Pre-'03 Marzocchi Fork damping modification

[Tom M]

The 2002 and earlier Marzocchi forks have a rebound damping cartridge in one fork leg and a compression cartridge in the other.  As has been pointed out by GuzziMoto, if you remove the spring from the compression side and cycle the fork leg up & down you will feel a point where the damping suddenly increases alot.  This is due to some oil bypass holes in the side of the cartridge being blocked by the damping rod piston part way through its stroke.  If you look at the attached pic you can see two of those oil bypass holes in the middle of the cartridge.  The one that's closest to the middle of the tube goes through both walls, the second one only pierces one wall.  When damping rod piston blocks the hole that goes through both walls it feels like a hydraulic brake was applied.

 

I attached a drawing of the stock cartridge and what I'm thinking of doing to lessen the hydraulic brake effect.  I actually like the way my fork works now but there might be room for improvement here.  Comments are welcome.

 

 

MARZOCCHI COMP CARTRIDGE REWORK.PDF

[GuzziMoto]

Maybe over this winter I will take the wife's forks apart for another freshening, and can take pictures of what I did. But if you re-assemble the forks with out the springs in them it is very apparent what the issue is. You can feel the lack of compression dampening. No wonder people did not like the red frame bikes, no compression dampening through most of the stroke and then it hits hard all at once.

[LowRyter]

gees, ...I like the dampening on my red frame.  It's set up a little stiff but seems to handle good.

[GuzziMoto]

As I said, if it is anything like the forks on the wife's red frame bike there is no compression dampening, until the forks go past the ports in the damper tubes (as pictured),then all the sudden you get full (lock) dampening.

Feel free to test this next time you are changing fluid by re-assembling the forks minus the springs and run the forks through the stroke. That is what I did and, while I did not think it was bad before, I was impressed in how much more controlled it was after. 

[Skeeve]

The 2002 and earlier Marzocchi forks have a rebound damping cartridge in one fork leg and a compression cartridge in the other.  As has been pointed out by GuzziMoto, if you remove the spring from the compression side and cycle the fork leg up & down you will feel a point where the damping suddenly increases alot.  This is due to some oil bypass holes in the side of the cartridge being blocked by the damping rod piston part way through its stroke.  If you look at the attached pic you can see two of those oil bypass holes in the middle of the cartridge.  The one that's closest to the middle of the tube goes through both walls, the second one only pierces one wall.  When damping rod piston blocks the hole that goes through both walls it feels like a hydraulic brake was applied.

 

I attached a drawing of the stock cartridge and what I'm thinking of doing to lessen the hydraulic brake effect.  I actually like the way my fork works now but there might be room for improvement here.  Comments are welcome.

 

Nice blueprint, but since I'm not that familiar w/ the insides of forks in general, & USD forks at all, is the piston stroking up from the bottom or down from the top relative to the tubes as displayed in the bp? I'm assuming the holes thru the tube wall function like the holes in the damper rod in conventional forks, and the piston is solid, forcing oil thru the holes? It would seem that rather than bothering with filling the "B" holes, that just adding additional appropriately sized holes in the 2nd wall & maybe going up to 10wt fork oil from 5wt would be easier for most folks [since adding metal is harder than removing it.]

 

Unfortunately, I haven't a clear idea of how the separate leg/circuits [rebound & compression] work at all in the 'zocchi's, but have read several times over the years here on V11LM that supposedly expert sources have said the Marzocchi's chosen by MG for our V11s aren't really very good, & not worth playing with for "serious" [ie, track] use, but what V11 owner fools themself that they're going to go out there & whip up on the J-brand crotch rockets? If there's something the shed tinkerer can do to signficantly improve the stock forks with little cost but the time invested, I'm all for it!

[GuzziMoto]

As I recall.....

There is dampening in the pistons on either end. The adjustable dampening is in the piston connected to the rod. That one is moving down through the tube under compression. The valve on the other end is fixed dampening. Neither really matters until the piston moves past all the holes in the tube as fork oil just blows through the holes instead of being forced through the valving. That is why the compression adjuster has no effect until the last part of the travel, where the fork nearly locks up. It is something of an adjustable hydraulic bump stop.

I do not claim to be a suspension expert, but in my opinion the forks on the V11 are not that bad. But the do have a correctable basic flaw in the compression side. The only have any noticeable compression dampening in the last inch or so of travel. The rest of the time the oil just freely blows through those holes in the tube. Closing up one or more of the holes to force oil through the valving, coupled with appropriate oil choice (with the hole closed up you may want slightly lighter oil as you now have dampening), can result in a decent fork. It is still way behind the Ohlins forks or even forks from most late model sportbikes. But those upgrades cost $$$, this is something you can do yourself for little or no money.

You could just use thicker oil but then you still have the issue that the last inch of travel is hitting that hydraulic bump stop, and with thicker oil that would be even more pronounced. Plus your adjuster has no effect on the first four inches of travel.

