H beam vs. I beam rods

[Alky V6]

I thought this would be an interesting subject to discuss. I have some interesting theories and ideas that pertain to this subject that some may find interesting.
I want to make it clear that I am not an engineer and don't have a lot of field experience building high output engines using different rod designs and seeing any significant number of connecting rod failures, but the question has been burning in my mind lately and I have come up with some interesting deductions that may pertain to this age old question.

Which is better in a high output supercharged application, H beam or I beam? And why?
Let's say around 1100 bhp and higher in a V6 engine, and keep the discussion mainly about steel rods.

 

[Blazer406]

I thought this would be an interesting subject to discuss. I have some interesting theories and ideas that pertain to this subject that some may find interesting.
I want to make it clear that I am not an engineer and don't have a lot of field experience building high output engines using different rod designs and seeing any significant number of connecting rod failures, but the question has been burning in my mind lately and I have come up with some interesting deductions that may pertain to this age old question.

Which is better in a high output supercharged application, H beam or I beam? And why?
Let's say around 1100 bhp and higher in a V6 engine, and keep the discussion mainly about steel rods.

Laz told me his take on this awhile back...... only big massively built i-beams for any major HP maximum effort buildup.....His take was the h-beams twist too much......

He mentioned one thing that stuck with me.... think of it like this...... a 1200 HP V8 is making 150HP per hole...... it only takes 900HP V6's to get to 150hp per hole...... once you step to a 1400 hp v6..... you need a similar strength rod that someone building a nearly 1900 hp V8...... you need lots of cross sectional area.....which means more weight.... these off the shelf relatively lightweight rods are asking for trouble.

Something to think about.....

 

[Alky V6]

That all makes perfectly good sense to me. I agree with all of it.

I also agree with the idea that the H beam rods twist more, but why would they twist more? What is it about the structural differences that would cause the extra twisting in a H beam over an I beam? Assuming the cross sectional area was the same.

 

[Alky V6]

The forces put on a rod are;
Tension
Compression
Bending
Twisting

It's easy to understand where the tension and compression loads come from, but what about the twisting and bending forces?
How do the tension and compression loads contribute to the twisting and bending forces on the rod?
How does the wrist pin contribute to bending forces? What about the big end?
In what planes are the bending forces put on the rod?
How well do the different rod designs handle the bending forces in certain planes?

 

[Alky V6]

If one were to put up two saw horses 10 feet apart and lay a steel I beam across them, which direction would you want to lay the I beam to give the best support when a heavy weight was layed on the beam midway between the two saw horses? Which direction would you want to lay the I beam so that the I beam deflected the least amount and resisted being bent by the weight?

 

[Blazer406]

If one were to put up two saw horses 10 feet apart and lay a steel I beam across them, which direction would you want to lay the I beam to give the best support when a heavy weight was layed on the beam midway between the two saw horses? Which direction would you want to lay the I beam so that the I beam deflected the least amount and resisted being bent by the weight?

I am going to try and keep this in as simple terms as I can as some people will loose interest in this thread if it is too complicated.

Referring to the "Manual of Steel Construction" most engineers refer to it as "The Green Book"..... it has tables for the properties of many structural shapes..... including I-beams. Two particular properties listed in the tables that engineers refer to..... to "pick" a particular beam size based on a load.....are "moment of inertia" and "section modulus" These values directly relate to the beams particular strength in a particular direction..... i.e. your hypothetical situation on the I-beam on the sawhorses above. There are separate values for relative strengths about the "x-x" axis and the "y-y" axis.

A general note: In the green book.... the "s" beam is what most people call an "i-beam". From this point forward... I'm going to refer to the beam as an I-beam so as to hopefully not confuse anyone.... I hope.

If you break an I-beam down into it's basic parts that make up a shape..... there are two "flanges" and one "web"..... with the web separating the two flanges. I-beams are significantly stronger if turned right for the given load..... in the case of the sawhorse example..... you would want one flange laying flat on the sawhorse.... and the web sticking up vertically... with the top flange parallel to the bottom flange..... make sense?

