Esk8 IQ: The Math and the Science of Electric Skateboarding – Part 3

Originally published at: http://dev.esk8.news/2019/01/25/esk8-iq-the-math-and-the-science-of-electric-skateboarding-part-3/

Hiyas, nice to see you gluttons for punishment again :). This is the final installment of Esk8 IQ. What does that mean to you, the reader? It means this is where we tie all the previous info together and make it useful. It’s important that you understand the aforementioned concepts or none of this will make sense. I am about to cover ERPM, KV, Voltage, Watts, Charge Rate, and some other stuff. So hang on to your whitey tighties, its about to be a roller coaster ride.

ERPM (Electrical RPM)

This is how you use motor KV and battery voltage to define the ERPM value. Remember this, its the holy grail of VESC configuration values.

`KV * Max Voltage * 7 = ERPMExample:   190 KV * 42.0v (10S) * 7 = 55,860 ERPMExample:   260 KV * 25.2v (6S) * 7 = 45,864 ERPM`

Why is ERPM important? The short answer is that all VESC’s and VESC based ESC’s have an ERPM limit, you want to stay below this limit or you’ll be the proud owner of a \$200 + charcoal briquette. A standard 4.12 VESC has an ERPM limit of 60k, the FocBox/Unity and VESC 6 can go up to 80k per some sources but its (IMHO) better to stay at or below the industry defined spec of 60k.

Maximum KV

`8600 / Max Voltage = Max KV`
`Example:  8600 / 42.0v (10S) = 204.76 KV (Max)Example:  8600 / 25.2v (6S) = 341.26 KV (Max)`

Now we will discuss Motor KV. This is the value that defines the speed and torque (Powaaaaaah!) of an esk8 when combined with the ERPM and Voltage values. When you define the Maximum KV of a build, you are saying that is the highest speed motor that can be used while staying under the Voltage and ERPM limits of the VESC. If you don’t believe this refer to my earlier declaration regarding charcoal briquettes.

Maximum Voltage

You can also attack the question from the other side by asking “What is the maximum voltage battery I can use with a given KV motor?”

`8600 / Motor KV = Max Voltage Example:  8600 / 170 KV = 50.58v Example:  8600 / 190 KV = 45.26v Example:  8600 / 260 KV = 33.07v`

Lets face it, sometimes a build just rises out of a box of leftover gear or something else less stellar and deliberate than the scientific method. It’s all good, as long as you have an understanding of the math behind the electrical components of an esk8.

If you find yourself with a couple of 190 KV motors laying around screaming “BUILD ME!!!” at you then do it. With the above listed formula you will define the max voltage as 45.2 volts, or 10s (as 10s is 42v max and is the nearest voltage rating below the max allowable voltage). Understand, that doesn’t mean you can’s use a 6s battery pack, it’ll just be slow. If you use a 12s battery pack there is a good chance that, at speed, you will pass the ERPM barrier and kill your VESC which will then proceed to take its revenge on you by throwing your non-reading, non-math understanding ass straight at the ground with absolutely zero remorse and will enjoy your blood and loss of teeth. Just saying.

Maximum Watts

If you find yourself in need of defining the Watts value, then here you go. I typically don’t need to do this but some people like it.

`Max Battery Amps * Volts per Cell * Cells = Max Watts Example:  60a * 4.2v * 12 = 3024 Watts Example:  80a * 4.2v * 12 = 4032 Watts Example:  60a * 4.2v * 10 = 2520 Watts Example:  80a * 4.2v * 10 = 3360 Watts`

Maximum Charge Rate

`mAh / 1000 = ?a * Maximum Charge Rate always = 1C Example:   9000mAh / 1000 = 9a Example:   12000mAh / 1000 = 12a * Charging at rates over 2a will lessen the lifetime of your pack * Charging at rates over 5a can be dangerous`

I want to discuss charge rates for a minute. Its a weird subject as some people reallllly wanna charge at 20a, I honestly just don’t understand. Whatever, to each his own.

I charge at a max of 4a, most of the time I charge at 2a or 3a. It’s safer for the battery pack to charge at lower amps and it extends the lifetime of the battery pack plus, low amp chargers are cheaper. I would rather have multiple boards that are charged in a couple of hours than one board that is charged in 20 minutes.

He really said that. I mean, he must have.

Gear Ratio

`Wheel Pulley Teeth / Motor Pulley Teeth Example:  36t / 15t = 2.40:1 Gear RatioExample:  40t / 15t = 2.66:1 Gear Ratio Example:  40t / 20t = 2.00:1 Gear Ratio  `

As you can see from the above examples, the change in gear ratios based on the number of pulley teeth is not dramatic in a mathematical sense. In a performance sense, it is. The difference in speeds (more) & torque (less) is massive in an actual scenario between 36t /15t (2.4:1) and 40t /15t (2.7:1).

With higher gear ratios you have more torque, with lower gear ratios you have more speed. The trick is to find the least amount of torque you need so that you have the most speed without a noticeable loss of torque. This is very difficult to do the first time you build an esk8, so start with 36t /15t (2.4:1) as it’s the most common and best compromise using available components. After you get some ride time increase the tooth count of your motor pulley(s) for more speed or increase the tooth count of your wheel pulley(s) for more torque. In other words, experiment.

Wheel Circumference

`Diameter * PI = Wheel CircumferenceExample:  90mm * 3.14 = 282.6mm `

You’ll need this for the next section.

