Help me not die, brakes suck.

[b264]

Definitely belt skipping. Try a 15mm wide belt and carefully align it.

@b264

Without going into tensioning, which is king, I would say check the alignment very carefully. A belt that’s not aligned well can slip, as @taz mentioned.

Something else that can cause anywhere from minor to massive slipping in a perfectly aligned and tensioned belt, is broken cords. If your belt has broken cords inside, you will typically not see any indication of this by inspecting it (sometimes you can though) but it will just skip like crazy. In that case, failure is imminent anyway and you need to replace it before many more rides. If you check multiple belts, some new, and the skipping persists…

Then check when the skipping is happening. If the worst skipping is happening any other time besides during hard braking, then I don’t know what’s going on. That’s always when the worst skipping happens.

Alignment – the best way I’ve found to perfectly align belts is

• get motor mount (without motor) snug on hanger, then loosen a tiny bit so it moves
• install wheel and pulley with no speedring (washer) between the hanger and inner bearing.
• push motor mount flush up against wheel pulley so they are touching each other
• keep those held tightly together while you tighten motor mount clamps
• make sure mount is flush against, and touching, the wheel pulley, and there is no gap or misalignment
• finish tightening mount clamp all the way
• make sure again that mount is flush against, and touching, the wheel pulley, and there is no gap or misalignment
• when you’re all done, take the wheel and pulley back off, add a single 1mm speedring (washer) and just like that, the pulley will be perfectly aligned and 1mm away from the motor mount plate
• if you need it further away when you are doing tensioning, then add more washers

And also make sure your pulleys are concentric and don’t wobble at all as @taz mentioned. That’s super important.