What are fatigue failure modes of concern?
Cracking a wheel axle
a disastrous failure for a climber
Rolling fatigue causing spallation of sharp metal chunks from the rim of the wheel
A potential disaster for both ribbon and climber
We need ~100% confidence that a climber will make it to the end of the ribbon
Fatigue allowables are always expressed at 50% confidence of failure
Allowable stresses are reduced to increase confidence