High kv low gearing VS Low kv high gearing

[Dragonfueled]

This is a question that has allways interested me. Let's say same vehicle one with high kv low gearing the other with low kv high gearing. What would temps ,effiency, etc be for bolth setups geared for the same speed.

One has to be better then the other LOL. I'm guessing each would have an advantage but where I'm not sure

[suicideneil]

Its the question which has been plaguing my tiny mind for a while now..

Assuming voltage was the same with each setup, and the motors in question were the same size (just different kvs), then I would personally favour the lower kv option, but then again it depends on what you're specific goal was.

If it was to have a sensible speed with good temps and runtime, I would wager the lower kv motor drawing less current would give better results, but only on the proviso that it could pull that taller gearing needed without warming things up too much- feigaos need not apply as tall gearing or a heavy load does them no good beyond a certain point.

If we were talking Medusa 1600 vs 2000 or something like that, then either route would be good- you'd need an eagletree to see which was the best though (and a stop watch).

I think Im taking the wimps route and gping for the 1800kv castle 1518 when its released- best of both worlds I suppose on my chosen 5s voltage...

For speed runs though (as a final thought), I'd say higher kv with with higher voltage and play with the gearing- cross fingers and pray esc and lipos dont go pop.... :/

[Dragonfueled]

I get the low kv more effeicent thing but wouldn't the high kv low gearing have more torque? Given that torque or kt are supposed to go up with kv and still be geared low. Not saying it would be more efficient but I would think it would have more torque simply cause of it being able to use lower gearing and have the same speed.

Let's go to extremes. Let's take car a with a 9000 kv motor geared for 60 mph and same car with a 2000 kv motor geared for 60mph bolth using 2s lipos. In this case wouldn't the 9000 be more efficient? Or would they bolth work out the same? I figure the 9000 would love the low gearing.

[johnrobholmes]

As long as you weren't saturating the motor asking for too much torque, two motors with different kv producing the same power will be quite similar. The 9000kv motor would have more bearing and drivetrain losses. The 2000kv motor may have a lower efficiency from being overgeared however. Ideally you would want to run a motor that is volted to spin near the max rpm, for performance.


On 2s lipo both setups would be drawing the same current at 60mph. This leads me to believe that the higher KV motor would actually be better for the situation, as the motor resistance is lower and would produce less heat.

[suicideneil]

Lower kv motors generate more torque than high kv ones, all other things being equal, or so I've been lead to believe...

Spinning it faster and gearing down would certainly generate more torque for certain, but if we're going to extremes, a very high kv motor would suck ALOT of current, geared down or not I would expect. Lower gearing would reduce the load on the motor and therefore reduce the current draw etc, but at any given throttle amount it would still draw more current than the lower kv motor pullig taller gearing I expect, so.. I dunno, I think its personal preference really so long as the kvs, voltages and gearing are fairly sane.

Too many variables and unknowns for my liking, one could spend a long time doing bench testing with dynos and simulated loads to find the real answer...

[Dragonfueled]

True but it's fun trying to figure it out LOL. So many brushless questions are out there now that it's getting popular. Nitro feels outdated to me. When there's no more questions it's just not fun anymore.

Let the debate roll on. We all know high voltage low kv is the best. I never hear of the best setup within the same voltage

[hoovhartid]

Originally Posted by Dragonfueled

This is a question that has allways interested me. Let's say same vehicle one with high kv low gearing the other with low kv high gearing. What would temps ,effiency, etc be for bolth setups geared for the same speed.

One has to be better then the other LOL. I'm guessing each would have an advantage but where I'm not sure

I feel that this all comes down to the motor. Its hard for me to find a high kv and a low kv motor, that run the same voltage without one of them having the rpms out of wack. Most motors have an RPM sweet spot. So which ever one is closest to that spot per the given voltage....wins IMO.

[Patrick]

I've run the 1515 2.5d (1650kv) and the 1515 1y (2200kv) on 6s geared for 50mph or slightly over in my truggy. And although I never ran them back to back and it's been a while since I used the 2.5d, I remember it did run a bit cooler than the 1y. Over all I think I preferred the 2.5d setup geared for that speed (not a big difference though), but haven't really done enough thorough testing to say for sure.

[BrianG]

This question has been talked about in depth before. It comes down to these things:

- Like JRH said, you don't want so much current that you are saturating the motor.

- You don't want to gear a low kv motor so high that the ESC has trouble getting EMF signals at low speed.

In any case, power is power. Whether you get it via a low kv higher geared motor, or get it via high rpms and gear down is roughly the same.

The extra torque you get from a low kv motor is lost when you gear higher since final output torque = motor torque * gearing. And yeah, a high kv motor can pull a lot of current, but only if heavily loaded/stalled.

Personally, I kinda like higher rpm motors geared lower - usually run them ~70-75% of their max recommended rpm. Startups are smoother and there's still a ton of power.

[SpEEdyBL]

One thing that you guys are forgetting is internal resistance. Take two motors of the same design, one's 2,000 kv and the other is 4,000 kv, the 2,000 kv motor is going to have four times the resistance. So if you over tax the 2,000 kv motor by gearing for the same speed as the 4,000 kv motor, you will create four times as much heat in the windings. Of course the 2,000kv motor will draw a lot less current under acceleration, but it will also take a lot longer to get up to speed, due to the lack of torque.

