Understanding Piston Formula to connecting rod angle

**Stanley91** · 04-08-2014, 08:32 AM

Hi

I have the following formula to calculate the angle a connecting rod of a piston should be

P3 is the length of the crank
P4 is the length of the connecting rod
and K42 is the angle of the crank

=PI()-ASIN(P3*SIN(K42)/P4)

I don't understand where the formula comes from could someone tell me? and whats the purpose of taking pie away at the start?

**zbor** · 04-08-2014, 08:54 AM

Try look wiki

**Stanley91** · 04-08-2014, 09:20 AM

One of the first things I tried, Wikipedia only explains how the crank angle is calculated but not how the connecting rod angle is calculated

**zbor** · 04-08-2014, 09:51 AM

Ah, yes, rod angle is harder to calculate :/

**Andy Pope** · 04-08-2014, 10:17 AM

Harder but still just trig, no?

Using the diagram on the wiki page, drop a perpendicular line from N to line OP to give 2 right angled trianlges.
You should then be able to calculate new lengths and angles.

**MrShorty** · 04-08-2014, 02:14 PM

As noted, the overall problem seems to be basic trig

whats the purpose of taking pie away at the start?

I expect that this is related to conventions in engine design and trig. If I remember my trig correctly, convention for the arcsin function is to return an angle from the first/fourth quadrant [ASIN(x) will be between pi/2 and -pi/2]. It looks like engine designers want this angle to be between pi/2 and 3pi/2 instead, which is based on how engine designers "overlay" the unit circle around the crankshaft.