A question about connecting rods 2

[dicer]

Ran across a discussion about the merits of I beam vs H beam con rods. It was a "which is best" question.
I would lean towards the I beam for the proper application of beam loading. I would like to see what you folks have say about this.

 

[ivymike]

The I design is easier to forge ('I' being the case where the middle of the 'I' can be seen when viewing along the pin axis, and H being the case where there is a small slot in the rod when viewed perpendicular to the pin axis). I don't think the 'I' generally enjoys any important advantage in regard to buckling.

 

[Lou Scannon]

Considering the plane of angular acceleration, the I-beam seems better. OTOH, the "width" of the rod is constrained by the bore spacing and the need to share space with the crank cheeks and other rod & main bearings. Thus perhaps making the plane of the rod beam longitudinal (i.e. "H-beam" design) improves the buckling strength in the inherently weaker (because the depth is constrained to be narrower) plane. Though I haven't done the bending/buckling stress analysis, my gut says a well engineered H-beam rod is stronger for this reason (considering that the flanges can be made as wide necessary for the required strength in the lateral plane).
BTW, I have a real world experience of rods being "squished" (shortened) very slightly, under the stress of knock, without any visible bending. I did a compression stress analysis that indicated this was indeed possible with cylinder pressures exceeding 2000 psi and the connecting rods in question. The plastic strain was approximately 0.020" in the worst rod, IIRC.

 

[patprimmer]

I thought I beam was easier to forge and H beam was easier to machine from billet.

I am of the unproven belief that the long thin ribs of the H beam are more prone to deflection under compression of the rod than is the centre of the I which is restrained by the shorter thicker ribs typical of I beams

Regards
Pat
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[Lou Scannon]

Good points Pat. I'd like to see the FEA on both designs.

 

[dicer]

I don't think the bending stresses have been considered.
I beams as in say a large gantry crane have the load situated in such a way that the flanges on the top and bottom are in compression and tension. So how accurate is FEA? Does it take into consideration the common equations for I beam vs H beam design? What is FEA based on? Old strain gauge measurements? Brittle Lacquer cracks?
I'm curious.

 

[Lou Scannon]

Not being a stress engineer myself, I perhaps naiively assumed that the latest methods (FEA) are an improved model over the macro empirical methods that were taught when I was in uni. Of course, as with anything, garbage in = garbage out.

 

[ivymike]

I suppose another important question to ask is whether buckling of a rod is a common failure mode. Seems like the only bent rods I've ever seen had a stuck piston or a flooded cylinder to blame. If you were to make a rod as light as you possibly could, perhaps the question would get more important.

 

[patprimmer]

Hydraulic lock is the main cause of bent rods, but once you get into high boost, reasonably high compression and exotic fuels that require very low air:fuel, like less than 5:1. This can get close to hydraulic lock and bent rods can happen if suitable rods are not used.

Regards
Pat
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[Tmoose]

In Charles Fayette Taylor's "Internal Engine" series from MIT press (Volume 2) there's a section on preferred con rod design. For the most part it is technically based on a collection of sources from various ( widely varying) industries from the 30s to the 80s. The bibliography is 122 pages.

Anyhow, his recommended (not mandatory) orientation is H-beam, like a Carillo rod. One of the explanations offered is heavy unsymmetrical loading can cause greater bending moments than inertial "whip".

The design details other than beam profile are more more important for durability, I'd say.
Most of the cracks in connecting rods I saw originated from the terrible profile ( a notch, really) to accept the rod bolt head in many stock rods. H beam rods almost invariably thread the rod and use a bolt coming up thru the cap. The resulting detail where the beam joins the big end is much more nicely shaped.

 

[larrylcoyle]

Like most real life applications, the "which is best" question evolves down to the actual application or end use of the rod. Obviously using a H-Beam Carillo rod in a slightly modified engine is not the "which is best" use of the rod for this application. You really have to allow for the actual end use application to determine the answer to your question don't you? Another major issue to consider is the weight of the connecting rod as this has a significant impact on the application. You are going to see the machined H-Beam rod to outweigh the traditional I-Beam rod. Talking about weight, the difference between a standard rod bolt on the Carillo rod and their SPS-CARR bolt can be as much as 28 grams per bolt. This is significant as well.

Larry

Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018

 

[patprimmer]

Apart from the head, I would have thought a bolt pretty much weighed the same as the metal it replaced, and all steel bolts of similar dimension were pretty much the same weight.

I have seen I beam rods that have cap screws from the bottom as well. It certainly does improve the strength at the shoulder which is a common weak spot on rods with through bolts.

To compare a Carrillo with an OEM I beam is not exactly a real comparison. To compare rods of equal quality and weight to see which is strongest is more relevant. Strength for bulk might also be an issue as is fatigue strength vs weight as it affects the material choice. The balance of these properties obviously need to be tied to the individual application.

Regards
Pat
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[dicer]

In my search, it appears, there has been no honest study other than FEA, on connecting rod bending forces. As far as compairing H vs I beam.
Does anyone have any photos of F1 con rods?

 

[GregLocock]

I don't have photos, but I do have some nice drawings by Tony Matthews of the conrods in Ferrari's F1 in 2000.

To my unpractised eye they look almost identical to a road car's.

"there has been no honest study other than FEA, on connecting rod bending forces."

I don't know what to make of that sentence. I've seen straingauged conrods going into engines, I rather imagine that the engineer involved discovered what he needed.

Cheers

Greg Locock

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[GregLocock]

http://me.engin.umich.edu/autolab/Projects/Completed/InCylinderFrictionMeasurement.html

Cheers

Greg Locock

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[ivymike]

I've seen piles of straingaged rods too, but most of the time the rod forces were of interest for reasons other than rod life.

 

[larrylcoyle]

patprimmer
You mention comparing a Carillo vs OEM I beam, how many OEM H beam rods are you aware of? The original question was comparing H beam to I beam. Someone mentioned Carillo and I do have direct knowledge of this product and others.

You statement regarding I beam rods using cap screws is the best solution when you can use it as limited by individual application rules.

Larry

Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018

 

[dicer]

Straingauged rods? How many where H beams?
I personaly only know of one application for a H beam rod in a old airplane engine, and it was chosen I think for the clearance space it provided for the 2 close articulated rods, and not so much because it was the strongest ie best as far as strength.
If you compair an H beam vs I beam con rod, looking at them straight on, that is in line with crankshaft centerline axis. H beam rods are always wider, so before jumping on the H beam band waggon, a fair comparison would be to have an I beam of the same width and flange cross section thickness. The I beam always wins, must be why all the OEMs' In large industrial engines win.

 

[patprimmer]

Larry, there are plenty of aftermarket or race quality I beams to compare to the Carrillos.

Regards
Pat
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[dicer]

Wow I wish there was an edit function.
I had someone at the door when I posted that last one. And looks like I really goofed up.
"Large industrial engines use them"