# Forces - Connecting Rod

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Hi everyone,

I'm trying to establish the relation F=Force of the head of the conrod=f(C=torque-couple of the motor) in a conrod.
I tried to do it myself but apparently, I donnot have the right result because of the way I project my force vectors.
I searched for an answer on google, ecosia, etc, but no one except this site (http://www.pats.ch/formulaire/cinematique/cinematique4.aspx) provides a result; but there is no explanation, no proof...

I'm okay with Ft = Mmot/r and the direction of the force.
Fs = Ft * cos(B)
however, I really donnot understand why Fs = Ft/sin(A+B).... I rather find Fs = Ft * sin(A+B) !

Could someone explain me why I'm wrong (or why the site's wrong) please?

Thanks,
CharlieCharlie

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Posted (edited)

Are you trying to get the maximum force on a conrod, or the average force on a conrod?

If it's the average, just divide the torque of the engine by the number of cylinders and then divide again by the radius of the crankshaft.

If it's the maximum... I think the easiest is to find the maximum pressure inside the cylinder, then divide by the area of the piston top to get the force.

EDIT: I'm assuming that you are talking about internal combustion engines.

Edited by StefanLazic

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Posted (edited)

Draw Fs on the right in the opposite direction. In other words, draw a free body diagram of the hinge. Perhaps that will make things more clear.

Fs as drawn is a force exerted by the hinge on the left rod, Fd and Ft as drawn as forces exerted by the right rod on the hinge. I guess this causes your confusion.

Edit: it could be the other way around, if the piston is driving rather than driven.

Edited by Bender