The Belt Flex Tester gives an indication of the risk of spontaneous cracking of the shoe soles due to flexing in wear. The flex tester features 3 free-wheeling crowned flexing rollers, 1 driven roller as well as a cycle counter. It is manufactured with reference to STM 459F standard.

The sole forepart is attached to an endless belt which passes around two rollers, the smaller of which determines the severity of the flexing action.  The endless belt usually accommodates up to siz sole units.