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Thread: Changing Marzocchi fork seals & oil - Shiver 750

  1. #16
    apriliaforum prov-nov
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    Good to know. will be interesting to see if the SKF seals I ordered have springs on both sides.

  2. #17
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    if they are SKF, extra springs or not... will probably be fine as long as you re-install them the correct direction (don't ask how I know haha)
    Don't be a jerk... jerk

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  3. #18
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    UPDATE: I got the SKF seal, took it apart and since it was apart and I had some time, I took the fork leg to ESP, the local suspension guru to give his opinion of the scratches. He ageed that most dirt bikes have way worse scratched and don't leak, but he said let me chuck it up in the lathe and use some fine sandpaper on it. Most of the scratches disappeared. When I got home I used some 2500 grit from a paint project and sanded a little more on the remaining scratches being mindful that I could go too far. After reassembly I pushed down on the fork a few times and once the initial bit of oil was wiped off with a paper towel, they seemed to be like new.

    I did have one minor hickup during reassembly. When I used a piece of PVC to push the seal in one of the springs looked a little damaged from the effort. Should have had a bigger diameter piece of PVC! So I looked at the spring in the recently failed seal and it was much thicker, and stronger, so I swapped it in instead. I'm sure it could add a little stiction, but I'm really more concerned about leaking at the moment. Seems to be fine. I'm about to go on an hour ride so I will get back to you all on the thicker oil I used. 5 weight instead of the "zero weight" . I also found the proper level per the dealer is 545 ml in the left leg and 535 in the right. I only have 500 in there now, so I will let you guys know what I think.

  4. #19
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    Just got back from my ride, boy is it hot! The 5 wt fork oil was not very compliant! No leaking, which was the goal, but way too much dampening. 2.5 weight seems to be the only option I can find. Even the dealer is suggesting 10 weight, well at least the parts guy, I havn't talked to service yet.

  5. #20
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    Nice to hear that they are not leaking, I changed mine 2 months ago and I have put motul 10w and it feels pretty good, it doesnt bounce around, but I cant really compare it to the stock because the oil in them was never changed so it is night and day with the fresh oil.

  6. #21
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    Ordered Motul 2.5, full synthetic. I plan on changing out the compression leg only to see the difference.

  7. #22
    apriliaforum expert Frodo's Avatar
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    Showa forks have compression and rebound damping in different fork legs. I understood that the Marzocchi had both in one leg and just a spring in the other. But disassembly shows it is not as simple as that.
    I first ran 7.5W in my Marzocchi forks and found way too much damping. From memory I used 2.5W and it is right on.
    The problem with specifying oil volume is that you don't know how much is in there before you add the new oil. With fully disassembled forks, that's fine, but for a simple oil change, you need to measure level. What is the level in your forks after adding the right amount of oil?
    Frodo
    2015 Shiver
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  8. #23
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    The oil level shouldn't effect damping unless you're running such a low amount that the damper isn't getting fluid. the only other reason oil level would mess with damping/spring rate would be more/less air volume. When the fork is squeezed, the air inside acts as a spring as well. More air = more spring... though i would think this has little effect compared to the overall spring rate of the forks themselves.


    edit: keep reading below. I'm wrong.
    Last edited by RadicalTireSkid; 08-19-2019 at 10:20 AM.
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  9. #24
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    The air spring can be significant, moreso the further down the travel you are. It's not linear, unlike a coil spring usually is. Using volume is fine if you do it the same way every time, or if you don't need to compare to anyone else. If you ever want to quantify the relative contribution of the air spring you should definitely use oil level measurements.

    I'd recommend looking up the actual viscosity specs for the fork oils you've tried. It makes comparisons a bit easier. Not all 5w are the same in practice, and the variation can be huge in the extreme. Full synthetic in particular I would expect to be firmer in feel, even as the temperature rises. This is because you want a more consistent feel in the high performance situations that motivated the development of high performance fork oils!
    2009 Shiver (White) | UK

  10. #25
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    Quite the ordeal with those seals.


    With my leak, the Sealmate took care of it. Not much dirt really, but there was some white paste, probably seal grease during assembly.

    No more leak!

  11. #26
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    Quote Originally Posted by IndelibleInk View Post
    The air spring can be significant, moreso the further down the travel you are. It's not linear, unlike a coil spring usually is. Using volume is fine if you do it the same way every time, or if you don't need to compare to anyone else. If you ever want to quantify the relative contribution of the air spring you should definitely use oil level measurements.

    I'd recommend looking up the actual viscosity specs for the fork oils you've tried. It makes comparisons a bit easier. Not all 5w are the same in practice, and the variation can be huge in the extreme. Full synthetic in particular I would expect to be firmer in feel, even as the temperature rises. This is because you want a more consistent feel in the high performance situations that motivated the development of high performance fork oils!


    PV = nRT from basic chemistry implies it would be linear.

    Anyway, using PV = nRT ... P1V1 = P2V2 (holding nRT as constant)

    so if we have a 50mm (2") fork and air volume is about the top 7" of the tube

    V = diam^2/4*3.14159*volume height

    Volume height is pretty much a function of fork compression

    V2 ~ results from fork compression so let's say the fork compressed 4"

    holding diameter the same, volume is only a function of fork compression between V1 and V2

    so P1*7*V = P2*4*V

    P2 = P1*7/4

    P1 = 14.7psia

    P2 = 25.73 psi a so a gain of 11 psi in the fork tube

    the only area effected by the pressure delta to change spring rage would be the cross-sectional area of the fork cap (roughly)

    so a 25.73 - 14.7 = 11 psi gain in the fork tube

    11 psi / fork cross sectional area = 11psi/(2^2/4*3.14159) = 34lbs per fork leg.

