H beam vs. I beam rods

[K1tom]

That's what I like,intelligent dialogue.
Glad you talked to Mike.
He is an insightful guy and a gentleman.
He does scare me sometimes though.

I'm looking at the pic's presented so far and I see a common problem:

Parallel beam sides.
I don't know if you remember your wave theory from physics class,but you have the start,the peak,the crossover point,the trough and the end at zero.
It's known that metals do tend to have a resonance "Q" or natural vibration.
Put a series of sensors along the beam of a set of rods with parallel sides and you will find peaks and troughs around 1/4 & 3/4 marks along the length of the parallel sections,when the natural resonance is excited.
The higher the inpulse applied,the greater the excitement or motion.
Looking at where Mike's rod failed in the pic',I would say it was at the 1/4 point of the most highly stressed end of the rod.
Shouldn't be hard to excite that resonance somewhere in the RPM band.
Couple that with the extreme loads .......
Two things might have saved that rod [and Mike's wallet]:

1.Greater broad face width on the big end [to withstand the swinging motion].

2.Non parallel sides to minimize any standing waves [lower Q].

I realize the wave issue may not sink in with most people,but the issue has it's roots in structural designs such as buildings and bridges,especially in earthquake zones....think Tacoma bridge.
Structural reinforcement doesn't work too well when it's wobbling around like a drunken sailor.

You will always get some vibration at the natural frequency of the part. The ideal goal is to never stay at a RPM where this takes place for any length of time. As for the taper beam rod, this design looks real cool and you would think it would even out the stress around the big end but actually, there is not much stress in that area just because of the way the beam flares out around the big end. At this point we have to look a couple of things. First you have two different reasons for a connecting rod to fail. In most cases, the rod will fail in tension (pulling) at TDC of the exhaust stroke. This is where the piston wants to continue up the cylinder walls and out through the cylinder head and the crank wants to pull it all back down. At that brief moment, everything has essentially stopped and you have a static pull that is equal over the length of the beam. The other time a rod will fail is due to overload / buckling. In both of these cases, most of the time the rod will break right below the wrist pin. With this thought in mind, I have to ask why would you make the thinnest part of the rod beam in what is one of the highest stressed parts of the rod beam? BTW, the swinging motion is a very small part of the stress on a rod and takes place several degrees past TDC.

Tom

 

[Alky V6]

You will always get some vibration at the natural frequency of the part. The ideal goal is to never stay at a RPM where this takes place for any length of time. As for the taper beam rod, this design looks real cool and you would think it would even out the stress around the big end but actually, there is not much stress in that area just because of the way the beam flares out around the big end. At this point we have to look a couple of things. First you have two different reasons for a connecting rod to fail. In most cases, the rod will fail in tension (pulling) at TDC of the exhaust stroke. This is where the piston wants to continue up the cylinder walls and out through the cylinder head and the crank wants to pull it all back down. At that brief moment, everything has essentially stopped and you have a static pull that is equal over the length of the beam. The other time a rod will fail is due to overload / buckling. In both of these cases, most of the time the rod will break right below the wrist pin. With this thought in mind, I have to ask why would you make the thinnest part of the rod beam in what is one of the highest stressed parts of the rod beam? BTW, the swinging motion is a very small part of the stress on a rod and takes place several degrees past TDC.

Tom

A couple questions, Tom.
I suspected the natural frequency of the rod might be part of all the stresses a rod sees. Is it common for the connecting rod to have a natural frequency within the typical rpm range of the engine? And what kind of rpm range have you heard of this occurring? Might it occur at multiple points in the rpm band?

The swinging motion stress coming into play after TDC. Is that due to the combustion pressures peaking at around that point?

 

[K1tom]

Tom is right. We had a very nice phone conversation and I sent him my old set of rods and pistons and also a brand new set for him to evaluate. I look forward to hearing his input about my Oliver rods.

