Threw A Rod Club Members- need some pictures - Miata Turbo Forum - Boost cars, acquire cats.

Welcome to Miataturbo.net   Members
 


General Miata Chat A place to talk about anything Miata

Reply
 
 
 
LinkBack Thread Tools
Old 03-03-2009, 08:40 PM   #1
Elite Member
Thread Starter
iTrader: (15)
 
patsmx5's Avatar
 
Join Date: Aug 2007
Location: Houston, TX
Posts: 8,797
Total Cats: 248
Default Threw A Rod Club Members- need some pictures

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!)
patsmx5 is offline   Reply With Quote
Old 03-03-2009, 09:05 PM   #2
Supporting Vendor
iTrader: (31)
 
Savington's Avatar
 
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 14,370
Total Cats: 1,332
Default

Name:  DSCN1293.jpg
Views: 2
Size:  63.2 KB
Savington is offline   Reply With Quote
Old 03-03-2009, 09:13 PM   #3
Supporting Vendor
iTrader: (31)
 
Savington's Avatar
 
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 14,370
Total Cats: 1,332
Default

**** my isp.
Savington is offline   Reply With Quote
Old 03-03-2009, 09:24 PM   #4
Elite Member
iTrader: (12)
 
Join Date: Aug 2006
Location: Central-ish VA
Posts: 4,449
Total Cats: 16
Default

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.
neogenesis2004 is offline   Reply With Quote
Old 03-03-2009, 09:28 PM   #5
y8s
2 Props,3 Dildos,& 1 Cat
iTrader: (8)
 
y8s's Avatar
 
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,038
Total Cats: 407
Default

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.
y8s is offline   Reply With Quote
Old 03-03-2009, 09:36 PM   #6
Elite Member
iTrader: (15)
 
Join Date: Dec 2007
Location: San Antonio, Texas
Posts: 4,889
Total Cats: 28
Default

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?
ZX-Tex is offline   Reply With Quote
Old 03-03-2009, 09:39 PM   #7
Elite Member
Thread Starter
iTrader: (15)
 
patsmx5's Avatar
 
Join Date: Aug 2007
Location: Houston, TX
Posts: 8,797
Total Cats: 248
Default

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 is offline   Reply With Quote
Old 03-03-2009, 09:43 PM   #8
Elite Member
Thread Starter
iTrader: (15)
 
patsmx5's Avatar
 
Join Date: Aug 2007
Location: Houston, TX
Posts: 8,797
Total Cats: 248
Default

Quote:
Originally Posted by ZX-Tex View Post
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?
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.
patsmx5 is offline   Reply With Quote
Old 03-03-2009, 10:05 PM   #9
Elite Member
iTrader: (12)
 
Join Date: Aug 2006
Location: Central-ish VA
Posts: 4,449
Total Cats: 16
Default

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?
neogenesis2004 is offline   Reply With Quote
Old 03-03-2009, 10:13 PM   #10
y8s
2 Props,3 Dildos,& 1 Cat
iTrader: (8)
 
y8s's Avatar
 
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,038
Total Cats: 407
Default

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
y8s is offline   Reply With Quote
Old 03-03-2009, 10:28 PM   #11
Elite Member
iTrader: (12)
 
Join Date: Aug 2006
Location: Central-ish VA
Posts: 4,449
Total Cats: 16
Default

Quote:
Originally Posted by y8s View Post
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.
When talking about a motor, is buckling not failure?
neogenesis2004 is offline   Reply With Quote
Old 03-03-2009, 11:00 PM   #12
y8s
2 Props,3 Dildos,& 1 Cat
iTrader: (8)
 
y8s's Avatar
 
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,038
Total Cats: 407
Default

Quote:
Originally Posted by neogenesis2004 View Post
When talking about a motor, is buckling not failure?
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.
y8s is offline   Reply With Quote
Old 03-03-2009, 11:07 PM   #13
Elite Member
iTrader: (24)
 
kotomile's Avatar
 
Join Date: Oct 2006
Location: Monterey, CA
Posts: 7,578
Total Cats: 40
Default

Is this a good case for shot peening?
kotomile is offline   Reply With Quote
Old 03-03-2009, 11:09 PM   #14
Supporting Vendor
iTrader: (31)
 
Savington's Avatar
 
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 14,370
Total Cats: 1,332
Default

Quote:
Originally Posted by y8s View Post
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.
o hey i did that
Savington is offline   Reply With Quote
Old 03-03-2009, 11:18 PM   #15
Elite Member
Thread Starter
iTrader: (15)
 
patsmx5's Avatar
 
Join Date: Aug 2007
Location: Houston, TX
Posts: 8,797
Total Cats: 248
Default

Quote:
Originally Posted by y8s View Post
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.
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.
patsmx5 is offline   Reply With Quote
Old 03-03-2009, 11:18 PM   #16
Boost Pope
iTrader: (8)
 
Joe Perez's Avatar
 
Join Date: Sep 2005
Location: Delicious and Moist.
Posts: 26,327
Total Cats: 1,926
Default

Quote:
Originally Posted by Savington View Post
o hey i did that
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.
Joe Perez is offline   Reply With Quote
Old 03-03-2009, 11:51 PM   #17
Supporting Vendor
iTrader: (31)
 
Savington's Avatar
 
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 14,370
Total Cats: 1,332
Default

picky-*** bitches.

Name:  DSCN1297.jpg
Views: 1
Size:  62.2 KB
Savington is offline   Reply With Quote
Old 03-04-2009, 12:47 AM   #18
Elite Member
Thread Starter
iTrader: (15)
 
patsmx5's Avatar
 
Join Date: Aug 2007
Location: Houston, TX
Posts: 8,797
Total Cats: 248
Default

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...).
patsmx5 is offline   Reply With Quote
Old 03-04-2009, 01:09 AM   #19
Junior Member
 
Join Date: Sep 2008
Posts: 218
Total Cats: 0
Default

Anyone ever break a forged rod? I know people have spun bearings, but anyone ever break one?
JimAtFSU is offline   Reply With Quote
Old 03-04-2009, 01:42 AM   #20
I'm Miserable!
iTrader: (2)
 
Join Date: Apr 2008
Location: monroe CT
Posts: 463
Total Cats: 0
Default

i bent some rods
Attached Thumbnails
Threw A Rod Club Members- need some pictures-photo0199.jpg  
1990miata1.6 is offline   Reply With Quote
 
 
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
Project Gemini - Turbo Civic on the Cheap Full_Tilt_Boogie Build Threads 59 12-15-2017 09:00 PM
OTS Bilstein to motorsports ASN conversion stoves Suspension, Brakes, Drivetrain 5 04-21-2016 04:00 PM
Odd Steering issue interestedofold Suspension, Brakes, Drivetrain 5 09-29-2015 02:42 PM
Bp/b6 Scat H beam rods Dparks7 Miata parts for sale/trade 1 09-28-2015 10:49 AM


Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -4. The time now is 12:02 PM.