Yes and very articulately stated - far better than my attempt at linguistic efficiency.

Well.. that is not correct..

I have no dog in this fight, but am seeing some suppositions that are interesting..

All steel alloys.... well really all alloys fatigue..

Web stiffness is a big consideration is design of columns or beams..

Is a con rod a column?

Yes..

Is it a beam?

Yes..

An H or I section obviously works... and will work to some very high standards..

It would be interesting to hear what a designer at Pankl, Arrow, Carillo, Pauter say about there selection for rod design..

Well, given infinite cycles, EVERYTHING fatigues. Steel alloys tend to be much more robust than titanium in this regard.

And given finite cycles everything...

con rods are interesting

radius of gyration .....

compression..

tension..

bending... (hopefully not)

There's 'infinite' and there's infinite...

Most steel alloys, under 'low' enough stress loads can undergo damned near infinite cyclical loading... but there is a threshold stress level where damage will begin to accumulate. Unfortunately that threshold level is pretty damned low - anything weight limited isn't going to be built to that spec unless its absolutely necessary.

Aluminum, (and some other metals/alloys, notably TI) don't have such a threshold stress. If you take aluminum and cyclically load it, it will fail and it doesn't matter how small the load is. Its a particularly annoying fact in the aerospace industry.

Note: I'm not an aerospace materials specialist... but I did stay at a Holiday Inn Express.... I mean I work with people who specialize in aerospace materials.

The discussion about the fatigue behaviour of Titanium is quite interesting and there is a huge amount of published data.

It is possible to show that compared to very high quality heat treated steels such as S300M that a typical 6Al4V does have a slightly increased fatigue crack growth rate but this criteria is not really valid as rods are not designed to support even short fatigue cracks.

It is fairly safe to say that once a fatigue crack initiates in a rod its life would be very short and the rod would be scrap.

It would be conventional to design a steel con rod to not have stresses which exceed the Endurance Limit of the material chosen.

4340AQ seems a common choice and quite good, although I think S300M would be better.

The fatigue endurance limit is the stress at which the material will never initiate a fatigue crack and a copmponent designed using this criteria would have an infinite fatigue life.

Titanium and Aluminium do not exhibit an endurance limit. If you keep cycling them they will eventually accumulate enough daamge to cause them to initiate a fatigue crack. If, howwever, designs stresses are low enough this may not be an issue.

A Pankl GT3R rod has a quoted racing life of 40 hours but Honda are confident enough to use a Ti Rod in a production NSX. Clealry the rod stresses must be quite different.

Without a significant amount of work and information regarding mean stresses and stress range it is hard to draw real conclusions about life but I tend to think that we are generally being over conservative and worry too much about well designed Ti rods.

In real I would worry more about galling between the rod and the big end bolt, and damage caused by this process would almost certainly cause surfae damage and subsequent fatigue failure.

Titanium does of course only have around 50% of the stiffness of steel components.

In broad terms the stiffness to density ratio of steel, aluminium and titanium are all the same.

I'd love to jump in here, but it's like 338am! -_-... And I depart shortly back to the South of Calif. Away I got from my beloved Northern California. Anyways, to be honest we use both I and H beam construted items. It depends ofcourse on the specific engine, H, V, I, etc along with bores and strokes, for example, some of the inline fours we developed have cylinder walls not capable of withstanding increased bores or simply become unstable with running at max bores. So depending on rules or regulations we might select a I or H beam on a given engine because weight or other aspects are a factor. In short I'd love to continue this, but now I really must run! Addictive read and cool subject! One I have rather enjoyed and the Pelican-Users who's wife hooked him up with a Pankle F1 connecting rod is pretty cool. ^_^

I'll try and post some of the past connecting rods from some of my past racers, F3 engines, or Japanese super endurence cars etc.

