Marz Fork service

[BrianG]

Ok.... got the springs from Max. I opted for a 1.10 set.

 

Now, what weight oil and how much??

 

Yes, I see the service manual says 400 cc's but I'd like the current parameter of "oil level" better. Anyone got this number?

[Guest vkerrigan]

I think you'll get different numbers and weights from anyone you ask, but I used 2.5w Motul synthetic, 425 cc per fork with my 1.0 springs and it works well for me.

 

Happy Trails..............vk

[andy york]

I used the 400cc amount of 7.5w synthetic and went for about 17-23mm of sag

 

andy

[Guest ratchethack]

Brian, no comprendo on the 'I'd like the current parameter of "oil level" better' part of your post. The factory spec'd level is as you stated. There is no standard answer to your questions. Are you trying to achieve some specific results by varying the volume and viscosity? What are your objectives, as exactly as you can describe? As far as optimum oil weight, 10w is factory spec.

 

A little more info por favor. What do you weigh? What's your riding style and what kinds of speeds & roads do you ride primarily? Looking to improve your fork performance for aggressive riding in the twisties - or what? Mostly solo or 2-up riding, or a percentage of both? Carry any gear on a regular basis? Have you changed out your rear shock &/or rear spring to correspond with the nearly double rate over stock increase on your fork? Have you set your sag properly front & rear? Compression & rebound settings? The more detailed info you can provide and the more clearly you can state both your starting point and your objectives, the easier it will be to get a response you can use.

 

I'm not busting your chops here , just trying to help put some additional focus on your inquiry. With any luck, maybe one of the more experienced chassis tuners out there will chime in, but there's gonna have to be more info to work with. I'll be watchin' with interest!

 

Rgds.,

 

Ratchethack

[dlaing]

I am also curious as to the details of Andy's and especially VK's mod.

VK, If I may dare ask, how much do you weigh, How did your sag turn out, did you have to add a preload spcer, and do you find that you can still get enough rebound damping out of the adjusters?

And since you went higher with the oil I would think it might bottom out on the compressed air a little too soon, or even prevent it from using fulll suspension travel.

But you said you are happy with it, so maybe it is fine.

I ask because my stock shocks feel like they are bottoming out on air.

And I was thinking of with a lower oil level.

But maybe I am bottoming on the damper rod or whatever the mechanical conclusion of the travel is.

Andy, did you simply pre-load the stock springs?

Thanks!

[BrianG]

No problem on the info. The more experience I can tap, the better!!

 

I'm about 220 lb in full gear. I like the twisty asphalt, but ride too much slab to call myself a dedicated Sport Rider. I guess I'm a Sport Touring type. My most recent ride is a CBR 900RR that has the full Traxxion fork revalve/spring and Penske shock treatment, with Heli-bars and a Corbin seat......... which should be somewhat revealing! One up, sometimes with Givi luggage, often too fast, and rarely to track-days (to get my ass whipped by my 20 yr old son). Currently on Michelin Pilot Road's, but LOVE the Pirelli Diablo's on the 'blade!

 

Most manufacturers spec fork oil capacity in terms of "remaining airspace" as measured from the surface of the oil, to the top of the collapsed fork tube, absent the spring. This is apparently so that the critical issue, which is the airspace-spring, is consistent and reproduceable regardless of the completeness of the draining process, which apparently can be challenging on some forks.

 

This is the first step in the optimization of this suspension. Max says it's a good enough fork if it's set up right. My suspension philosophy is informed by Max's, being; the best suspension is the one that best keeps the rubber on the road. That means the more compliant, the better. Max calls it "the cushy ride". This makes sense to me, and is comfortable on the road, and sucessful on the track. Given proper springing (15-25% Free Sag and 25-35% Loaded Sag) enough rebound damping to kill the second bounce cycle, and little to no compression damping, will be the ideal, front and back.

 

The rear suspension is next. Typical Jap suspension is overly soft front springs and overly firm back spring. I'm not sure if this is the Guzzi norm, but the goals for the back will be the same as the front. Next winter's project, as I have still got a PC III to install, and a rear fender to excise.

 

Your thoughts??

