I have tested flywheels to destruction and its all down to the shape. If you cut a flywheel in half you would see how the outer edge is poorly supported as the shape is reduced from a thick vertical to a thinner vertical.
Any low revs tends to bend the edges outwards. Any higher revs causes the edges to break off as they try to bend outwards.
This reduction in thickness means the edge is poorly supported and only stays together because it is complete. Any hairline crack breaks the support and the edge simply bends outwards and breaks apart very quickly within the bending moment.
Any casting flaw is equivalent to a hairline crack.
The cheaper flywheels are sometimes poorly cast.
Keyways are notorious for causing shafts and casting to fail. Usually the crack/failure is attributed to the keyway having square cut bottom corners. The solution is to relieve the stress area with rounded corners.
Cheap flywheels tend to have poorly cut keyways.
Seeing flywheels explode and how the tin casings cannot hope to contain the flywheel parts as they fly outwards is quite an experience.
Flywheels can be lightened, but only if they are made to a good design originally.
Brian P.
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