narrow rims on a bmx bike will bend VERY quickly in comparison to wider rims. the sidewalls of the rim should've EVER roll back on themselves if you're running the right tire pressure. most modern BMX rims have enormous sidewalls, allowing for brake pad adjustability and making them that much stronger. mountain bikes are different from BMX bikes. a BMX typically will have about 90psi per tire, whereas a standard freeride/downhill will have around 40-60psi, somtimes more, sometimes less. the 90psi keeps the rim protected from most situations that will cause a flat spot.