max backspace?

With everything else being the same making an axle longer does not increase it's strength. Yes there will be more twist along it's length than a shorter axle but they will both reach their yeild point with the same torque being applied. The one advantage a longer axle might have is if it was subjected to very high shock loading for a very short duration the twisting may act as a shock absorber preventing the metal from reaching it's yield point.