I have the 4 leaf spring from Peter Mulberry in my 4/4 for 6 years because the factory spring was completely limp. Over the years and about 60,000 km, the Mulberry spring also sagged a bit. Now it is so that these springs deform but their spring rate remains. So I decided to continue using the springs. Instead of buying a new 4 leaf mulberry spring, the lowering blocks on my 4/4 were removed (only 4/4 have lowering blocks from "Sport" on). Now the height of the car is perfect, it would be a little too high with all new springs and no lowering blocks. The axle now sits in the center of the spring and not above it like with the lowering blocks. This gives me a much more controlled cornering feel, driving is much more fun. If I would combine lowering blocks (in the sense of keeping it) with a panhard rod I have a measure that worsens the driving and another measure that should improve it again. Then rather omit both parts and achieve a good result. Another advantage is that I have gained more suspension travel. For the undisturbed rebound I had to remove half of the upside sandwich rubber bump stop and install slightly longer shock absorbers. As it is now, my 4/4 rides to my great delight. I also have the feeling that the sagging at least until now came to an end.
