during the discussion on the other crank thread one of GR's comments that the amount of overlap between the big end and main journals made the crank stronger to withstand cracking. He quoted various other engine cranks etc.
OK so to the various mini cranks and as we know a large journal crank "sometimes" will crack more than a small journal crank.
Now most of the cranks will crack/break thru the web between the center main and No.2 big end, so as GR said if the overlap between the two journal diameters is greater ,the risk of breaking is less.
So here are the dimensions of various mini cranks and the amount of overlap between the two journal sizes.
Now going by these dimensions a big journal crank should be stronger. The overlap in a 998 crank is less than a 1275 so they should be weaker and more prone to breaking. A 1100 crank ,which are very prone to breaking has a overlap of .039", and if you stroke a 1275 big journal crank the stroke ends up .125" more[shift the center line by .062"] and the overlap ends up at .1525"
The other dimensional difference between a 1275 and a 998 crank is the distance [thickness] between the thrust face at the center main and the face of the big end. This cross section area is where the crank breaks, and a 1275 is less than a 998, but only by .020". The web is wider in the taper from one side to the other and would give a greater/stronger section.
The other thing different between the cranks is the cross sectional dimension of the webs between the 2 big ends.
So if "stiffness" is a issue with the B/J cranks breaking, then a B/J crank should be the strongest. IE the most overlap and the greatest supported area.
The" solution" to the problem is to grind/roll a big radius from the journal face to the thrust face in the assumption that this is going to stop a crank shaft from cracking/breaking thru the web. Now if the crack intersects between the center main and the big end then why is the big end not also radiused.
The other point is how/where/why does the crack originate. A radius will relieve a stress point but the idea of a radius assumes that the start of the crack is originating at the brg face and exiting thru the opposite ends of the web.
If this is the case then the ONLY way this is going to happen, is for the crack to be bent longitudinally[maximum point of stress] and this is not going to happen as the mains stop that.
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