broken axles

Threaded axles always break on the inside of the cone on the heaviest loaded side. Cassette hubs tried to solve this problem by moving the cone 20mm. A freewheel hub has a distance of about 40mm from the dropout to the inside of the cone cassette hubs have a distance of about 20mm from the dropout to the inside of the cone. This makes the side opposite the cassette under more stress since the distance from the dropout to the inside of the cone is about 30mm this reduces the stress on the axle to 20/30 or about two thirds. So far so good however  a zero dish wheel removes this advantage. One partial solution is to use a solid axle these are about 30 percent stronger. Another partial solution is to use a hub with a brake boss. In shimano hubs the spline brake hubs move the cone out 5mm the six bolt hubs and the threaded tandem hubs move the cone out 10mm. The best solution is to get rid of the axle threads. Threads create what engineers call a stress raiser. This information was hard to find because engineering handbooks assume you are not stupid enough to load a bolt on the threads however threads increase the stress by a factor of 3.3.  Hi E bullseye and Phil Wood got rid of the axle threads. They also used larger diameter axles this makes them much less likely to break.