Maximum Speed (MPH & KMH)

`Motor RPM * Gear Ratio = Wheel RPM * Wheel Circumference = MM/Min MM/Min * 0.000037 = Maximum MPHMM/Min * 0.000060 = Maximum KM/HExample:7,980 * .42 = 3351.6 * 282.6 = 947,162.16 MM/Min 947,162.16 * 0.000037 = 35.04 Max MPHExample:7,980 * .42 = 3351.6 * 282.6 = 947,162.16 MM/Min 947,162.16 * 0.000060 = 56.83 Max KM/HMax Speed w/ Efficiency35.04 * 85% = 29.78 Max MPH56.83 * 85% = 48.31 Max KM/H `

This is the mot difficult formula to use as it requires the use of the efficiency variable. You will need to experiment with this value to figure out what is most accurate for you. Your efficiency value will change based on riding style, proficiency, wheel type, substrate type, etc. I usually use 88% and it’s almost always close.

Honestly, I wouldn’t worry about too much. Its about how fast you ‘feel’, the actual data is almost irrelevant until you start tweaking settings to get more speed on your street board or more torque on your mountain board.

This is essentially all I have to give regarding the math and science of esk8. These articles are the product of many hours of research in building electric boards and the culmination of much experimentation by me and many people in this community.

I hope you have enjoyed reading this as much as I have enjoyed writing it. If you have then buy me a beer next time you see me, if you haven’t, [redacted].

* Damon says I’m not allowed to talk like that in public, but I feel like you get the idea :).

24 Likes

Great article but seems to be a mistype around this bit, afaik eprm limit on a focbox is the same as a 4.12 but fairly dependable to 100k

Vesc 6.6 and derivatives go to 150k(?) and lastly unity goes somewhere around the same place as the 6 but ran out of testing ability with a maxed out 270 kv motor iirc

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It’s about spec’s, not what people ‘say’ it will do…hence the 'Math" part of the title. If the specifications of individual ESC’s are higher than the specifications quoted, please link them to me so I can update the article.

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The max has not been actually performed by Jeff, but he can calculate it it an states the limit of the Unity

Frank has confirmed the erpm of VESC6 a few times in the other place

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I not Watts guy either but max watts should be calculated with a voltage closer to nominal, under load it will never be 4.2v

How close to nominal depends on many factors though.

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Well shit. I just bought some 6374 190kv motors. I have been up to 40 mph with my current 12s battery & 190 KV 6355 motors w/ FocBoxes w/o issues but idk if I wanna roll the dice again after reading this…

Kinda already knew about this I guess. But there were only 190 KV left in the recent group buy. Still I probably should not be doing 40 mph speed runs. Also gearing for such a high top speed keeps me under the ERPM limit except when I am doing an all out speed run.

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Thank’s a lot for these maths : my max speed test run at 59.02 kmh quiet perfectly matched your calculation :
datas of my setup :

• 220 KV motor / 42V (10s battery) => 220 x 42 = 9240 ERPM
• pulleys 16t/60t => ratio : 16 / 60 = .26666
• Wheels diameter : 150 mm => Wheel Circumference = 150 x 3,14 = 471 mm

So let’s apply the maths :
Theoretical Max Speed = 9240 x .26666 x 471 = 1 160 544 MM/Min
Theoretical Max Speed = 1 160 544 x 0,00006 = 69,63264 kmh
Max speed with your 85% efficency variable = 69,63264 x 85 / 100 = 59,187744 kmh
Great !

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Welcome! Level matched with Builder’s Forum.

Thank’s … but the problem for building is I have more 4 feets than two hands and two feets

Otherwise FYI, my setup is an All Wheels Drive (= 4 motors), so maybe such a setup better matches the 85% efficiency variable than a dual…

As a noob I must qualify this question with “I don’t know shit” but howcome so many people do 12s4p and 190KV motor builds? I just finished my first build and went with TB 12s4p and TB 6355 190KV motors as was suggested this was fine by a lot of people. TB even sells their 12s4p and 6374 190kv motors together in a bundle online, are you saying this is really dangerous?? Not trying to stir the pot but also I’m just genuinely conernced!

12s4p and dual 6355 is a great build. You get a bit more with 6374, but I don’t think I’ve heard anyone who doesn’t think 6355s are a great option.

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Naw, @BillGordon is correct, 6355s are great especially with 190kv motors. @torqueboards bundles them for that specific reason.

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6355 and 6374 are almost indistinguishable in power provided they are setup the same.
The difference is that the 6355 will heat up faster.

I think he’s concerned about why 190kv motors are often used and bundled with a 12s setup when the math in this article proves that the they exceed the max safe kv for that voltage.

8600/50.4 = ~170.6 max kv

8600/190kv = 45.2v (lower than 12s)

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Yeah that’s right in the article it says with regards to 190KV motors:

That’s why I’m confused =/

Hey Mike, the 7 should be changed to rotor-pole-pairs?

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This pretty much goes against what every DIY enthusiast/elitist has said to me regarding DIY vs prebuilts lol.

I don’t see how, but that’s because you didn’t say what you’ve been told

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Oh no I’ve had no problems I was just using that bundle to exemplify my confusion about the article and what everyone’s said about 12s and 190kv compatibility. Even @mmaner said in the article

but then also said

So I’m just utterly confused now