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Originally Posted by squeeforever
On a side note, Brian, could you explain as to why the speed calculator is so far off on my old setup. I was running a Lehner XL3100 on 12 cells in a E-maxx with 13/66 and it says only 30 something MPH....I know it was at least 50....
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Well, I'm not sure, but these are some possibilities:
- My "program" assumes 1.2v/cell for NiXX and 3.7v/cell for LiPo. As we know, these batteries have more voltage at a full charge: ~1.35/cell for NiXX and 4.2v/cell for Lipo. That does make a difference. Of course, we assume that the voltage will drop under load, but once the vehicle is no longer accelerating as heavily, that load is reduced. Plus the motor is running at higher efficiency at high speed requiring less current. So the voltage sag will be less pronounced.
- There might be some discrepancy between the motor kv ratings published versus the real world values.
- BK Wanderer motors are listed with their "loaded" kv values. Exactly
how much load? I dunno. I generally use the unloaded ones for best case speed. Vehicle weight definitely has some say here.
- Tire diameter can make a large difference. Even duct-taped, there has to be
some expansion, although small. Even .1" ballooning can account for 1.5 mph.
For example, if I have an Emaxx, in second gear, with 51/15 gearing, using 5s lipos (3.7v/cell), an 8xl motor (loaded kv=1875), and 5.5" wheels, I get 34mph.
Now, on the same emaxx with the same gearing, using 4.2v for the lipos, unloaded kv value of 2084 (according to the Feigao chart), and 1/2" of tire balooning, I get 50.78mph.
Quite a difference! The latter is best case, while the former is more conservative. The real speed is somewhere in the middle.
Edit: I forgot to add that the calculator simply takes the total battery voltage, multiplies by the kv value, divides by the total gear ratio, multiplies by the tire circumference (which changes based on ballooning factor), then converts inches per minute to mph. That's it. Nothing magical. I've checked it numerous times and everything looks correct. You can check the source code if you want - it's just javascript. Math doesn't lie, so there's gotta be something different in the motor specs or something. If anyone can see a mistake, by all means
please let me know and I'll fix it.