canadamaxxer
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posted January 10, 2005 02:46 PM
Ok , that part of it is simple physics. Let's make some assumptions for easy math: the bearing in question is .75 in wide and the curved surface is 2 in long. That makes it 1.5 in square. 20 psi divided by 1.5 in square is 13.333 pounds. That means the bearing and journal are held apart by 13.333 pounds of force. Anything greater than 13.333 lbs of force would cause the clearances to decrease IF the oil was compressible. When the combustion event happens, it pushes down on the rod with 100's of lbs of force. If pressure was the sole factor involved here, the bearings would touch every time the cylinder fired. Since it is so much more complicated than that (non compressiblity of the oil, oil wedge formation, etc) there is no contact (under normal conditions).
  
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entropy
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posted January 10, 2005 04:09 PM
hmmmm... if a 4.6mm stroker motor is making tq= 120 ft-lb (@ about 8k rpm) , thats 10# per inch of stroke X 2.4" = 24# total force on the journal; divide by 1.5 in sq = 15.8psi pressure on the journal at max tq. my motor is running about 22-24 psi at 4000rpm. Looks like I am covered (or is my math fukkered???)
hmmmmmm.....
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canadamaxxer
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posted January 10, 2005 04:39 PM
Entropy,
I think your math is sound but it doesn't take into consideration the actual pressure of combustion. The peak force is probably 900+lbs. Imagine having a compression gauge in place while the cylinder fires. How many psi would you likely see? I suspect probably 1000+ psi for a split second. That pressure is spread over the area of the piston and pushes down onto the rod and eventually to the crankshaft.
Here is a very interesting thought: all of your engines' horsepower is transfered hydraulically through your engine oil....and this is solely possible because the oil is non compressible. The pressurization is there to provide the non compressible column of oil...and the leakage is built in to allow splash lubrication of other components (pistons and cylinder walls) as well as to circulate out the hot oil for fresh stuff.
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Ra12r
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posted January 11, 2005 07:38 AM
Is there a higher flow oil pump gear available?
Entropy, if you math is correct then ultimately there is NOT alot of cushion for drag racing forces or DYNO. Resistance to accelerating crank rotation plus Combustion forces would increase the PSI that is occuring between the journal and the bearing. This force would be the highest between 3000-8000 rpm's. IE: the major acceleration(wheelie range) portion.
Hypothesis: Bearing damage occurs more at lower RPM's.
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posted January 11, 2005 08:25 AM
quote:
I write all of the above info and all you can do is twist my words into that?????
quote:
So basically what yer sayin..is that shimmed oil relief valves blow up motors.
What I said was that if the relief valve cannot flow enough to keep the pressure at the calibrated pressure (WHEN IT'S OPENED BY HYDRAULIC PRESSURE......WHICH SHOULD NEVER HAPPEN,ANYWAYS,UNLESS SOMETHING IS WRONG INTERNALLY AS WELL....WHICH IS ALSO WHAT I WROTE!!!!!), the pressure will climb and that climbing pressure is bad!!! I have seen backyard butchers put 3 or 4 washers in behind the spring to attempt to raised the pressure on a sacked out engine (which obviously will not work). If the oil filter plugged on that engine, you would see some interesting shit happen......
Think about this: you go to your local auto parts store and buy a shift kit for your auto trans in your musclecar. What is in the kit? A bunch of washers, or a bunch of springs??? The answer, of course..... is springs. Why install a washer to increase pressure when a spring is available? You wouldn't do for valve springs and yet it's fine to do it for this??? I guess it could be argued that since that relief valve should never open, then the washer doesn't hurt anything.....but then why have the valve at all??? You might as well cut a length of steel tube and replace the spring with that.
The washer behind the spring is an old school trick ...and it probably is fine, but there are a lot of old school tricks that are no longer though of as anything other than butchery (using powdered cleanser to seat piston rings used to be commonplace...but not anymore). It is up to the builder to decide......after all even service manuals are opinion....and sometimes the engineer who wrote the manual knows less than the guy who is up to his elbow in the engine.