Recent Formula 1 rod designs...
http://forums.pelicanparts.com/uploa...1286371100.jpg
http://forums.pelicanparts.com/uploa...1286371127.jpg
http://forums.pelicanparts.com/uploa...1286371164.jpg

Looks like a combination between I and H.

Chris_seven, I am intrigued by your comment that Aluminum, Titanium, and Steel all have nearly identical stiffness to density ratios. Are you saying there is no real benefit to using Titanium as far as stiffness to total weight is concerned?

My understanding was that a particular part must be designed to withstand a particular stress and so needs a certain strength. When building a part out of Aluminum at this strength, there will be a greater volume of material than with Steel, and the area moment of the cross section will be greater in certain instances. This makes the Aluminum part stiffer for a given strength, even if there must be more of the Aluminum, which offsets some of the savings in mass, though not all.

Flieger - you are on the right track... Al, Ti, and Steel may have the same stiffness/density ratios, but the geometry comes into play.

There are times when a larger part using a lower density material that allows for a physically larger part results in a lighter part. Its certainly not as simple as taking the current design and changing the metal.

In my materials science class, we looked at some composite materials to withstand a large compression load. One particular design was very counter-intuitive. It worked by using a physically large block of lightweight material to disperse the load, reducing the stress to levels the material could withstand. In the end, it was lighter and stiffer than any of the conventional metal designs.

Now, in the case of engine rods, I doubt you will see composites any time soon, the loads are too multi-directional. For the time being Al and Ti will remain the top-end.

This thread is great. All I see is "Blah, Blah, blah blablablah, blah bbbbbblahh, blah." Way over my head but I use Pauter rods... LOL

Funny...I bought the R&R rods to build a motor. Ran out of time. Bought a motor. It has Pauter rods. Nothing to read into here. Just...randomness.

Max,

I am sure that Titanium Rods are used because they are lighter but size for size they will not be as stiff as a set of steel rods so it all depends on your design criteria. The point I was making is that just looking at strength/weight isn't the complete picture.

I have not seen many Ti rods that seem to have a much bigger cross-section or MOI than a steel rod so they will deflect more for the same inertia force - obviously the lighter rod will help - but weight must be the most important criteria.

Obviousy Honda are confident that they have designed a rod where the stresses are low enough not to worry about fatigue at least for the life of the NSX engine.

If space is not an issue then obviously a bigger cross section can help to get the stiffness back and a simple comparison of the I beam depth shows, at least qualitatively' the effect of geometry and weight.

Some of the advanced composites have huge stiffness to density ratios and I am surprised no one has tried Metal Matrix Composites yet.

They can have densities similar to aluminium with E values greater than steel and tensile strengths of around 900GPa. They can be a bit directional but I think some of the vacuum casting or powder meatallurgical techniques now being used can help in this area.

I would ceratainly think they would make great for pistons as the hot strength can be remarkable good.

It could also be possible to investment cast rods so that they had a hollow section and reduce weight in this way.

The reduction on RPM that F1 has introduced will probely mean that these type of developments won't happen. The 24000 rpm that was being considered a few years ago may have needed these types of approach.

Thank you, Chris. So if one were to "give back" some of the weight savings and have a dimensionally larger rod than needed on a strength basis, the rod would be stiffer in Titanium or Aluminum. The stress dictates the mass of material needed, and Aluminum and Titanium are less dense than steel, so they will be stiffer for that mass. The masses of the rods (with equal strength) would be very similar, but the stiffness would vary due to the differing cross-sectional area and area moment. Is this correct- that the mass would be "the same" if all the metal rods were of equal strength on an ultimate yield stress basis?

Are you saying the mass moment of inertia about an axis parallel to the crankshaft be the same if the rods are simliar mass but the Aluminum or Titanium rods have that mass distributed further from the axis of rotation?

This is why I have always understood the Engineers when they say that you cannot just change the material, you have to change the design, as well. Porsche found that out with Aluminum and Carbon monocoque experiments on the 956.

Just plain beautiful....

Priceless!!