[Fot] Racing rod design

[MadMarx]

Hi Michael,

 

I add a little graphic about stiffness:

 

Bild

The graph was taken from the web but it shows very clear that the stiffness
of an I beam and a H beam are in different directions.

I beam  stiffness is perpendicular to the crank, H beam parallel to the
crank axis.

 

There are 3 major forces in a rod:

Pulling at TDC

Compression at LDC

And bending from the rotation

 

For the first two forces the area of the rod is the important factor. The
shape is not important.

For the bending forces the shape gets important.

If you attach a rope to a wall, take the other end in your hand and moving
your hand quick up and down (like a crank stroke) the rope will swing up and
down too and shows a bended line. A rod is like this rope and gets bended
caused by the rotation of the crank.

For the stiffness of the rod beam the orientation if the beam design is very
mandatory. Like a beam in a building there is a strong axis and a weak axis.

The top graph shows how the “I = stiffness value” is affected by the
orientation of the H or I.

That means an I rod can bear more bending forces than a H rod. An that leads
to a better resistance of an I rod against fatigue.

 

Crank flexing is there but compared to the 3 main forces so small that it
doesn’t influent the total result.

If it would, then all engines would suffer from many bearing failures
because at least to transfer the flexing from the crank journal to the rod
beam the only contact way would be the bearing and that means the bearing
gets forces on its edge and it would bear this for long and fail quickly.

 

I had a look to the Manly rod catalogue.

All I-beam rods seem to bear higher power than the H-beam ones by same or
lower weight. I think that is a clear statement of a rod designer who builds
both types. The H-beam is the cheap brother of the I-beam and fails earlier
on heavy use is my guess from all I have learned in the last few days.

 

But I’m open for other thoughts.

 

Cheers

Chris

 

 

 

Von: Michael Porter [mailto:mdporter at dfn.com] 
Gesendet: Mittwoch, 22. Juli 2015 00:19
An: MadMarx
Betreff: Re: [Fot] Racing rod design

 

On 7/21/2015 11:24 AM, MadMarx wrote:

Hi Guys,

 

 

In US forums I found an interesting opinion:

 

H-beam rods are for low rev high torque engine (turbo, compressor)

I-beam are for medium torque engines with high revs

 

 


It's going to depend upon four variables--Young's modulus for the specific
material used, the section modulus of the shape used, the mass and the
finish treatment.  In a piston engine, the peak bending load is going to be
at the point of highest downward pressure on the piston combined with the
largest angular deflection of the rod from perpendicular.  That's going to
be somewhere around 75-90 deg. of rotation after TDC on the power stroke.  

If all other variables are the same, then section modulus matters, and
generally, what determines the section modulus with regard to bending is the
beam strength and resistance to torsional deflection (even though the rod
runs in only one plane, torsional deflection has to be considered since the
crank throws are heaving up and down and bending the crank between the mains
under both varying piston pressure and inertial forces, and those forces are
translated to the rod at roughly 90 deg. to the plane of rod rotation).

For that reason, I think the H-beam rod would have a lower stiffness in the
vector approximately 90 deg from its plane of rotation, as compared to an
I-beam, all other things being equal. With forces parallel to the plane of
rotation, the H-beam ought to be stiffer.  So, maybe, the difference is not
exactly rpm, but how the crankshaft behaves at that rpm.  If it's twisting
and bending, the rods will be subject to twisting and bending loads more or
less normal to the plane of rotation--the vector where the H-beam has the
least beam strength, in addition to the expected bending loads parallel to
the plane of rotation.

With a theoretically perfect crankshaft, one that does not deflect under
load, the H-beam rod would be stronger and less prone to deflection in all
cases, I'm presuming, because its section modulus would be higher, and its
beam strength higher in the plane of rotation.  It might be that the H-beam
is still actually superior in all cases, if properly finished, is no
heavier, is of the best material and is not subjected to bending/torsional
loads beyond its limits angular to the plane of rotation. But, to know for
sure, one would need to know the actual section modulus of the specific
shapes to be compared, and to have some idea of the crankshaft behavior at
the speeds required.  And, it would probably be helpful to do some detailed
analysis of the failure mode of the rod in question.  Signs of fatigue from
torsional flexing or side bending loads would be helpful to know, and would
inform any decision about what to buy next.


Cheers, Chris.







-- 
 
 
Michael Porter
Roswell, NM
 
 
Never let anyone drive you crazy when you know it's within walking
distance....
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