Sorry for the "wall of text" but this blew my mind when I learned it for the first time and wanted to share.
Forgive my ignorance, but how do you pull the wrist pin out of a piston during the exhaust stroke? A 4 stroke motor isn't producing power and is only moving because of the momentum generated by the combustion stroke...
Meters per second of piston velocity (MPS)
MPS = 2 x (Stroke mm/1000) x RPM / 60
2 strokes because it goes up and down per revolution. /1000 to go from mm to m. /60 to go from minutes to seconds
So stock stroke is just over 50mm and a stroker crank takes that to 55
15.8 = 2 x (50/1000) x 9500/60 Stock piston velocity at 9500RPM
17.41 = 2 x (55/1000) x 9500/60 Stroker piston velocity at 9500RPM
10% increase in piston velocity with the stroker crank at the same RPM.
For figuring sake let's pretend that it takes exactly 180 degrees to go from peak velocity up stroke to down stroke per 360 degrees of crank rotation. Now in actuality the event isn't linear like that and the numbers are even more "peaky" but this works good for figuring.
9500/60 = 158.33 rotations per second
1000/158.33 = 6.32 ms per rotation
6.32/360 = 0.018 ms per degree of engine rotation
0.018 x 180 degrees = 3.24 ms per 180 degrees of engine rotation
Piston travelling at 15.8m per second mid stroke and then in 3.24ms it's travelling at the same speed in the opposite direction.
Traveling at speed X
, decelerating and reversing course to accelerate again and
travel at the same speed is equal to accelerating once but double the speed. So at a peak velocity of 15.8mps
we're taking the piston from 0 to 31.6mps in 3.24ms.
1g is 9.8mps in one second of acceleration
31.6mps/3.24ms = Y/1000
1000*31.6/3.24 = 9753
Y = 9753 (How fast we would be going in mps if we accelerated at 31.6mps per 3.24ms for a full second)
9753/9.8 = 995g's
For a nice round number let's say our piston weighs 100grams. At 995g's that piston exerts 99500grams
(99.5kg's or 219.36lbs) of force. Lots of "pull" stress there on a non combustion stroke.
Now for fun let's find out what our 55mm stroker does at 17.41mps
34.82mps/3.24ms = Y/1000
1000*34.82/3.24 = 10747
Y = 10747 (How fast we would be going in mps if we accelerated at 34.82mps per 3.24ms for a full second)
10747/9.8 = 1096g's
That same "hundred gram piston" is now exerting 109600grams of pull stress on the con rod during an exhaust/intake
stroke (109.6kg's or 241 lbs)
Let's reduce the RPM to 7500. If we skip through the blah blah and get to the answer (using all the same formulas above)
8ms per rot, .022ms per deg, 3.96ms per 180, 13.75mps velocity, Y = 6944, 709g's.
Theoretical 100g piston with the stroker crank is now 70.9kg's.
Point is: by reducing the RPM by 11% we see a stress reduction of 35% in this one aspect. Reason why RPM's are reduced when
increasing displacement, stroke, bore, etc.