Twas a joke at Ra's expense...tis why he was a penis in his reply.
Jokes are far funnier when they dont have to be explained. Needless to say...it had nothing to do with what you typed.
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canadamaxxer
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posted January 11, 2005 12:43 PM
YCIS: I guess this is another time when sarcasm is difficult to show off with the printed word....No harm, no foul, it's all good......and it helped to stir up the thread....so it's good for all of us!!!
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canadamaxxer
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posted January 11, 2005 01:12 PM
quote:
Is there a higher flow oil pump gear available?
Entropy, if you math is correct then ultimately there is NOT alot of cushion for drag racing forces or DYNO. Resistance to accelerating crank rotation plus Combustion forces would increase the PSI that is occuring between the journal and the bearing. This force would be the highest between 3000-8000 rpm's. IE: the major acceleration(wheelie range) portion.
Hypothesis: Bearing damage occurs more at lower RPM's.
Ra:a couple of things to add:
1) if the oil pressure was the sole thing that prevented contact, combustion pressure would pound out the bearings in a matter of minutes. You asked what was the force to overcome the oil pressure....I showed you that combustion pressure is Waaaaay beyond the simple hydraulic answer...so there has to be something else....that something else is the wedge effect. I dug out my old text book from school, and it terms this effect "Hydrodynamic lubrication". As bearing clearances increase, the ability to maintain this wedge becomes more difficult. The spec quoted was "if bearing clearance is doubled, five times the amount of oil is thrown off the bearing (versus proper clearance)" This means with double the clearance, five times the amount of oil (volume)has get to this bearing at all times or it will be starved. This is why proper clearance (neither too tight or too loose) is so important.
2) Bearing damage is probably more likely at high rpm's (primarily rod bearings). There was a spec of "a small block engine spinning at 6000 rpm will carry 5 litres (over 5 quarts) of oil in a vortex around the crankshaft" discussed in school. If oil starvation and loss of oil pressure are likely to happen, it would be when there is very little oil left in the sump area. That is the reason for windage trays and crankshaft scrapers in oil pans...and also why dry sump is such a good idea. There is something to be said for low RPM's and heavy loads. That is why it is suggested to avoid turning on the nitrous at low rpm. Crankshaft speed is too low and the combustion events are too large....and something has to give. Of course the connecting rod is typically the weak link. When you see a dude on a Harley pulling away from a light and is in 3rd before he gets through the intersection, you have to understand that the load is higher than the guy who "screams" (LOL!!!) the bike through the same intersection.....luckily for those dudes the bottom end on most hogs are pretty tough (typically a full roller bottom end if I remember correctly) so they probably can take it. A more likely cause of failure at high rpm is as a result of super high piston speeds causing either the rod or the rod bolts to stretch temporarily, which can cause a loss of the hydrodynamic lubrication (in the case of the bolts stretching) and therefore starvation..... which shows up as a bearing failure, or the piston clipping either the valves (the failure is obvious) or the head (which can begin the bending of the rod).
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entropy
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posted January 11, 2005 04:12 PM
this thread just gets better n' better
go-canadamaxxer-GO!!! 
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canadamaxxer
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posted January 11, 2005 04:16 PM
Thanks Entropy!!!
You guys are making me dust off shit in my brain that I thought was long gone and forgotten!!!
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Ra12r
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posted January 12, 2005 06:31 AM
Very good picture you painted Canada. There two good events that leads to possible/probable oiling compromise.
1) Forces during Low rpm launch. a)combustion forces pushing down /crank inertia from drivetrain acceleration resistance (equal and opposite to a point).
2) Rod bolt stretch increasing journal clearances resulting in decreased oil layer thickness.
