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Originally Posted by Wally
You probably need to plot it on some graph paper to appreciate what happens. On short rod ratio's the acceleration curve in particular is gnarly around TDC, while long rod ratio's it is smooth. Remember that although the crank may be travelling at a fix speed, the piston isn't because the rod is not operating at a fixed angle.
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Yes, I have graphed it on paper, I drew a triangle with one side being half the stroke, the other being the length of the rod, and the unknown side being the distance of the piston pin centre to the crank centreline. I put the angle from TDC between the unknown side and the half stroke side if that makes any sense. I also made a spreadsheet using the sine rule to calculate and graph piston motion, and my results tend to contradict what I've read elsewhere. Particularly what I read here
http://www.stahlheaders.com/Lit_Rod%20Length.htm. It says
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Long Rod is faster at BDC range and slower at TDC range.
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and
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Short Rod is slower at BDC range and faster at TDC range.
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But my own calculations say that the motion will be identical at BDC and TDC, just in opposite directions, or negative, if that makes sense. I think that web site might be wrong, or saying something that is different to what I'm saying.