[Guest vkerrigan]

dlaing................I weigh about 240 lbs w riding gear (Aerostich Darien) plus the usual protective equipment. I went with the suggestions of Ken Zeller @ Evoluzione in Simi Valley back when he was involved with Guzzis. With my weight and riding style he suggested a 1.0 spring set w 15 mm preload shim, 2.5 w fork oil (Motul) and, after re-checking my notes, I used 415 cc per fork instead of the 425 cc I mentioned in the previous post. As I said, I'm very happy with the "road feel" of the bike now. I was able to turn my compression damping WAY down to one and sometimes two clicks above full CCW.

I sure don't claim to be any suspension guru, but I'm very happy with the mods done to the front end. But, as many have said before, once you improve the front, it highlights the rear's deficiencies. When I get some money ahead, I'll probably do something with the shock as well.......................vk

[Guest ratchethack]

This might be a good place to chime in with a closely related question. A number of posts on this topic lately have made reference to the idea of varying fork oil volume as a way to adjust the "air spring" effect in the fork. Dave just mentioned the possibility of "bottoming out" on the low volume of compressed air.

 

This just doesn't compute for me , but maybe I just need to be better schooled. I've rebuilt and "kitted" a number of forks over the years, and using the shop manual as a guide, I've had the Marz fork on the Guzzi dismantled down to the cartridges and fully laid out on my shop bench twice in the last year - once to inspect the cartridges while I was changing out the oil, and once to replace the seals.

 

Just looking at it on the bench for a pedestrian level of understanding of how it works, it appears that the "operating volume" of air in the forks is sufficiently large relative to the oil volume that changing the oil volume by adding or subtracting a few tens of ml of oil would have a negligible effect on changing the "air spring" effect.

 

Now this is almost certainly different with other forks, but I can't see how it would be possible to bottom out these forks on what appears to me to be a relatively large remaining air volume after the forks are fully compressed. It would seem much more likely that bottoming occurs prior to the mechanical limit of the springs, and that a "hard" limit is reached according to length of travel by a rapid ramp up of hydraulic pressure approaching full "hydro-lock", so that bottoming is achieved by design via compression valving of the oil.

 

If it were possible to bottom out the fork on compressed air, regardless of oil level, I would expect the seals to blow out long before air pressure could get high enough to limit travel to zero.

 

Am I missing something here, or maybe I'm just way off on visually guestimating the relative volumes involved??

[BrianG]

Ratchethack, the current generation of forks, generally known as cartridge forks, as well as some of the older type, do use the air-space as a continuously variable overlaying spring to suppliment the main suspension spring. Varying the volume of that airspace alters the rate of rise of the airspring suppliment, to the point of hydraulic lock if sufficient space isn't available for full fork travel. Varying this airspace by 5 mm creates a noticable difference in ride quality, to a racer. I have not noticed much difference with less than 10mm change, but that much is definitely notable.

 

FYI, the note I got from Max this morning suggested that "Fork Oil" is not appropriate for most cartridge forks. He suggests specifically identified, synthetic "Cartirdge Suspension hydraulic oil" as made by Spectro and others. They use 2.5 wt and 5 wt. I guess we need to check the specs on the Guzzi recommended oil. I suspect that I'm going to 5 wt in any case.

 

vkerrigan, what have you got for front sag numbers now? Free and loaded, please?

[Guest vkerrigan]

BrianG...............The sag numbers taken at the time of installation (11/03) were: free @ 22 mm and loaded @ 35 mm. I would also throw in a +/- 1 mm fudge factor as well. Since you went with a 1.1, your free number may be a little different but I would think the loaded would be similar. It sounds as though you already have your suspension pretty well sorted for your criteria. Hope you're as happy with the end result as I with mine. ...............vk

[Guest ratchethack]

Thanks for the further education on cartridge forks, Brian. If I understand you correctly, what you're saying is that the "working air spring" in the fork is contained within the cartridges, rather than within the larger open volume of the fork tubes?

 

This would explain why I've been confused by what others were saying, also why fork seals on cartridge forks don't blow out under excess air pressure.

 

I add to my education here every day (OK, not every day. But at least once a week)

 

Thanks again.

 

Rgds.,

 

Ratchethack

[Guest ratchethack]

Brian, after reading your post and Max's recommendation, I did some deeper checking into Spectro fork oil. A few items:

 

As mentioned previously, Guzzi specifies 10w "Fork Oil" for the Marz fork on our bikes. I've been using Spectro 10w "Fork Oil", but I've been considering going to a lower viscosity.

 

Spectro also offers a separate product called "Cartridge Fork Fluid".

