Wow, I figured range would go down from changing to a bigger wheel.
Interesting, I’ll change to 8 inch soon as my 6.5s are slowly becoming bald.
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Interesting! You might have dialed in your motor current draw going to a larger tire, lowering your RPM. A larger tire should actually increase the current draw, so you may have been well below your motors maximum efficiency during acceleration which can contribute quite a bit if you are stopping/starting often.
Also at cruising speed it might seem confusing, but if your motor is spinning at higher RPM outside of the efficiency range, even if it uses less current you will lose range in the long run by being outside of the ideal 75% load efficiency range. You might be experiencing this by moving to a bigger tire where you motor pulls more current, but you travel farther per rpm if that makes sense.
That being said, you make a good point about going over bumps. A smaller tire transfers much more of the force you feel from a bump towards the back of the board. The contact point on the tire for the same size bump will be closer to the bottom of the tire with larger wheels, which means more of the force is used to “lift” your board up as opposed to “pushing” it backwards.
I almost think this might be a major factor as well.
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Bringing this thread back as I’m curious to see how many wh/km I’m using
I’m running 16/40
10s5p 30q
190kv 6374 bkb senseless motor
I ride pretty hard, 70-90% throttle most of the time. I’m only 140lb so I shouldn’t be running alot
Plz help so I can get a realistic range on every esk8 call
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Will you help me out? I’m running 6354 190KV 2450W motors. I weigh 280 lbs. I wanna have a cruising speed of 25 mph with 31 mph top speed. I live in Arizona and everything is flat and about 1 mile between stops. I’ve got 10s4p 30q setup with 120 (more like 112mm) wheels and 15:32 gear ratio. Should I simply change to 38t wheel cogs? Where would my sweet spot be for efficiency?
I don’t understand what “75% of load efficiency” means. in my understanding of the most efficient way to drive a motor it’s at a balancing point where copper losses are equivalent to iron losses. so if u spin ur motor on the vesc tool at full speed and look at the current drawn you will likely see a very low current draw. That’s ALL your iron losses at that speed including the small bearing and air friction losses. Generally a skateboard spins the motor relatively slow and a gear ratio of maybe 3.5:1 is a limit due to space with pulleys. If you were able to change the gearing somehow to let’s say 10:1…assuming the gearing itself isn’t awfully inefficient, you’ve now greatly reduced the current needed to produce a certain amount of torque at the expense of greater iron or switching losses. The ideal ratio of these losses would depend on your use and if ur doing a lot of hard accelerating or if u more so sit at top speed.
Unfortunately a motor that has many magnets and teeth, and greater iron losses per rpm, isn’t a substitute and you won’t get reduced copper losses. Actually you can get a hair less as with many mangnets they can be a hair thinner and your airgap diameter can subsequently be a hair wider making a bigger lever but not much. More magnets and teeth seems of benefit for better control and cheaper production as less magnetic material is needed
I know @MoeStooge uses metal gears and with those he has room for a much higher gear ratio but that’s with little metal gears which I believe don’t last long besides being very loud
Assuming pulleys are used with the ideal tension…I’ve read they’re even more efficient than a chain when under loading although generally I read of them being a bit less efficient.
Lately I’ve been obsessed with making a jackshaft allowing two gearing reductions using two belts before getting to the driven wheel. Maybe with a skateboard the sharp bends of the belt with the small pulleys is a bigger loss than typical w a belt
If we can spin our motors faster they’re very likely going to be more efficient.
every motor regardless of the winding (Kv) has a most efficient rpm based on how it’s used and balancing the iron and copper losses. If you’re mad about torque, more gearing is more efficient, if you’re crazy for flying at top speed always maybe less gearing would show better. But look at ur no-load current draw and see it’s a pittance and realize how much room you have to trade copper for iron losses if only u had a bigger gear ratio.
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Super short answer for your weight…yes.
More torque for your weight will help get.you up to speed easier…
Now about your preferred top speed that may change and a decision of gearing for speed vs gearing for efficiency.
With that said at 38t gives you more options to play with gearing… A better option IMO for someone your size would be a 12s battery and then back to gearing vs eff.
The 75% load is the optimal load (power) an electric motor should be run at to maintain the efficiency balance you are talking about. There are many factors that contribute to efficiency losses at lower and higher loads, many of which you mentioned in one way or another. Gearing has an affect on the current draw for any given speed, so if you are designing for efficiency, you need to look at the speed you want to have high efficiency at and attempt to maintain ~75% motor load by adjusting gearing.
You are thinking what I am saying, in one way or another.
