When a stretch belt exceeds wear specifications, it’s no longer able to grip the pulleys and will start slipping.
Stretch belts can take loads off of the main accessory belt drive, allow for better packaging, and solve NVH problems that can occur with a longer serpentine belt. Less belt means less tension and fewer noise and vibration problems.
When inspecting a stretch belt, always measure the depths of the grooves with an inspection tool since the differences between a worn belt and a good belt can’t be seen or felt. The new materials used to make a stretch belt might crack or separate like older belts before the grooves are worn.