Wheel spin (how many revolutions?)

Joined
Mar 8, 2018
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Location
Rhode Island, USA
The Newton meter torque drag is a great number to know but really how about something simple like my front wheel spins for 4 minutes and my Chinese 1000w dd rear hub motor will go 2 full revolutions before stopping with a one hand spin of the lucky wheel!

How many revs for others out there guys? Like a high quality Grin 9C? And I mean with a tire on it. Then we can talk spin down from 30 mph etc.

Thanks. Jan
 
On a somewhat related topic ... Not to thread jack my own thread ... Has anyone found that digital speedometers are not indicating speed very well compared to a Cycle Analyst version 3.1. .... Seems a rechargable speedometer I got in yesterday was way off in spite of setting the wheel circumference to 2032mm ... Then I changed it and speed didn't seem it be affected ... I have a few more of a difference model arriving that I will test next ... And I guess it's time to carry my GPS and check that too.
 
Sounds like something is wrong 2 spins is not much is it , it may have something to do with the hall sensors or their wires being shorted.out.
I don't use a DD hub so can only guess its not right.

Different issue , my rear non powered IGH hub spins about 30 revolutions before stopping same hub I read someone gets 7 revs before stopping.
 
For geared hubs, it depends on on the clutch, temperature (affects the grease), freewheel drag and possibly usage. With a bare new motor, you can try turning the axle with your fingers and you wonder how it ever spins. Add the angular momentum from the wheel and I get anywhere from a few seconds to tens of seconds.
 
It runs just fine but I did figure out when I bought another external speedo that I have the pole pairs count set to 24 when it really should've been 23 for the cycle analyst and I'm actually going further and faster than I thought but 8.2 watthours per mile over 40 miles is pretty good I think, it's just always been stiff (eddy current losses) although with power off it rolls like a bike with a freewheel. I just received another new one so it's time I set it up and give it a spin and maybe this time get a side cover all the way off to have a better look inside and maybe get some heavier power wires into the hub. Is that hub a clutched or direct drive? A clutched hub is going to spin very freely so it is going to definitely have less drag.
 
I'm going to connect a gram spring scale to the tire and see how much force it takes to turn the wheel ... that should be an easy way to measure drag. I usually run the motors I test at their lowest speed and how many watts it takes to turn it then at higher speed and see how many watts then ... 1 to 6 watts to get it to turn at low speed and 60 to 180 watts at full/high speed is what I have seen.
 
eCue said:
You might need a fish scale by the sounds of it :D
Naaaa it has rotational drag not stiction ... it turns easily just has what you might call speed drag which is typical with magnetic field eddy currents. Actually my brother thought it was side load on the bearings ... he sees a lot of that in poorly constructed wheels for motorcycles.
 
What is happening is you get drag from the magnets in the direct drive motor. Its called cogging, and gets stronger if any phase wire is shorted, or regen braking is engaged.

Your cogging sounds about normal to me, not exceptionally strong, or weak. It varies by motor brand, and even by individual motors sometimes. One motor I had could even fail to rotate when you pushed the bike at first, but ran fine. That was an extreme example.

One thing about cogging, it does increase the faster you spin the motor, so it resists a lot more when decending a mountain at 30 mph, than it does if you are just riding along at 5 mph, like you do if something broke, or you just ran out of battery.

To get good coast when you need it is easy, just tickle the throttle a tiny bit, less than 1/4 throttle, and the motor will then turn easy, without using up your battery. At low throttle, the motor will turn a certain rpm, but if you are already pedaling or coasting that speed, it will not use more than a few watts. if it draws 50w that sounds like a lot, but its not. One amp hour of 48v would last one hour at 50w draw. So a few min coasting, or pedaling down hills at low throttle will not blow your range.

So when you go down a big hill, and want to coast faster, open the throttle a crack. When you are screaming down a hill and would not like to hit 50 mph, leave the throttle off and it will slow you some.
 
