gave it a topspeed run over a distance of 400m straight, slight wind, measured avg speed back and forth in an upright normal riding stance, i.e. I brought the board up to max speed and pressed “record” in both directions once. before the runs I measured the voltage: 40.2V!
avg speed “->”: 43.4kph
avg speed “<-”: 43.4kph
so pretty consistent, also with all other runs I ever did with my vanguard! also, so far I never felt like there was anything holding this motor back - it always goes straight to top speed and stays there. difference of wind in my back vs. wind in my face for example: nada!
now, off to crunch numbers:
ESC: VESC in FOC mode, battery limit 50A, motor limit 70A
Motor: SK3 6374 168KV
Gearing: 16/36T with 15mm belt
Wheels: Flywheels 90mm 75A
Voltage: 40.2V
Theoretical max speed: 50.9kph @ 6753 motor rpmAverage speed: 43.4kph @ 5758 motor rpm
= efficiency of 85.2%
and thats in foc mode that is known to have slightly less top speed, so it compares with your 65% first run.
I now really wonder where this pretty big gap comes from? in theory my board should be a lot worse: extremely soft big urethane chunks vs. hard slim urethane and a gates gt3 15mm belt vs. nothing!
btw, I carved home after that test and measured the urethane temperature when I stopped at home with a laser temperature gun: 47°C, ambient temperature 23°C.
So I’m at 72% of the noload and ur at 85% to a similar speed.
Actually ur big wheels probably have better rolling resistance. Soft is a good thing as long as it has lots of rebound, especially on a less than smooth surface.
Other than that your motor is much bigger with a stator of 5374 maybe and mine add up to 4750. And then the lack of gearing beyond that. If someone did a test with a similar sized stator without gearing it would be more revealing of the difference between pulley and hub. But your test does show these little motors are being pushed beyond what they’re best at for sure.
Sadly I’ve broken 2 escs in 2 days. One was my old Chaka one on FOC and one of the new ones I just put on from elkick lasted less than 2 charges and that was on bldc! I think they’re “mildly” broken as the Chaka one doesn’t show a failure code.
Didn’t have time to do a temp test. Of course the speed test here didn’t kick it up anything other than warm to the touch. from what I could tell from the short period I was riding the temp takes longer to rise likey partially because of the steel lack decreased thermal conductivity and also maybe the airgap is helping. Have to test more when I can but for shorter hard runs the bell stayed way cooler
you shouldnt compare apples with oranges (bldc vs. foc). to make it comparable, its hub foc 65% vs. belt foc 85% and its pretty much the same topspeed too. if i went BLDC, my top speed would rise for the same voltage and then its probably 92% vs. 72% or so - either way the gap of roughly 20% would probably stay.
but you have 2 hubs and I just have a single motor and your gearing is basically 90KV vs. 168KV?
got a cycle gps app - when I press start it records distance and speed and when I press stop it tells me max and average. that app was a bit weird though, because the velocity kept “fading in” when I looked at the data. obviously this was bullshit since I hit start when I was at a stationary speed and during the whole run the motor frequency didnt change one bit. my guess is that its some smoothing kalman filter working behind the scenes of that app. I simply checked the speed graph and extracted the last stationary speed, which corresponded pretty much to the max speed (as expected, since my speed simply didnt change in those 400meters). perfectly flat pavement and motor frequency absolutely stable.
thats easy to do when you have 2 guys with the rider going in a circle. where would my fixpoint be when i shoot a video from ego perspective? wont going to happen! also, I really trust gps, so far it has always given me pretty accurate results - I dont know anyone who wouldnt trust a gps distance and speed estimate tbh unless its within the typical gps error of roughly ±12m.
I mean, in the end my 85% just fits what most people have reported so far - I just wanted to make sure that these values were actually correct and not some downhill peak measurements. its in my own interest to be accurate.
on a sidenote: we both base our max theoretical speed on a correct motor kv. did you ever check with vesc live monitor and motor erpm, if your KV value is actually correct?
sorry, I wont enter any discussions about gps measurements. Im working with stock car gps, differential gps and odometry on a daily basis and its more accurate than what we would need it to be here in this case.
