the torque is the same in newton meters, but since the radius is different at the wheel edge and at the axle, the force is different. you appear to be confusing force with torque.
Happy afternoon everyone
If you remember just one thing from my reply, let it be this:
Loads by themselves have no meaning, the resulting stresses from loads have.
Elevated loads don’t automatically translate to larger stresses or structure failure, to make conclusions first calculate the stresses.
For 12mm axle, the stresses resulting from this moment will be ~45MPa, which is less than 10% of yield stress of any nice axle steel (or even Titanium)
Exactly, this isn’t even go entirely to the hanger/axle.
To make it closer, you can degrease your bearings and load them with sand and shitloads of red loctite, then test your torque by all means.
I don’t really sure what was the purpose of this thread to begin with, but any serious consideration of this topic have to include stresses and not just loads.
PS
I asked the OP nicely to stop this “elevated load” apocalypse of hub motors in another thread, but I guess he didn’t listen.
PPS
I can argue that standard motors/mounts apply larger loads on the hanger than hubs. The hangers see additional bending moment.
PPPS
I want to start a thread discussing loads and fatigue loads on hangers to demistify the issue and to supply some kind of design criteria for new hangers/motors/whatever. (But this thread was out earlier)
Happy Sunday,
Dani
8 Likes
I wish you would simply post the information here since we are already having the conversation. In your opinion, what minimum design features are necessary to prevent all future wheel separations with direct drive?
First get your definition right as far as hub motor and direct drive lol. They are designed and setup differently.
2 Likes
@professor_shartsis please use some annotated figures to explain your case, otherwise whatever you write on this forum will generally be ignored.
I didn’t go into the details discussed above, but if the stator is directly mounted on the axle, then axial stresses beyond what the material is able to handle will lead to failure. And like @dani said, the torque doesn’t matter but the stress. If you use a larger dia axle, the stresses go down.
In case of direct drive( I am talking about eLofty) the stator applies torque on the 14 mm square shaft at the very back using a square hole.
2 Likes
Irrelevant
1 Like
I would think an acceleration force, in the direction of movement.
1 Like
他妈的你在说什么 
1 Like
I’m just disappointed there were no animated graphs. 
1 Like
6 Likes
That’s what I thought reading when I was through this. And in the case the acceleration isn’t moving the board forward because of say a wall I’d assume the wheels would loose traction and spin before the force would be great enough to bend or break the axel. Of course I have no idea about any of the math on this and am no expert on physics but that’s just what made sense to me.
lol you guys are funny
Except the lever isn’t fixed to the pivot.
If you take a hub motor off its axle, launch it to space, don’t 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 “the stator” and the part that typically moves “the rotor” 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 doesn’t twist, it still experiences internal stress, and if its insufficiently strong it will “yield.”
1 Like
TLDR. You’re missing something, because according to your math every hub axle out there should be failing, but obviously isn’t, and they’re not made out of unobtanium either.
1 Like
i can’t tell whether you’re still denying a hub motor applies twisting force to the axle or truck. if you’re 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 it’s smaller radius)?
Not sure if serious or just trolling. I wasn’t trying to infer that there’s no twisting force being applied, just that you’re 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, it’s from a brushless (switching) motor, so it’s 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 doesn’t 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.