High kv low gearing VS Low kv high gearing

[johnrobholmes]

A low KV motor would be less efficient when geared taller at a given voltage however. A 3 turn motor on 10v would have more losses than a 2 turn motor on 10v. When both setups are geared for the same speed on the same voltage, you have more copper losses with the slower, higher turn motor.

[shaunjohnson]

well i'm a newbie at this but even i can figure out this.

vehicle requires X watts to move at 60mph.
IT DOSENT MATTER WHAT KV YOU USE weather it be a 1kv motor on 60,000volts or a 9000kv motor on 8.4v...it will still draw X watts to get the vehicle to 60mph.
if the motor wasnt built to take that punnishment..then it frys..but not without trying to draw X watts.

there is no such thing as a low KV motor giving more tourque.
yes they will most likely cogg less as would a 4 pole vs a 2 pole motor (4 pole FTW!!) but both motors will still draw Y watts to get the vehicle X watts or 60mph.

the only thing playing a part here is the friction losses in the tranny and motor.

make sence?

[suicideneil]

This is getting far to technical and... pedantic almost, with arguements and counter arguements for and against either option; time for a more highly accurate method of helping me choose.

**Coin toss**

80mm 2000kv wins!

Unless I get lucky enough to score the castle motor for beta testing, I'll order from Mike when he gets them back in stock.

[johnrobholmes]

It is very pedantic. It really boils down to testing each individual ESC, vehicle, and motor combination. There is no telling where we are falling on the efficiency curves, or where the losses are.

[SpEEdyBL]

The problem is that people don't READ until they COMPLETELY understand what others have written. My previous posts I know are 100% correct, because I have the formulas AND actual data to back it up.

It could be that the original question is vague. Because if it is refering to two different motors of the same size, but different wind to change the kv, OR two motors of different size to change the kv, there will be two VERY DIFFERENT outcomes. I was refering specifically to motors of the same size, and others seemed to be using facts that would only apply to the bigger motor/smaller motor comparison, which has caused some confusion.

[sleebus.jones]

Originally Posted by SpEEdyBL

My previous posts I know are 100% correct, because I have the formulas AND actual data to back it up.

Ok, then how do you explain Castle's stance on torque here:

Originally Posted by Castle Creations

A high power capable brushless motor in electrical engineering and physics terms, has unlimited torque. We live in “the real world” so technically for us that’s not totally true, but – a brushed motor has a torque level that due to its design has an upper limit, regardless of how much power is being applied to it. That limit is low enough that you can see it clearly on an average track On the other hand, a high power brushless motor’s limit to torque in an RC vehicle is not within the bounds of the motor itself so much, but rather falls on the ability of the battery to deliver current to it. We generally don’t describe these motors in terms of “one has more torque than the other”, but rather “the 7700Kv motor is faster and draws more current than a 5700Kv motor in the same vehicle”. It draws more current, because it’s making the car go faster and doing more work than the 5700 motor is. As long as the batteries used are very good at supplying current without an excess of voltage depression (low internal resistance is good) both motors will appear to have the same torque, even though one is much faster than the other. Battery technology is constantly improving, and the first thing you’ll notice when you use a very good battery pack (or perhaps trying a Lipo pack for the first time) with these systems is a more “punchy” feel when you accelerate. The faster you set up the car to go at full throttle, the more reliant you are on good batteries to flow that current into the motor and maintain acceleration performance. So think of torque as a function of battery capability only.

[SunnyHouTX]

I agree with Castle Creations' post there from experience. What I also found was that you don't need as "good" a battery for the lower Kv motors to develop torque. For example, a 2000mAH battery is enough for a 3500Kv motor to produce plenty of torque. The same battery will make a 7700Kv motor sputter/cog/bog down. So it seems that, given the appropriate battery configuration for each motor, both motors will produce plenty of torque.

[BrianG]

I can agree with both viewpoints. Let me explain.

Torque formula (oz-in): torque = (1352/kv) X amps
Divide that by 192 to get ft-lbs, which is what most of us are used to.

So, a higher kv motor does reduce torque, but usually the increased current of a high kv motor offsets that. Example:

2000kv motor A at 80A = 0.218 ft-lbs torque.
4000kv motor B at 160A = 0.218 ft-lbs torque.

For the same gearing, doubling the kv will also increase the amperage. Maybe not exactly double like above, but you get the idea. Obviously, speed will be greatly increased as well.

Now, let's look at the effects of gearing. Let's say both motors are geared with a total 10:1 ratio.

Motor A: Torque is multiplied by gear reduction, so motor A torque at the axle is 2.18 ft-lbs of torque. Let's say this motor and the 10:1 ratio gives you 40mph.

Motor B: Torque at the axle would be the same 2.18ft-lbs of torque, but speed would be much higher because of the higher kv. More work is being done because of the higher top speed.

However, if you gear down motor B so the total speed matches that of motor A, the amperage will be reduced, but the ratio is higher. Let's say you geared down to 20:1 ratio, and the motor now pulls only 80A (as a result of lower gearing). Motor torque goes down to 0.1408 ft-lbs, but when multiplied by the gearing of 20:1, gives you the same 2.81 ft-lbs of torque.

So, gearing is absolutely crucial. We can't just look at motor torque by itself because most of us run them in something (kinda boring to watch a motor shaft spinning without anything attached ), so this argument of "motor" torque is irrelevant.

This all leads me to the conclusion that as long as you meet the following requirements; A) maximum motor rpm is kept withing the higher side of the efficiency curve, and B) vehicle weight/gearing is not set such that the ESC is not seeing the appropriate minimum motor rpm for proper operation.

So as long as your setup is within these guidelines, there is no advantage either way for general running. But if you are just looking for top speeds only with little low speed running, lower total motor rpm geared higher is ok as long as you don't mind potential stuttering at initial take-off. If you are looking more for low speed performance with little to no top speed running, a higher rpm motor geared down is better.

Phew! That was a long post.

[yuri]

Plain and clear, as everything should be, but where is that higher side of the efficiency curve? Any graphs about efficiency?

[BrianG]

There is a program around here made by GriffinRU that has motor efficiency curves vs rpm. You probably don't have to be totally exact, but you will see that efficiency curve shape are more or less the same for most BL motors, but the efficiency max value will be different for the better motors. Not sure if all motors are listed, but there are quite a few.

[SunnyHouTX]

Excellent post Brian!