patsmx5

iTrader: (16)

I was doing some numbers a while back trying to calculate the tensile load placed in a miata connecting rod at various RPMs while taking the engine RPM, stroke, rod ratio, piston weight, and other things into consideration. In my calculations I need to know the smallest cross sectional area of the connecting rod so I can find the working stress in the rod (stress = P/A, but I have no A).This needs to be fairly accurate, and I don't even have a rod to attempt to somehow measure. I've got some accurate data on piston weight, now I just need the smallest cross-sectional area, which is where the rod will break.

So with the slew of recent thrown rods, and my need to raise the boost, can I get a few of yall to take closeup, well focused pictures of your rod fracture with a tape measure or rule level to the rod for scale? Then I could blow up the pic, and approximate the area pretty well using some calculus.

Thanks in advance. It's for a good cause! (more boost!)

Savington

iTrader: (31)

Savington

iTrader: (31)

**** my isp.

neogenesis2004

iTrader: (12)

Thats such a clean break on your rod sav. It almost looks like your power level just stressed some sort of already present defect to the point of failure. But what do I know.

y8s

iTrader: (8)

sav, take another picture but actually focus on the rod fracture instead of the piston. actually both pieces--the small end side and the big end side.

ZX-Tex

iTrader: (15)

My broken rod is so beat up it would not work for this. But I have a bent one I would sacrifice to the cause if it could be cross-sectioned with a hacksaw for example.

So where are you headed with this? FEA? Euler column bucking? Tensile load?

patsmx5

iTrader: (16)

Thanks sav. I tried blowing that pic up 500% and printing it out on 4 different pages and I'm gonna have to agree with y8s. The piston is well focused even at 500% magnification, but the rod right where it fractured ain't.

patsmx5

iTrader: (16)

Wat? I dunno, I was gonna try to calculate the tensile load put in the rod at the top of the exhaust stroke. From what I understand tensile load is max here and rods often fail from tensile loading, not compressive loading. And tensile loading is a function of RPM, not power. Yet people claim that power bends their rods. So I wanted to run the numbers and see what kind of tensile stress the rods see at various RPMs. Might could setup a function in mathematica and make a pretty plot too.

neogenesis2004

iTrader: (12)

I heard on the Discovery channel during a program about how modern steel changed brige building that steel has something like a 3x stronger tensile strength vs its compressive strength. If that is true (why would Discovery lie!?!?) then how would a rod fail in a tensile load before a compressive load?

y8s

iTrader: (8)

rods will never* fail in compression.

grab a wire hangar and straighten it out into one long rod. then push on it from both ends. how'd it fail? compressive fracture? nosir. it buckled.

the thing about tensile failure in a cyclic loading environment like a motor is that it exploits existing structural weaknesses. if there's a casting flaw or a surface irregularity, it can grow very fast into a crack. then say you miss a shift and yank the motor to 8 grand at high vacuum and it yanks on that irregularity and forms a small crack after few rotations and then you're living on borrowed time.



*ok almost never

neogenesis2004

iTrader: (12)

When talking about a motor, is buckling not failure?

y8s

iTrader: (8)

it is "a" failure, but not strictly a compressive failure. a compressive failure will show up as a fracture at an angle from the direction of compression. the material will shear or crumble where the molecules can no longer hold on to each other.

in more ductile materials, the cross section will just bulge and split.

kotomile

iTrader: (24)

Is this a good case for shot peening?

Savington

iTrader: (31)

o hey i did that

patsmx5

iTrader: (16)

So is buckling the result of induced shear stress from compressive loading? If so it would want to shear at a 45* angle too, but with the rod shape constantly changing I guess it would fail differently.

And sav, that pic won't work. I got it blew up and outlined the rod where it failed. The quality isn't the problem, but the camera was a bit off center in the pic, and when I outlined it with it magnified 5x it's pretty obvious one end is 10% wider and longer than the other, though the two halves should be symmetric. I guess if possible, back up, then zoom in and try to be right over the center of the rod and snap a pic. Thanks again.

Joe Perez

iTrader: (8)

The plot thickens. How about strain hardening and necking, followed by buckling and fracture?


About a year and a half ago, I missed a 2-3 shift and actually managed to shift back into first. The engine made a really interesting sound. I'm pretty sure that the clutch was slipping wildly, and sadly I wasn't logging at the time so I have no idea how high it actually spun.

That afternoon I ordered MazdaComp motor mounts.

Had me on pins & needles for a while, but within a week or so I was back to hammering on it at 13PSI and shifting at 7k. And I've not thought about it again, until this recent rash of rod breaking-ness.

Savington

iTrader: (31)

picky-*** bitches.

patsmx5

iTrader: (16)

Thanks, that pic works. I printed it out on grid paper and started adding up the number of squares to approximate the area. This is ridiculous how many boxes I have to add up, but it will be very accurate. Probably within .1% of the actual area of what I printed, which is probably 98% accurate. So should be within 2% of accurate, which is good enough. Will finish adding **** up tomorrow and post the actual cross sectional area of Sav's crazy shaped rod, which may indeed be a factory defect (or maybe all miata rods have this defect, more to come...).

JimAtFSU

Anyone ever break a forged rod? I know people have spun bearings, but anyone ever break one?

1990miata1.6

iTrader: (2)

i bent some rods