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Thread: Changing Marzocchi fork seals & oil - Shiver 750

  1. #31
    apriliaforum expert Frodo's Avatar
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    The workshop manual specifies:
    Fork oil quantity (right stem) (Marzocchi) 530 cm³ (32.34 cu.in)
    Fork oil quantity (left stem) (Marzocchi) 545 cm³ (33.26 cu.in)

    Right fork: Oil level (from sleeve rim, without the spring with preload pipe and with stem at end of stroke) 65 mm (2.56 in).
    Left fork: Oil level not specified.

    Note that the air gap is specified at the end of the travel (which seems to be the usual way to do it). Adding the spring will reduce the air gap further.
    The manual says the travel is 120mm, so the air gap (without spring) moves from ~180mm at the start of the stroke to 64mm at the end. So the air compresses to a third. I would think that this adds significant springing at the end of the stroke.
    Last edited by Frodo; 08-17-2019 at 04:13 PM.
    Frodo
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  2. #32
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    A compression ratio of 3:1 gives about 95 lbs / 43 kg, per leg.

    If you're doing this seriously, you will most likely be measuring the force required to compress the fork, as I imagine each one is unique in terms of how the air gap changes with the spring in or out. There must be a reason the spec is usually with the spring out, though...

    Otherwise, it's enough to do it by feel. Add / remove a few mm (~ 5 cc) or so at a time. It might be best to start low (not too low!) and just squirt some juice in the open cap when necessary, no need to remove the fork legs.

    Measure the level at the next change if desired, springs in or out, it's only for your reference.
    2009 Shiver (White) | UK

  3. #33
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    Quote Originally Posted by IndelibleInk View Post
    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.


    I also feel like it may be better just to slightly under-fill rather than overfill from a shock characteristics standpoint. Sry about the math mistake. Was trying to whip something out during a short break haha.

    Maybe that's part of the recipe of how my friends get there dirtbike shocks redone so that it's good for the trails but doesn't bottom out on jumps. Just fill with a bit more fluid... but not so much that you blow the cap off or blow fork seals haha. Light fork compression will yield similar spring rates while full compression will yield much larger spring rate.

    Anyways.. this is all good stuff. Learning learning. Thanks for the post!
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  4. #34
    apriliaforum expert Frodo's Avatar
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    Quote Originally Posted by IndelibleInk View Post
    There must be a reason the spec is usually with the spring out, though...
    Easier to measure the air gap - no spring in the way.
    I reckon the engineers would design the air gap with the spring in, then set the specs with it out.
    Frodo
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  5. #35
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    Sorry for the big delay, but I had some pre-cancerous thing cut off my forehead, so my helmet and bandage were in conflict. That meant no riding since mid July. I've been told that all fork oils do not really have the same viscosity even though we think we are getting 5 weight when the bottle says that. So on the advise of ESP, the local suspension shop/guru, I ordered Motul 2.5 weight for the compression leg. When I changed the seal I originally put in 5 weight for both sides and the resulting ride was quite stiff. I pulled only the right leg and put the lighter motul 2.5 weight in and it feels much better now. So the left leg still has 5 weight and the right 2.5 weight. Not sure if it is just that I want it to be better or it really is, this is all seat of the pants stuff. I will say the 5 weight was without a doubt on the harsh side though.

    I took a 35 mile test loop up Big Tujunga Cyn and back on Angeles Crest (LA area) and it seems to work well. It is certainly different than stock, I'm just not sure it is better. The official weight called for is 0, but no one I contacted has heard of such a thing. I've read a couple of blogs where people have added 5 or even 10 weight, I can't imagine how that could work based on what I experienced. For now I think I'm going to quit playing with it and just ride it.

    Frodo, thanks for your experience on the fork oil change, that helped me. When I measured oil level with a cable tie, the original level, ( in the leg that was not leaking), was 3/4 inch. So that is what I've been using. The only way to do it by volume is to disassemble everything to make sure all the old oil is gone, and I just don't want to spend the extra time at this point.

    At least it doesn't leak!

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