Anyway.... to give you some idea of the relative strengths of i-beams in general if you want to compare the "values" of section modulus or moments of inertia.... you can easily see how much stronger the i-beams are if loaded correctly....

Looking at a small 4" x 7.7 I-beam.... (7.7# per foot).... the moment of inertia in the "strong axis" which would be loaded like above with one flange laying flat on the sawhorse.... is 6.08 in^4. But if you loaded it on the weak axis.... the moment of inertia is only .764 in^4.

This should give you some sort of "feel" for how much stronger an I-beam is in one direction vs. loading it in the other direction.

In the case of the "twist" that I mentioned in my post above.... I would think you would find after some lengthy engineering analysis....that the "h-beam" rods are weaker when the load moves from loading one axis to another.... i.e. a twisting load....

One observation is that when you compare the two different rods and how they are installed in a motor.... the i-beams have their "flanges" turned 90° from the way the h-beam rods are when installed in the motor. How does that come into play? I am not real sure without some 3-d modeling..... which I don't have time to do at the moment...

I hope I didn't loose anyone...

 

[cruzn57]

additionally

another stress factor is stretch, when the piston reaches TDC, the rod pulls the piston back down, it was going up, now must do a 180 degree, on the power stroke its blown down, on the other stroke its pulled down
I'm reading this in hopes of learning more!.

 

[Alky V6]

Excellent, Blazer406! That confirms my thinking. I didn't realize the differences in strength between the two orientations was so great. Very important.
The big question is, which axis is going to see the highest bending forces in a typical very high output internal combustion engine? Which axis would have the greatest chance of seeing higher bending forces?

Other forces contributing to the loads on the rod are;
Vibration
Resonance (ordered vibration)
Component natural frequency vibration, such as the phenomenon experienced with simple driveshafts. This would also factor in the material of the rod.

cruzn57. The piston attemting to stretch the rod at TDC on the exhaust stroke would be putting the rod under tension. Tension is the term for stretching.

 

[Alky V6]

What forces during the operating cycle of the rod could contribute to bending forces being imparted on the rod? Wouldn't it involve the bearing surfaces at either end?
What direction would the bending forces be in?

 

[Mike Licht]

What forces during the operating cycle of the rod could contribute to bending forces being imparted on the rod? Wouldn't it involve the bearing surfaces at either end?
What direction would the bending forces be in?

It depends on the crank, in the case of a stock crank that twists quite a big the loads are quite a bit different than than a forged crank.
Mike

 

[Alky V6]

It depends on the crank, in the case of a stock crank that twists quite a big the loads are quite a bit different than than a forged crank.
Mike

Let's assume a 1200+ bhp V6. All the best parts. This is a max output situation. High cylinder pressures.

 

[Alky V6]

Let's concentrate on the piston wrist pin. I think that is the area that would have the greatest chance on imparting a bending force to the rod.
High cylinder pressures.
Maybe a wrist pin size that is undersized for the amount of cylinder pressure that will be seen. Not to mention, the occasional tuning mishap that might cause a spike of pressures above normal.

Under load, how does a wrist pin deflect?

 

[Alky V6]

Here are more interesting questions to ponder.
When a rod failure has occurred, what axis does it appear the bend is in?
When a person polishes a stock rod in an attempt to control stress risers, which axis does the process focus importance on?

 

[Alky V6]

More questions.
OEMs that employ serious engineers have typically relied on what rod design over the last century?
When you catch a glimpse into a serious OEM high performance engine, or a purpose built over-engineered high performance engine, such as F1, which rod design is most often seen?

