what kills con rods?

[MotoJouni]

You're missing out inertia of piston and conrod which is of considerable figures, and I'd guess especially at higher revs and small throttle openings not fully compensated by gas pressures.

I think you are right. The effect of inertia does it actially for two times every revolution, the piston speeds up and slows down every 180 degrees. The total result is the combination of both inertia and gas pressures.

Some Laverda rods are coppered for reducing notch sensitivity, which is of course softer and then more easy to scratch than the steel under it.

I once did a very littlebit mathematical modelling of SFC 1000 crankshaft. The piston changes it's direction very rapidly at TDC but makes smoothened direction change at BDC. I think this conclusion is made for better gas exchange but leads to bigger forces at TDC.

 

[Gravelroad]

[...]Some Laverda rods are coppered for reducing notch sensitivity[...]

The copper plating is applied before the bearing surfaces of the rod are rougly machined, then case carburizing follows. The intention of the plating is to have only the bearing surfaces carburized and not the entire conrod surface. An entirely carburized rod would be quite notch sensitive, you're right on that one, but that has nothing to do with the softness of copper.

 

[MotoJouni]

^I have understood that it also fills small notches. At least little notches filled with brass can decrease notch sensitivity, not knowing does the electrolytic copper actually have that kind of effect.

 

[Dellortoman]

At inlet stroke: tension
At compression stroke: compression
At power stroke: compression
At exhaust stroke: compression

Not very shure of conditions on engine breaking. I think that's a calculatory question.

It's not quite that simple. There are inertial forces, which have longitudinal and lateral (sideways bending) components as well as forces induced by gas pressure in the cylinder. If you're interested, there have been plenty of academic studies on the subject. Just google "dynamic forces in connecting rods". Here's a typical paper http://www.academia.edu/5349972/Dynamic_analysis_of_loads_and_stresses_in_connecting_rods

 

[MotoJouni]

It's not quite that simple. There are inertial forces, which have longitudinal and lateral (sideways bending) components as well as forces induced by gas pressure in the cylinder. If you're interested, there have been plenty of academic studies on the subject. Just google "dynamic forces in connecting rods". Here's a typical paper http://www.academia.edu/5349972/Dynamic_analysis_of_loads_and_stresses_in_connecting_rods

Thanks. I have seen a couple of some similar documents some years ago and then I didn't understood half of what was said on them. It would be interesting to see how the con rod force is altered on different load conditions. Also in this paper only one load condition is considered.

The effect of inertia does it actially for two times every revolution, the piston speeds up and slows down every 180 degrees.

This is not true as I expressed. The momentum accelerating and braking the crank happens for two times on every revolution, but the change from tension to compressive force on con rod and vice versa happens once in a revolution.

At inlet stroke: tension
At compression stroke: compression
At power stroke: compression
At exhaust stroke: compression

I think this was from Graham Bells book, but now I think I or some translator have mixed some things up like piston force and conrod force for example. But that seems to be true at least (from the book) that the biggest force happens for a con rod's lower end is when the inlet stroke starts at or very near TDC and is a tension force. It could be seen from Dellortoman's link on page four or 618 on figure 3a. In a super charged drag race engine things could be different.