Connecting rod shape 3

[jriordan]

I cannot find any analysis of a I-shaped connecting rod vs. a H-shaped. The H-shaped rods seemed to be used in the more expensive applications, but I cannot figure out why it should be preferable.

 

[MaxRaceSoftware]

a very simple analysis =>

H-Beam design is used were engine expected not have any
chance of damaging-detonation , its stiffer in that plane
and would increase damage

H-Beam = NASCAR , etc

I-Beam = more flex in that plane, more forgiving , so used more in Nitrous classes where exact tuneups are harded
to get quickly, and more detonation chances

the H-Beam is better when things are perfect or close to

Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[patprimmer]

"H" shaped are easier to machine from billet, especially before CNC mills were common, while "I" shaped is probably a bit easier to forge.

The "H" style was made popular by the excellent quality of the rods made by Carillo from about 30 or 40 years ago, and the fashion continues, probably as it is "obviously different" to stock rods.

Technically they are equal strength to "I" rods under tension, which is were virtually all rod failures occur in normally aspirated engines.

Under high compressive loads, like in high boost forced induction systems, "H" beams have a slight dissadvantage as the long thin ribs can flex and allow the rod to bend.

Regards
pat

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[strokersix]

In school one of my professors spoke of torsional stiffness of connecting rods as a forgotten design consideration. He said that there are some engines where torsional excitation of the piston/rod/crankpin system can be an issue. Unfortunately I don't recall which engines exhibited the problem but I suspect they were larger than automotive type. A tubular connecting rod construction can be used to provide acceptable stiffness in axial, bending, as well as torsional load cases. I suspect the real reason is the manufacturing method used for automotive "H" versus "I" section rods, as Pat stated above. Anyone know of examples of engines that had trouble with connecting rod torsional stiffness?

 

[framorgal]

I have designed a conecting rod for sport aplication, the first design was a H shape rod, but due to the hight engine speed and load supported, the material stress of this design was very close to the limit, for this reason I tryed to design a second one but with I shape.
The result was that for the same weight, the I shape was around 20% stiffer, and the weigth distribution more closer to the ideal.
I'm agree with Pat arguments.

Paco

 

[EnglishMuffin]

In "The Internal Combustion Engine in Theory & Practice", Vol 2, C.F. Taylor says:
"The theoretical loading on the connecting rod is easily calculated ..... the actual stress on the rod column may be far greater than such a calculated value on account of unsymmetrical loading, vibratory stresses (including the piston in rotation), and stress concentrations. Heavy unsymmetrical loading comes from the piston axis being out of line with the rod axis, more or less common to all engines. It has been found that such loading may cause greater bending moments in the rod than "whip", and it is one argument for orienting the "H" section of the rod 90 degrees from the usual direction. This design also simplifies forging and machining in many cases".

 

[StefanH]

Check out this ferrari F1 rod (picture is from Desmo at AtlasF1, so thank him for it, not me!):

 

[Andy330hp]

Picture doesn't display, is there a link?

 

[enginesrus]

I agree with that English Muffin.
So all arguements aside. How many failures have you guys/gals, seen in both styles? As someone mentioned Carillo rods were (are) the best. And they have always been
the 90deg I or "H". I have no experience with H but I have seen lots of "I" rods bent and in pieces.

 

[patprimmer]

No one said Carillo is the best. I said they were excellent. I think they were the best, but some have closed the gap and maybe even surpassed Carillo. It depends somewhat on application. What is best in a diesel truck is not always what is best in a F1, or a top fuel dragster or a sprintcar.

Crower, Manley, Argo, Precision Automation and Robotics, and even Eagle make some excellent rods now.

The F1 rod is an "I" beam, and I expect Ferrari could afford Carillos if they thought they were better suited to their engines.

The Ferrari rod probably takes nearly twice as long to machine as compared to a Carrilo.