Now I would like to compare the probability of 1) vs 2). I do not feel that #2 is more responsible for bearing failure. I base this on other engines that spin at higher rpms. IE:1000's, 750's, and 600's. I personally shift my 600 at around 14,000 all day without failure. The limiter is even higher. zx12's and busa's all have limiters that will stop over rpm's. I am not saying that stretching cannot occur, but I am not observing stretching failures with current metalurgy technology. The overall weight of the piston rod combination is greater in a stock zx12 than in what many of us are using. Personally i have carillos and JE pistons and that combo is lighter than stock REDUCING the reciprocating mass. IE: less forces to stretch the rod bolts. However my same combo is producing MORE combustion forces to combat inertia at low rpm's. IE: greater acceration/torque while oil pressure is still low.
I would be very interested in a higher flow oil pump gear!!!
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canadamaxxer
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posted January 12, 2005 09:38 AM
Ra:
I understand what you are saying and have a couple of points to add:
1) The ability of the engine to be run at high rpm has to do with reciprocating mass, but only indirectly (for the most part). The stroke at which the reciprocating mass is articulated is the major factor. Think about it: the majority of the engines that really scream are short stroke mills....they get a lot of their displacement from the bore. The opposite side of the coin is the monster Harley V-twins that can't rev very high at all....as a result of a huge stroke which creates a phenomenal amount of piston speed at high rpms (the benefit that most if not all harleys have is the full roller bottom end and connecting rod big ends that are one piece....the crank is disassembled, the two rods with bearings installed, and the crank reassembled. There are no bolts to be the weak spot.). If those mills rev much beyond 6000 rpm they will most likely pop.
2) If rod bolts don't distort under load, then why does every competent 12R engine builder say to NEVER reuse the rod bolts? To retorque a fastener that has been partially stretched is very possible, but in this case will show up when the forces are pulling the rod apart (high rpm), not pushing it together (low prm). The benefits of Carrillo Rods and JE pistons (I have them too) have to do with weight, but also in the quality of the metalurgy of the rods AND the fasteners.
Piston speed for a given engine at a given RPM is a constant regardless of the mass of the components. The addition of the mass to the velocity (and keep in mind the piston and rod have to start and stop twice per stroke....so it's actually positive and negative acceleration, not velocity) creates a force. Obviously the faster the engine turns, the greater the force is. If that force is greater than the tensile strength of the fastener, it will stretch. The funny thing is that steel is far more elastic than anyone can imagine (other than engineers and metallurgists) and can snap back to what appears to be what is was ( at least to the naked eye). I have personally repaired/replaced dozens of engines (with built in rev limiters) that have destroyed themselves by having the rod bolts stretch. How it happens most often is missed shifts (shifting 3rd to 2nd instead of fourth...it happens) or broken driveline components (clutch failure??). The limiter will try to stop the over rev, but it is not a perfect science....and when it is a mechanical gearing issue (missed shift) there is nothing the limiter can do.
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Ra12r
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posted January 12, 2005 07:44 PM
Thinking about rod bolt stretch, I wonder if the "torqueing process" is the real stretch on the bolts. That is what I always interpreted when reading about stretching or stretch rod bolts?!
Secondly, as rpms/piston speeds increase the time for the direction changes decreases and approaches zero. This creates more of steady motion. Also, the pull force that you mentioned only would occur during the "intake" phase. However, the downward forces to "pull" a piston would not be that much. At least not enough to stretch the rod bolts.
Ernestly, many of our conclusions for "what blew up the motor" is conjectur at best. I say this become spun bearings still don't make alot of sense to me. Especially when the oiling system is "intact". It is kinda like measuring life forces. To do it you have to kill your subject. If we could somehow watch the failures occur, then boy that would really be a break through........
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MadMike
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posted January 12, 2005 09:07 PM
RA, on thing to remember on your pistons being lighter, they might be slightly lighter but they are larger and thus more force is being created,
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canadamaxxer
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posted January 13, 2005 07:03 AM
quote:
Thinking about rod bolt stretch, I wonder if the "torqueing process" is the real stretch on the bolts. That is what I always interpreted when reading about stretching or stretch rod bolts?!
