just some connecting rod info (read the links)

[grumpyvette]

http://www.vips.co.uk/demos/mech/con_rod/vm_anim.htm

http://www.grapeaperacing.com/tech/connectingrods.pdf

http://www.rustpuppy.org/rodstudy.htm

BOTH designs can be made extremely strong

both designs tend to have rod BOLTS fail before the RODS THEMSELFS

rods seldom fail in COMPRESSION, its the (CRACK THE WHIP) EXHAUST stroke where the piston is not working against any significant compression to slow it down as it approches TDC where the rods stretch and bolts tend too fail

I tend to use the 7/16" rod bolt (H) style rods even in SBC as they tend to have more rod to block and rod to cam lobe clearance and the larger dia. 7/16" rod CAP SCREW STYLE bolts that screw directly into the rod body are significantly stronger than the comon 3/8 rod bolts that use nuts on the bolts on the (I)beam or stock style rods.

but (I)beam rods are also manufactured that use the 7/16" cap screw rod bolts

almost any aftermarket rods are at least some improvement over the stock production rods strength

another thing to consider is that FORGED pistons with AFTERMARKET floating piston pins can be SIGNIFICANTLY STRONGER AND LIGHTER IN WEIGHT than cast/pressed pins thus greatly lowering stress at high RPM levels

its generally a failed valve train (no rod/piston combo can repeatedly try to compress a bent valve without major problems) or detonation thats the cause of problems if the rod bolts and lube system are not at fault!

things to read

http://www.hotrod.com/techarticles/82378/

http://www.vips.co.uk/demos/mech/conrod.htm

http://www.me.metu.edu.tr/me426/notes/Conrod/sld020.htm

http://www.stahlheaders.com/Lit_Rod%20Length.htm

http://www.autoink.com/SAHome.html

http://www.victorylibrary.com/mopar/rod-tech-c.htm

http://www.carrilloind.com/pdfs/10777_eprint.pdf

[Brad-ManQ45]

Thanks Grumpy - I've been wondering what would be best for a TT Chevy and now I know...

[Brad-ManQ45]

Thanks Grumpy - I've been wondering what would be best for a TT Chevy and now I know...

[pparaska]

This doesn't seem correct to me:

For compressive loads' date=' it is a little different.

When a rod fails from compressive loads, it bends.

The compressive loads are not straight; this makes

the shape of the rod critical to strength. The design

of an H-Beam rod helps them resist bending more

than an I-Beam design. Well, let me clarify that a bit,

an I-Beam rod can resist bending about the same as

an H-Beam, but the H-Beam can do it with less

weight.[/quote']

I'm sorry, but I have a big problem with those statements.

 

Assumption: The bending loads on a rod are due to friction in the rod bearing/journal interface, the piston pin/rod/pin bore interface, and rotary inertial effects of rotating the rod about the crank journal. The direction of that bending load is in the plane described by the length direction of the rod and a line connecting the bores for the rod bolts at the rod/cap interface. (In other words, a plane parallel to the work bench top when you lay a connection rod on it.)

 

Bending resistance of I-beam vs H-beam rods:

 

An I-beam is designed to resist bending by having the flanges resist tension and compression, and the web that connects them (the vertical part of the letter I) resisting the resultant shear between the flanges. That means that you use an I-beam to EFFICIENTLY resist bending by having the bending be in the plane of the web. Here is a picture of the I-beam, with the parts labeled:

 

Note that an H-beam rod has the flanges at 90 degrees to the typical bending load direction (plane), hence the flanges would be in shear and the central web would be in bending. Since the moment of inertia (the geometrical/mass parameter of the beam that contributes to bending stiffness) of an I-beam is much larger in the plane of the web than in the plane parallel to the flanges, the bending resistance of an I-beam rod would be much larger than an H-beam rod.

 

See the example on this page for and explanation: http://www.engineersedge.com/column_buckling/column_ideal.htm

I'm saying that the I-beam is designed to be used to resist bending about the axis Ix in that drawing. An I-beam rod follows that design philosophy. Note that you'd have to thicken up the web of an H-beam rod to make it's moment of inertia (Iy) for this loading condition equivalent to the I-beam's Ix.

 

An H-beam rod therefore has the flanges and webs in an inefficient direction for bending.

 

Buckling:

 

The resistance to buckling of a column is directly proportional to the moment of inertia of the column. For a column with moments of inertia of the cross-section that are different in different directions across the cross-section, you would use the lower moment of inertia, since it would have the primary buckling mode preference. Since the I and H-beam rods both have a high and low moment of inertia, you would have to look at the lower moment of inertial values for each to see which would go into column buckling first. You'd need the dimensions of the cross sections to calculate those.

 

The above ignores things like web and flange buckling and crushing. Maybe this is the where the H-beam rod design is superior?

