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Ok, what snaps a con rod!

Old 06-24-2007, 07:42 AM
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j_blaze
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Default Ok, what snaps a con rod!

Wow, I ported an rtr engine yesterday and it was screaming! It took about five minutes with the dremel, put it back together and the 21 was screaming like a 28, no joke. I will defineately be porting again in the very near future. Enough of the good stuff, Heres what happened: I was on my last run on the street with my truggy, I had installed some 1/8 on road buggy tires to not wear out my dirt tires, good idea right? It was, for about half a tank, until I was going to do one more high speed pass to make sure I was not too lean and check temps when the engine just stopped, no bog, no stutter nothing, just a quick stop. So I checked the temps, 248, I leaned it 1 hour and dropped it on the starter box. Won't turn over, so I try to manually turn it over and it is jammed. I thought it was flooded so I loosened the plug and nothing. Then my heart began beating, took her out, pooped off the backplate and woah! who put all these aluminum chunks in my engine? and wheres the other half of my con rod?

I'm guessing excessive rpm's from too lean engine with small tires on a smooth surface did the trick, is that right? I just want to verify with 100% accuracy so I know what not to do. I blew a con rod before during break in - left receiver off WOT - on a brand new engine but this one had about 3 gallons through it. Any feed back is greatly appreciated!
Old 06-24-2007, 09:46 AM
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SAVAGEJIM
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Default RE: Ok, what snaps a con rod!

What will cause the conrod to fail catastropichally is typically the axial forces along the length of the conrod. Such forces that cause these failure stresses are usually caused by trying to start a brand new engine that has not been warmed up or pre heated and more often, from hydro-locking. From your description, the conrod split midway between the crank pin and the piston pin. This is indicative of compressive failure. If you collect the conrod pieces and try to put the conrod back together like a puzzle, you will find that these pieces are shorn at a 45degree angle to the lengthwise centerline of the conrod. This is proof you experienced compressive failure, not an overrev failure.

Another cause for conrod failure is overrevving. The compressive forces along the conrod in this case is not such a major factor, although it is still a significant factor since every power stroke the conrod expeiences both compressive and tensional stresses. In an overrreving situation, it is at the crank pin and conrod journal for the crank pin that will see serious forces. The inertia of the heavy end of the conrod against the crank pin will cause that journal to expand and round out if overrevved. This journal at this end of the conrod will therefore expand untill it stretches and snaps and the crank pin is free of the conrod. This will always happen before the conrod is pulled in half and splits between the crank pin and piston pin. Under this kind of failure (which is caused by tensional forces by the way), if you collect the conrod pieces and try to reassemble them like a puzzle, you will see that the conrod actually stretched like a rubberband before it snapped.


For any of you with a brand new engine, whenever you start it up, always pre-heat it. Also, as a precaution and to help relieve some of the conrod compressive stresses, lossen the glow plug a quarter turn when starting a new engine.
Old 06-24-2007, 09:57 AM
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Default RE: Ok, what snaps a con rod!

Depends on where it broke, what the crankpin looks like and what the con rod bushing looks like.
Old 06-24-2007, 10:28 AM
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SAVAGEJIM
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Default RE: Ok, what snaps a con rod!

That's correct.

Overrev:
If the crank pin journal is rounded out and the crank pin shows damage all the way around the pin, this is indicative of overrevving. This of course is when you experience an overrevving engine; you cannot mistake the very very high pitch scream of an overreving engine when you hear it.

Compressive:
If the crank pin jounal shows an ovaling or impression towards the top of the journal, and the crank pin shows damage on the right half of the pin (looking at the crank as the pin is in the TDC position), this is indicative of compressive stresses.
Also, a conrod that has split between the the crankpin and the piston pin, the pieces will have 45degree angles in relation to the length of the conrod centerline. Reassebling the pieces will consistently display this 45degree angle on the pieces.

Tensional:
If the crank pin jounal shows an ovaling or impression towards the bottom of the journal, and the crank pin shows damage also on the right half of the pin (looking at the crank as the pin is in the TDC position), this is indicative of tensional stresses.
Also, a conrod that has split between the the crankpin and the piston pin, when you put the pieces back together, you will see that the conrod has actually lengthend from tensional forces before it snapped. The conrod under sufficient tension will actually go through a plastic phase and stretch very briefely before it fails.

In reality, you could see these signs together in a engine that has failed. An engine that is suffering from too tight a piston sleeve can show both tensional and compressive damage signs. The conrod crank journal will also appear ovaled out at the top and bottom since the crank pin is applying significant stresses when pushing the piston up and when pulling it back down.

This is a guideline to determine what might be the cause of the conrod failure.
Old 06-24-2007, 03:53 PM
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Default RE: Ok, what snaps a con rod!

If you ran lean the bushing in the big end probably rotated, seized and finally blew out. It happens.
Old 06-24-2007, 09:43 PM
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Default RE: Ok, what snaps a con rod!

A 2 stroke conrod basically only ever experiences compressive forces and never tensional forces. Tensional forces are only felt in 4 stroke engines at the end of the exhaust stroke and during the first half of the intake stroke. These compressive forces come from two things. First (and most obvious) is the pressure on the piston from combustion. Maybe less obvious is the fact that the piston has to change it's speed from zero at the top and bottom of the stroke to a maximum speed roughly halfway up the stroke. That means the piston has to be accelerated. In a .21 engine at 30,000 revs that means the piston goes from a standstill to roughly 5000 feet/minute (~60 mph) in 1/2000 of a second. The piston gets accelerated (by the rod) at around 10,000 G's. On the powerstroke the combustion pressure does all the accelerating so it's compressing the rod.

