Mike,
Did you upgrade the stub axles to the chrome moly ones? The stock ones are a failure waiting to happen and I am glad it was the inner one that failed on ERA535. If the outer axles go it is a bigger deal what with the resulting fire and all.
I built my rear end myself and did not go with upgraded axles....it did cost me more in the long run.
You asked why the caliper and disc? Did I say it was a hard launch?
The Goodyear Eagle race tires did not help the situation.
Pat