Limitless How does motor RPM impact speed running?

[dure16]

How does motor RPM impact speed running?

For example, if you can gear for the same speed with all three of these motors, how do you choose which one to use? Assuming motor size is the same for all three.

1000kv, 38T pinion, 24T spur
2000kv, 27T pinion, 34T spur
3000kv, 21T pinion, 40T spur

 

[Tex Koder]

How does motor RPM impact speed running?

For example, if you can gear for the same speed with all three of these motors, how do you choose which one to use? Assuming motor size is the same for all three.

1000kv, 38T pinion, 24T spur
2000kv, 27T pinion, 34T spur
3000kv, 21T pinion, 40T spur

I believe it has to do with on several factors..
1. Main thing is how quickly you want to get your vehicle up to speed (let's say 130+MPH) ,given >
2. How much available road length (let's say 1,500 ft.)
3. How many AMPs your ESC can provide (along with lipo capacity) in a given amount of time ,before it overheats.

Each of your motor/pinion combos will play a part in the above variables.

 

[LibertyMKiii]

How does motor RPM impact speed running?

For example, if you can gear for the same speed with all three of these motors, how do you choose which one to use? Assuming motor size is the same for all three.

1000kv, 38T pinion, 24T spur
2000kv, 27T pinion, 34T spur
3000kv, 21T pinion, 40T spur

Missing a key detail here.
What voltage does the person want to run?

I get what you are after is gearing vs KV, but I'll shortcut you to something more important to look at:

If I assume 32v for all 3 setups you have to also consider how the motor is made and what it is rated for.
In general, the 3000kv will drive the amperage up very early because of the KV. It could also risk going over 50k rpm easily.
The 1000kv will have low amperage but also cannot handle a lot of amperage because it is designed for high voltage instead. (only 1/5 scale motors might be an exception)
The 2000kv is the only motor spec'd for 8s voltage.

Pictures are worth 1000 words sometimes.....
Here is an example of a TP4070 motor (780kv) motor wire. You can see why it is rated for high voltage and low amperage. The black jacket around the wire was loose/baggy on it because there are so few strands of wire compared to the higher KV variants.

 

[dure16]

Main thing is how quickly you want to get your vehicle up to speed

Does the higher or lower KV motor get the vehicle up to speed faster?

Here is an example of a TP4070 motor (780kv) motor wire. You can see why it is rated for high voltage and low amperage.

Thanks. So running lower voltage on a low kv motor, with high gearing, can cause too much stress on the motor?

 

[LibertyMKiii]

Thanks. So running lower voltage on a low kv motor, with high gearing, can cause too much stress on the motor?

Right, as you can see the wire on that motor would not have been happy if I had run 4s voltage and 600+ amps.
That 780kv was rated to 125amps max.

It always comes down to running the right motor for the voltage.
If you are looking at 8s that will be a motor in the 1100-2400 kv range. If it is a 40mm can I would argue that it is 1600-2400 kv range.
only on the high end of that KV range if it is a motor like a CM series or the castle 1721 which both can handle high RPMs and amperage.

 

[#ARRMArcfanboy4life]

So running lower voltage on a low kv motor, with high gearing, can cause too much stress on the motor?

I heard that somewhere on the forum this week.