Bolt stretch is an essential part of ensuring that the fastener never comes loose and also maintaining sufficient clamping force. Many torque settings these days are a minimum torque spec and then the fastener is turned another certain # of degrees. This is called torque-angle. The benefit of doing this is the the inconsistencies of friction between individal fastener is less of a concern at the lower initial torque settings. This creates more even torque because in theory all of the fasteners end up at the same tension regardless of the lubricant used. We are starting to see the use of bolt stretch gauges that actually measure the length of the bolt during tightening. The ideal torque is when the fastener increases in length by a specified amount. There is elasticity engineered into the bolts. If that elasticity is overcome, the bolt never returns to its' original length, and is out of calibration. This can happen from overtorquing, stretching due to overrevving, or reuse on rebuild.
quote:
Secondly, as rpms/piston speeds increase the time for the direction changes decreases and approaches zero. This creates more of steady motion. Also, the pull force that you mentioned only would occur during the "intake" phase. However, the downward forces to "pull" a piston would not be that much. At least not enough to stretch the rod bolts.
I think you are misunderstanding the physics of what is going on inside the engine. For an engine to complete one cycle, the piston has to STOP four times. Sure as the RPM increases, the time the piston is stationary becomes less, but it still has to stop. This means there are positive and negative acceleration forces happening every stroke. When the piston reaches the top of the bore...and stops....and then begins to reverse direction, the laws of physics come into play. The inertia of the piston wants to keep it going in the direction is was going (traveling up the bore), and to stop the piston, the rod has to begin to pull down on the piston (as the piston/rod reach and pass TDC). If either the rod or the bolts are weaker than the force of the piston (what is it mass X acceleration??? Where's Bean's when you need him?)moving up, something has to give. Some high performance guys wills actually build their engines knowing that this will happen (especially the rod "growing" in length) and set the deck height so that the piston just lightly contacts the head at maximum RPM.
quote:
Ernestly, many of our conclusions for "what blew up the motor" is conjectur at best. I say this become spun bearings still don't make alot of sense to me. Especially when the oiling system is "intact". It is kinda like measuring life forces. To do it you have to kill your subject. If we could somehow watch the failures occur, then boy that would really be a break through........
I can agree with this one partially. Using some of the new techniques out there (oil analysis), it becomes more possible to determine the cause of failure. Also a good mechanic can decipher many of the symptoms to determine the root cause. I will admit, there are engines out there that fail for no obvious reason, and there are engines out there that, by all rights, should have failed and are still running. It would be wild to build an engine (with cut out windows and easy access oil pan, etc) for the express purpose of making it fail, but no one I know has the millions of dollars needed to put on that kind of scientifically controlled experiment. All we can do is control the factors available to us....For the average person: primarily the quality of the oil used and the frequency of oil changes. For someone who wishes to dig deeper...a high volume oil pump is the bees knees.
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dougmeyer
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posted January 13, 2005 08:44 AM
I should probably keep my mouth shut here, but there are several misconceptions floating around in this thread. I don't have time to elaborate right now, but I'll get back to you after I've had several espressos some morning........
Doug
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posted January 13, 2005 08:48 AM
Get that man some coffee...I need entertainment!!!
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canadamaxxer
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posted January 13, 2005 10:52 AM
 Edited By: canadamaxxer on 13 Jan 2005 10:52
I found some information on bearings and lubricants online (from http://www.machinedesign.com/BDE/mechanical/bdemech6/bdemech6_16.html):
Bearing Lubrication
Most bearings operate with a fluid film -- oil, another liquid, or a gas. By far the largest number of bearings are oil lubricated. The oil film can be maintained through pumping by a pressurization system -- in which case the lubrication is termed hydrostatic. Or it can be maintained by a squeezing or wedging of lubricant produced by the rolling action of the bearing itself -- termed hydrodynamic lubrication. If loads are too high or speeds too slow, the hydrodynamic action begins to break down a condition referred to as boundary lubrication.
