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Math question

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I’m not sure if this is the right place to ask, but I have a question for fellow math enthusiasts.

The formula for calculating piston primary inertia force:

(0.0000142 * piston assembly weight * RPM^2 * stroke)

My question is, what does 0.0000142 represent? I’ve only ever been told “it’s probably just a constant” but I’m not satisfied with that. Does it represent the periods of deceleration and acceleration at TDC and BDC?

Thanks all!

i actually don't know this so I'm asking for help, love great questions like this

It'll be a derived number which is used as an approximation for a specific set of standard calculations, just as 5252 is an approximation used for the HP-torque-rpm equations. You don't mention if it was an imperial or metric calculation - that's important.

Exactly what is it being used for as it's missing some critical information, such as the rod length:stroke ratio, for anything other than a broad estimate, or mean value?

You need specific rod/stroke data to do it properly. I might actually have a spreadsheet somewhere from a first or second year assignment where we did velocity/force components through an engine cycle. One number will not cover peak force for all applications but will probably be close.

Weight is in lbs. The correction factor is:

(crankshaft radius/connecting rod length) added on upstroke and subtracted on downstroke.


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