# Thoughts on Kv selection

Motor Kv selection seems to be a common question, and after lots of lurking I think I might have it generally figured out. Hopefully, I’m not wrong on the subject and maybe this thread will be worth adding to the stickied guide list.

When you design your board, you generally have a top speed in mind, and will pick your motor, gear ratio, and wheel size to hit that target. So basically the question “what Kv should I pick” really boils down to

1. should I pick a high Kv motor and gear it up/use small wheels or
2. should I pick a low Kv motor and gear it down/use larger wheels

The confusion comes from the fact that Kv is essentially constant/Kt, i.e. a high Kv motor will have a low Kt torque constant. Now some people think that this means high Kv motors have less torque than their low Kv siblings, but this is not the case, because the effect is exactly offset by the changes in gear ratio that you will make in order to achieve the same top speed.

Let’s try a real world example, let’s say 50 kmph top speed target, with a 10s LiPo: Configuration 1: a 200 Kv motor at 36 V turns a 7200 rpm. Assuming 83mm wheels, that’s 36V *200rpm/V *3.14 *83mm *60min/hr 1km/1000m 1m/1000mm = 112 kmph with no gearing. To get this down to spec, we have to gear down to 2.24:1. Now Kv 200 rpm/V 23.14radians/rotation 1min/60s = 20.93 A/Nm, or 0.04778Nm/A. Assuming the battery maxes out at 50A, the max torque at the wheel is therefore 0.048Nm/A *50A 2.241= 5.38Nm

Configuration 2: a 300 Kv motor at 36 V turns a 10800 rpm. Assuming 83mm wheels, that’s 36V *300rpm/V *3.14 *83mm *60min/hr 1km/1000m 1m/1000mm = 169 kmph with no gearing. To get this down to spec, we have to gear down to 3.38:1. Now Kv 300 rpm/V 23.14radians/rotation 1min/60s = 31.4 A/Nm, or 0.03185Nm/A. Assuming the battery maxes out at 50A, the max torque at the wheel is therefore 0.03185Nm/A *50A 3.38= 5.38Nm

So theoretical torque is the same regardless of what Kv we choose, because we will just pick a different gear ratio to compensate. How do we pick a motor then? Well, have you ever noticed how two sk3 6374’s with lower Kv’s have lower wattage ratings listed by Hobby King? I think that is because the way you actually achieve a lower Kv for a given motor housing/stator is by winding the stators with thinner and therefore longer copper wire. Long, thin wire has higher electrical resistance than short, thick wire, wasting more energy in the form of heat losses, which not only decrease energy efficiency, but also cause increased wear on the motor.

So in short, the old adage that a low Kv/high Kt motor is good for torquey builds and bad for top speed is kind of off the mark, since your pulley gearing changes all that anyway. You should always pick the highest Kv available as long as you can find motor/wheel pulleys to support the intended top speed.

Well… Except for the fac that the VESC (pre v6) seems to have an erpm limit of about 60k, so if your Kv is too high relative to your battery voltage, you risk damaging your speed controller. Also there’s some thought that maybe needs to go into the possibly higher mechanical losses associated with a more dramatic gear ratio, maybe some weird inertial effects due to increased rotational mass of a larger wheel pulley too, I dunno, haven’t hammered it out. But I bet these are minor effects, so in the end they can probably be ignored.

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as you say you can do adjustments with gears and it largely has the same effect as kv and then there’s the limit of how big a pulley you can fit, and you dont want to use one smalller than 17 supposedly.
with the lower kv you have thinner wire that gets hotter per amp but with a motor having a very high kv and very thick wire, it takes more amps to make the same torque. the heat balances out in terms of getting the same amount of torque out. but lower kv motors do have a lower power rating and it could be related to that or it could be the high speed and high kv motor are able to increase their speed and of course speed and torque is power. it seems an easy win and just go faster and you have more power, but there is the other motor major loss besides copper losses, iron losses being eddy currents and hysterisis which increase with speed. you can design a motor for more speed but the design generally used by us probably has an ideal ratio of speed to torque that is an equal balance. and an ideal most efficient speed to torque ratio. not that youd want to sit at one speed all day

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I have had two setup ups with same size motor one 270kv and 140k geared to same top speed.

