lol you guys are funny
Except the lever isnât fixed to the pivot.
If you take a hub motor off its axle, launch it to space, donât secure it to anything, and apply power, not only will the rotor/tire turn in one direction, but the stator will turn in the opposite direction. this is because there is a force acting between the energized winding aka âthe statorâ and the part that typically moves âthe rotorâ with permanent magnets. just because you attach the stator to something, like a skateboard axle, does not mean the stator no longer experiences a force when the windings energize.
the torque experienced by both the rotor and stator has the same value in newton meters (newtons force measured at the end of a 1 meter lever)
the force transmitted to the wheel edge and the axle have different values, because the radius differs.
^in this case the torque per amp is 0.127 newton meters per motor amp or newtons of force measured at the end of a 1 meter lever attached to the center of the rotor.
0.127 newton meters per motor amp * 120a motor current gives 15.24 newtons at the end of a meter lever or newton meters
but to determine the force measured at an 84mm (42mm radius) wheel edge
we take 15.27 newton meters / 0.042 meters wheel radius = 363.57 newtons
which is:
81.4lbs force = 363.57n * 0.224 pound force per newton
or to determine the force applied by the stator to an 8mm axle:
we take 15.27 newton meters / 0.004 meters axle radius = 3817.5 newtons
which is:
855lbs force = 3817.5 newtons * 0.224 pound force per newton
^even if the axle doesnât twist, it still experiences internal stress, and if its insufficiently strong it will âyield.â
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TLDR. Youâre missing something, because according to your math every hub axle out there should be failing, but obviously isnât, and theyâre not made out of unobtanium either.
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i canât tell whether youâre still denying a hub motor applies twisting force to the axle or truck. if youâre denying it please explain the issue to @dani then, he seems to think 15.27 newton meters will bring a 12mm axle to close to 10% yield stress ie 10% of failure⌠how about an 8mm axle (which experiences greater force due to itâs smaller radius)?
Not sure if serious or just trolling. I wasnât trying to infer that thereâs no twisting force being applied, just that youâre seemingly overstating it, and missing the fact that not all of that equal and opposite force is going into twisting the axle. Some (most) of it is going to the ground in the form of acceleration, and on top of that, itâs from a brushless (switching) motor, so itâs not a constant force.
In fact, once the axle is fully stressed (windings are energized), all the energy is transferred to the vehicle because the ground doesnât move, but the axle remains under stress as long as the windings are energized, proportionally to the equations I described above.
Constant motor current = constant torque
The question I asked @hummieee you quoted was in fact a rhetorical question.
Again, not sure if serious or just trolling. True, but not all of it is to twisting the axle, how much of that torque is being converted into acceleration. This is where the energy is being bled off. Also, how many electric motors run at 100% power all the time?
It doesnât matter because if an axle isnât engineered for 100% power it will fail.
false. with 75kv, 120a motor current, 8mm axle, the axle experiences 15.27 newton meters or 855lbs force, about 17 times more than the normal force from standing on it.
Disengaging the shart, truly you are allknowing. 
@malJohann work/energy (joules) is force (newtons) times distance (meters), so even though the force placed on the axle is great, the distance that it twists under stress is very small (unless thereâs failure), so the energy transferred to the axle is small as well, even though the force and stress can be quite large.
WellâŚyes.
Like maybe everything that you set your eyes on.
But when we are talking about fatigue it means infinite life cycles.
Let it goâŚ
For 8mm axle itâs around 150 MPa. Which is higher but not too bad.
Let it goâŚ
@dani iâm very interested in your opinion about if say, someone proposed to not have the 12mm steel axle go all the way through, and instead, effectively, the aluminum truck became the axle, how would the aluminum part do under lots and lots of stress cycles, considering aluminum has no endurance/fatigue limit (in contrast to steel)?
My 2cents for what itâs worth:
I think most can benefit from a simple thought exercise. Imagine a direct drive installed onto a hangar with the rotor connected to the wheel and the stator connected to the hanger.
If I were to hold the rotor (or the wheel) and allow the stator (the hanger) to hang freely - Now, applying power to the motor, the stator along with the hangar will turn w.r.t. to the rotor. The opposite holds true, which is the intended function of any motor - The stator remains stationary while the rotor spins.
On to the magnitude of torque applied onto the hanger, another thought exercise. Imagine the same scenario as above, but this time, i hold the rotor with my left hand and the stator with my right hand, resisting motion in any direction. Now, applying power to the motor, I get torque on my left hand and a reaction torque on my right hand. If I were to slightly release pressure on my left hand, the motor will now spin slightly, but I would still have to resist the torque on my right hand, albeit, slightly less.
This being said, torque can be applied to either the hangar or the wheels, and hence I agree with @professor_shartsis on this front.
Disclaimer: I am not here to address the calculations made as I am far too lazy to run through the entire thread, though Iâd like to share my thoughts on what I think the FBD will look like.
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Aluminum is softer and weaker material.
The yield stress of Al6061-T6 is ~240MPa (for example). In comparison good steel is ~900-1000MPa
For aluminum axle to make sense it needs to be at least 3 times thicker (to be as stiff).
As for fatigue, if the stress is low enough it really doesnât matter
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Hmm you say as for fatigue, if the stress is âlow enoughâ you say it doesnât matter. Iâm confused what does it mean then when a material such as aluminum has no endurance / fatigue limit?
From your graph: Hub Motor / Direct Drive Axle Stress Safety Discussion
When will the part fail if the stress is say 5ksi?From practical point of view of course.
And will you live to tell?
I donât know but with 14 magnets my understanding is thereâs 42 torque ripples per wheel rotation in BLDC mode