 

Now, the Spectro applications are stated thusly (from goldenspectro.com):

 

Spectro Fork Oil is recommended as a replacement oil for all O.E.M. fork units of conventional or inverted design. (no specific fork mfgrs listed)

 

Golden Spectro Cartridge Fork Fluid is recommended to replace all O.E.M. fluids and is excellent in both upside-down and conventional forks. For use in Kayaba and Showa upside-down forks. (of course no mention of Marz)

 

The two products are evidently so different that they use for the most part entirely different rating parameters (except for viscosity index).

 

Example: the 10w Fork Oil has a viscosity index of 111.0. The Cartridge Fork Fluid comes in "Very Light 85/150" and "Light 125/150" and has a viscosity index of 150, regardless of Very Light or Light. (a ~40+% difference!)

 

The Fork oil products use a coefficient of stiction rating (cSt) at 40C and 100C, where the Cartridge Fork Fluid uses a rating I never heard of (SSu) at 100F and 210F. I assume that never the twain shall meet for comparison purposes.

 

Other than viscosity index, there appears to be little other than flash point to be able to compare one to the other. Sorry, but flash point ratings are a little difficult to translate to fork performance

 

The viscosity ratings would indicate that the Cartridge Fork Fluid is way out of range for what I'm interested in, but for all I know, the parameters that can't be compared could be even more important to focus on.

 

So what's the right Spectro product for our Marz forks?

 

At the risk of touching off a low flash point "Fork Oil Thread", I join with you Brian, and throw myself upon the mercy of the Forum Petroleum Engineering community

[andy york]

And as I check my notes oops (brain fart)I ain't got no notes

I went with the 1.15 Springs. I believe I might fall into the aggressive rider catagory( at least thats what the local authorities say)

I'm about 220 also and I have hard bags installed on the LeMans so the bike weighs a fair amount more than stock. I prefer a stiffer ride than most but I have also been asked by others what I have done as they liked the feel.

I don't remember the exact length of spacers used but at the time they seemed quite long compared to stock.So long in fact that I could just barely get the nut back on the "top" of the threaded rod. It took 2 of us to get the job done .I also lent towards the racer side of sag. 7.5wt synthetic as opposed to 10w by Guzzi.

After all was put back, install a couple of ny-ties for measuring purposes and off for some riding. All was good ...only under severe front brake only stopping did I get the ny-ties within 1/4" from using all the fork travel.

I like it as well as some other folk who have had the pleasure of being Guzzied.

And it does bring out the fact on my bike that the rear is undersprung cause she wallows in the corners from the back.That will be fixed next week with some new hardware

 

andy

[BrianG]

Thanks for the further education on cartridge forks, Brian.  If I understand you correctly, what you're saying is that the "working air spring" in the fork is contained within the cartridges, rather than within the larger open volume of the fork tubes?

 

This would explain why I've been confused by what others were saying, also why fork seals on cartridge forks don't blow out under excess air pressure.

 

Ratchethack

44224[/snapback]

 

Um.... I'm not quite sure what you are meaning by the air being "contained in the cartridges", but it's the air space above the oil in the fork tube. If you recall my previous blathering, the "measurement" used to define the amount of oil is taken with the tube collapsed in the fork, but absent the spring. I think the "absent the spring" part is to make measuring easier.

 

Anyway, the measured air-space (less the oil displacement of the spring which we'll ignore for the purposes of this discussion) defines the volume of the "working air spring" at full stroke compression. People have been bantying around 110mm for that number.

 

If you look in there when you have the oil level set and you extend the fork to the free sag point, that defines the "0-value" air spring volume, which is about 100 mm added to the compressed space (given 20mm free sag, out of 120 mm total travel).

 

So that means that we are talking about 110 mm measured air-space at compression + 100mm of tube extension at the free-sag point, for a near 2:1 compression ratio, meaning 2 atmospheres, or roughly 30 lbs of pressure at full compression. Perhaps the seals can/do tolerate that much pressure momentarily. I do know that they blow oil like a bugger when they leak!

 

Anyway, if I have confused the air-spring situation sufficiently, it's only payback for you screwing up my idea about the fork-oil.

 

I was happily going to use Golden Spectro Cartridge Fork Fluid, but now that seems wrong. I'll pop Max the question quoting the Guzzi Service Manual info and see what he suggests. This is gonna hurt!

[helicopterjim R.I.P.]

Not that it matters but I understand you Brian.