Quantcast
  1. XenForo is here. Problems or comments on the new site? Please report them.
  2. New stuff: Our new Classifieds have launched. Check them out!
  3. New stuff: You can now tag users in posts using the @ symbol, and they'll get a notification that they've been mentioned. Try it out.

Does negative air "fight" positive air? Pike

Discussion in 'Downhill & Freeride' started by sam_little, Feb 23, 2006.

  1. sam_little New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    May 18, 2003
    Messages:
    784
    Location:
    Portland, OR
    Hi. So I was reading the owners manual on my new Pike 454 last night. I like the concept of negative and positive air, and I understand what it does in terms of ride characteristics and such. However, I am unclear as to whether or not the negative air "fights" the positive air. By cranking up the negative air pressure to max rec. for my weight, and doing the same for the positive chamber, would I get the same effect as if I just kept the pressure in the positive air chamber a little lower and didn't worry to much about the negative?

    I'm sure I could test all of this out on my own, but I'm trying to decide whether to keep or sell the fork (which is newly won), so I don't want to mount it. If by cranking up both pressures I can keep the fork "bottomless" yet supple, then I think I'd rather keep the fork and ditch my 05 Pike team.

    Anyone feel like explaining the wonders of neg/pos air? I did a search and found only a thread response saying, "I guess you don't understand the merits of negative air." :rolleyes: Well, neither do I, so far.

    Thanks a bunch!
    #1 Feb 23, 2006   
  2. ride_fast New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Jul 10, 2005
    Messages:
    325
    Location:
    boulder, co
    i'm prety shure the negitive air fights the positive air but then again im not really shure. i had a reba and i would have the pressure levels balenced and then i would ride on it and it would slowly get less and less travel until it would like die and have no suspension and then i would have to like pump it back up beccause the positive air leaked into the negitive air. it really sucked
    #2 Feb 23, 2006   
  3. zedro New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Sep 14, 2001
    Messages:
    4,183
    Location:
    at the end of the longest line
    no, the negative pressure allows you to create a much better 'spring curve' than a single positive chamber would. Air springs are non-linear and their behavior varies according to initial pressure, air volume, and the change of volume (and resulting temperature). With only a positive chamber, you end up with either a shock that sags properly but blows through travel, or the opposite with too little sag and too stiff but doesnt bottom. The negative chamber allows for you to run a higher positive pressure to resist bottom out while it countacts that pressure in the initial stroke to get a nicer sag (remember the air springs arent linear so the positive chamber becomes more dominant as the negative approaches zero through the stroke).

    Rear shocks can better get away with single pressures since they have bigger volumes and shorter strokes (and get help from the geometry of the suspension), but it's real hard to get a nice feeling fork without the negative air spring to balance things out. Using the opposing pressures and varying their initial volumes gives limitless tuning, but can also be a pain in the ass to setup properly and maintain the tuning as well.
    #3 Feb 23, 2006   
  4. sam_little New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    May 18, 2003
    Messages:
    784
    Location:
    Portland, OR
    Ahhhh. I love that "click" you get when something finally makes sense. The above is what I was confused about, as I failed to realize that the negative air would in fact be compressed (right?) through the stroke, thus being rendered neutral.

    Was negative air technology first applied in MTB shocks, or elsewhere? Cool stuff, it seems, if you don't mind spending some time tuning the shock.
    #4 Feb 23, 2006   
  5. zedro New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Sep 14, 2001
    Messages:
    4,183
    Location:
    at the end of the longest line
    it's not actually negative pressure, it's a positive pressure applied in the opposing direction (or the other side of the piston); the chamber is actually expanding so the pressure is being reduced to zero. The increase in pressure in the positive chamber happens at a different rate than the decrease in pressure in the negative chamber and thats what gives the desired effect. Also some manufacturers specify a minimum 'negative' pressure so the chamber does not turn into a vacuum (ie. an actual negative pressure).
    #5 Feb 23, 2006   
  6. OGRipper Active Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Feb 3, 2004
    Messages:
    9,454
    Location:
    NORCAL is the hizzle
    Professor Zedro is in the house and class is in session! What he said.

    The key for me is that negative air allows you to run higher positive pressures without lots of stiction. So you get good small bump compliance and also a progressive rate with good bottom out resistence.

    I love my new pike 454 air. :love:
    #6 Feb 23, 2006   
  7. sam_little New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    May 18, 2003
    Messages:
    784
    Location:
    Portland, OR
    Yeah, I thought about that a bit more after I posted the follow-up. Still, the effect is the same, though I didn't appreciate that the chambers change volume at different rates. Thanks for the good info.
    #7 Feb 23, 2006   
  8. zedro New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Sep 14, 2001
    Messages:
    4,183
    Location:
    at the end of the longest line
    no the volumes do change at the same rate, but the change in pressure vs. volume does not. For instance the pressure could increase by 10psi for the first inch, increase by 15psi for the next inch and 25psi for the last inch. Now change that initial volume (say add more oil) and the progression might go 10-17-30. This is what makes it tricky.
    #8 Feb 23, 2006   
  9. vitox New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Sep 23, 2001
    Messages:
    2,940
    Location:
    Santiago du Chili
    negative air actually fights the preload that occurs with air pressure in one single chamber.

    i think zed explained the rest pretty extensively, bottom line is you NEED some way to remove the preload or youll end up with sub-par performance, for the reasons zed exposed earlier.

    afaik there are 3 routes, first is the one englund and the original SID (98) used, which is a spring set in a way that it makes the fork compress (opposite to the air chamber which makes the fork extend, hence "negative").

    another way is the negative chamber (dual introduced with the 99 sid), to my knowledge RS was first out with this design so ill explain it based on that, the negative chamber is on the other side of the main piston and expands when the fork compresses, you fill it up from the underside, through the rod that connects the bottom bolt to the air piston. the air chamber is comprised of stanchion, underside of air piston, and bottom-of-stanchion-plug.

    third method is the self adjusting negative, soloair in rs speak but manitou had a similar device before in the nixon and in shocks its been present for years. it works much like the system open brakes use, when the fork is extended, both chambers are connected, but when you compress it a bit, the connection is broken, hence you have the same pressure in both chambers always (at the top of the travel), loose some adjustability for the sake of user friendliness, in the case of the nixon you also gain that instant travel adjustment feature.
    #9 Feb 24, 2006   
  10. Tootrikky New Member

    Rep  |  Likes:
    0   |   0
    Joined:
    Jul 31, 2003
    Messages:
    773
    Location:
    Mount Vernon
    Just got a 454 Air u-turn....it's freaking amazing, super supple yet ramps up perfectly. Wondering about a World Cup now!
    #10 Feb 24, 2006   

Share This Page