I beam versus H beam rods

David Redszus · Post by **David Redszus** » Mon Feb 11, 2008 10:57 pm

af2 wrote:
curl'd wrote:
Stef wrote:H-Beam are good for high compressive (buckling) loading i.e turbo or supercharged engines. Can't pull as much weight out of them though.

So which way does a rod bend and then break? Is it along the thrust or pin axis?

If the above question is answered and we know the sectional dimensions of an H and I rod, the problem then becomes one of simple mechanics.

Stef · Post by **Stef** » Tue Feb 12, 2008 2:59 am

I've seen both types of rods fail in both axis for various reasons. The critical buckling load is determined generally by the Rankine formula (or modifications thereof). Effective lengths for the rod are normally K=1 for the thrust axis and K=0.5 for the pin axis. But, the pin axis value is not strictly correct and could be as high as K=0.65 (the shape of the transition to the bigend strongly influences this).
Elastic or plastic failure may occur on either axis (or both) depending on rod geometry, cylinder pressure, rpm, etc.
An H-Beam rod is a convienient (for low volume) way to improve the strength in the possibly weaker pin axis for engines with high cylinder pressures and means you don't have to do a rigid analysis for each application which is time consuming and hence costly.

Ed-vancedEngines · Post by **Ed-vancedEngines** » Tue Feb 12, 2008 3:07 am

I am wondering if some of these connecting rod and rod bolt issues are coming after bearings get hot and the heat on the rods is what may be fatiguing them? Is anyone checking these cranks for heat marks on them when there are failures?

Ed

Stef · Post by **Stef** » Tue Feb 12, 2008 3:21 am

At the end of the day even the most rigid analysis of a connecting rod is worthless if the rod and bolt are made from poor quality materials.

mallow415 · Post by **mallow415** » Tue Feb 12, 2008 10:38 am

Concerning the Scat i beam rods with 7/16 bolts.
Would you replace the bolts with better arp bolt after 1 season in a dirt track motor?

theMoportal · Post by **theMoportal** » Fri Feb 15, 2008 12:12 am

Perhaps this might shed some light?

rskrause · Post by **rskrause** » Fri Feb 15, 2008 5:59 am

I am not a Pro. But I think primary rod failures are extremely rare and occur due to defects in the metal (which may be acquired if the rods are nicked, or due to manufacturing defects, etc.) not to the cross sectional design. Obviously, rod bolts do fail, but I suspect even most of these failures are due to improper assembly or defects.

Richard

curl'd · Post by **curl'd** » Fri Feb 15, 2008 4:38 pm

To themoportal, those pics are confusing to say the least, perhaps this one will shed some light on it >

Click on it twice to full-size the image.

theMoportal · Post by **theMoportal** » Fri Feb 15, 2008 7:37 pm

Did I say "shed some light"?

I should have said "stir the pot" -- much more accurate.

Mike Thompson · Post by **Mike Thompson** » Sun Feb 17, 2008 9:14 pm

I have used the inexpensive eagle h-beams in my SBC makeing close to 1000hp with nitrous. They are stronger than you think.

Watch, I'll bust a couple now.

afrfox · Post by **afrfox** » Fri Feb 22, 2008 11:19 pm

forgive my ignorance, but which one is the best???