In the intrest of brain storming about rod design , I have to respectfully disagree with Tom on which rod design is best for high HP blown applications.
Im certainly not an engineer and have no basis for my opinion other than common sense!! While under severe pressure from high Hp I would think that the the rod tries to buckle under the pressure. The beam is weakest through the side of the beam and that is what will buckle under the load!! Check all the pics attached to this thread and that is the direction they all bend first!!
I have bent 5 sets of steel rods along my path, 2-carrillo , 2-oliver and 1-crower. All bent along the short side of the beam , Carrillos were the worst , Olivers were second and the Crowers were only slightly bent. When I look at the side of each of the beams and imagine which one is strongest there is no doubt in my little mind that the H beam Carrillos are the weakest ( no material preventing buckling down the side of the beam) , Olivers are second simply because the width of the I beam is narrower than the width of the crowers.
My common sense "hillbilly brain" looks at this like this. If you were to set a 5000# load on top of a 8' long wooden 2 x 4 standing straight up , there is no question that it will buckle along the short side!!! Now make it a 4 x 4 and it most likely will hold the weight. So from that I get that the side of the beam (width and mass) is possibly the most important part of the beam in a high HP application!!

Thats my story and I dont see anyone convincing me otherwise!! Mike

Mike,

You are correct about changing from a 2X4 to a 4x4 but you have also doubled the amount of mass. From a rod design point of view, no one wants a 1,400 gram connecting rod. I am not trying to convince you one way or another and you found something that works for you so don't change until you have some other problem. I am only presenting facts from an engineering stand point and from looking at and examining 30 years of broken parts (several differnt designs). What you may or may not know is from 1981 until 1988 Oliver made a conventional I-beam rod and then switched to their Parabolic design because engines were getting to the point they were making a lot more power and the old I-beam would no longer stand the loads.

Tom

 

[K1tom]

A couple questions, Tom.
I suspected the natural frequency of the rod might be part of all the stresses a rod sees. Is it common for the connecting rod to have a natural frequency within the typical rpm range of the engine? And what kind of rpm range have you heard of this occurring? Might it occur at multiple points in the rpm band?

The swinging motion stress coming into play after TDC. Is that due to the combustion pressures peaking at around that point?

All parts in the engine will vibrate when excited at their natural frequency and different parts of the rod will vibrate at different levels due to the different amount of mass. I.E., the cap will vibrate at a different frequency than the beam. This becomes a real can of worms due to the somewhat dampening effect of the crank, rods, wrist pin etc. Several years ago when we were making rods for Winston Cup cars, we looked at this and from what I remember at that time, the rods were vibrating at something like 5,000 RPM. I also remember some inline 6 cylinder endurance engines that had a severe crankshaft vibration that took place around 6,000 RPM. In the inline engines, the teams had to change the rod bolts after every race or they would fail the next time out and they also were having transmission failures. When they straightened out the cranks, they could run several races on the same bolts without any problems and they never broke any more transmissions.

Tom

 

[BlownV6]

Mike,

You are correct about changing from a 2X4 to a 4x4 but you have also doubled the amount of mass. From a rod design point of view, no one wants a 1,400 gram connecting rod. I am not trying to convince you one way or another and you found something that works for you so don't change until you have some other problem. I am only presenting facts from an engineering stand point and from looking at and examining 30 years of broken parts (several differnt designs). What you may or may not know is from 1981 until 1988 Oliver made a conventional I-beam rod and then switched to their Parabolic design because engines were getting to the point they were making a lot more power and the old I-beam would no longer stand the loads.

Tom

I believe this is the older style your reffering to. I can see where the newer style leaves more material at those outer flanges because this style machines more material out closer to the side of the beam. In a v6 what is your opinion about how much HP this style is good for?? My thinking is that this is a pretty stout rod capable of some pretty high HP numbers. I have someone that is intrested in buying a set of these and I may be telling them completely wrong!! Thanks Mike

 

[K1tom]

I believe this is the older style your reffering to. I can see where the newer style leaves more material at those outer flanges because this style machines more material out closer to the side of the beam. In a v6 what is your opinion about how much HP this style is good for?? My thinking is that this is a pretty stout rod capable of some pretty high HP numbers. I have someone that is intrested in buying a set of these and I may be telling them completely wrong!! Thanks Mike

LOL! That is an antique. Yes, that is one of the old beam designs and I have seen a few of them blow the center right out of them and that is on a n/a Busch engine with a 390 CFM 4 barrel. Based on what I see in the picture, it is an old Busch Grand National rod circa 1986 or 1987. Did you get this in the package from Rhyne?

Tom

 

[K1tom]

A couple questions, Tom.
I suspected the natural frequency of the rod might be part of all the stresses a rod sees. Is it common for the connecting rod to have a natural frequency within the typical rpm range of the engine? And what kind of rpm range have you heard of this occurring? Might it occur at multiple points in the rpm band?

