Broken rod on stroker crank - Page 2 - Kawasaki Z125 Forum
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post #11 of 29 (permalink) Old 09-18-2019, 11:59 AM
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I think your RPM was too high (Greg_E is on to something). What does the manufacturer recommend for a max RPM?

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post #12 of 29 (permalink) Old 09-18-2019, 12:23 PM
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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...





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post #13 of 29 (permalink) Old 09-18-2019, 03:06 PM
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Man that is very unfortunate. Sucks.
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post #14 of 29 (permalink) Old 09-18-2019, 04:28 PM
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Quote:
Originally Posted by crakerjac View Post
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...



Sorry for the "wall of text" but this blew my mind when I learned it for the first time and wanted to share.

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.
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Last edited by Kawasaki Brad; 09-18-2019 at 04:31 PM.
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post #15 of 29 (permalink) Old 09-18-2019, 04:55 PM
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@Kawasaki Brad , I really want to throw out a sarcastic gif/meme, but I followed through with your math and yeah... holy crap, the piston is basically going 0 to 70 MPH in 3.24ms. That's cooking...


Edit: Screw it, sarcastic GIF
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post #16 of 29 (permalink) Old 09-18-2019, 05:19 PM
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That's exactly it, and with essentially no forces to check that pulling force... You get broken stuff.

It still could be a manufacturing defect.

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post #17 of 29 (permalink) Old 09-18-2019, 07:33 PM Thread Starter
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I put the high compression piston back in it with the stroker crank, it ant nothing to write home to momma about, I am going to play with the tune some more, t/b parts said they would discount the replacement parts 50%, I am thinking about buying the 67mm bore this time and send the piston to have a forged one made
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post #18 of 29 (permalink) Old 09-19-2019, 09:41 AM
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I'd bet that Wiseco already has a piston that fits if they can give you the measurements needed to find it.

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post #19 of 29 (permalink) Old 09-19-2019, 12:54 PM
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This also is a great illustration of why lighter engine internals are used. Thanks for the info, pretty shocking indeed.
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post #20 of 29 (permalink) Old 10-21-2019, 01:17 PM
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Any updates? We are still waiting on pictures!


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