What precisely makes a great fork spring?
In a perfect world the suspension of your motorcycle would react differently every time. The top half of the spring would be supple and would react flexibly to absorb small bumps and irregularities instantly. As the spring compresses, it becomes more rigid so even large bumps and holes in a road surface will not cause the suspension to bottom out.
For cost-efficiency reasons, new motorcycles are equipped with linear springs. Compressing the top half of this type of spring takes just as much force as compressing the bottom half. The result is that the top of the spring is in fact too rigid and the bottom too supple. Take a look at CSR Suspension fork springs.