    So yea, I guess it can have a bit of an effect. Would really need to look at real world numbers of V1 and V2 when fork is compressed. So if you way overfill the fork tube... yea... spring rate skyrockets. but if you're within 10%-20% fill volume... shouldn't make much of a difference.
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  12. #27
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    P2 = P1V1/V2 - since V2 is only dependent on fork travel in your analysis, P2 remains inversely proportional to stroke - that's non linear.

    People measure this stuff al the time on shock dynos and the like. Plenty of graphs to peruse for those so inclined.

    E.g.
    Name:  air spring.gif
Views: 26
Size:  5.0 KB
    https://www.peterverdone.com/archive/oilheight.htm
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  13. #28
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    Quote Originally Posted by IndelibleInk View Post
    P2 = P1V1/V2 - since V2 is only dependent on fork travel in your analysis, P2 remains inversely proportional to stroke - that's non linear.

    People measure this stuff al the time on shock dynos and the like. Plenty of graphs to peruse for those so inclined.

    E.g.
    Name:  air spring.gif
Views: 26
Size:  5.0 KB
    https://www.peterverdone.com/archive/oilheight.htm
    good point. inverse relationship. but really it comes down to what the overall change in volume is of compressed vs uncompressed.


    I should also correct myself where less air = more spring. as in less air means a larger delta in Volume and a larger pressure differential.

    So yea.. we got into a tangent. LOL

    now I'd like to know real world volumes just to understand how accurate we should be with the fill levels when rebuilding shocks.
    Last edited by RadicalTireSkid; 08-16-2019 at 11:50 AM.
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  14. #29
    apriliaforum expert Frodo's Avatar
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    I fitted air caps (normal tyre caps) to the forks of my very first bike (silver XL250) as the front springs were weak. Made a huge difference. The big benefit of air is that it is very progressive as resistance increases significantly at the end of the stroke. Yes, this will be most pronounced when the air gap is small.
    From memory the Shiver manual specifies oil volume for one leg and air gap in the other - I'd like to know the air gap in both. I think I posted what I set my air gap at in my post. The springing is fine and the cable tie method shows that I never bottom out.
    Frodo
    2015 Shiver
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  15. #30
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    EDIT: indeed, air assisted forks were a thing for a good while back in the day; the only bike I rode with it was a Z1300, but I'm pretty sure the damping was shot. I think for a while MX / Enduro bikes were running air only, i.e. no coil springs, like modern mountain bikes. Most teams are switching back to old-fashioned non-assisted "coil-oil" on the moto side, though, as they're just as adjustable and less finicky / more durable, whilst the weight penalty isn't as much of a problem when you don't have to pedal.


    AIR GAP:
    You do need to be more precise the smaller the air gap is, because of the inverse relationship. The link I posted mentioned needing to be accurate to "a few millimetres" in a fork - at roughly 2 cc per mm in a 2" bore, there's plenty of margin for volume measurement error. I'd also wager an accuracy of 5 cc is far easier to achieve (repeatably) than under 2 ish mm, especially with the meniscus effect. So it's probably also fair to say a shock requires greater precision because of the lower overall volume.


    I think your numbers might have been excessively conservative as well. 4" travel is probably fine, puts it right where you'd be for "firm" braking on the road. 7" air gap is maybe a little large. For the Showas, it was just under 6" - I expect Aprilia asked Marzocchi to work to a similar spec, but that's obviously not a guarantee of any kind (EDIT: Frodo's thread states a 65 mm air gap! But the design is quite different from the Showas, only one leg has a full spring for instance). And there is a precedent for the forks not being filled with the correct amount of oil from the factory, too. Luckily, for a given fork travel figure, the ratio of extended and compressed volume is the same no matter the cross section, as long as that cross section is consistent along the entire length of the (uncompressed) air volume.

    The other thing is you set the pressure ratio to 7/4, when it should have been 7/(7-4) i.e. 7/3.

    Taken all together, with a 6" air gap you'd be looking at more like 30 psi (gauge) at 4" travel and so roughly 95 lbs equivalent air spring effect.
    To test the sensitivity, raising the oil level to the actual 5.8" air gap stipulated (5 mm change) yields ~ 33 psi and 103 lbs. I.e. a 3% change in oil height gave an 8% increase in "support" near the bottom of the stroke.
    Raising the level only 1 mm more gives 105 lbs - hence I'd say the recommendation to work within a mm or two seems sound.

    Compare 4" travel with 4.5" travel (12.5 % extra travel)
    Starting with the 5.8" air gap: 103 lbs vs. 160 lbs (55% extra)
    And with the 6" air gap: 95 lbs vs. 139 lbs (46% extra)

    Bearing in mind that the 0.99 kgf/mm coil spring contributes around 222 lbs of force (extra, above preload) after being compressed through 4" of stroke, rising (ideally linearly) to 250 lbs at 4.5", it's clear the air spring is very important in tuning the progressive feel of the fork.
    Note also that a 0.95 kgf/mm spring, the next softest usually available, requires 213 lbs to compress that same 4", or 240 lbs for 4.5".


    So the long and the short of it is that, theoretically, a change of air gap of the order of 5 mm is at least as effective as a rate up or down on the fork spring when near the bottom of the travel; meaning it's another option to explore if you like the fork everywhere but at the bottom, or only like it at the bottom of the stroke, as examples.
    Last edited by IndelibleInk; 08-16-2019 at 03:53 PM.
    2009 Shiver (White) | UK

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