MOPED ARMY

steve, draw a circle on some paper the same diameter as the stroke, draw a horizontal line at tangent to the top of the circle, draw another line 2mm below that

connect where the second line intersects the circle to the center

break out ye old protractor and measure the angle

This is a better drawing....

To do that with a calculator, divide the mm spec by half the stroke, subtract that result from 1.0, take inverse cosine of that. For example 2 mm and 60 mm should come out to 21 degrees.

I'm not sure if this method is absolutely correct because it doesn't account for the angle of the connecting rod, but when near TDC the connecting rod is approximately parallel to the cylinder bore anyway so that would be a minor factor.

Lets try that again.

See attached

mk i tried that and it didnt seem work... the pictorial way devised by crackstar, is really helpful but not something you really want to be doing while tearing your bike apart. can't there just be a simple ratio for "for X degrees flywheel is rotated, the piston moves X mm"? or is it because as the piston reaches TDC, and BDC, that ratio changes... im using the .gif at wikipedia to better envision this. pretty sweet moving diagram of a two cycle engine.

Cool diagram. You will note that even though the crank is turning at a constant speed, the piston slows down and stops as it approaches TDC (thus the term, "dead center"). The ratio of degrees of crank angle to piston position is not constant throughout the cycle. In fact the first degree you turn the crank the piston may not move at all.

Neglecting the possible effect of the connecting rod, it is approximately a sine function. That can be solved by drawing a circle on paper or by using the trig functions on a calculator.

yea man i love that diagram. sometimes i find myself staring at it for hours. yes that makes sense now, the piston slows as it reaches TDC. I think those things up top that i suggested would be AVERAGE stroke:rotation ratio?

ahem... what would those trig functions be? For a mechanical engineering major im pretty damn bad at math. that, and i dont always want to think. Im sure that circle trick is the right idea, but you can easily mess shit up and be innaccurate when you do a pencil and paper trick like that.

aha, didnt even notice that. thanks a lot hombre.

Cosine (theta) = adjacent/hypotenuse

The rest is just algebra.

Mechanical engineering? Go do it in AutoCAD and let the dimension tool find your exact angle....

There has to be a computer with autoCAD on it somwwhere on that campus

we have pro/ENGINEER im sure they're pretty similar. My stroke is 1.64 inches= 41.656 mm... thats true, I could draw circles with tangent lines etc in a matter of seconds. Maybe I'll make a chart of timings in mm and respective degrees, for future reference.

You need to know the dimensions of the connecting rod to find the exact answer. The approximate solution (single cosine or a circle on paper) just uses the stroke length.

You need to know the dimensions of the connecting rod to find the exact answer. The approximate solution (single cosine or a circle on paper) just uses the stroke length.

For small crank angles, I agree.