 

[SS/GN]

I have noticed most hi HP rods are H beam(look on summit)Polishing stock rods has been proven to show no benefit just something done in the old days.Good thread Don thanks

Kevin

 

[Alky V6]

I have a book that covers the 50 greatest motorsports engines from 1913 to 1994. The title of the book is Classic Racing Engines by Karl Ludvigsen.
I was curious and thumbed through the book and found that only one of the engines used a H beam rod. The reason why a H beam rod was used in this case, it was "favoured by some designers for high speed engines because they place less mass at the periphery of the rod." No mention of strength attributes.
Peak power of this particular engine was at 11,000 rpm.
The engine, the remarkable 1984 Renault EF4 1.5 Liter twin turbocharged V6.

 

[SPOOLFOOL2]

Great thread.

It is interesting that the preferred way to carry a load in a building is with a "I" beam, but the preferred way on a steel conecting rod is an "H". I wonder if there are differences in parasitic drag with air or oil that my be involved. It "may" also be that the load on the H beam transfers better to the ends of the rod then the I beam. On a side note. My guess is that the "I" beem is so common in production cars, because it's cheaper to manufacture than an "H".

Mike Barnard

 

[GARY HARVEY]

Beam ground (polished) rods are not going to contribute much but if you fulfill the equation and shot peen, you will improve the overall strength and shear resistance of a rod.
In spite of the fact that in today's market place you can purchase high performance rods for reasonable cost and not have to deal with historical methods of rod preparation.
There are other process's that have advanced rod technology btw if you would like to fall back onto prepping your own rods, which I sometime use.

 

[Alky V6]

Great thread.

It is interesting that the preferred way to carry a load in a building is with a "I" beam, but the preferred way on a steel conecting rod is an "H". I wonder if there are differences in parasitic drag with air or oil that my be involved. It "may" also be that the load on the H beam transfers better to the ends of the rod then the I beam. On a side note. My guess is that the "I" beem is so common in production cars, because it's cheaper to manufacture than an "H".

Mike Barnard

Where do you get your information that the 'preferred' con rod is a H beam?
The book I mentioned in my last post covers the most exotic engines ever built. I'm sure money was no object in their development. The con rod design, arguably the weakest link in the engine, picked for 49 out of 50 of the engines, was an I beam design. Do you really think that all these engine designers would put an inferior con rod design in their exotic creations?

It's interesting that in this same book, the author specifically states what rod design is used in most, if not all of the engine descriptions. Where he possibly didn't mention the rod design, there is a cutaway drawing of the engine where you can cleary see what rod design was used. It appears that the rod design is of some importance. At least it seemed to be for the author.
I'll do some reading of the other engine descriptions to see if there is a mention of the attributes of using an I beam con rod.

 

[Alky V6]

Beam ground (polished) rods are not going to contribute much but if you fulfill the equation and shot peen, you will improve the overall strength and shear resistance of a rod.
In spite of the fact that in today's market place you can purchase high performance rods for reasonable cost and not have to deal with historical methods of rod preparation.
There are other process's that have advanced rod technology btw if you would like to fall back onto prepping your own rods, which I sometime use.

Please don't mistake my mentioning of con rod beam polishing to think that I feel this method to be valid in todays world. I brought up con rod beam polishing to show where the importance was placed by the early performance engine pioneers in trying to make a production con rod live under high stress use. Obviously, those flange surfaces of the beam of the rod were the first areas to develop stress cracks under hard use. That goes to show where a lot of stress is placed on the beam of any con rod.

Todays specialty con rods are a far cry from the days of polishing OEM con rods where a much better material and manufacturing procedure is used, and care is taken to distribute the proper amount of mass of the rod in strategic high stress areas of the rod.

I brought up the polishing of the I beam only to illustrate where the early engine builders felt were the high stress areas of the con rod.

So, if the flange of an I beam rod is considered a high stress area for a connecting rod, would you want to add mass, better the design and/or material there at the flanges to help control the stress forces? Or, would you take away mass, as a H beam rod would be doing, and move the majority of the mass to the more central area of the con rod as hp levels were increased?