car springs vs trailer axle springs...apples vs pickles.
Two completely different operating environments. Our cars/trucks with rear leaf springs simply follow around corners with little if any side loads.
We all know the tremendous side loading conventional trailers leaf spring operate in.
Some trailer newbs have said they place a bottle jack under the spring to jack that wheel. Doing so can crack that spring or stress that spot and break later down the road.
There are several reasons a spring may crack. I've had my share of trailer suspension issues in the '70s and '80s when I hauled for a living. Crack or broken springs happen.
With the availability of trailer suspension parts I don't carry suspension parts now nor back in the day. Waste of time...money....and a paying payload cheater.