Here is a research study from CalPoly confirming the 75% load target, with a lot more technical detail as to why this is the case. There is also a book called “handbook of electric motors” that goes into, effectively, the most detail available to the public on electric motors if you are really interested. The author is hamid toliyat. He talks a lot about motor sizing and efficiency considerations.
http://www.itrc.org/reports/pdf/r06004.pdf
In regards to belts vs chain vs gears:
The reason belts are less efficiency in most cases is because they are never always the right tension (during riding they stretch and compress, heat up and cool, etc) That coupled with the very large surface contact area between the belts and pulleys, the fact you are constantly bending the belt as it travels over the pulleys thousands of times per minute adds up to an energy sap.
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Firstly, I agree with Accrobrandon about getting yourself a 12s battery. It will give you more RPM to work with and in general give you a more powerful board with less stress on the motors.
I have a very similar setup except I run 12s with AT wheels. Unfortunately i wont be able to give you a definitive answer on what is the best gearing setup for your board, but I might be able to help.
What is your top speed right now? Start by getting a gearing setup that gets you to your ideal speed and then you can adjust for efficiency. The consequence of the 75% load rule is that you will probably have to be cruising at below your top speed (which is fine, since you want to cruise at 25mph).
The next thing you do is go and ride around and get some telemetry somehow that shows your motor amps (from which you can find motor power) or motor power directly if the telemetry tells you. I think there are a few bluetooth modules or maybe the VESC can even store telemetry (ive never tried). Compare the power to 75% the motors maximum rated power, or 2450W each (~5000W total) and shoot to get your power output around 3750W. My hunch, though, is that at 3750W you are going to need to gear for some ridiculous top speed to even get to 3750W motor power so you will be SOL.
Something to add is that most of the motors we use can never actually maintain that power output, so something rated for 2450W is probably only good for 2kW constant output. Just something to keep in mind.
Lastly, the reason I don’t even try to get high efficiency with my board is because I use huge motors to get low end torque. I probably won’t be able to get to 75% load at reasonable cruising speed since my board makes so much power. That’s one of the many reasons boosted board went with ~40mm motors. They were able to utilize as much of the power those motors could make while making sure they kept the motor load near that 75% mark during fast cruising so they also got good range from a small battery. I remember feeling the motors of my friends boosted board recently after a cruise and they were very warm, but not hot. Mine were barely warm, meaning his board was probably at a better motor load than mine was. Its a trade off im willing to make to be able to absolutely smoke pretty much any prebuilts.
The moral of the story here is that if you can properly use the power from your motors at the 75% output level you will get higher efficiency. However, its uncommon we can do that because our boards make a LOT of power meaning our motors put out way too much power to properly use at that 75% output level without having to gear to some ridiculously high cruising speed that actually requires that amount of power.
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i read/skimmed the paper but it leaves me feeling empty. Stating a motor is most efficient at 75% of its rated power begs the question how are they rating the motors? I know in esk8 there’s no standardized rating method and if you live in Australia or Norway the ambient temp will greatly effect what power can be produced as the higher temp will increase the winding resistance.
I have heard if you’re under load and that load sinks you to about 80-85% of your no load speed that’s an ideal Load and speed but that’s also vague.
But why bother with vague numbers when the details can be figured or at least an ideal single speed and torque output can be figured, using that info when on an electric vehicle which necessitates constantly changing speed and torque is another element to add.
Tell me if I’m wrong but as I understand it there’s only two losses in a motor, copper and iron, and we can figure the ideal motor speed and torque looking at those. Copper losses are directly related to torque production and how we use these motors copper loss is by far the largest loss. Iron loss or switching losses, come from the switching magnetic field and the eddies and hysteresis produced. Iron losses are tiny for us! Across the board if u want a more efficient drive u need to drop the amount of torque you’re asking of the motor thereby decreasing copper losses. Simple as that. If it’s 100kv or 300kv it doesn’t matter and the heat to torque ratio will be the same, the motor km, and it’s only through gearing that you can reduce the torque the motor has to produce and gain efficiency. This is the case for the vast majority of motors. Adding that gearing results in higher motor speed and greater switching losses which are eddies and hysteresis. eddies being exponentially produced and hysteresis being linear and at some far off point of adding greater gearing the iron losses will be greater than the copper. Actually when they are equal…THAT is the most efficient production of power from the motor. Unfortunately for us it likely requires spinning the motor maybe some way off speed that no practical belt drive can get to. I know @MoeStooge here uses some really high mechanical gearing and he’s likely got the most efficiently run motor because of that. The gearing of his solid gears on the other hand is likely inefficient. Belts and chain can be very efficient but there isn’t the room on the wheel for a giant cog. A jackshaft could be done and not that hard but no one does that. Would love to see a belt driven board with a jackshaft. So two belts, let’s say 3:1 ratio on each, and then multiple them together and you have a 9:1 ratio. That would likely be close to most efficient and also capable of much more power by weight or volume.
