The new 375/465 spring was a HUGE improvement over the 400/450!
Not as stiff on road and much better performance offroad. The 400/450 was too strong (given my weight and setup) and overpowered the front, causing it to bottom out offroad. But the new 375/465 allowed the rear wheel to travel instead of pushing that weight forward onto the forks. So the fronts only had to do their own work, instead of the rear's too.
I noticed that the spring felt a lot stiffer at the beginning of the ride but softened up as I worked it on the trail. I expect it to continue breaking in over time and just get better.
BTW, I didn't add any preload to the spring. I just locked it down with a little tension on top of the spring and that was enough for me. With my setup (no bars, no side boxes, 200 lbs riding weight + 30 lbs in top box) I could probably use an even softer spring, but I'm much happier with this one onboard.
Here are my spring recommendations based on total weight (rider/passenger/luggage):
- light spring - Total weight less than 250 lbs. I don't know what spring rate this would be. I'd have to call Progressive springs and see what's available.
- 375/465 - Total weight between 250-400 lbs. More travel makes it better off-road. Highly adjustable for varying load.
- 400/450 - Total weight between 300-400 lbs. More appropriate for on-road riding.
- 450/500 - Total weight over 400 lbs (I think this is the spring rate CSC offers).
So for most people riding 1-up or light 2-up the 375/465 is the one to get. The spring is longer than the 400/450 so you get more travel overall. And you can set it either lighter or stronger to allow for more variation in rider and cargo weight (just changing the preload as needed).
If your total (rider/passenger/luggage) weight exceeds 300 lbs, the 400/450 spring would be fine but provides little or no advantage over the 375/465.
Around 400 lbs total, I would probably try the 450/500.