV motor have higher torque. I'm by no means an expert in these things. Running conditions play a huge part of creating motor heat from resistence. Mostly trial error and a temp gun until I get to a sweet spot with the ESC and Motor.

 

[Freelander]

I just changed motors from a 2050 to a 1650 which should potentially get only 80 percent rpm. On the same gearing we did not see only 80 percent of the speed. It was still 90 percent of the speed achieved with the 2050 motor. I think the lower kv motor worked less hard to get to it's maximum rpm whereas perhaps the higher kv motor never achieved maximum rpm. The lower kv motor has a bigger can both length and diameter and stays cooler. I assume it has bigger windings and therefore generates a stronger magnetic field meaning it has more torque(?).

So I see this as a win. I lost speed but not as much as you'd think but overall I gained reliability and headroom to now gear taller and go well beyond what the 2050 was capable of since I feel like it was at the limit of it's performance.

I do have much higher kv motors but they're generally in much much smaller cars. Like I have a 5700kv in a 4lb. Touring Car I run on 2S. It's fun.

 

[cfelix10]

I just ordered a 1650kv Max 6 combo for my Mojave. Is that overkill? I was going to run a 22t pinion. Is that a good choice with that motor combo?

 

[LibertyMKiii]

Lower RPMs is always a better thing within a reasonable range. Obviously if you go from 1900kv down to 500kv that might be extreme. Gearing can compensate and most lower KV motors are larger, which will dissipate more heat.

 

[MambaSenton]

I just ordered a 1650kv Max 6 combo for my Mojave. Is that overkill? I was going to run a 22t pinion. Is that a good choice with that motor combo?

Yea should run fine I’m on a 25t with a little lower kv motor

 

[Hector_Fisher]

So what if volts and load is the same? Does going to lower kv, and gearing up to regain some wheel speed increase load/generate more heat then less load (smaller pinion) on a higher kv? If I lower my kv but keep the volts and load the same I should run cooler, even at maximum throttle, correct?

Load on the motor (power/wattage) is a product of voltage and current, so you can't maintain the same load unless you make the exact same conditions (same pinion, voltage, motor, battery, vehicle weight, same throttle input, etc). So essentially the exact same truck.

Load aside, theoretically if you have a truck with a 2050kv motor in it and only change the motor to a 1400kv one (same can size), the truck will have more torque and less heat, but run slower. In a simplistic view, changing KV is effectively the same as changing the gearing.

Up to a point, taller gearing (22t instead of 20t) and larger kv allow for greater maximum power through the motor. More power = more current = more heat. But like @LibertyMKiii said, it all comes down to balance and being within reason.

 

[LibertyMKiii]

Load on the motor (power/wattage) is a product of voltage and current, so you can't maintain the same load unless you make the exact same conditions (same pinion, voltage, motor, battery, vehicle weight, same throttle input, etc). So essentially the exact same truck.

Load aside, theoretically if you have a truck with a 2050kv motor in it and only change the motor to a 1400kv one (same can size), the truck will have more torque and less heat, but run slower. In a simplistic view, changing KV is effectively the same as changing the gearing.

Up to a point, taller gearing (22t instead of 20t) and larger kv allow for greater maximum power through the motor. More power = more current = more heat. But like @LibertyMKiii said, it all comes down to balance and being within reason.

Also in the RC world most of the motors of lower KV will be larger motors than original.
Unless you are directly comparing a Neu or TP Power motor of the same size.

In general I see people comparing RTR motors vs HobbyWing motors (or replicas like Rocket etc).

 

[jondilly1974]

Motor size is more indicative of heat than KV, if all else is equal. So many moving parts to these discussions so it’s really hard to boil down in a super simplistic way, but can size is as close as you can reasonably be. There will be exceptions, but overall it’s the best indicator. Some of that is surface area to disperse heat. Some of it is larger and stronger magnets can produce more work.

 

[Hector_Fisher]

Also in the RC world most of the motors of lower KV will be larger motors than original.
Unless you are directly comparing a Neu or TP Power motor of the same size.

In general I see people comparing RTR motors vs HobbyWing motors (or replicas like Rocket etc).

Or surpass motors, that have a kv for every color in the rainbow

 

[LibertyMKiii]

Or surpass motors, that have a kv for every color in the rainbow

Same with Castle. Each KV is a different motor size with the exception of maybe 2 of their motors.

 

[UPPERDECKER]

Sorry I should have mentioned the motors in question. They are the same size, same 1717 can actually, going from the 1650kv to the 1260kv. Also running on pavement, quite hard.
The truck itself won't be changed other than pinion. Same weight same volts.
So I should expect a double down effect for running cooler with a smaller pinion and lower kv? Even running it full tilt? Something is telling me this motor isn't going to like turning full trigger.

From castle;
NOTE: The 1717-1260kv has a lower wattage/power rating compared to the 1717-1650kv; as such the 1260kv will run hotter than the 1650kv when geared for the same speed. For higher power applications (IE speed and drag setup), the 1717-1650kv is preferred with appropriate gearing.