The swinging motion stress coming into play after TDC. Is that due to the combustion pressures peaking at around that point?

The swinging motion stress comes from the starting and stopping of the side to side motion and in some cases is refered to as beam whip. Compared to the other forces being so much greater, I do not worry about this.

Tom

 

[The Radius Kid]

Mike, when you say short side of the beam, what does that mean? I need pictures.
Are you talking about what would be one of the flange sides on an I beam rod?

I believe Mike is talking about the inside or trailing face of the narrow side of the rod as it rotates away from TDC.
The long side would be the side facing the outside of the block at the piston leaves TDC.
Correct Mike?

 

[The Radius Kid]

You will always get some vibration at the natural frequency of the part. The ideal goal is to never stay at a RPM where this takes place for any length of time. As for the taper beam rod, this design looks real cool and you would think it would even out the stress around the big end but actually, there is not much stress in that area just because of the way the beam flares out around the big end. At this point we have to look a couple of things. First you have two different reasons for a connecting rod to fail. In most cases, the rod will fail in tension (pulling) at TDC of the exhaust stroke. This is where the piston wants to continue up the cylinder walls and out through the cylinder head and the crank wants to pull it all back down. At that brief moment, everything has essentially stopped and you have a static pull that is equal over the length of the beam. The other time a rod will fail is due to overload / buckling. In both of these cases, most of the time the rod will break right below the wrist pin. With this thought in mind, I have to ask why would you make the thinnest part of the rod beam in what is one of the highest stressed parts of the rod beam? BTW, the swinging motion is a very small part of the stress on a rod and takes place several degrees past TDC.

Tom

I don't disagree with what you're saying,but it would seem to me that the ideal rod would be a basic triangle shape with sculpted ends.
In reality,the mass you'd carry would make the transient response of the engine slow and limit the RPM range more than necessary due to the extra attendant loads applied to the rotating assembly.
In a properly designed rod,I agree *if* the rod fails,it should break just below the wrist pin - the point where there's the least amount of material.
In the case of Mike's Titanium rods,the breakage appears to have happened toward the big end.
Vibration and resonances concern me because a standing waves have a time to dissipation,depending on the Q of the material,shape,etc.
That vibration may still be disippating as the side loads on the rod increase while the piston is travelling downwards.
The combination of the two may be enough to snap the rod under heavy loads.

This is for the guys:

Take a ball of Brass 4" diameter and strike it.
It gives you a "thunk" at best.
That's a low Q situation.

Take the same ball of Brass and work it into a round,flat,thin dish.
Suspend it on top of a threaded pin with a piece of felt to rest on and you have a cymbal.
You know what happens when you strike it with with a drum stick.:smile:
That's high Q.

I will agree with Tom on one other thing: M300 is more of a panacea to me for rods.
If it's too hard it approachs a brittle situation,forsaking toughness I think I'd rather see.

 

[951le]

Tom,you've showed us how the I beam is a weaker design.What about the Parabolic versus the H beam

 

[BlownV6]

I believe Mike is talking about the inside or trailing face of the narrow side of the rod as it rotates away from TDC.
The long side would be the side facing the outside of the block at the piston leaves TDC.
Correct Mike?

You know I love you man , but what the HE-- are you talking about???
The short side is the short side!! The beam has 4 sides 2 long sides and 2 short sides!! If I need to explain this any furthure Im going to have to pour me another drink!!! Better yet Im off to bed!! Mike

 

[The Radius Kid]

You know I love you man , but what the HE-- are you talking about???
The short side is the short side!! The beam has 4 sides 2 long sides and 2 short sides!! If I need to explain this any furthure Im going to have to pour me another drink!!! Better yet Im off to bed!! Mike

Aw man...my bad.
Gimme' a big hug!
Short = narrow face.
Gotcha'.
I was actually describing the same face [I think] ... just picking one of the two.
Say Hi to Linda.:smile:

 

[The Radius Kid]

I think you are all misunderstanding the reason for an "offset" or "on center rod". An offset rod has the beam moved over on the big end and the reason is to put the beam of the rod in the center of the cylinder bore. I have attached a picture of this. The problem with a Buick block comes in because it has a split journal crank and has a web between the two journals and this web moves the rod pin over on the crank and no matter what you do, you cannot move the beam over far enough to put into the center of the bore without hitting the beam with the counterweight of the crank. In the case of BlownV6, he as an odd fire crank which has a common pin (two rods on the same journal) and this changes everything. When the rod beam is not in the center of the bore, the piston will tilt on the power stroke (this is compounded on a turbo charged engine due to the high cylinder pressure). This tilting does two things. #1 it will side load the rod which puts a bending load on the rod beam and can ultimately cause rod failure. #2, when the piston tilts in the bore, it upsets the ring seal causing a reduction in power.