A better comparsion of low kv vs. high kv would be comparing two different sized motors with the same resistance. For example, a neu 1521 vs. a neu 1512 and gearing for the same speed.

[sikeston34m]

Originally Posted by SpEEdyBL

One thing that you guys are forgetting is internal resistance. Take two motors of the same design, one's 2,000 kv and the other is 4,000 kv, the 2,000 kv motor is going to have four times the resistance. So if you over tax the 2,000 kv motor by gearing for the same speed as the 4,000 kv motor, you will create four times as much heat in the windings. Of course the 2,000kv motor will draw a lot less current under acceleration, but it will also take a lot longer to get up to speed, due to the lack of torque.

A better comparsion of low kv vs. high kv would be comparing two different sized motors with the same resistance. For example, a neu 1521 vs. a neu 1512 and gearing for the same speed.

I gotta disaggree with you on part of this Speedy.

The lower kv motor will make more torque than the higher kv motor.

The magnetism created by using more turns is greater, thus the torque is also greater.

[starscream]

I've run many many setups and I personally have not seen any performance advantage of either as long as we're not talking in extreme's.

I mostly run 1/8th conversions so I try to run setups that keep the rpm similar to the nitro engine used for the vehicle as this tends to keep gearing options more feasible.

I've had success with motor setups ranging between 24k to 40k but if I had to choose I'd go with a lower Kv simply because gearing up with pinions is easier to manage, bigger pinions tend to mesh better than smaller ones, changing pinion gears is a lot easier than changing spur gears, and I run mech rear brakes so motor spacing from the brake calipers is crucial and easier to obtain with larger pinions.

[SpEEdyBL]

Originally Posted by sikeston34m

I gotta disaggree with you on part of this Speedy.

The lower kv motor will make more torque than the higher kv motor.

The magnetism created by using more turns is greater, thus the torque is also greater.

Not entirely true. Lower kv motors make more torque per amp than high kv motors, but high kv motors will draw way more amps. Also, when you use a smaller pinion gear, you also increase torque at the wheels.

I have tried both the Novak HV4.5 and the HV7.5 - two different kv's, but otherwise identical motors (listed at5,000 kv and 3,000 kv). The amount of torque the HV4.5 has over the HV7.5 is quite a lot. The 7.5 with a 14t pinion takes forever to spool up and is still slower than the hv 4.5 with a 9t, which is a rocket. Not to mention that a 14 tooth on the HV7.5 heats it up way to much. I guess if i was just shooting for 30 mph, the the lower kv motor would be better, because of the power required just to spin the motor, if that answers the question.

[sikeston34m]

Originally Posted by SpEEdyBL

Not entirely true. Lower kv motors make more torque per amp than high kv motors, but high kv motors will draw way more amps. Also, when you use a smaller pinion gear, you also increase torque at the wheels.

Let me explain this another way.

A high kv motor compensates for it's lesser torque with lower gearing. Lower gearing multiplies Torque, does it not?

A high kv motor does draw more amps than a low kv motor under NO load.

How many amps a motor pulls depends on how loaded it is. This applies for ANY motor with ANY kv. The higher the gearing, the higher the load is for the motor.

A lower kv motor has MORE torque. Torque is it's ability to overcome resistence. There is a way to measure it. This statement is true.

The lower kv motor is geared higher because it doesn't need such a low ratio to multiply torque.

Now back to the original purpose of this thread.

High kv low gearing VS Low kv high gearing.

Total Watts of power delivered is what needs to be considered.

A Lower Kv motor geared higher will pull more amps. If it were possible to match two setups, one high kv low geared, and one lower kv high geared where they drew exactly the same amount of amps on the same voltage.

They would deliver the same Total Watts of Power. They would be the same!

Watts = Amps X Volts. Higher voltage setups trade amps for volts in this equation. Wire size is limited in RC. We don't use cables the size of our thumb. The largest wire size most use is 12 Ga. 12 Ga. wire has it's limits. The greater the length of the wire OR the smaller the gauge wire, the greater the resistence AND the greater the voltage drop. This hurts efficiency.

The Total amperage drawn is what creates heat especially in wiring that is being pushed to it's limits. Heat is wasted energy.

The final voltage delivered to a motor, after voltage drop values due to wire size is considered, is enough to affect overall performance.

Any differences in the setups would stem from differences in overall efficiency. Lower kv motors tend to be more efficient because higher voltages are used with them.

Also Lower kv motors are more efficient for 2 reasons:

1. It doesn't have to spin up to such a great rpm.

2. The phase switching on the ESC is less.

[BrianG]

I agree. The torque thing is 6 of one, half a dozen of the other. High kv (lower torque) but geared low (high ratio) is roughly equivalent torque-wise to low kv (higher torque) with taller gearing (low ratio). So, really the only thing to look at IMO is ESC smoothness. They seem to start off better when gearing is such that there is less mph/rpm, which means lower gearing.

So, I tend to shoot for a higher overall rpm and gear down. Hasn't failed me yet and all my setups run cool as a result.