Does this mean it has a weaker top end?
How does this translate to driving the truck?
My plan is to lower my top speed to enable more use of the trigger without lowering power output, my goal is to lower motor temps. I ran my gear through a calculator and honestly I'd be great with half of the speed I have available now.
Another thought, how would this impact battery performance? Does the hot 1650 pull and waste more? Or will the 1260 pull more but waste less through heat on the can?

Thanks for the feedback guys. Feasting on food for thought here.

 

[Hector_Fisher]

IMO, you're delving into the physics of how different KV motors operate. If that's the case (and it seems that way) I would suggest reading up some articles on how KV affects the motor. You might consider starting here and working around the interwebs:
https://www.arrmaforum.com/threads/low-kv-high-gearing-vs-high-kv-low-gearing.15873/

Returning back to the physics equations, if castle says the 1650kv can handle more power, it's probably because it's overall internal resistance is lower (possibly due to larger wire and fewer windings in the motor), and therefore the heat generation at those extreme power ranges (higher amperage) is less than the 1250kv motor-- but I'm speculating there.

Overall, I'm fairly certain that you won't experience too much of a difference on the Mojave with either motor, as long as you could get a pinion that fits and will meets your speed requirements. Honestly, you'll probably want a good motor fan either way, which should rectify heat generation in either motor.

 

[jondilly1974]

Sorry I should have mentioned the motors in question. They are the same size, same 1717 can actually, going from the 1650kv to the 1260kv. Also running on pavement, quite hard.
The truck itself won't be changed other than pinion. Same weight same volts.
So I should expect a double down effect for running cooler with a smaller pinion and lower kv? Even running it full tilt? Something is telling me this motor isn't going to like turning full trigger.

From castle;
NOTE: The 1717-1260kv has a lower wattage/power rating compared to the 1717-1650kv; as such the 1260kv will run hotter than the 1650kv when geared for the same speed. For higher power applications (IE speed and drag setup), the 1717-1650kv is preferred with appropriate gearing.

Does this mean it has a weaker top end?
How does this translate to driving the truck?
My plan is to lower my top speed to enable more use of the trigger without lowering power output, my goal is to lower motor temps. I ran my gear through a calculator and honestly I'd be great with half of the speed I have available now.
Another thought, how would this impact battery performance? Does the hot 1650 pull and waste more? Or will the 1260 pull more but waste less through heat on the can?

Thanks for the feedback guys. Feasting on food for thought here.

I believe the 1650kv is just made with better components than the 1260kv. Honestly I don’t even know why they even make the 1260kv.

 

[LibertyMKiii]

The higher KV motors have a higher amperage rating due to the way they are wound and have more wire in most cases. IT is misleading the way castle wrote up the specs to obtain a higher power rating for the 1650kv because in reality the 1260kv motor is likely a 10s motor and should have a similar power rating, but Castle does not have a matching ESC for over 8s so they tweaked the specs for that motor to make it fit what they sell.

In general I like lower kv with more gearing to obtain X speed versus using KV, but there is limitations and certain power bands.

This is an interesting graphic that seems to be true. somewhere around 24-38k rpms is the peak power band. I believe for the larger the motor the band shifts down, but have not seen clean data like this on it.

 

[Freelander]

That to me looks like your target RPM for a motor at a given voltage for the best results is 30,000. That puts a 1650 at 6S in a real sweet spot. You aren't pushing the max 40,000+ rpm and losing torque you aren't under utilizing the motor you know what I mean?
Actually it's all in there freelander: that might be true in this specific case but no clean data exists for the effect on the power band for different size cans and a host of other variables which might affect performance.

Thank you for grounding me. I jumped to conclusions.

No problem. It helps if you read the whole comment before replying.

I'll try and do that in the future.

 

[Dan B.]

I just ordered a 1650kv Max 6 combo for my Mojave. Is that overkill? I was going to run a 22t pinion. Is that a good choice with that motor combo?

I think that combo is just about perfect. That’s what I run in my Mojave. I’m running a 21t pinion and the motor got hot running in grass. I added a dual fan setup to the motor and now it’s fine. I would think your 22 would probably be ok with fans as long as you don’t run in high grass or go mud bogging with it.

 

[ARRMA-GET’M]

I believe the 1650kv is just made with better components than the 1260kv. Honestly I don’t even know why they even make the 1260kv.

In My personal experience since being in the hobby since 1990., the newer Castle 17 series 1650kv is wound with thicker gauge copper wire than what the older 15 series 1650kv motor was so it can handle higher currents there for able to hold 8s power and the amperage ratings associated with high gearing for speed runs or pushing a 40 pound 1/5 off road truck. Perhaps not a direct comparison to the 1260kv (castle too?) but it is good to know. There’s not a lot of info on the web about brushless motors and kv science as far as hobby grade electronics go if you’re searching “hobby grade brands” and hobby related electronics. Manufacturers really don’t want to talk about the nitty gritty. You’ll have to really alter your searches and think OUTSIDE of the hobby mindset to find honest true facts and answers.