Why H-beam rods?

If the problem was static loading, I could design an I-beam rod myself
and order an extrusion from a catalog.

The horizontally-disposed, simply-suported center-loaded beam that you
describe has been illustrated hundreds of times in strength of
materials texts.

If you work in a high rise building, your weight is probably being
supported by a horizontally-disposed I-beam attached to
vertically-disposed I-beams that resist the wind.

However, the connecting rod's supports are definitely NOT simple, both
ends are moving constantly, so the I-beam or H-beam is exposed to
cyclical bending loads as the piston thrusts against the cylinder
walls, then tension and compression loads as the crankshaft starts and
stops motion at the ends of the stroke.

As I mentioned previously, the tension loads are the most severe
problem for the connecting rod designer.

As I thought about the subject further, I realized that the H-beam is
somewhat similar to a thin-walled gothic cathedral in concept.

The architects wanted to build cathedrals as high as possible, while
having as much open space inside as possible. They also wanted to
reduce the amount of stone to the minimum required.

High winds in a storm would actually cause the tiled roof of the
cathedral to move side to side and the solution to keep the cathedral
from falling down was the masonry flying buttress, which is loaded in
compression.

The small end of the connecting rod is "seeing' loads similar to what
the roof of the cathedral experiences, and the two side caps are like
buttresses.

The central part of the rod is like the main structure of the building.
It would be bending backwards as the crankshaft turned, if not for the
bracing of the buttresses.

The forward buttress would be loaded in tension, the trailing buttress
in compression on the down stroke, then the loads would reverse on the
upstroke.

 

Short version . . .

H-beam rods are stiffer than I-beam.

That's why Udo had to use production rods in his high-output BMW airheads.
Factory rods will flex with the crankshaft.
Carrillo rods will saw it in half.

 

Where do the bending loads on the connecting rods come from? The ends
are pivots.

 

For the same weight of of the same alloy with the same forging or
hardening processes applied, occupying approximately the same operating
space, that is probably true.

Hopefully, not much flexing is going on with a more modern design. The
airhead was originally designed as an aircraft engine in the 'teens of
the last century, and has since seen yeoman automobile and motorcycle
service.

If the crankshaft breaks or if the rod breaks?

There are older technologies which don't adapt well to more modern
technologies. A fan of wooden ships once told me about a failed attempt
to replace an oaken mast with an aluminum extrusion. He said that 17th
century ship technology worked just fine if you stuck with 17th cntury
materials.

 

You are correct that bending loads do not come from the piston pushing
against the side of the cylinder due to the angle of the rod. Because
of the pivoting action those forces go straight from the wristpin to
the crank journal.

To see the bending loads take a rubber hose or something else that will
bend without much force. Hold one end and let it dangle vertically.
Keeping that end stationary grab the bottom end and shake it back and
forth. The inertia of the hose will have the mid section lagging
behind the motion of the end. When you reverse the direction of the
end the mid section will keep going till it flops to the other side and
gets dragged back. Look at the direction the big end of the rod is
traveling at TDC and BDC. The H beam is better suited to take these
loads.

As for the tension and compression loads, the reason a rod is more
likely to break when under tension is in a big part due to the fact
that the acceleration forces on the piston are greater at TDC than BDC.
To see why picture a rod just a bit longer than the crank throw. When
the crank is 90 degrees ATDC the piston is near the bottom of its
stroke. During the next 180 degrees it hardly moves. In the next 180
degrees it has to go all the way to TDC and back. My gut feeling is
that the rods work equally as well in tension, but the H would be
better in compression since any bend in the rod would weaken it under
compression.

Bruce