It's amazing how a company with such a high end reputation for quality suspension, in many fields, can produce something as crap as these forks. But, as those in business will understand, if someone asks you to make something, why not. And clearly, when Ducati asked Marzocchi to make some upside down forks for the Monster and SS range in 1994, the word most emphasised was cheap. Ie, "Yes, well, they're great, but do you have anything cheaper?"
So what we got were these 40mm forks with a damper cartridge arrangement that is fixed into the fork tubes by the bottom edge of the tube being rolled to fix them. I've often poked quite a bit of fun at them, but it's not really Marzocchi's fault if it's what Ducati wanted. As always, it's amazing the crap Ducati have managed to get away with over the years.
Functionally, the forks have normal style rebound damping, but compression damping in the final 1/3 (or so) of travel only. This leads to quite a bit of initial dive, the sort of thing that catches you out when you grab a handful of front brake at low speed while working through traffic for example.
They were fitted to the Monster (M400, 600 and 750) and SS ranges (400, 600, 750 and 900SS CR) from 1995 until 1999. I have record of two different dimension springs being fitted, but the overall rate calculates at much the same. Being way too soft, and two stage to try to overcome it.
My set of these came from Minnie originally, before I fitted some 41 mm Showa non adjustable forks to her. I did replace the springs at one point, but they've been unused for quite some years now.
I had a little time and curiosity, given my other fork playing, so decided to drag them out and have a looksee.
As I found them, they were fitted with 0.85 kg/mm rate springs fitted with 33 mm of preload and filled with oil to 100 mm. Why they had 33 mm of preload I don't know, but it seems way too much. So I tested them again with 17 mm of preload and the oil level set to 140 mm. The preload tubes in this instance were pieces of 32 mm OD orange electrical conduit. This is much the same size as the original tube, in both OD and thickness. With a wall thickness of 3 mm, it does have a noticeable impact on oil level, especially when compared to the Showa thin wall steel tube preload spacers for example.
Then I dug up some original M600 springs from my stash of oldies. I couldn't find any original preload tubes (long since cut up no doubt), so I cut a piece of orange conduit 120 mm long, which I'm pretty sure is the original length. This gives 13 mm of preload on the original spring. The std oil level is 90 mm, so I began with that, then added more preload (another 20 mm piece of orange goodness), then dropped the oil level to 130 mm with the same extra preload. The orange conduit volume also had an impact here. I calculated that the 20 mm long piece of conduit added the equivalent of 5 mm of oil height to the fork internal volume. Which I'm mentioning as I didn't think of it when originally adding it, but later went "ah" when it appeared the air spring rate may have changed a touch. As ever, it's all the errors you introduce without thinking that catch you out.
And so to a graph. Blue is how I found it: 0.85 kg/mm spring with 35 mm of preload and oil set to 100 mm. Black is with the preload reduced to 17 mm and oil dropped to 140 mm. Red is with the original spring, preload and 90 mm oil level. Green is with preload increased 20 mm and 90 mm oil level. Orange is the same as the green, but with the oil level dropped 40 mm to 130 mm.
The orange and black curves show much more linearity, which is what I'm seeing a lot more of with oil levels reduced by 40 or so mm. The shape of the yellow curve is also influenced by the spring rate, which changes from soft to harder at around 70 mm of compression. With the preload set to 33 mm, this corresponds to 40 mm of fork compression, or just a bit more than the static sag would hopefully be. I have found that the static sag load seems to be in the 45 kg range, conveniently occurring around 35-ish mm of compression for the black, green and yellow curves.
If I add the M900ie 'final' curve, with the 0.85 kg/mm springs running 5 mm of adjuster added preload and oil level set to 155 mm in purple, the orange and black look a bit light on at the top end. Given the black and purple springs are both allegedly 0.85 kg/mm the difference in overall gradient is a bit odd. I'd probably want to go back up in oil height by maybe 20 mm or so to increase the air spring.
Splitting them up for clarity, first a 20 mm increase in preload with the original spring. The difference in static sag with this change should be in the region of 15 mm.
Then a 40 mm drop in oil level. It's only in the last 1/3 of the travel that you'd notice any difference on the road.
The difference a heavier spring makes to the mid travel range is shown in the next graph. Similar end points, but a difference in the middle of around 8 mm for a given load. Black is 0.85 kg/mm, orange original.