Occassionally my cranks feel like they very suddenly slip when I stand on my pedals on the 53 t ring. Thinking that this might be due to a worn chainring I purchased a replacement but noticed that the new replacement has teeth that appear same as the old one,in that some ofbthe teeth are rounded and som are flat/ramped. Why are the teeth all different shapes like that and how can you tell if a chainring is worn enough for replacement.