H - Beam vs I - Beam Rods
 

All high end rods seem to be an H - Beam construction and I have always wondered what is wrong with an I - Beam.

I looked on the web but can't really find any good arguments one way or another.

I beams are like stock rods, so they don't look as cool as the H beams. From an engineering stand point, the I beam is a very efficient structure. I would bet the H beam matches the I beam in moment of inertia, but I have not looked this up so it is speculation on my part.

The H beam (like R+R) has a greater area moment of inertia than an I beam on the axis that the rod will rotate. Stiffer per weight so it transmits power better to the crank or can be lighter. Trade off is less stiffness in the other direction, where there are not so many loads (loads along an axis parallel to the crankshaft axis).

How do Pauters fit in?
 

Max

You mean the I beam (like Pauter?) is stiffer in the rotation axis for its weight than the H beam (e.g., Carillo)? And what is a stock rod? Or do I have these mixed up?

How does "area" MOI differ from MOI.

If you will forgive that question from a non-engineer.

MOI seems difficult to calculate for other than simple disks. I once asked one of my ME buddies if there were a simple way to just measure MOI rather than try to calculate it for complicated shapes. He didn't know off hand, though we agreed that for something which just rotates you could wind a string around it, put a weight on the string, and see how long it took for this or that (say one revolution, or X drop of the weight) to happen, fudge for friction, and get a number.

For rods there is MOI and reciprocating weight as well. Even more complex calculation for MOI?

Or is it half-way simple, like rod big end weight contributes to the MOI of the crank? And the rod doesn't really have a separate MOI?

If Pauter is neither H nor I, but X or something, this leads to the question of why they chose that design. It is said that the Pauter has less windage. They obviously are strong enough for the job, and light enough (until you get into titanium), and because they cost less than the Carillo (maybe Carillo clones have arrived and are as good?) a lot of us have purchased them. And Pauter has engineering skills.

But how they are strong enough for their size (and especially weight) in radial plane bending, where they are thin compared to both stock and Carillo styles, is something I've not seen explained.

At least to my satisfaction. That's a slippery concept since I don't have the training to understand this stuff in depth (pun intended). But the concept that the deeper the beam the stronger it is in bending (in that plane) seems understandable enough, and I've seen it in action when I built my house, which was my introduction to such concepts.

Area moment of inertia is a measure of how far out from the centroid the area of a cross section is distributed: a hollow shaft is stiffer than a solid shaft for the same strength (and weight).

This is an "H beam connecting rod" to me:
http://forums.pelicanparts.com/uploa...1285367479.jpg

I consider this an "I beam connecting rod":
http://forums.pelicanparts.com/uploa...1285367520.jpg

(Photos courtesy of R+R)

The mass moment of inertia about an axis parallel to the crankshaft will be pretty close to equal between H and I rods of equal strength. Just think of spinning the rod around the big end, little end, or middle, they should all have a similiar trend.

Because of the distribution of that mass, though, the H-beam rod will be stiffer at an equal strength.

Absolute measurements of mass moment of inertia are not really necessary. You know less is better and without changing the metallurgy, the only way to do it is to re-shape the mass so that it is centered on the axis. Because the axis of rotation changes throughout the stroke, you have to trade off putting the mass on the centerline of the rod (like the Pauter) or you can have a stiffer rod, or lighter rod for the same stiffness, by putting the mass in an I-beam or H-beam. The linear inertia is not effected.

The best balance between stiffness and mass moment of inertia may be a hollow elipse or rectangle cross section. Due to the increased stiffness, material can be removed from the total and therefore the total mass will be less. This would partially offset the new distribution of the mass and help reduce the mass moment of inertia.

Some interesting discussion on this topic:

Finally, an analysis of I-Beam rods vs H-Beam rods • Speed Talk

H and I ( or W,M,S, and HP) sections are similar... a quick check of properties in AISC MOSC ASD (green) which is what I have handy.. reveals that while the shapes are very similar, the properties are different..

Which makes me wonder if the OP Is referring to the cross section of Pauter rods, which more of a cross, than the similar sections of the R&R or Carillo rods..

Yes, Pauter is neither I nor H in my nomenclature.

The Pauter design is called a cross beam design. This type of rod was developed way back in World War One for aviation applications. The major problem with the design is poor tortional contol of pistons, especially larger diameter/ heavier. The Pauters seem to have a good reputation for reliability, but I perfer an A or H beam designs over the WW1 stuff.

H Beams BTW were developed in WW2 and are currently used in Nascar/ Formula1 programs

Very entertaining debate, finally turning a bit rude :eek:

I agree with max's definition of H and I beam and having spent some time looking, I think I prefer the I beam but would agree that they don't look as good as the H - Beam.

I looked at the Oliver Connecting Rods Website and I think their arguments about pin oiling make some sense.

Below is a used Ti connecting rod made by Pankl and Piston made by Mahle for one of Michael Schumacher's Typo 049 Ferrari F1 engines (given to me by my wife as a gift).

Note an I beam is chosen for the design.

http://forums.pelicanparts.com/uploa...1286137542.jpg

Nice wife! :eek: :)

Is that an optical illusion or is that rod really that big? How do they get that thing to do 19,000 RPM?

Optical delusion:

Next to GT3 Mahle piston and its Pankl Ti rod (also an I beam) for comparison :

http://forums.pelicanparts.com/uploa...1286147659.jpg

I noticed the GT3 rod is also an I beam. What do the factories know that we don't?

My guess is that given Titanium's propensity to fatigue, the I Beam has a longer work life, as the H beam design appears to have significantly more stress risers surface area along the spine of the rod than the I beam. Additionally, the higher stiffness and lighter weight of Titanium likely mitigates much of the moment of inertia problem when comparing steel/moly rods between the 2 designs.

We are comparing apples to oranges between Titanium and Steel/Moly.

Can you please explain more about your stress riser area theory? I do not fully understand how the two differ appreciably in this aspect.

The reveals that are in the sides of the H-Beam rods result in a much thinner material thickness than anywhere on an I-Beam rod. My thought is that these margins would be prone to stress fracture and subsequent crack propagation.

I think I may have misappropriated stress-riser as the term for my purpose. Nonetheless, moly steel won't fatigue, Titanium will and fatigue is a function of cycles and material dimensions. In an H-Beam rod, this is not a good thing for Titanium.

Before the debate of I vs H gets out of control, both types of rods are actively used in Nascar and Formula 1. Both yield acceptable reliability. The thoretical arguments do not seem to translate to one design over the other in real world applications.

Also, the Pankl GT3 rod is a design of economy. The "net forging's" intent is to minimize machining, yeilding an economic, yet robust product. It's not easy to do and my hat goes off to them.

OK, I see. Less buckling stiffness because of the area moment means more strain, more fatigue.