macribs said:
I remember I too was under the impression it would be an easy peasy task to have insane e-power when I joined.
If I can do it, then it is easy-peasy. Large people just need more power, torque, stronger frame, etc. for the same result as the cyclist weight guys, since performance gets back to power-to-weight. For street riding it can be done with a hubmotor, but for offroad other than mild flattish trail riding it's gotta be a mid-drive for a big guy (some hubbies can excel at that too). I make it easy-peasy, because I freely share advice to avoid the potential pitfalls, so all you need is metal working capability either yourself, a friend, or hired.
markz said:
So, get the highest KV possible...
Not necessarily, and I did forget to mention an issue with highest Kv hubbies (the high Kv 3 phase hubbies that is), which is low motor inductance. It's harder on controllers, so you can't run at the margins. eg If guys tried to run significant current using 24s so commonly mistaken as an appropriate voltage wtih 100V component controllers, the 2 turn 40mm stator high Kv motors I've been since 2008 would mean quick controller death.
I say "not necessarily", because first, a motor is the same motor regardless of Kv. If controllers were available at any voltage desired for the same price, then slow wind motors would be fine, though extreme voltages require special considerations. That's not the case though, since we have very real voltage limits. The same motor wound to a higher Kv has shorter thicker copper, so it can handle higher current for the same heat created in the copper, period. They're all limited in voltage by the controller limits. That's why I accurately refer to slow wind motors as "slower lower power motors".
Note that the above is over-simplified a bit, because it ignores the other portion of efficiency reduction of the motor itself. You have copper losses from resistance in the windings, which is current squared times resistance (often referred to as I2R losses), and are dominant. The other portion is iron core losses, which increase with rpm, but you can take a raw motor and spin it up no load, and that's your max iron core losses, no-load current X voltage. (copper losses are minuscule at such low current).
markz said:
...with the smallest possible wheel diameter. Of course do the calculations for your desired top speed, etc etc etc. Then it runs cool.
Cooler than with a larger wheel at the same performance, but not necessarily "cool". Temperature gets back to heat generated vs heat dissipation. Wheel size is your
only means of varying a hubbie's gearing, and all electric motors require proper gearing. Our motors aren't running a conveyor belt in a factory at constant speed. Our needs are intermittent and even with the same rider loads (not just weight) can vary greatly. For a no temperature worries high power system, which also gives you essentially absolute reliability and highest overall efficiency, then you need to set up for low system stress under the harshest conditions expected. That recommendation could be different for a long commuter where it could be advantageous to give up some low end performance and efficiency in favor of max efficiency at cruise, since stress free under max conditions means a bigger motor that has more core loss than an otherwise identical smaller motor (more energy to spin the motor at cruise). It would have to be quite flat and slow or very aerodynamic for that scenario to ever pay off.
I got tired of a slow leak in the well worn 19.25" 5" wide tire I've been running. Since my motor has proven to run at low stress, and I want to set a crazy high well documented speed run (higher than the 172kph, 107mph, I've already hit) before going down to a more practical voltage and top speed, I am upping the tire size to 20.5". The fact that I've lost 50lbs (23kg) since May helps ensure continued low system stress despite the 6% gearing increase, so I'll still get to enjoy some performance increase from the 12% or so increase in power-to-weight ratio. The lower net loads even means I can turn the controllers up a bit higher without worry, for even better power-to-weight.
Sorry Madin that I've participated in going so far off topic, especially since much of this stuff deserves its own thread for more visibility, but I believe some needed addressing starting with Rix's "best" comment. I'm done now. BTW you've done a beautiful job with the motor mods regardless of my feelings that the manufacturer should have made most unnecessary.
John
^^