Fixing this is something you can do yourself, or you could pay a suspension guy to do a basic set up to make what you got as good as it gets.

If I was paying someone to do a thorough revamp of my V11 forks I would have to at least consider upgrading to '06 GSXR 750 front end as that would have much more capability in the end, on many levels. In fact that is almost exactly what I did for my Daytona except it was an '04 GSXR 1000 front end.

[Tom M]

I did the modifications and put the forks back on the bike over the weekend.  It'll be a while before I get to try them out since it's cold and snowy out there.  I think I'll ski instead

 

It took my welder about 30 seconds to fill those two holes with his TIG setup. 

 

My goal is a nice supple fork for the small hits but no bottoming except for the biggest hits.  I feel like I was pretty much there before making this change, but I'm hoping that this change makes the compression damping transition from very little damping to full damping a little smoother.  We will see...

 

FWIW when I installed stiffer springs a few years back I dialed in the preload and oil height to get the right sag and get full travel out of the fork only on the big hits.  While doing that I found my fork wasn't very supple even with the compression adjuster backed all the way off, so I drained the fresh 7.5wt oil out of the compression side and went to 5wt.  It worked.  I'm certainly no expert but the fact that it worked told me that there is some damping going on even when the damping rod piston is above all 3 of the stock bypass holes.

[Skeeve]

As I recall.....

There is dampening in the pistons on either end. The adjustable dampening is in the piston connected to the rod. That one is moving down through the tube under compression.

 

Thanks, that's the info I was lacking that's relevant to size/placement of the holes...

 

The valve on the other end is fixed dampening. Neither really matters until the piston moves past all the holes in the tube as fork oil just blows through the holes instead of being forced through the valving. That is why the compression adjuster has no effect until the last part of the travel, where the fork nearly locks up. It is something of an adjustable hydraulic bump stop.

I do not claim to be a suspension expert, but in my opinion the forks on the V11 are not that bad. But the do have a correctable basic flaw in the compression side. The only have any noticeable compression dampening in the last inch or so of travel.

 

That's because you don't need any compression damping unless you take a >50% of stroke hit... Compression damping interferes with the suspension's ability to react to small bumps...

 

The rest of the time the oil just freely blows through those holes in the tube. Closing up one or more of the holes to force oil through the valving, coupled with appropriate oil choice (with the hole closed up you may want slightly lighter oil as you now have dampening), can result in a decent fork. It is still way behind the Ohlins forks or even forks from most late model sportbikes. But those upgrades cost $$$, this is something you can do yourself for little or no money.

Yep. The "little or no money" part is the bit I like. As I like to say: "If it's for free then it's for me!" 

 

You could just use thicker oil but then you still have the issue that the last inch of travel is hitting that hydraulic bump stop, and with thicker oil that would be even more pronounced. Plus your adjuster has no effect on the first four inches of travel.

 

See, that's the real problem: the compression damping coming into play shouldn't be an "all or nothing" scenario, it should gradually have more play, so that it is more seamless in operation. Let's say the fork has 5" [125mm] of stroke: the escape holes [since that's what they are] for oil in the tube outer wall can be divided into zones. Since the 1st 1" [25mm] is lost to sag, there won't be any holes for the 1st 2" [50mm] of stroke length; that's so the 1st inch lost to sag and the 1st inch of actual stroke used have the maximum play without any compression damping f/x [in this range of small bumps, rebound damping is all that's needed, & not very much of that, lest the forks "pack down."] So, at the 2" mark, there would be the max amount of open wall space [how one determines that will come up later.] Once the piston moves past the holes & occludes them at the 2" mark, some compression damping will come into play, but it will still need to be mild, so to appear more seamless in action and not upset the handling mid-stroke. At the 3" mark, as the piston move down, it will occlude the next set of holes, and move into the "deep stroke" range where compression damping is beginning to become a real need. At the 4" [100mm] point, the piston is moving into the last 1" of travel, and covers the last holes in the outer walls, so that any remaining oil is forced thru the piston damper(s), for maximum effect [hydraulic bump stop time.]

 

Now, how to decide about the sizing of the holes? Basically, it's time for phi. Nature loves phi, & therefore, so should you. The holes in the sidewall should progress in a 1.6x ratio. What is the area of the holes in the pistons? Use 1.6x that for the hole(s) at the 4" stroke point. At the 3" stroke point, use 1.6x the area of the holes at the 4" stroke point. For the 2" stroke point, use 1.6x the area of the holes at the 3" stroke point. Chamfer all the holes after drilling them, btw: not only do we not want any burrs left behind to screw up any seals, etc., but we want to minimize any shear f/x and get the damping medium [fork oil] to flow smoothly thru the holes when it needs to. Adjust the viscosity of the fork oil to suit.

 

Ride on!