Regards
pat

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[EnglishMuffin]

One of my only experiences with a bent rod was where a piston disintegrated in one of my engines - all that was left was the pin, but it had hit the smashed up piston pieces and the rod had become bent. What I think you are calling the H style, and Taylor is calling the 90 degree orientation, would have helped a bit, but the rod probably would still have bent. I once made some very large billet CNC machined 7075 Aluminum rods for a guy with a turbocharged pulling tractor. I made them with a simple rectangular section, but he had a detonation and one got bent. So it's not true to say that rods always fail in tension due to inertia forces, although my general impression is that racing guys do usually have tension failures. If you run the numbers, however, it is not obvious, at least to me, that this should necessarily always be the case. It also seems that the buckling strength is generally likely to be less than the tensile strength.

 

[EnglishMuffin]

My last sentence should have included the qualifier "in the part of the rod we are discussing". A lot of the tensile failure pictures that I have seen seem to be related to the big end (such as bolts or upper part of the bearing housing).

 

[Tmoose]

I agree with English Muffin.
A decade or so magnufluxing passenger car, motorcycle, and race car rods suggests crappily designed or poorly executed transitions between I/H-beam and big-end or wrist pin end have brought WAY more rods to their demise than the orientation of the beam section.
I think Taylor also points out the (Carillo) H beam lends itself nicely to the threaded rod instead of bolt-n-nut design, which tends to improve the beam-to-big-end transition a lot simply by removing the whopping notch for the bolt head.

 

[evelrod]

I have always used whatever worked best in the application at hand. In my case I have broken both Carillo "H" beam and Cosworth "I" beam rods on more than one occasion, unfortunately. Pat has it right on---the Carillo rods were "different" and due to quality construction and strength (compared to OEM stuff of the 50's and 60's) were perceived by the rodding public as 'superior'! My first choice has usually been Cosworth. Certainly NOT because of cost but, instead, overall weight was my main concern. The Cosworth rods could be machined to a lighter (in some cases MUCH lighter) weight. I still like Carillo and should I get even crazier than I already am, I will probably use them in my vintage mini. Not a big concern at it's present level of tune!

Rod

PS: The only rod I have ever seen 'bent' was from an early Chrysler Hemi drag engine. I think it lost fire and hydraulicked a cylinder. You will have to ask Pat on that one. V8's are a little out of my area.

 

[patprimmer]

Rod

The only noticably bent rod I ever saw was an OEM unit out of a 308 CI Holden V8. Similar story. It had a poorly installed water injection system, which siphoned water into a cylinder while parked on a slope. It hydraulic locked and bent a rod.

My comments re "H" vs "I" for high boost applications is precautionary, as to me the chance of the long thin ribs on the "H" are somewhat more susceptible to flex, and allow the beam to bend.

Regards
pat

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[EnglishMuffin]

There appears to be a definite difference of opinion here. It would appear to me that if the rod fails in bending due to off center compressive forces or even elastic instability, it is more likely to bend in the plane of the gudgeon pin (brit speak), since no significant moment can be applied to the rod in the orthogonal (pinned) direction, so surely the H design would be superior in this regard, as claimed by Taylor. MaxRaceSoftware and patprimmer appear to hold the exactly opposite view.

 

[patprimmer]

EnglishMuffin

Yes I hold the opposite view.

Even though the ribs on the "H" style are somewhat deeper, which should increase stiffness in the plane of the gudgeon, the ribs are so long and thin, that they are inclind to flex at the outside edge, alowing the beam to bend.

An 'I" beam puts a lot more metal in the area of the outer fibre when the rod is being bent in the plane of the gudgeon.

Much of this is based on intuition and anecdotal evidence.

I would really like to see some finite element analysis of a few structures. I might be converted.

Regards
pat

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[EnglishMuffin]

It sounds as though you are talking about lateral buckling of the compression flange, which is covered in Roark to some extent in the beam section. Might be interesting to run the numbers for a specific Carillo rod, or as you say carry out an FEA buckling analysis.

 

[EnglishMuffin]

By the way, I am somewhat confused about the I and H terminology. On this site :

http://www.billzilla.org/engcrank.htm,

the H style is defined to be the more conventional design, like the Ferrari rod in StefanH's post, and the I style is the type with the rod cross section rotated through 90 degrees. That's the reverse of what I thought. Which is correct ?

 

[patprimmer]

My understanding is that a Carillo is a "H" beam and a traditional OEM rod is an "I" beam

Regards
pat

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