Now over-rev to 40,000 and the piston acceleration increases to over 18,000 G's which is almost double what it had at 30,000. Just where the rod will break depends on the design and it's weakest point. Because it's at an angle to the crankpin at maximum load it's bending first in one direction then the other so it's being flexed at a very high rate and aluminium fatigues fairly quickly.

Old 06-24-2007, 09:55 PM
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Default RE: Ok, what snaps a con rod!

The pieces are too small to collect, but it looks like the whole bottom of the rod exploded, the part that connects to the driveshaft. The cylinder is also showing a little damage...

I'm not sure if I should buy the parts or just a new engine, probably the later...

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Old 06-25-2007, 01:44 AM
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Default RE: Ok, what snaps a con rod!

That looks exactly the same as the rod in my nitro hammer wich broke due to a lean run.
Old 06-25-2007, 11:05 AM
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Default RE: Ok, what snaps a con rod!

It failed due to user error. 1) Flat out high speed runs on pavement is just not good for the engine. Once the engine hits top speed back it off. There's no sense in just letting it scream for the entire length of your street. 2) You put tiny buggy tires on a truggy. That will cut your rolling resistence and reduce your final drive ratio letting your engine top out in half the time.
Old 06-25-2007, 11:11 AM
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Default RE: Ok, what snaps a con rod!

Over revving is what usually snaps a rod...it can only take so much...
Old 06-25-2007, 10:43 PM
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Default RE: Ok, what snaps a con rod!

The pic posted by j_blaze show signs of overrevving and/or crank journal bearing siezure. In either of these cases, the journal will be the first to fail because the main shaft of the conrod actually has more strength due to the fact that there is more material there.
Old 06-25-2007, 10:51 PM
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Default RE: Ok, what snaps a con rod!

by downunder

Now over-rev to 40,000 and the piston acceleration increases to over 18,000 G's which is almost double what it had at 30,000. Just where the rod will break depends on the design and it's weakest point. Because it's at an angle to the crankpin at maximum load it's bending first in one direction then the other so it's being flexed at a very high rate and aluminium fatigues fairly quickly.
You're right about the 2 stroke conrods experiencing compressive forces at all times. I was thinking about 4 strokes when mentioning conrod tensional forces.

EDIT: Now that I think of it, a 2stroke engine conrod could see tensional forces. After the exhaust port opens and the pressure on the piston surface drops to zero as the exhuast gases exit, the piston must still drop down further to open the intake ports and still further to BDC. During this arc of the crank travel, the intertia of the rotating moving parts contines to turn the crank abd thus the piston is actually being pulled down at this point. This is where the conrod sees tensional stresses. Is it as significant as the compressive forces from combustion? No, but it is still tension.

As for the conrod experienceing a bending moment, are you referring to Euler buckling? I didnt think the frictional forces at the crank pin and/or piston pin would form a strong enough couple to cause a significant bending moment on the conrod since they are supposed to be lubricated from the fuel. But then, the inertia of the contod itself might cause a bending when very rapidly changing directions inside a very high revving engine.
Old 06-26-2007, 08:15 AM
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Default RE: Ok, what snaps a con rod!


ORIGINAL: SAVAGEJIM



EDIT: Now that I think of it, a 2stroke engine conrod could see tensional forces. After the exhaust port opens and the pressure on the piston surface drops to zero as the exhuast gases exit, the piston must still drop down further to open the intake ports and still further to BDC. During this arc of the crank travel, the intertia of the rotating moving parts contines to turn the crank abd thus the piston is actually being pulled down at this point. This is where the conrod sees tensional stresses. Is it as significant as the compressive forces from combustion? No, but it is still tension.
The rod is still under compression when the exhaust port opens because the piston is still moving quite quickly but has to slow down to zero at BDC. The only thing that can slow down the piston is by the rod pushing against it. The arc of crank travel is quite large but the vertical distance moved by the crankpin is quite small.

The only time the rod tends towards being in tension is as it approaches TDC on the compression stroke where it has to slow down the piston. However the inertia of the piston is pretty much counterbalanced by the compression pressures on the piston plus ignition occurs at around 20 degrees before TDC and that sudden pressure rise does a really good job at slowing down the piston .
Old 06-26-2007, 06:45 PM
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Default RE: Ok, what snaps a con rod!

Wouldnt the conrod experience this same tentional tendency phenomenon close to TDC also occur the piston approaches BDC as well? Afterall, it is the same circle.

Im not certain if piston inertia would contribute too much to conrod compression after the exhaust pressure drops to zero on the down stroke. Most of the intertia would be in the crankshaft and flywheel due to their higher mass than the conrod and piston. Also, friction on the cylinder sleeve, although it is supposed to be very small from the fuel's lubrication, will actually begin to pull on the piston.
It is therefore these the crank and flywheel that begin to do a significant amount of work on the conrod and pull it as it to begins to pull the piston down. Also, the piston is also doing work as it is coming down as it is pushing some of the fuel-air charge into the cylinder through the now open intake ports.
Old 06-26-2007, 07:25 PM
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Default RE: Ok, what snaps a con rod!

In the RC cars, I have seen catastrophic rod journal failures occur mostly during instant deceleration from maximum rpm. Of course every car rod I've seen tossed was also aggrivated by slightly lean high speed mixture settings which I assume was causing low frequency vibration from high speed pre-ignition. Everything in this hobby has its absolute breaking point and there is nothing that has a mean time between failure of infinity.

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