The differences in kv I don’t think are relevant as we are both at max throttle and hitting the same speed. So as u say FOC to FOC and at the same speed. So 65 to 85%. The differences between motor size are large, yes I’m in two but they add up to 47x50 stator and urs is much larger. The best I can hope for is adding more copper and maybe do the femm program to possibly find the magnets aren’t as big as they best…and next time the 4747 bit bigger stator. The real test would be to get motors the same size and one with gearing and one with just a low kv maybe. There are a lot of different tests possible but this would reveal how the increased load of a hubmotor would be more inefficient.
flying start for me means you cross the start line already with stationary max speed? at least thats how I did it: hit max speed, hit record, hit stop before I had to brake down, gps trace done.
tried to export runtastic data for the last 30minutes, but you cant export from the app, only from the account and it looks like the data that is stored there is very sparse. more like a pointlist for a visual route drawing than high frequency gps trace data. got just these screenshots for now … or a link to my runtastic events which show the velocity profile at least - not sure if you can access it: adidas Running | Running Plans to Get Fit (check the last stationary 200m - 43.44kph is actually the smallest value of the bunch of pretty much constant values. a few 0.x kph dont really matter in this anyway. on my way back the plateau is a lot shorter cause I had to overtake a bicycle. in the end only the obvious plateau matters though imo.).
its totally relevant, since you base your max theoretical speed calculation off the KV value and your current measured voltage. let say you think you got 90KV but in fact got 80KV motors - that means your max top speed is actually only 88.9% of what you think it is.
Yea kv is relevant for sure in this test. But the size of the motors being so different it’s not a test that reveals the inefficiency of Hubmotors. All we’ve found are the max speeds really. And how inefficient we are at that speed. For my best results I bet I’d need like a 50kv and then the motor at like 15mph would surely be more efficient. My motors are pretty small
we can agree on that - in fact to measure the efficiency you’d need the energy you put into the system and the energy you get out of it, i.e. the energy that works against air drag, tire deformation & co.
my experience though is: a motor will just always get to its max speed - low percentage inclines, low percentage downhill, wind in your back, wind in your face - all these things dont seem to make a difference - the motor somehow always reaches its desired rpm, which in my case on 2 boards is pretty much always 85% of the theoretical max speed. ive never experienced a slowing down of the motor because of a small incline or some extra wind thats helping or working against you.
not sure how to explain it better - its a really weird feeling - like a car that has an electronic speed limit installed (like some german cars at 250kph) -> marching straight to that and then staying there.
the only time when a motor didnt reach this constant rpm was, when my gearing was too large for a 6355 motor, but Im still not sure if the vecs powered down from heat or the motor just didnt make it. once I changed the gearing a little, it also always went to its max speed, which also was about 85% of the max theoretical value.
question remains though, why would the current hub motor only reach 65% of its max rpm in foc when all belt drives tend to get to 85% (not sure anymore in which thread quite a few people reported 85ish %)? do you think its under too much load, even with 2 hubs? is the heating up weakening the magnets or causing some significant extra resistance? do you know how much current you draw at top speed? maybe your battery is not strong enough and you have significant voltage sag?
could try 1 vs 2 vs 4 hubs to check if its a load issue
could try an ice cooled motor max speed test to rule heat out (like take it out of fridge with truck attached -> on board and gogogo )
could try heavy parallel batteries to rule voltage sag out (you can also make voltage sag visible if you record vesc live data)
torsional torque is “momentum x lever length” (lever = basically rotor radius), so its linearly increasing with diameter? wouldnt that mean that motor A is as strong as 2 motors B with half the diameter of motor A?
probably not that easy, since you can fit more iron and copper into a larger diameter too!
)
could try heavy parallel batteries to rule voltage sag out (you can also make voltage sag visible if you record vesc live data)