The relevance of motor kv with gear ratios taken into consideration?

[b264]

@tech.shit

bigger motors have more torque

While I have experienced this in person, this is a controversial topic which has its own thread even.

Do bigger motors deliver more torque at the same motor current?

Today It was suggested to me that people who upgrade just their motors to larger motors get more torque with the same motor amp settings. backed up with experiences of both bikes and esk8s doing this and feeling more torque. My understanding is that motor torque is proportional to motor amps x Kt. (torque/amp) and that Kt is inverse of Kv. (it’s 1/Kv) so a larger motor of the same kv maybe able to handle more motor amps, but with the same motor current it’s torque should be the same. A…

It’s fair to say not everyone agrees with that.

[MysticalDork]

Generally, going for a really massively overkill motor on a lower-average-power build (like dual 80100s on a street board, for example) will lose you some efficiency, because of the extra friction, eddy, and hysteresis losses. It’s best to match the motor more closely with the intended application.

Not to mention higher cost and weight.

I’m not using wattage numbers, because those are irrelevant. But bigger, beefier motors, that can handle higher current, are worse at low load.

[Evwan]

@b264

While I have experienced this in person, this is a controversial topic which has its own thread even.

I really wanna make a scientific test with this. Maybe a motor dyno or something

@Audiobahn1000

Ok so say we DO have an ESC and a battery that can handle any motor we throw on the board. Then what would be the advantages/ disadvantages of using a high power motor over a lower one? Like, would a board have better range if it had dual 2000w motors vs dual 5000w motors or is there no disadvantage at all besides price and weight to using larger motors?

Once again, that wattage number means nothing. If you’re running a motor on 20s, it’s going to have twice the wattage of a 10s setup at the same duty cycle % and same current. If you have a setup that can dump a lot of current into a motor, the larger motor will be better as it should have less resistance and it would have more thermal mass to deal with the extra heat of increasing current.

[poastoast]

@ShutterShock come do your dyno thing

[ShutterShock]

Ughhh

It’s on a list somewhere haha

Also there already is a motor dyno thread

[Evwan]

I just dont see why a larger motor would have more torque. If it’s more efficient, the actual number would just be negligible and it shouldn’t be more noticeable.

[MysticalDork]

@Evwan

I just dont see why a larger motor would have more torque.

For the same motor current, at the same KV, it wouldn’t.

A larger motor will have more torque capability, because you could push it harder before it saturates or otherwise reaches its limit though.

[ross_forp]

Up to somewhere between 6-10k rpm, higher kv, same size, same no load speed factoring gearing will run cooler on account of the lower copper losses but at some point spinning the motor faster and faster will cause the iron losses to exceed the copper losses, defeating the efficiency advantage of spinning faster, but you’ll have to measure it experimentally to be sure what that point is.

[Evwan]

Well, yeah, that’s what I’m saying. FWIW, in most cases a decent 6374 motor will max out the current capabilities any esc, especially if the rider isn’t a fatass.

[ross_forp]

@Evwan

I just dont see why a larger motor would have more torque. If it’s more efficient, the actual number would just be negligible and it shouldn’t be more noticeable.

It’s covered in a different thread, but the lower resistance same kv motor has the same torque for the same motor current, but the lower resistance one has more motor current for the same battery current, so the larger motor gets more torque at the same speed for the same battery current.

[Evwan]

Isn’t the difference in resistance almost negligible unless you’re going from something small to something really big?

#derail

[MysticalDork]

Exactly. I wouldn’t expect the difference between a 6355 and a 6374 to be more than a handful of percent under most conditions.

I could totally be talking out my ass though, it happens.

[ShutterShock]

@MysticalDork

Exactly. I wouldn’t expect the difference between a 6355 and a 6374 to be more than a handful of percent under most conditions.

However another consideration is that larger motors take longer to get heat soaked - not a problem for some people, but definitely a problem for larger riders or hilly areas

With motors overheating, braking gets reduced too.

[ross_forp]

@Evwan

Isn’t the difference in resistance almost negligible unless you’re going from something small to something really big?

#derail

I think it could be noticeable…

@ross_forp

But to summarize the answer to the OP’s question, 10-12lbs additional thrust @ 15mph is plausible by dropping the motor resistance from 70mOhm to 20mOhm with a larger, same kv motor and changing nothing else. I assumed 2 motors, 12s, geared for about 40mph, with 30a bat and 80a motor limits per motor. At 15mph, they’d be hitting the battery current limit and not the motor current limit, and essentially the lower resistance motor has more motor current at that speed than the higher resistance one on account of lower I^2R losses (both have less than 80a motor current available at that speed on account of the 30a battery limit, but the lower resistance motor’s motor current is higher).

[Evwan]

70mohm to 20? That’s a huge difference.

[Evwan]

Sometime this week I can give you the resistance values of a flipsky 190kv 6355, 6368, and 6384

[Pedrodemio]

Since things are already confusing, throw cooling into it

A bigger motor with poor cooling can have lower continuous power capability than a smaller one with good cooling. At the same time the bigger one might also have lower efficiency due to higher rpm related losses, the power necessary to just overcome all kinds of friction is not small, specially on bigger motors

A big advantage of big heavy motors is thermal inertia, they can handle a huge torque spike for seconds or minutes before overheating

In truth, all depends on your riding style and no definitive recommendation can be made

[Katarsis]

How did this setup work out for you?

[Evwan]

Currently running 205kv on 16s. Soon to be 18s. It’s loud, fun, no problems yet.

[Katarsis]

Awewome! I have the same satup in my new build.