Making bias tires with a 60 natural 40 sbr N299 tread compound. 4.5 phr 6PPD and Wingstay combined. Two phr blended wax. 1 TMQ 2 sulfur 2 TBBS. Some tires are cracking horribly within 3 months of being mounted and only mounted on a rim, never used, loaded or flexed. Any ideas about what...