 

 

PS: Did some [literally] back of envelope figuring, and an all-drilling, no welding option using the existing holes in the tube and basing the size & placement on an approximation of a phi-progression would be to use a #1 machinist's drill at a point .925" up from the bottom of stroke [adding in the 2.13" ref from bottom of tube, that would be ~3.06" up from the bottom of the tube], and the same drill at another .925" up from there, or at ~3.98" [heck, let's call it 4"] from the bottom. Then, use a letter N drill at ~4.9" up from the bottom. All these holes would only be thru one side of the tube; I'd space them out so none of them are in line w/ other holes, for strength. Remember, you'd be adding some void space, so you'd likely need to increase the fork oil viscosity slightly. Don't forget to de-burr & chamfer all the holes for smoother operation. Sorry for the 90% of the rest of humanity, but I don't have any metric drills & don't feel like looking up what the metric equivalents to my SAE machinist & letter drill sizes would be. ["That will left as an exercise for the reader..."]

[emry]

The main problem with velocity squared dampening (holes) is that they do a poor job of being able to react to slow speed movement (round bumps, often not enough damping) while not being harsh during high speed movements (square bumps, when it can hydraulically lock). The fact that this system is also positionally sensitive (damping changes based on fork compression) make matters even worse. Think about going over a series of bumps spaced closely, the first no damping, the next more, the next tons... (Cartridge bleeds are used on very long travel suspensions though, baja cars, monster trucks, etc.)

 

In most motorcycles a fairly linear damping curve is desirable, shims do a good job of this and the holes in the valve can be sized to act only during the fastest of movement when velocity damping can be useful to prevent bottoming.

[GuzziMoto]

I don't see how going towards a damper rod fork (holes or a hole in a tube to bleed fork oil through) is a good thing, but if Skeeve wants to go that way I wish him luck.

KTM has had a bitch of a time getting progressive dampening to work. Not sure they have gotten it yet 100%. I prefer to reduce the amount of bleed off through the holes and let the valve stacks perform the dampening. That is what they are there for. 

I am not a fan of progressive spring rates nor am I a fan of progressive dampening.

By the way, the moving piston is the normal low speed dampening and it is externally adjustable. The other valve at the bottom of the cartridge is the high speed dampening and it is not externally adjustable. The bleed off through the holes in the tube will diminish the effectiveness of the low speed valving and the external adjustment until the suspension movement is fast enough that the oil cannot all go through the holes. It is as if there are three stages of dampening instead of two. The low speed stuff is handled by the bleed holes until speed rises high enough to force oil through the valving in the piston. The bleed holes are large enough and there is enough of them that their dampening rate is very low. Almost no resistance to compression is provided by them. As the speed rises and the valving in the piston comes into play there is the potential for normal dampening in what I think is a pretty small window.  Then, if the speed gets high enough the bottom valving comes into play. That applies until the fork compresses past the bleed holes, at that point the dampening rate shoots up dramatically and the are just the two standard valving rates being applied as the bleed holes are out of it.

It is not a great system but it can be improved on as mentioned. I would try to find a balance between reducing the bleed off and fork oil weight, but if you want to take the valving in the piston out of the equation and focus on the bleed off of fork oil ala damper rod method you certainly can.

[LowRyter]

what is the big complaint?  does someone have Ohlins envy?

 

I like the handling & ride. 

 

BTw- the Ohlins aren't top line Ohlins either.  You can always go for more and more.  Or perhaps gut it all but the springs and weld a shock to the fork ala Dr John.  That would be cool.

[GuzziMoto]

The complaint is that the early V11 forks have little to no compression dampening as delivered. If that is not an issue for you then you need not worry.

If you ride in a manner that having compression dampening would make things better then this does concern you.

It has nothing to do with Ohlins, although they are nice. But even run of the mill Showa and KYB forks have real compression dampening.

Yes, there is always better options, but what we are talking about here is an option that is free to little cost and gives you the compression dampening that the forks do not have in stock form.

Again, if you have no need for compression dampening then don't worry about it.

[LowRyter]

well everyone needs compression dampening, sometimes the jolts are little stiff for bumpy roads around here but the control is good with little brake dive.  all in all not a bad compromise.

[Skeeve]

I don't see how going towards a damper rod fork (holes or a hole in a tube to bleed fork oil through) is a good thing, but if Skeeve wants to go that way I wish him luck.

Who said that was what I wanted? I'm just talking about modifying the existing setup in a logical manner, not that I think it's better than a proper cartridge set-up like Gold Valves which have been shown to work well.

 

Don't be putting words in my mouth, when there's never enough room for pie to begin with!

[GuzziMoto]

I was equating drilling holes in the cartridge tube to bleed off fluid to the holes in a damper rod set up. Very much the same idea. I would rather increase the amount of fluid required to pass through the valving then decrease it as the holes in the cartridge would do.

To each their own, and I meant no offense.