Tom

I know the difference all too well....but at least we're on the same page.
Your #1 point is the one that I focus my attention on.

 

[K1tom]

I don't disagree with what you're saying,but it would seem to me that the ideal rod would be a basic triangle shape with sculpted ends.
In reality,the mass you'd carry would make the transient response of the engine slow and limit the RPM range more than necessary due to the extra attendant loads applied to the rotating assembly.
In a properly designed rod,I agree *if* the rod fails,it should break just below the wrist pin - the point where there's the least amount of material.
In the case of Mike's Titanium rods,the breakage appears to have happened toward the big end.
Vibration and resonances concern me because a standing waves have a time to dissipation,depending on the Q of the material,shape,etc.
That vibration may still be disippating as the side loads on the rod increase while the piston is travelling downwards.
The combination of the two may be enough to snap the rod under heavy loads.

This is for the guys:

Take a ball of Brass 4" diameter and strike it.
It gives you a "thunk" at best.
That's a low Q situation.

Take the same ball of Brass and work it into a round,flat,thin dish.
Suspend it on top of a threaded pin with a piece of felt to rest on and you have a cymbal.
You know what happens when you strike it with with a drum stick.:smile:
That's high Q.

I will agree with Tom on one other thing: M300 is more of a panacea to me for rods.
If it's too hard it approachs a brittle situation,forsaking toughness I think I'd rather see.

Regarding the triangle shaped beam, the simple answer is NOPE. In 30 years of examing rods both broken and unbroken, in 99+% of the cases where a rod broke and there is no sign of detonation, the rod fails just below the wrist pin. Detonation tends to break them off just above the big end. Based on the photos that were posted, we cannot tell where Mike's rods broke. The lab sectioned the beam just below the pin and knowing this lab, I suspect they did this to look at the fracture surface under a SEM (scanning electron microscope). I have received Mike's rods and spent a little time looking at them today. We may know more shortly.

Tom

 

[BlownV6]

From my experience with rod failures , I am quite convinced that at the HP level that I am at , the beam needs to be as thick as possible to help stabalize the beam structure. Much like modern thinking on pushrods , bigger , beifier & badder is better!! Forget about the weight!! Provide what ever it takes to try to stabalize the column/beam from having any harmonic action!! The harmonic action is transmitted throughout the entire rotating assembly and will help lead to bearing and crankshaft type failures.
Mike

 

[lazaris]

From my experience with rod failures , I am quite convinced that at the HP level that I am at , the beam needs to be as thick as possible to help stabalize the beam structure. Much like modern thinking on pushrods , bigger , beifier & badder is better!! Forget about the weight!! Provide what ever it takes to try to stabalize the column/beam from having any harmonic action!! The harmonic action is transmitted throughout the entire rotating assembly and will help lead to bearing and crankshaft type failures.
Mike

Finally. X2!!! More important than I or H beam.

 

[Alky V6]

From my experience with rod failures , I am quite convinced that at the HP level that I am at , the beam needs to be as thick as possible to help stabalize the beam structure. Much like modern thinking on pushrods , bigger , beifier & badder is better!! Forget about the weight!! Provide what ever it takes to try to stabalize the column/beam from having any harmonic action!! The harmonic action is transmitted throughout the entire rotating assembly and will help lead to bearing and crankshaft type failures.
Mike

I agree, Mike. It's similar to the strategy used with driveshafts. To help raise the natural frequency point of the shaft, the diameter is increased, wall thickness is increased, or shaft length is lessened.

