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Originally Posted by Chris is me
If I remember correctly, I once read a really, really old CD thread that said 81% was determined by taking one of 229's drive bases, running it to top speed, and measuring that speed. Ratio of that top speed to actual speed became the default "speed loss constant".
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I seem to recall reading that somewhere too, but I wasn't sure.
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I'm sure this number is somewhat correlated with efficiency, wheel size, tread type, etc etc.
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It's a lot more than "somewhat". In the real world, the top-speed to theoretical-speed ratio is correlated with playing surface, wheel type (e.g. pneumatic or solid) and tread type, chain vs belt, chain or belt tension, wheel alignment, wheel runout, torque-dependent friction in the gearbox and drivetrain, and speed-dependent friction factors such as large airfoils.
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For most practical purposes it's "good enough".
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It's quite useful as a rule of thumb. Whether or not it's good enough depends on what you're using it for. Students need to understand the limitations.
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Efficiency in JVN's calculator is used to calculate motor loading, not speed.
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That is true. In a physics-based model, the torque-dependent friction (gearbox "efficiency") would affect top speed.