in a bmx bike and how can you know that it is sealed or unsealed?
phosters answer is correct on how the bearings look different as far as performance goes sealed bearings spin a lot long and spin very smooth. sealed bearings require little maintenance and last a long time. unsealed bearings should be cleaned and re greased at least every couple of months.
sealed bearings are encased and unsealed are caged or loose ball bearings. if you can see the actual ballbearings they arent seal. if they are enclosed they are sealed.
