V-belts

The problem we experienced (and I'm guessing your problem too) is that most belts with kevlar/aramid cords typically also have a much stiffer section. The belt manufacturers do this so the rest of the belt can handle the increased tensile and breaking strength that the kevlar cords allow. This causes them to need much larger wrap radii, and they'll get brittle/crack/slip if used around smaller pulleys. The bending and cracking you describe is exactly what you see in these cases. If your mower originally had basic nylon-cord belts, there's a good chance the pulleys/sheaves are too small for standard kevlar-cord belts. When this is the case, they also need to be uncommonly tight to not slip, because the stiff section doesn't grip down in the smaller sheaves nearly as well. That may explain why yours are getting so hot. I don't have my belt tables in front of me nor know what size of belts you have, but I'd guess a B, C, or 5V section. In which case, if using full-section kevlar belts, your pulleys probably need to be around 8" diameter at the very least to effectively transmit power and not crack or slip. If you go to cogged belts, you'll probably be ok down to 4" or 5" pulleys.

Try cogged ('X-series') belts and I suspect your problems will go away. Cogged belts also help keep the sheave groove from getting packed with crapulence - which is why we use them on sawmill equipment at my current job. And make sure you're getting the right series of belts too: It's a pretty common mistake to put conventional (A/B/C) belts on sheaves meant for narrow-section (3V, 5V, 8V) belts (or vice-versa), and they won't grip right and cause grief.