OEM springs are about twice stronger than they need to be because manufacturers know about them getting weaker every year. Obviously manufacturers want them to last many years. After about 20 years or so OEM springs get down to 50% strength and start slipping. If you install extra new spring, you'll get 150%, which is not needed. Same about HD spring. I am not sure if "HD" springs are really stronger because standard spring works fine for 20 years.
I was always buying cheapest springs on ebay for all my bikes.
I was always buying cheapest springs on ebay for all my bikes.