Hydrostatic lubrication: The main virtue of hydrostatic lubrication is that it can accommodate heavy loads at low speeds because it does not depend upon relative motion to maintain the lubrication film. Instead, lubricant is supplied from a special pump and feed lines to the bearing. The oil is fed through flow restrictors, which generally are stationary. The flow restrictors automatically adjust the oil flow for the applied load. Another advantage of this lubrication system is low deflection in certain load ranges, making it preferred for many high-precision machine tools. The disadvantage of hydrostatic lubrication is its high cost and complexity.
Hydrodynamic lubrication: This form of lubrication occurs more or less naturally in properly finished, sized, and lubricated holes and shafts. Essentially, rotation of the journal causes it to drag lubricant into a wedged-shaped channel generating a load-carrying pressure. The lubricant in this wedge creates sufficient pressure to keep the journal riding on the oil film. This form of lubrication is generally preferred because it is simple and dependable. Also, the lubrication action improves as speed increases, which in most applications goes hand-in-hand with an increase in loads experienced as speed increases. Its main drawback is an inability to carry heavy loads at low speed and appreciable wear under frequent stops or starts, or motion reversals.
The oil for hydrodynamic lubrication can be fed from an oil reservoir. Or the bearing can be made of a porous metal impregnated with oil that "bleeds" to the bearing surface as the shaft rotates. Most porous-metal bearings, however, operate under boundary or mixed-film conditions.
Boundary lubrication: This form of lubrication is essentially a breakdown of hydrodynamic action. At high loads or low speeds, the pressure of the hydrodynamic film cannot prevent metal-to-metal contact. So the opposing surfaces partially ride on an oil film and partially rub together as surface high points come in contact. Lubrication is provided by lubricant decomposition products or surface-active additives which form a thin, soft, solid film on the metal surfaces and prevent metal-junction adhesion.
Boundary lubrication is not the most desirable operating mode, yet at times it is completely unavoidable. It is found mainly with slow-moving loads where the cost and expense of a hydrostatic system is not warranted. Hinge bearings in aircraft landing gears, for example, do not move fast enough to develop hydrodynamic films, yet hydrostatic systems would be too heavy, costly, and cumbersome.
Also, I found a nice picture showing a side view of a bearing journal (including the wedge effect...which could be shown better than this pic IMHO): http://www.autosite.com/garage/subsys/07-01a.asp
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dougmeyer
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posted January 13, 2005 01:53 PM
You've partially redeemed yourself with the citation of the effect of hydrodynamic
lubrication. This explains why it isn't a balance between load and oil pressure (which, of course, would be totally insufficient at high loads) that keeps the shaft and bearing from touching. The hydrodynamic wedge forms what is essentially a very slipperry, solid (due to the non-compressability of liquid) wedge shaped "bearing" made of oil that is constantly re-supplied with cool material (oil).
But wait! There's more........................
Doug
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aliveagain
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posted January 13, 2005 03:44 PM
expresso ready?
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canadamaxxer
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posted January 15, 2005 06:59 AM
tick tick tick......times a wasting.....
IF I'm wrong about anything I sure would like to know about it this year......it seems kinda weird to pop into a thread, say "you're wrong....stay tuned for more info", then only give half an answer (and the "citation of the Hydrodynamic effect" was done many posts ago, thank you), and then never bothering to return. I have researched every comment I've made and can back it all up, so I would love to find out about any opinion differences.
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dougmeyer
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posted January 20, 2005 07:55 AM
Sorry,
I know that's impolite, but the physics won't change any time soon. I don't have time right now but I will get back in. Relax, man.take those red gloves off. You French Canadian or something ? 
Regarding hydro lube- you mentioned it but kind of glossed over the fact that it is the PRIMARY factor in why a babbit bearing actually works under load.
Doug
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canadamaxxer
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posted February 03, 2005 09:46 AM
Edited By: canadamaxxer on 3 Feb 2005 09:46
French Canadian? OUCH!! that hurts!!!
