Maximum centrifugal load for hub motor wheel

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hias9   100 W

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Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 09 2020 7:15pm

I have a 45mm magnet height hub motor with Hubsinks inside a 24'' DH rim with DH tires and DH tube, so significant weight on the outside and spokes are only 13/14ga butted.
When using torque mode on the ASI controller with 25% field weakening current, no load speed is very high. I stopped at about 25% throttle where the 24'' rear wheel was already at slightly above 2500rpm (300kph).
If there is no traction at the rear wheel, the motor could possibly accelerate the wheel to more than 2500rpm where I am not sure if the wheel can take the centrifugal forces.
What would you say where the limit is that the wheel can take?
Unfortunately I cannot limit rpms in torque mode. There is torque mode with speed limiting, but it is not an option in this case.

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neptronix   100 GW

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Re: Maximum centrifugal load for hub motor wheel

Post by neptronix » Dec 09 2020 8:40pm

2500rpm in a 24 inch wheel would correlate to over 100mph.. so centrifugal forces may be the last of your worries.. might want to turn the field weakening down a hair, lol.
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hias9   100 W

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Re: Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 09 2020 8:47pm

The problem when I set max field weakening current lower is that there is significantly less power available in the field weakening area and top speed would also decrease significantly. Yes, no load wheel speed at 20% throttle was a bit above 2500rpm which is 300kph or 186mph. I did not want to go further ;-)
Centrifugal forces are what I am worried about because when the rear wheel loses traction completely it could possibly spin even faster than 2500rpm and I am not sure if the wheel can take those forces.

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neptronix   100 GW

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Re: Maximum centrifugal load for hub motor wheel

Post by neptronix » Dec 09 2020 9:05pm

If you're spinning your motor to 2500rpm and that's only 20% of your top speed then you've got things set about 10x faster than they should be. That motor should do no more than 70mph tops even on the highest voltage the controller can take with no field weakening. Unless you have a 500v battery and some crazy fets you're forgetting to mention :P

Where are you getting this RPM measurement? this can't be right even with 25% field weakening.
"Love and compassion are necessities, not luxuries. Without them, humanity cannot survive." - Dalai Lama

My first major build: 1.6kW 8T MAC motor on a Trek 4500 MTB.
Monster MTB: Leafmotor 1500w @ 4kW on a Turner O2 full suspension.
The monster scooter: Heavy duty Cannondale semi recumbent - under construction.
Blue Dream: Maxaraya FS semi recumbent and high efficiency mid-drive - under construction.

hias9   100 W

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Re: Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 09 2020 9:36pm

I am using torque control, not speed control. If I twist the throttle by 20% it results in 2500rpm. I guess no load speed at the max field weakening current it is set to (87.5amps) would be somewhere around 3000rpm at full throttle, but I don't want to try it.

No load rpm without field weakening are about 950rpm (depending on battery SoC). That's about 70mph. Kv is around 11.7.
45amps of field weakening result is a no load speed of about 1400rpm
70amps are already 2100rpm.
87.5amps max field weakening is what it is set to and twisting the throttle by 20% already results in more than 2500rpm no load.
RPM measurement is from the ASI controller and correct.

I did not increase field weakening current because I want a higher no load speed, but because it gives significantly more torque on higher speeds.

On the Sabvoton I used before 50 or 80amps of field weakening resulted in only about 1300rpm because of erpm limit, but not on the ASI.

Of course I don't go anywhere near 186mph. That is no load. I am just worried that maybe spokes will fail when the rear wheel spins that fast when it has no grip.
Are there any common values on how much centripedal forces a wheel can take?
Last edited by hias9 on Dec 09 2020 10:44pm, edited 1 time in total.

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Re: Maximum centrifugal load for hub motor wheel

Post by goatman » Dec 09 2020 10:44pm

did you rewire the motor to delta?

hias9   100 W

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Re: Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 09 2020 10:45pm

No, still wye not delta. It's just the fw that causes these no load rpms. Without fw Kv is about 11.7

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Re: Maximum centrifugal load for hub motor wheel

Post by goatman » Dec 10 2020 12:23am

which ASI controller?
im curious because im considering to use Grin Technologies 4040 all axle hub motor for a boat conversion. i wouldnt need to rewire the motor if i knew which ASI you use.