Only change between setups was kv and gear ratio.

Performance was similar. Hard to give a winer without data but it feels similar.

The low kv setup is much preferable because it means I can have a larger motor pulley, this makes belts last really long. The motor pulley last much longer too. Also the wheel pulley is smaller so the pulley and belt have more ground clearance which helps them get damaged less form uneven ground or rocks.

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What pulleys do you have the low Kv motor set up with?

Originally was 20/39 = 1.95:1 Later I changed to a new wheel pulley. … 18/36 = 2:1

The 270kv setup was 4:1 ratio!

I should mention I’m on 12s with 97mm wheels

I have not had a belt brake since changing to the low kv which was over a year ago.

Did not know that belts broke that often. That’s very good to know

What wheels and motors are you using? Going 36T:18T with a dual 6384 setup (@170KV) on 100 mm wheels.

They don’t if your setup is good. If there is any slip between the belt and the pulley during hard braking it will shred belts pretty quickly.

The small motor pulleys 12t to 15t wear out really fast, once the teeth are worn they slip and that will kill your belts. Idler pulleys can help to reduce slip also.

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Racestar 5065 140kv, 97mm flywheel clones. But will upgrade to 100mm boa’s soon. The racestar motors have proven very reliable even with lots of heavy rain usage. I would like bigger motors but I use the board to commute and its already so heavy to carry around.

i know someone who uses belts on his bike and has had the same belt with for like 6 years. is there possibly a better belt and are yours kevlar? his latest bike has a belt with a pattern that is even new. i dont know it on hand but i doubt motorcycles are changing their belts so often. seems weird still no one has belts that last.
does the belt ruin the cog or the cog the belt or what or both.

I would say yes great in theory but not really practical with street set ups. I think the sweet spot has been found with the 200 kv mark. With larger wheels say pneumatics on a mountain board then yes you have some wiggle room pulley wise but then factor in size of the wheels…too much thinking…

I’m drinking.

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I use Steel pulleys for the motor. It’s so much stronger than the usual alu. And hold so much longer…

After 2000km on my Trampa AT, the steel pulley still looks like new, and never broke a belt yet.

Here’s the comparison chart between 100kv and 1:1 ratio vs 200kv and 2:1 ratio. The electrical resistance on the 200kv is 1/4th the resistance of the 100kv assuming the same size motor and same copper volume because the 200kv winding wires are half the length and twice the cross sectional area compared to the 100kv.

The most obvious difference is the huge boost in efficiency while accelerating with the 200kv motor (green line top left chart) caused by the 75% reduction in motor heating (purple line, top middle chart). Also notable is the reduction in battery amps across the chart with 200kv (purple line, bottom right chart) for identical acceleration performance (yellow and red lines, bottom left chart)

Both charts assume 50v, 100mm tire, 60a motor amp limit, 60a battery amp limit, same size motors & copper volume.

(edit: click to enlarge)

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don’t have time to read this now, But looks good!

Thanks @professor_shartsis

Where are my bloody glasses?

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@dareno if you click it there’s a bigger version.

@professor_shartsis thanks for this by the way but alas even the bigger version has me needing visual enhancement. I will find them though because this is a good resource!

So i guess high KV with gearing is the winner?

This i did not know! just started playing with my first build, better go back and correct the speed limit.

Yeah, it’s probably the way to go. I don’t have a food understanding of the magnitude of things like eddy current and bearing losses, but in the absence of such knowledge, the best we seem to know is that higher kv = higher max current, and assuming you gear for the same top speed, that extra current is directly proportional to torque. So sk3 6374 200 kv with 80A current is capable of 8/7x more torque than the 150 kv, 70A version

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What about increasing kv and reducing voltage to achieve same results? Like instead of using a 12s with 200kv we use 10s and 240kv.

What would be the difference on those setups? Would there be any differences in efficiency or torque?

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