EPDM and Neoprene Belts
Automotive belts are typically made of neoprene or EPDM. Vehicles manufactured before the early 2000's had neoprene belts. These belts did not last as long as EPDM. EPDM belts last longer because the material used has more elasticity, making them less vulnerable to cracking.
Belts ride a little above the pulley to allow dirt and debris a pathway out of the belt system. This can be a problem because as an EPDM belt wears, the gap between the pulley and the ribbed belt begins to decrease. This loss of clearance prevents dirt and debris from leaving the belt system. This is known as hydroplaning. It may not be noticed, because of the belts ability to resist cracking, it appears to be fine.
This is why a belt wear gauge is used for diagnosing automotive belts. Even a 5% loss of material will result in a loss in belt performance. A belt wear gauge measures the depth of the belt. While measuring a new EPDM serpentine belt, the gauge raises above the ribs. Notice while measuring a worn belt that the gauge sinks into the belt. This indicates a need for replacement.