It was never my intention to simply gloss over the hydrodynamic wedge info...there are only so many words to describe the same thing. The problem (IMHO) with this topic is that it's very hard to keep your eye on the ball when guys post in throwing tangents into the mix. The basic thread is about oil pressure....and bearing clearance is the main controlling factor with that (given all other things being equal)....but all-of-a-sudden we're on about the pressure required to overcome a given oil pressure....and the shit hits the fan.
I may have come across a little harsh in my last post...and for that I do apoligize. It's just that I have years of info in my head and if something is incorrect, I want it out and the correct info in.....ASAP.....
....and oh yeah, I can't type worth shit without the gloves on!
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Ra12r
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posted February 08, 2005 05:54 AM
Canada,,,,,,,,maybe you are not wrong?! I say this because bearings get burned or spun while other bearings do not. This (in some cases) definitely shows that oil pressure was sufficient at the time of failure. However, the "hydrodynamic!?" properties were overcome in "ONE" rod bearing that was NOT under any more forces than the other rod bearings.
Besides, we were imformed that running our bikes "LOW ON OIL" would improve Horsepower.......... and Canada, that was NOT your suggestion. That is why I make it a point to focus on facts and truth not blindly worshiping certain celebrities.
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canadamaxxer
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posted February 08, 2005 08:18 AM
quote:
Canada,,,,,,,,maybe you are not wrong?! I say this because bearings get burned or spun while other bearings do not. This (in some cases) definitely shows that oil pressure was sufficient at the time of failure. However, the "hydrodynamic!?" properties were overcome in "ONE" rod bearing that was NOT under any more forces than the other rod bearings.
Besides, we were imformed that running our bikes "LOW ON OIL" would improve Horsepower.......... and Canada, that was NOT your suggestion. That is why I make it a point to focus on facts and truth not blindly worshiping certain celebrities.
Ra, this is still a very interesting topic, and I do appreciate the compliments.
Here is the million dollar question: what is the oil pressure at the failure site at the time of the failure?
If you think about a hydraulic circuit (think of an inground lawn sprinkler system), where is the pressure usually measured? near the source, right? Now what happens to the pressure on the gauge when the first sprinkler head in a line of 6 heads is broken off? IF the volume to the gauge section of the circuit is high enough...nothing. If you could measure the pressure of the flow on the broken head it would be close to the pressure on the gauge, but what about further down the line? the answer is that the pressure is reduced on the other heads (how much the reduction is, is dependant on the size of the hole spewing water, and the volume of water available).
Let's make this a little more real. The oil in an engine is pressurized by the pump, fed through oil galleys and passages and (typically) to the main bearings (let's ignore the rest of the oiling system...camshaft, etc). From the main bearings, the oil flows through the crankshaft oil passages and to the rod bearings. If main bearing clearance is too high (to a point), things are probably still ok, as there is volume present to maintain the oil wedge, but......is there enough oil volume left over to properly feed the rod bearings? of course the rod bearing fails if there isn't enough left over to maintain the wedge....and we blame the rod bearing. When we overhaul the engine, we replace all of the bearings and start fresh....and the true culprit is never found because it's in the garbage can already.
That is one possiblity, here is another: as the engine ages, and after many cold startups the rod bearings are pounded out a bit and worn and now take more oil to maintain the wedge (remember that doubling the clearance increases the required volume by 5X). During a situation of high load and RPM, the oil volume available to the bearing is slightly less than what the bearing needs, so there is SLIGHT contact....but that wears the bearing a little more...which then ups it's required volume....but there is no more oil to be had.....so there's more contact...and eventual failure.
Add to these possiblities design choices or flaws....as an example: the ZX12R crankshaft feeds oil to 2 rod bearings each on main journals #2 and 4. Most automotive engines I have worked on feed oil from 2 main bearings per journal...so there is always lots of volume. If makes me wonder why no crank builders have picked up on this and have drilled the cranks for more oiling.....
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