07-05-2020, 12:26 PM
I'm no metallurgist but I have always thought the small fillet radii used on A7 cranks to be an important factor, often aggravated by poor regrinds back in the day. Whatever the stress is in the journal it's concentrated at least 50% higher in the fillet. The myriad bending actions Robert alludes to are no smaller for want of crank support.
Other than that the greatest enemy is 'enthusiasm' - over-advanced ignition adds to stresses massively and yes, high compression ratios; 6 or 6.5 used to be reckoned the highest that would give reliable service.
I was taught that ferrous metals have a fatigue limit - that is to say there is a level of stress below which they will give infinite service. I was also taught there's many a slip 'twixt cup and lip...
Other than that the greatest enemy is 'enthusiasm' - over-advanced ignition adds to stresses massively and yes, high compression ratios; 6 or 6.5 used to be reckoned the highest that would give reliable service.
I was taught that ferrous metals have a fatigue limit - that is to say there is a level of stress below which they will give infinite service. I was also taught there's many a slip 'twixt cup and lip...