spinningmagnets said:
because you should never run a motor anywhere close to saturation
Agreed, but...builders around here will continue to raise the amps until it becomes painfully obvious that they are running way too hot (wasted battery amps). If we can list the max amps per "X" on a given hubmotor, then builders can decide if they want to overheat a smaller hub occasionally, or make the jump to the next size up. Due to "bang per buck", these seem to be the direct drive hubs of interest?
28mm MXUS V2
35mm Leafbike 1500W
45mm MXUS 3000W V2
50mm QS 205/50H V3
Those aren't the best "bang per buck" DD hubbies...not even close.
The measure you need to look at is the phase-to-phase resistance, because that tells you the heat generated for a given current. Core losses are small enough to ignore except at high rpm where BEMF forces lower current, so they really only come into play when looking at efficiency at cruise and having no relation to current limits.
So you have heat created in the copper as one part of the equation and is easily calculated. The other is how much heat the motor can dissipate, but since the motors you list have quite similar outer shell dimensions they are very comparable in heat dissipation with the larger motors having a slight edge due to greater width at the perimeter.
Rules of thumb would be useless, because load, terrain, riding style, wheel size, and even length of a typical ride are all critical parts of heat equation. That's before considering motor or bike mods to increase heat dissipation.
Saturation is a useless number for hubmotors, so teach people something useful instead. ie Copper losses (heat generated in the copper) is current squared times phase-to-phase resistance, and copper resistance goes up by about 0.4% per degree C of temperature increase in our operating range. Tie that in with the baseline heat dissipation of an unmodified motor in Justin's testing and you get both sides of the equation.
If you really want to give people something useful to ponder, point out that going from a 26" wheel to a 29" wheel increases heat generated by 24% for the same acceleration, so putting DD hubbies in 29ers should always be discouraged. In fact, geared hubbies should be discouraged for 29ers too, since no manufacturer is installing a different gear reduction for use in the larger wheels. That's small compared to going down from a 26" to a 20" wheel, which decreases heat by over 40%. If you compare results for a 20" wheel to a 29", for the same acceleration over twice as much heat is generated in the motor. That discussion isn't complete without pointing out that changing the winding turn count of the motor will make exactly 0 difference in how much heat is generated.
Note that these differences are actually low, since with greater heat comes higher temperature, and in turn higher resistance, which creates even more heat. If a 20" wheel is too small for your taste, fine, simply use the smallest wheel size you can live with. Just make your decision armed with accurate info. I've seen people debating the weight of tires and rims as if it really matters, but that effect is insignificant compared to the wheel diameter, which is almost never discussed.
