I know this sounds weird, but here goes. Can serpentine belts, the kind that replaced the old v-belt fan belts shrink or get smaller?
Not that I’ve ever heard of. They can stretch, despite the fibers in them, but not shrink.
Any rubber belt shrinks as it heats up. That’s the characteristic of rubber. When V-belts were used you could have a slightly loose belt. And when the engine was started cold the belt would squeal from slipping. The slipping of the belt would create heat where the belt would shrink and the squealing would stop.
Yes they can shrink. I just replaced an S-belt from my Honda because my old one shrank. Despite what others have reported, it sounds counter intuitive I know, but the process is known as Volatile Loss during curing and post curing. In this case, depending on the belt manufacturer, the polymers and plasticizers engineered into the designed product vary, to the extent that over time they evolve out of the product resulting in a lower volume of material.
I believe this also ‘contributes’ to pulley failure as more tension is placed upon the pulley components over time. You can also analogize to other rubber/polymer based products that shrink over time, construction products; roofing, expansion joints, etc… .