Marzocchi End of Travel Observation

[celler]

I have always noticed it takes a big hit to use the last 1 1/2 inches of travel with the 48mm Twin chamber forks. Recently just did an oil change and before I drained the oil from the inner cartridge I decided to stoke them though full travel. What I noticed was that about 1 1/2 inch from the end forced ramped up a lot. upon pushing on it I heard some fluid get ejected. Then upon stoking the rod though full travel again everything was normal though the stoke. Seemed like in use the cartridge pulled in some excess oil? What do you all think? I'll be sure to pay attention to this in future oil changes. It is possible on my last oil change I did not totally poor out the oil in ICS area and this is the that ended up back in the cartridge.

[twowheels]

Oil migrates between the outer and inner chambers ... you can verify this by carefully draining all of the oil out of the outer chamber and between the male and female legs and measuring it. I consistently retrieve 280ml where 310ml has been put in.

[celler]

So over time without any hard hits that fully bottom the suspension oil level goes up in the inner cartridge effectively increasing the ics spring preload until a hard hit expels it out fully. This would explain right after the oil change I went out for a ride and used all but 1/2 inch of travel and before the change same trails about 1 1/2 inch unused. Off for a longer ride tomorrow. I'll gather more observations.

[GMP]

Interesting topic so I'll bite. All CC forks do this to a point. When the end of the cartridge with the seal enters the bottoming cone, oil pressure increases dramatically. The cartridge seal is directional like most seals and is installed to keep oil in the cartridge, it's less effective at keeping higher pressure oil out of the cartridge. So some oil is forced past the seal. If you bottom hard enough to cause the ICS piston to move up to the shaft recess, it will of course purge the extra oil. If not you tend to build up additional preload on the ICS spring from the extra oil in the cartridge until it is purged naturally or manually during a service. How easy this happens will depend on a few things like bottoming cone length relative to the ICS position at which it purges, and how often and hard the cartridge enters the bottoming come. It will not limit travel, just make it stiffer from more effective ICS preload. Some think this is why KYB SSS forks come set up with free play in the ICS, to allow for this oil migration. If you otherwise set the fork up stiff enough so it's not constantly dipping into the bottoming cone this should be minimized.

[Jakobi]

One thing I've noted with the Marzocchi 48mms PFP is that if you overfill the inner cart you can indeed have them hydro lock to an extent, in that the ICS cannot purge the fluid fast enough and chokes. Quite often I'd overfill to allow several purges during the bleed process. If trying to push them through a full stroke you'll hit significant resistance until it can slowly purge out. I'm not 100% certain as it's been a while since I've been in a set, but it's possible that as the ICS piston would move up in the cartridge it would block off the drain ports on the side which would add to the choking. Letting the leg extend and stroking again would allow further travel.

I've also seen some tuners modify the ICS shaft and relieving it to allow the purge to happen sooner and flow a greater volume of fluid past the seal.

[GMP]

Jake the upper oring is visible through the holes after a good purge, so its hit or miss if its causing a restriction, I'd say likely not the root cause. Also there is enough volume over the piston to allow some air compression during a purge, it will not stop it. Seems like you could remove the upper oring like in the KYBs, have you tried it? I do the same, extra oil, for a higher velocity purge, and yes you get a couple good farts from the cartridge. I attributed this to the compression of trapped air leading the compression of the ICS spring due to the stiction and high preload of the ICS, why a rapid compression is needed. It might be interesting to do some measurements of the shaft recess, seal position, calculate piston travel and determine if there is indeed a mismatch in the design. Wouldn't surprise me as the PFP system is a way off with most of the adjustment unusable. I for one am done with tinkering with this mess of a fork, have two sets on life support and thats more than enough. I call it the 100 hr fork. Last good set on my buddys bike shedding anodizing pretty bad now at 100 hrs. Also had an interesting problem with high stiction, almost binding in one cartridge, like a damper rod was bent. Turned out to be the outer oil was so fouled it was causing stiction of the ICS piston. No more than a total cleaning and oil change solved it.

[celler]

Great discussion and matches my observations and experiences. I went out this week with freshly purged cartridges and that used the majority of travel in my test ride but I noticed fairly quickly in the ride I was again not using the last 1/12 inches of travel. So seems as soon as near full travel is used the cartridges get filled with extra oil resulting in a stiffening at the end of the travel. A big hit will compress the final travel but is not used in normal conditions. I have noticed dropping off about a ledge and flat landing is enough to penetrate the last 1 1/2 inches.

[Jakobi]

My set didn't run as much preload on the ICS as yours Glenn, so I could probably manage to over fill them a bit more than you before finding it too hard to get the piston assembly into the cart and the threads to take up. I never experimented with removing o-rings or modifying the shaft on mine so can't comment on that.

I really liked these forks in theory, but the design and implementation left room for improvement. I've seen so many cases of these where they have worn out prematurely or began fouling oil at a rapid rate. While it's understandable that wear and tear (and shit) happens when things are in use, the limited support and parts availability and prices associated with them doesn't lend to them being a sustainable option.

One thing I found very interesting was that when my set were fouling it was very noticeable in that you'd feel their performance taper off over the course of a single ride. On the other hand I've had other forks run well past the point where the oil should have been changed, yet still feel in better shape than the marzocchis did after only a couple hours.

[GMP]

I'm with you Jake. If things were different it would be interesting to implement a few engineering changes. I have been able to get + 1-2mm longer adjustment from the compression shaft but to really make a difference you need a shorter ICS spring. Piston mod to channel trapped air you observed to the center of the piston/seal area for a better purge, the excellent spring perch bushing mod that was done by another member, and last but certainly not least more releif on the bushing lands. Steve and I have talked about trying a Husky KYB upper as a replacement but never got to try it. Getting back to the point, if you can valve/spring/adjust oil level of the fork stiff enough with a reasonable amount of comfort, it should spend less time dropping into the bottoming cone, and pump up less, so more consistant. You want to use all the travel on a ledge drop or something like it occasionally, where it will self purge, but not partially and regularly. This may sound counter intuitive to some who feel the fork is too stiff originally, which is why I mention the idea of dropping the ICS preload range with a shorter spring. With stiffer base valving, you need less cartridge pressure (less ICS force) to prevent cavitation. This is whats different about this fork from other CCs, lighter valving and higher pressure behind it.