Been driving/riding an EV with a high clogging 6000-12000 watt motor for about 17 years yup 50 -60 watts at full speed is what this hub motor draws and about a couple of watts and low speed ... I can go 5-6mph at 50 watts on level road without pedaling so one of my SunPower solar panels could keep me going all day long! As for 50mph downhill the output FETs would slow you down at about 38-40mpg as they start feeding back into the battery even with the controller turned off so in a crazy case like that I have a three pin connector I can just pull!
 
Talking about rotation drag ... the first eBike DD hub drive I built for a older friend took it to Florida for the winter and lived on his boat near the salty water. Yeah not a good idea for the bike but even worse for the almost water tight hub motor. By spring time the rear wheel wouldn't turn at all so he packed it in his Santa Fe and shipped it north. Apparently rain water got into the hub possibly through the axle wire hole and did a number on the neodynium magnets. Has anyone else seen this happen?
IMG_20180509_184702~4.jpg
 
JanComputerman said:
... Has anyone else seen this happen?
rare earth magnets come in many flavors, where neodymium magnets are especially prone to corrosion, unless proper protection from the elements is done by way of plating or conformal coating. The magnets will, rather quickly, corrode if unprotected.
(remember kids, always use protection)
Since I've rarely opened up motors I have yet to see this in hub motors, but in the early days of neodymium magnets use in linear motors (speakers) it was an unfortunate irritant to have purchased many units (speakers) that self-destructed by way of corrosion or demagnetization in less than a years time.

BTW, ferro fluids were developed some 40 years ago to help cool the magnet structures of neodymium speakers to help prevent demagnetization from heat. (Trivia from a trivial mind)
 
Judging by the water marks it looks like it had completely submerged the bottom magnets. Fortunately the hall senses were on top! I hammered the rotor and stator apart, yeah it was stuck together, removed excess rusted magnetic particles, coated it with anticorosive black paint, put it back together and got it running again with what I figured was a slightly bigger air gap. Not sure how well it is going to stand up as the forces pull on the crumbling magnets but I figure in a few weeks it will be back for another teardown and maybe an epoxy coating.
I'm thinking I may want to paint my magnets before it can happen again to my motor should it get wet or moisture enters the motor.
 
drill tiny drain holes around the outer edge of the covers, and water that gets in will get out so you won't have that big a problem.

See Justin_LE's video about water and hubmotors; it's in a VEVA presentation a few years back here on ES and on the http://ebikes.ca blog page and youtube.


https://www.google.com/search?num=100&newwindow=1&ei=WB0vW5u8B8mEoASakZP4Dw&q=VEVA+presentation+justin+hubmotor+water&oq=VEVA+presentation+justin+hubmotor+water&gs_l=psy-ab.3...59322.60273.0.64623.5.4.0.0.0.0.265.565.0j2j1.3.0....0...1c.1.64.psy-ab..2.0.0....0.zEjXP0i8lRk
 
For a hub that is so well sealed it seems counter intutive to drill holes to let water out that shouldn't be in it in the first place. I think after seeing how badly the magnets we're damaged that it may make a lot more sense to get some sealer over them to protect them better. Drilling holes will just let water in quicker as well as out from spinning action but keeping water out should be first priority. Interestingly the stator laminations we're not rusty ... silicon steel ... But the nickel plated magnets we're basically reduced to mounds of magnetized filings.
 
Seriously--go watch that video, and read the stuff Justin posted about the tests done in it regardign water ingress.


Basically there's not really any way to completely totally seal up a typical hubmotor against water ingress, so the best thing you can do is to give the water that does get in an easy way out.

The longer stuff stays wet in there, the worse the corrosion is going to be.

If you have water that gets sucked in via the wires, and the hub is really well sealed everywhere else, the water is not likely to be able to get out via the wires again, so instead it's giong to chemically react with any and every surface it can, until the water is used up (meaning, it *looks* dry inside, because the water isnt' water anymore, it's corrosion along with bits of the motor).
 
Sounds like some desecant is needed inside the hub, certainly plenty of room for some in there. I guess the real test is if mine is getting damaged because I have only been out in fog no rain. Might be good to check out my windings too since I am running the Grinfineon 40amp controller but limited to 30amps. I can feel the power wires to the motor get warm in my fingers on 1200watt pulls.
 
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