Air pressure versus volume spacers- Mtbr.com
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  1. #1
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    New question here. Air pressure versus volume spacers

    Noob question. When would I want to adjust air pressure versus add/remove volume spacers? It seems there is some overlap between these two methods of adjusting fork performance, but I don't quite understand the difference.

    Thanks in advance.

  2. #2
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    Very basic terms: Air pressure for setting sag, volume spaces to adjust for how often or how hard you bottom out.

  3. #3
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    Intense 951 Evo and Santa Cruz Megatower

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    Vorsprung tech Tuesday and Shockcraft cover two ways to view them.

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    Now I know what you are getting at with changing the air pressure for setting sag.

    But, Air pressure adjusts spring rate, which in turn affects sag.
    You shouldn't sacrifice the correct spring rate for sag.

  6. #6
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    Quote Originally Posted by DouglyssRuns View Post
    Noob question. When would I want to adjust air pressure versus add/remove volume spacers?
    As a newbie, you might want to relax, sit back, and work with your more basic adjustments until you understand how they interact.

    But to answer your question, let me talk a little bit about air springs. Forgive me if this is too basic. Your fork is a sealed system, and the pressure on the inside is what keeps it extended. Now if you get on the bike and push the fork down, the pressure increases. Push it down more, and the pressure increases more. If you put 50 pounds downward pressure on the bars, the fork might move compress 2 inches. If you put 100 pounds pressure on the bars, the fork might compress 3 inches. If you put 150 pounds pressure on the bars, the fork might compress 3.5 inches. The spring rate is increasing. Weight to distance is not a linear relationship. Of course these numbers are just made up, but you get the idea.

    So if you put a volume spacer in there, the spring RATE will increase. Using the same numbers in the above example, lets say that 50 pounds will still give you 2 inches of travel. But with the spacers, 100 pounds might only give you 2.75 inches.

    See where this is going? Add spacers, and keep the same initial pressure, and you'll still get the same suppleness initially, but the fork will be MORE resistant to compression the further it goes into its travel.

    Bonus: On some systems, a rider might want MORE volume, not less. Suppose he's a lightweight and isn't using up much travel. Greater volume will decrease the spring rate and allow him to get more into the travel of the suspension. Of course there are a LOT of other considerations, that's why I suggest that you work with your fork the way it is, and see what you can do with it.

  7. #7
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    Air PRESSURE is the primary concern.
    Sag is a ridiculously coarse and crude method of estimating the correct pressure, but its a good start.

    You should adjust your pressure to your desired spring rate, to permit good travel without bottoming, or at least bottoming harshly.

    The volume spacers adjust how progressive the spring is.

    Springs come in two main styles. Progressive and Linear.
    The post above does some explaining with that. A coil spring can be linear or progressive. Assume for now a spring has a rate of 1lb/mm (just for the sake of explanation). If you place a 1 lb weight on that spring, it compresses 1mm. If you add 15 more pounds, the spring will compress another 15mm, and be 16mm compressed. A linear spring follows this "linear rate" until it is completely compressed.

    A progressive spring goes as follows. (And how progressive the spring is depends on how it is created, but lets assume a constant curve progressive)
    The first pound of force compresses the spring 1mm. But, to get it to compress another mm, I must add 2 extra pounds. The next mm of travel then takes 3 extra lb. To compress this spring 5mm total , we need to add it up this way:
    MM 1, 1 lb
    MM 2, 1lb + 2lb
    MM 3, 1lb + 2lb + 3lb
    MM 4, 1lb, +2 lb, + 3lb, + 4 lb.
    MM 5, 1lb, + 2lb, + 3 lb+ 4lb, + 5lb for a total of 15 lb to compress this spring 5mm! The Linear spring only took a total of 5lb to compress the same 5mm.

    Air springs are ALWAYS PROGRESSIVE due to the fact of compressing a set quantity of a gas results in an increase of pressure. (Feel free to look into 'ideal gas laws' and/or combined gas laws)

    Those tokens allow you to be able to give yourself a more progressive, or less progressive spring rate.

    As air suspension has a few drawbacks, such as suppleness and small bump compliance due to "stiction" (static friction) due to the many air seals required to prevent air leaks! As a result, many people run a lower air pressure (lower spring rate) to make the suspension more supple and small bump compliant. (Since a lower spring rate takes less force to compress as stated above). The problem is, in order to make air suspension nice and supple, you dont have the true proper spring rate in there, and you can bottom out very easily. You can combat this by adding these air spacers. They cause the spring rate increase more rapidly, (increasing progressive-ness). This in theory allows to to run a lower air pressure (softer initial spring rate) to allow better small bump compliance and compression of the suspension, while giving you a firm spring rate near the end.

    Where is the right setting? That's up to you, and what you want the bike and suspension to feel like!

  8. #8
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    Air pressure changes how the fork feels throughout the entire amount of travel. Volume spacers only change how the fork feels at the bottom of the travel.

  9. #9
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    Check out YouTube for the SkillswithPhil channel. He recently posted a video explaining pretty much what you asked about. Complete with cool animations/graphics.

  10. #10
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    Quote Originally Posted by offroadcmpr View Post
    Air pressure changes how the fork feels throughout the entire amount of travel. Volume spacers only change how the fork feels at the bottom of the travel.
    Incorrect. It makes the most difference percentage wise at the bottom, but reduced volume changes the entire stroke.

  11. #11
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    Quote Originally Posted by DouglyssRuns View Post
    Noob question. When would I want to adjust air pressure versus add/remove volume spacers? It seems there is some overlap between these two methods of adjusting fork performance, but I don't quite understand the difference.

    Thanks in advance.
    Coming from me, who barely uses suspension...

    My fatbike has a 4 yr. old 120mm Bluto. I weigh 190-ish #. With no volume spacers, at about 25% sag (I set sag from an aggressive position, not just sitting), the fork felt mushy. It dove under braking, it wallowed when I pedaled, it bottomed out when I landed from even mild jumps. I fiddled with the compression damping, but it didn't help - it actually felt better nearly locked out.

    Add 3 tokens (I went as high as 5, I think - 4 might still be optimum), still 25% sag (not sure what pressure). Fork doesn't dive, it supports me when I pedal hard, and only bottoms out on really hard hits. It handles so much more precisely. Compression damping is 1 or 2 clicks from wide open. 1 for the rough stuff. 2 for smoother stuff.

    -F
    It's never easier - you just go faster.

  12. #12
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    Quote Originally Posted by DethWshBkr View Post
    Incorrect. It makes the most difference percentage wise at the bottom, but reduced volume changes the entire stroke.
    Is it actually noticeable at the top of the stroke? I wouldn't feel a 1% change at the top of the stroke, but a 20% change I probably would.
    Fox has their handy little air spring curve graphs, but without actual numbers on the X and Y axis it's not as clear. The first 20%-30% looks similar though.

  13. #13
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    For the yellow line you would boost up the pressure to prevent bottom out. This will bring the mid portion of the curve up higher. (At the cost of a comfortable beginning pressure). Personally, I give up plushness for support and like running less spacers. But it can feel like a board on flat trails.

  14. #14
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    Quote Originally Posted by offroadcmpr View Post
    Is it actually noticeable at the top of the stroke? I wouldn't feel a 1% change at the top of the stroke, but a 20% change I probably would.
    Fox has their handy little air spring curve graphs, but without actual numbers on the X and Y axis it's not as clear. The first 20%-30% looks similar though.
    It will make a difference as soon as the suspension begins to compress. Not much at the start, but yes.

    Ignoring temperature - P1 * V1 = P2 * V2

    Their chart does show this.

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