 

[K1tom]

From my experience with rod failures , I am quite convinced that at the HP level that I am at , the beam needs to be as thick as possible to help stabalize the beam structure. Much like modern thinking on pushrods , bigger , beifier & badder is better!! Forget about the weight!! Provide what ever it takes to try to stabalize the column/beam from having any harmonic action!! The harmonic action is transmitted throughout the entire rotating assembly and will help lead to bearing and crankshaft type failures.
Mike

You are right and I agree but you should hear the phone calls I get from people that won't use a rod that is 20 grams or a crank that is 2 pounds heavier than a different brand because they think they will never win with the extra weight. When trying to move 2,500-3,000 pounds of car, a few grams here or there simply does not make much difference on the track especially when they are running a bracket program where they are competing against their own time.

 

[Alky V6]

Just for clarification I'm going to assign names to the 3 axis involved in decribing the rod in 3D space.

Let's lay the rod on a flat piece of paper on the top of a bench. The large end towards us, the small end away from us.
Bending over the rod and looking straight down onto the rod, let's draw a line from our left to right on the paper. That will be the x axis.
Still looking straight down onto the rod, let's draw a line perpendicular to the first line. From top to bottom of the rod. That will be the y axis.
Taking the pencil and standing it straight up on the paper through the bore of the big end of the rod, the pencil will illustrate the z axis.
Pairing 2 of the axis names, we can now describe a plane. For instance, the piece of paper would illustrate the x-y plane.
If one were to take another piece of paper and lay it on an edge perpendicular to the first piece of paper with the edge oriented from top to bottom of the rod, that would be the y-z plane.
If the second piece of paper were turned 90 degrees so that the paper were held straight up on one edge, but now the edge traveled from left to right across the rod, still perpendicular to the first piece of paper, that would illustrate the x-z plane.

Using these titles to describe the axis and planes, I believe the most common failure of a rod is seen with the rod bending along the x-y plane.

The picture of Robert's rod would be bending along the y-z plane. Bending along this plane is more prone to occur due to offset between the crank pin and the cylinder bore centerlines (explained earlier in the thread). Hence, the orientation of the beam flanges as with the H beam rod is more preferred in an offset crankpin situation because the H beam resists bending along the y-z plane better than the I beam rod.

If the orientation of the flanges on the beam of the rod dictates which plane the beam will resist bending the best. What does that say of the other plane?
If a H beam is orientated to resist bending along the y-z plane the best, what is protecting it from bending in the much more common failure plane, the x-y plane? remember the building construction example explained early on in the thread?

 

[K1tom]

Just for clarification I'm going to assign names to the 3 axis involved in decribing the rod in 3D space.

Let's lay the rod on a flat piece of paper on the top of a bench. The large end towards us, the small end away from us.
Bending over the rod and looking straight down onto the rod, let's draw a line from our left to right on the paper. The will be the x axis.
Still looking straight down onto the rod, let's draw a line perpendicular to the first line. From top to bottom of the rod. That will be the y axis.
Taking the pencil and standing it straight up on the paper through the bore of the big end of the rod, the pencil will illustrate the z axis.
Pairing 2 of the axis names, we can now describe a plane. For instance, the piece of paper would illustrate the x-y plane.
If one were to take another piece of paper and lay it on an edge perpendicular to the first piece of paper with the edge oriented from top to bottom of the rod, that would be the y-z plane.
If the second piece of paper were turned 90 degrees so that the paper were held straight up on one edge, but now the edge traveled from left to right across the rod, still perpendicular to the first piece of paper, that would illustrate the x-z plane.

Using these titles to describe the axis and planes, I believe the most common failure of a rod is seen with the rod bending along the x-y plane.

The picture of Robert's rod would be bending along the y-z plane. Bending along this plane is more prone to occur due to offset between the crank pin and the cylinder bore centerlines (explained earlier in the thread). Hence, the orientation of the beam flanges as with the H beam rod is more preferred in an offset crankpin situation because the H beam resists bending along the y-z plane better than the I beam rod.

If the orientation of the flanges on the beam of the rod dictates which plane the beam will resist bending the best. What does that say of the other plane?
If a H beam is orientated to resist bending along the y-z plane the best, what is protecting it from bending in the much more common failure plane, the x-y plane? remember the building construction example explained early on in the thread?

The simple answer is, an H-beam is more prone to bend in one axis and the I-beam is more prone to bend in the other. Like I said earlier, MOST rods do not break in compression they break in tension. In a case of detionation or an actual overload / buckling failure, the rod will bend in the direction of least resistance regardless of the beam type.