 

I've heard the internet/magazine rumor that H-beam rods appeared because of the ease of maching an H-beam rod over an I-beam rod with common milling equipment. Since racers wanted a billet rod for higher material quality and properties, the H-beam billet rod came before the I-beam billet one.

 

Maybe I'm just really confused...

[grumpyvette]

PETE I can easily see your area of confusion, but the MAJOR stress loads on a connecting rod are NOT

"due to friction in the rod bearing/journal interface, the piston pin/rod/pin bore interface, and rotary inertial effects of rotating the rod about the crank journal."

those are MINOR loads,compared to the compressive loads durring the power stroke and the TENSION LOADS durring the exhaust stroke.

OR in many cases the compressive OVERLOAD generated when a valve gets struck and bent at high rpms effectively making the rod and piston into a hammer trying to force the bent valve into the cylinder head to get clearance to rotate.

next Id point out that by far the most comon failure is the rod BOLTS stretching under TENSION at high rpms on the exhaust stroke,that distorts the larger end bearing ,causeing it to bind on the crank or the bolts themselfs to fail under tensive stress, once that occures the rod tends to come loose of its journal and be beat to a pulp by lack of clearances and a 50lb-70lb plus crank hitting it , with impact loads in the thousands of ft lbs per impact and at up to hundreds of times in several seconds while it has no place to go while part of it is jammed into the heads or block, once that happends, the rods pounded into a pretzel almost instantly.

the cross sectional area of the two rod bolts rarely comes close to the cross sectional area of the connecting rod itself ,so as a result the rod bolts tend to be the weak link in the chain as they say.

 

"The con rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and relaxing with every rotation, and the load increases rapidly with increasing engine speed. Failure of a connecting rod is one of the most common causes of catastrophic engine failure in cars, frequently putting the broken rod through the side of the crankcase and thereby rendering the engine irreparable; it can result from overheating, a physical defect in the rod, lubrication failure in a bearing due to faulty maintenance, or from failure of the rod bolts from a defect, improper tightening, or re-use of already used (stressed) bolts where not recommended."

 

remember those discusions on (QUENCH) where I recomended a .038-.042 clearance between the piston and heads be maintained, well MUCH of that distance is clearance that is filled when the rod stretchs under tension, at higher rpms, in fact it can come close to being less than 1/2 that distance in the upper rpm ranges due to rod stretch

next time you purchase connecting rods consider that the 7/16" rods have approximately a 20% advantage in larger and stronger cross sectional area vs the 3/8" designs and that cap screw rods tend to have about a 20% greater area of thread engagement vs the nut style rods

picking a QUALITY rod design manufactured with good materials with 7/16" rod bolts of either type is vastly more important than just sellecting the cheapest deal available

 

info

http://em-ntserver.unl.edu/Mechanics-Pages/Luke-schreier/unzip/Tension%20and%20Compression%20in%20Connecting%20Rods%20VI.htm

[pparaska]

Grumpyvette,

 

I fully agree the compressive and tensile loads are large.

 

Note I said BENDING loads when I named the bearing and pin friction and rotary inertia.

 

I then went on to mention compressive loads as they relate to buckling.

 

I agree with the rod bolts being the failure point.

 

I was questioning only the part I quoted which only dealt with the failures

in the rod material during compressive loading, as it relates to I vs H beam.

[desert dog]

If the engine is designed correctly or sufficiently, the side loads and friction related load are fractional compared to the force of the controlled explosion on the other side of the piston.

 

This is why rod length and piston skirt length dimensioning vs. deck height is critical for a free-revving engine.

 

And regarding mechanical interference (dropped valve, piston fragment) the first offset impact usually bends the rod.

[dr_hunt]

This brings up an important fact regarding turbo charged cars where the piston actually see's resistance from incoming pressure on the exhaust stroke at valve overlap. According to Corky Bell's book, this lessens the tension load by alot and actually means that turbo'd engines actually only see 20% more stress than the same engine NA although the HP output can be as much as 3HP/cu-inch. Also a fact is that the tension load on a NA rod at 6K rpm vs. 7K rpm differs by about 135%!!!!! That folks is why they come undone and why 7K is a magic number for most V8's. Oldsmobiles and Pontiac V8's usually stay around 5K to 5500!

 

So, knowing that the rod bolt cross sectional area is significantly smaller, while being constructed of a superior strength material is crucial to longetivity. Therefore consider it insurance to use the best fastener you can buy or IMO the best and longest rod you can afford!

[pparaska]

Yes, I understand those bending loads are low.

 

The real issue for rod material failure (as opposed to rod bolt failure, which I agree is more likely in many cases) is the case of compression loading on the rod beam, and the issue of buckling failure, not bending of the rod from the (relatively low) bending loads I mentioned.

 

The answer I'm still looking for is for the same weight of the beam section for typical I and H-beam rods, which has a lower critical buckling load level? I believe the failure is probably more from local buckling of the flange and web, than from an overall "column" buckling failure.