High kv low gearing VS Low kv high gearing

[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.