Justin rewired the motor to delta and used a phaserunner

viewtopic.php?f=39&t=108411

ill jump out of this thread so i dont sidetrack it.

hias9   100 W

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Re: Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 10 2020 12:55am

BAC4000.

But as I mentioned, these rpms are of course without load. While riding I don’t reach half of that value.
And if you want to run it deep inside field weakening area for a long time, I don’t think that would be very efficient.

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serious_sam   10 kW

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Re: Maximum centrifugal load for hub motor wheel

Post by serious_sam » Dec 10 2020 6:22am

hias9 wrote:
Dec 09 2020 9:36pm
Are there any common values on how much centripedal forces a wheel can take?
So apparently BAC4000 has "Field Weakening allows 1-200% RPM Boost", implying 300% wheel speed. So based on that, your claim of 300km/h is realistic. viewtopic.php?t=96264#p1409824

If that was my bike, I wouldn't run it over about 130km/h (with your wheel size and motor Kv). At that speed, the BEMF from the motor is about at the limit of the mosfets (~100V). Any higher than that, you are relying on the controller to ensure the mosfets don't see above that voltage (using field weakening magic). In my understanding, if there is a glitch (you lose battery power for example), the mosfets will see the full BEMF from the motor, and it's highly possible that you'll fry the controller in that situation.

That's one reason that Nucular controllers don't allow over 95V BEMF, no matter how much field weakening you apply. It just won't allow the wheel to spin faster than that. Yes it puts a limit on the advantage of field weakening, but it also makes a fail-safe controller that won't destroy itself in the case of a power failure.

So, I wouldn't be worried about centrifugal forces on the wheels, since I wouldn't be running 300km/h wheel speeds to start with (unloaded or otherwise).

As for your question about common values on how much centripedal forces a wheel can take, I would respond with a question: what tyres are you actually running ? That'll be the weak (mechanical) link. Not the motor. Not the rim. If they're bicycle tyres, I'm absolutely amazed that they didn't disintegrate at 300km/h. I understand that there's no load/hysteresis creating heat, but still, soft rubber and non-metallic ply, amazing.

hias9   100 W

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Re: Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 10 2020 8:59am

It probably allows even more than 200% rpm boost, but I am not going to try it with rim, tire and tube installed.
I am also a bit worried about the spokes because they are only 13/14ga butted spokes.
Yes, that is what I run the motor top speed on the flat. BEMF is only slightly above mosfet limit when I am going downhill fast on a road, but that’s rarely happening. No problems so far.

Why I am worried about centrifugal forces is because the rear wheel could spin that fast while it loses grip or is up in the air.
At the moment I use a 24‘‘ Maxxis DHF DH tire on the rear which has good grip, but the knobs wear out pretty quick when going fast.
It was spinning for a few seconds at slightly above 2500rpm no load and maybe around 10 seconds at 2100rpm. That’s quite loud and the Hubsinks create lots of wind ;-) No visible damage to the tire and it did not behave differently while riding.

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Re: Maximum centrifugal load for hub motor wheel

Post by 999zip999 » Dec 10 2020 9:30am

Safety first
You need a bulletproof Kevlar suit for riding. You need a bulletproof glass room for testing your motor. I can just imagine the rubber coming off the rim and wrapping up in the spokes.
I used to work on pedego bikes and when they first got there fat bike 4 inch tire a used to blow off their cheap rims all the time. Nightmares. Bad rim lock.

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Re: Maximum centrifugal load for hub motor wheel

Post by Tommm » Dec 10 2020 10:36am

Consider yourself lucky you didn't blow up your bike frame and possibly lose an eye from this "testing".

hias9   100 W

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Re: Maximum centrifugal load for hub motor wheel

Post by hias9 » Dec 10 2020 11:48am

I did not expect that setting fw current to 25% would already make the motor spin that fast. After I took a look on my mobile and saw the rpms in the app I did not increase throttle further and stopped immediately. However I felt confident going up to 2100rpm no load after that :D

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