Achieving full ride height on air spring forks

twentysheets
twentysheets Posts: 5
edited May 2008 in MTB workshop & tech
Hi,
I've just purchased a Rock Shox Revelation 426 Air U-turn which has both the nagative and positive air pressures valves. I have two questions re the set-up:

1) When I set the positive and negative air pressures both to 120psi (I'm about 68 kgs) and have the U-turn travel set to max, I'm not achieving the full ride height. ie The figures on the side read 115 mm rather than the full 130mm (no weight is being placed on the bike). If I drop the negative air pressure by 20 to 30 psi then the fork extends. Is this normal? And why can't I achieve full ride height with the pressures set for my weight?

2) The recommended air pressures differ between the manual and the printed decal on the side of the fork. Anyone know which is correct?

Your help would be much appreciated!!

Comments

  • nicklouse
    nicklouse Posts: 50,675
    run less neg air till bedded in.

    also forget the figures set pos. air to give required sag and then the neg air to suit riding style.
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  • Thanks for the quick reply., and the advice. I'll give that a go, I'll drop the neg air pressure while wearing in the fork, then up it over time. Do you think I'll eventually get up to the full ride height with equal pressures. I get your point though about not going by the quoted pressures. Cheers.
  • dave_hill
    dave_hill Posts: 3,877
    You use the negative pressure to tune how sensitive the positive spring is - imagine that the positive spring props the fork up and the negtive pulls it back down again.

    You will find that the forks will become more supple as you bed them in, but if they are anyhting like my Pike 454s, you will need to run a touch more negative than positive anyway otherwise they develop a top-out "clunk" as they extend.
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  • That's interesting because at the moment, the more negative air pressure I add, the less travel, or overall ride height I get.
  • dave_hill
    dave_hill Posts: 3,877
    Which is as it should be! The more negative spring you have, the further it pulls the fork into its travel!

    Just to get an idea of what is happening, empty both chambers completely. Now, pressurise the positive spring only and you should find that the fork extends right out to its full travel.

    Now start adding pressure to the negative. As you approach the same pressure as the positive spring, the fork should start creeping down. What you are doing is effectively setting the bump sensitivity.

    I have my Pikes set at 150lbs positive, 160lbs negative. At full travel, the 140mm marker is just showing above the wiper when I'm off the bike.
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  • Thanks for the info.

    My revelations have 130mm travel, when I add around 120psi positive and 110psi negative, I'm only just seeing the 115mm marker at full travel with no weight on the bike. Would you say this is normal?

    Cheers
  • dave_hill
    dave_hill Posts: 3,877
    Thanks for the info.

    My revelations have 130mm travel, when I add around 120psi positive and 110psi negative, I'm only just seeing the 115mm marker at full travel with no weight on the bike. Would you say this is normal?

    Cheers

    I wouldn't be happy if that was the case. Take 'em back.
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  • supersonic
    supersonic Posts: 82,708
    Let all the air out of the negative. Then all out of the positive. Refill the positive. Then the negative.

    Remember the two chambers oppose one another, so in theory you can alter the prssure in one by changing the other (fork moves as its sags or extends, chamber gets bigger/smaller, pressure must change). Start from scratch.