If I may....you are correct - the wire from which the new spring has been wound is of uniform cross-section, but there are two distinct coil spacing or "pitches":
- a short section with closely spaced coils at the upper end of the fork;
- a much longer section with wider spaced coils.
Thus, these are actually
dual rate springs rather than true "progressive" springs which would have a continuous variation in coil spacing all the way along the length of the spring.
Anyhow, the tightly spaced coils form a relatively "soft" spring -
until the coils are compressed or collapsed or "bunched" at which point that shorter portion becomes very stiff. That bunching would occur over a fairly short fork travel (not sure how much - maybe...1/2 inch or less). Once the tight coils have bunched, the widely spaced coils really start to deflect until all of the available fork travel has been consumed. This layout allows the fork to absorb small bumps very easily, but ensures that the fork can still deal with a really big bump.
Basically, the dual rate spring is a compromise and like all engineering compromises, there are advantages and disadvantages in the design. The advantage is that on a smooth road, the bike should ride very softly with little energy (ie. bumpiness) being transmitted to the rider. However, on a bumpy road, the bike has less travel available to absorb the larger deflections and bottoming may occur more easily than if a full length linear spring were used. Also, the XS650 has a relatively primitive damping mechanism which cannot adapt to the two spring rates available from the dual rate springs. The damping therefore, is also a compromise.
Anyhow, as I have said, the original spring were badly fatigued and had lost a good deal of their original stiffness. Wither new dual rate springs, the bike now rides very nicely and I am happy with it.
Pete