18-09-2019, 11:20 AM
Hi Colin
Centrifugal force at revs is very great and is simply calculated. At the top and bottom of stroke for an instant the con rod and piston assembly experiences the same force as if it was part of the crankpin. This is increased about 25% at the top of stroke and reduced at the bottom due to the short con rod swinging over the outside of a curve or the inside respectively. On the exhaust stroke there is little gas pressure to oppose the piston rod assembly and the upward force is a maximum. It is hard to believe but at just reasonable revs on a long stroke engine on the full throttle firing stroke the net peak down load is about the same as the peak upload on the exh stroke. At racing revs the upward force on exh stroke considerably exceeds even the full explosion downforce of low revs. On older cars particularly a lot of running is at lowish revs and the shaft of the rod concentrates the load on middle of the bearing. So the top half fatigues first. It is the greater upward thrust of two middle cyls esp at closed throttle which bows and contributes to breakage of raced cranks. (it pays to slow from 60mph in nuetral!)
Centrifugal force at revs is very great and is simply calculated. At the top and bottom of stroke for an instant the con rod and piston assembly experiences the same force as if it was part of the crankpin. This is increased about 25% at the top of stroke and reduced at the bottom due to the short con rod swinging over the outside of a curve or the inside respectively. On the exhaust stroke there is little gas pressure to oppose the piston rod assembly and the upward force is a maximum. It is hard to believe but at just reasonable revs on a long stroke engine on the full throttle firing stroke the net peak down load is about the same as the peak upload on the exh stroke. At racing revs the upward force on exh stroke considerably exceeds even the full explosion downforce of low revs. On older cars particularly a lot of running is at lowish revs and the shaft of the rod concentrates the load on middle of the bearing. So the top half fatigues first. It is the greater upward thrust of two middle cyls esp at closed throttle which bows and contributes to breakage of raced cranks. (it pays to slow from 60mph in nuetral!)