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Inrunner driven mountainboard, 2 stage transmission and both rear wheels driven at same time (same shaft)…
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They elaborate quite thoroughly on their testing methods on page 9 if you are interested, but it’s in theory no different than a dynamo-meter for a car. They look at input power, output power, and apply a load with a hydraulic piston pump which is attached to a beam (moment arm) which pushes against a load cell.
Their experiment was about as good as it gets for [practical use of AC electric motors.
Not too sure what you mean here, but if you care about efficiency you have to design a powertrain around your goals, or in our case the speed you want to go. The purpose of choosing a cruise speed you want to have high efficiency at is so you can design your gear train to get your motor to its most efficient range. Or, choose an appropriate motor for your gear train.
Its not quite that simple.
Dropping torque does not necessarily improve efficiency. It is dependent on load (HP) which varies with and is a function of torque and RPM. Since we are picking a single RPM to design for, the torque must vary. Lowering torque just lowers power output, which lowers efficiency. (unless you are above 75% load, which would increase efficiency until you get to 75% load and then start decreasing it again). If you pick any RPM and redo your design you’ll find the same 75% rule applies.
Here is a graph showing motor torque vs speed, with various efficiency lines (follow the red line to see continuous output rating) At stall (upper left corner) efficiency is terrible, but as we reach 200 Nm of torque, we are at 94% efficiency (max). 200nM is virtually EXACTLY 75% this motors stall torque of 270 nM. Notice as you decrease torque but increase motor speed the power is increasing to a point where efficiency starts dropping again.
The losses you are talking about do indeed produce the highest efficiency when they are equal, and it just so happens to coincide with most electric motors peak efficiency being around 75-85% load. This isn’t always the case, some really good industrial AC motors are even better (95+% load).
There are some good responses here:
Including one about rotor slip, which is a key feature of reducing losses and increasing efficiency:
And here is my last and probably most impactful reference. This is a document written by Excel Energy (Market cap 36 billion USD, 11,000 employees) which highlights the appropriate methods for choosing and implementing electric motors.
Here is a screenshot on page 21:
Beyond that I can’t really explain it much further, and if you aren’t convinced I would try to replicate the experiment carried out at CalPoly somehow. Its input to output power, so you could use the VESC to read the input power, and somehow replicate a dyno for your motor.
And just to be clear, belts and chain are inefficient ways of transmitting power. They don’t really pull that much energy out of our boards, but objectively they are not ideal. Belts are better than chain, but still not a good as gears. Belts/chains have the advantage of being inexpensive which is important for us.
With over a hundred years of auto manufacturing knowledge, we chose to use gears (spur, helical, herring, etc) to transmit power in the overwhelming majority of vehicles for many reasons (one of which is their efficiency)
I hope this makes sense.
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Smart!
Also Smart!
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This is relevant for me again. Thanks for all the explanation. Let’s see if I get it right.
To be most efficient I need to make the targeted cruising speed work at 75% of motor power (or is it the amount of power I use to accelerate to the cruising speed that needs to be at 75%?).
In our cases, with what parts we normally use (high power motor in my case here 3500W) that means that we properly need to gear it to an insane top speed, and most likely that doesn’t make sense for us?
Anyhow… I would like to do the calculations you suggest. But need some help.
My two motors are rated 3500W each. That is 7000 all together or if we say they exaggerate 6000W. So 75 % of that is 5250 and 4500W
At cruising speed they draw around 15-20 amp all together. At accelerations it’s between 60-80 for both most of the time.
If I calculate that into watt (amp x V) I get between 1000 and 2000 watt when cruising and 3000 and 4000 W when accelerating.
So that would be inefficient when cruising and closer to efficient when accelerating. if we go by the 75% rule.
My setup has an actual top speed of 57kmh / 35.5 mph. Acceleration feels a little slow in the beginning. When reaching around 25kmh / 15 mph acceleration kicks in better and it take off.
My thoughts are that I don’t need the 57kmh top speed and a little better acceleration from zero would be nice.
So I am thinking about changing gearing from 18/60 to 16/60. That would give me around 50kmh top speed and better acceleration.
What would that do to efficiency? As I understand it it would lower efficiency right?
To be honest I am pretty satisfied with the distant I get. I was hoping for 70km and I get over 100km. But I think this discussion about efficiency is interesting and would like to know more and maybe better the efficiency of this setup if it makes sense.
Rest of the specs:
14s. Battery 90A discharge capable. 6” wheels. Motor amp set to 65A. 140kv motors.
From this does it sound like I have understood it correctly?
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It’s easy to get peak efficiency… just gear the board for the lowest top speed you will accept.
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