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Why are the suspension curves s-shaped, rather than just increasing? I understand that air spring curves will steepen a lot when the residual air volume reduces (curves should have a singularity towards zero residual volume), but why do they start off steeper and then become flatter before becoming steeper again?

(In more mathematical terms, why does it have an inflection point?)

Is that actually the case or is it just drawn like this by the artist who created the manual?

Air spring characteristic curves from FOX manual

Note: it would be really interesting to see actually measured curves with values & units rather than just these qualitative charts

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    I think you're seeing the negative air chamber equalize with the positive air chamber in the initial parts of the stroke. But I'm not sure about that
    – Paul H
    Dec 27, 2023 at 16:57
  • @PaulH Sounds possible. At first, I thought then that curve should extend into negative stroke, but yeah the location of zero is kinda arbitrary & depends on the reference frame of choice - could be mech extension limit, could be fork extension at 0 load, could be at sag with rider. so yeah ..
    – Apfelsaft
    Dec 28, 2023 at 8:31
  • Shock curves generally don't start at sag. I am at least confident in that. Also, if you're asking about rear shocks and not front forks, the kinematics of the rear linkage can some into play as well, since rear real travel is measured vertically and not along its path.
    – Paul H
    Dec 28, 2023 at 14:20
  • @PaulH Wouldn't the negative air chamber decrease initial force, hence making the slope flatter instead of steeper at the start? Regarding your second comment, the graph is labeled as "air spring curves", not "rear suspension curves". The linkage design shouldn't be a factor.
    – MaplePanda
    Jan 4 at 6:06

1 Answer 1

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So I thought a bit about it, and it's basically fleshing out Paul's comment:

When setting the sag, we use e.g. 25& travel, and the force is then the static load distribution of the bike (throw in some cosine of head tube angle for the fork or some kinematic transmission ratio for the rear shock).

When lifting the bike, the fork goes to some neutral position where forces of pos/neg chamber (and maybe wheel weight) cancel each other out. In order to extend the fork beyond that, you'd have to pull the wheel down, and then you actually act against the negative chamber, and pulling further compresses the neg chamber even more. The force increases in negative direction. And compression of the negative chamber is also a progressive air spring hence we get the curve in the other direction.

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