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I have a question about suspension forks that have a pre-load knob only on one side. I believe having it on one side only is not uncommon, and the other side is often equipped with a lock-out knob. My question is: why is there a pre-load knob only on one side. Particularly for rather simple (low-end) coil suspension forks having only one knob seems to imply that you are pre-loading only one of the two coils in the fork, while the other spring would remain unchanged.

1) Is this assumption (that only one of the two coils gets pre-loaded) correct?

2) Why is it done like this? (Would this be done to have the "sag distance" still be covered by the other coil?)

3) Doesn't pre-loading only one of the two coils cause an unfavorable asymmetry?

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    Forks often are totally asymetrical inside. I have an old fork with knobs on both sides, but one of the legs is completely empty, the knob is just for the optics ... Still works flawlesly after 15 years oft nearly maintenance free abbuse.
    – linac
    Commented Apr 23, 2015 at 23:19
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    Thank you, I had assumed forks must be symmetric as I expected high friction from the deformation upon bending in an assymmetrical configuration. But o.k., it really comes down to how much deformation is actually happening. If the fork is very stiff, as it seems to be according to what I learn from you here (and from the answer below), then the deformation is so small that it does not matter. Interesting.
    – Nicklas
    Commented Apr 24, 2015 at 2:42

2 Answers 2

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  1. Your assumption is correct yes, only one side gets "preloaded".

  2. It's done like that mostly to save on expense and because a preload adjustment on both sides would be overly fiddly when the adjustment can be accomplished with one knob.

  3. Asymmetry, yes. Unfavorable, not so much. Forks are designed to be very stiff where they are not supposed to be suspending. That being the case, the two sides should always move as a single unit, thus only one preload is necessary because ultimately the forks preload is going to be an average between the preload of each side. That means expense and effort can be saved by putting adjustments on only one side that will ultimately affect the fork as a whole.

As you can see from the shot below, the internals on a fork may be very different by side. Suspension forks are designed with those stiff arches allow each side to move only as a unit.

Fox Shock internals

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  • Thank you very much! This is a very interesting and helpful explanation and information. By the way, I also have another, even "more affordable"(some may say: "cheap") suspension fork, bought just now, which has no lockout knob, but a preload knob on both sides. Would this imply that I can preload both springs (separately), and that this fork actually is symmetrical with two coils, right?
    – Nicklas
    Commented Apr 24, 2015 at 2:29
  • I would guess that is the case, yes.
    – Deleted User
    Commented Apr 24, 2015 at 15:08
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Since this question has been resurrected back to the top of the feed and the existing answer doesn't fully clarify the internal operation of suspension forks, I'll try to pick up the slack.

Firstly, any good suspension system (in general) has two main components: a spring and a damper. It's a common misconception that suspension forks are just repackaged pogo sticks. (Extremely cheap forks will indeed omit the damper, but that's a edge case).

  • The spring stores and releases energy in a controlled way.
  • The damper dissipates energy in a controlled way.

These work together to control the suspension's movement and dissipate the energy of bumps and other road irregularities. Accordingly, your observation that

[...] suspension forks that have a pre-load knob only on one side. I believe having it on one side only is not uncommon, and the other side is often equipped with a lock-out knob.

makes sense--one leg contains only the spring (controlled by the preload knob), and the other leg contains only the damper (controlled by the lockout knob). It is not necessary to duplicate these components in each leg; there is only one spring in the fork.

Due to the large vertical spacing of the bushings in each leg (shown in the picture attached in the other answer), the friction due to this asymmetry actually isn't that bad. The uppers and lowers are designed to slide freely with minimal friction regardless of how the fork is loaded--consider that the fork needs to operate correctly even as your entire body weight is slamming down onto it when you hit a bump.

So in short:

  1. That assumption is incorrect, there is only one spring to preload.
  2. The fork is stiff enough that the one spring can smoothly control the entire assembly's movement.
  3. There is a significant asymmetry in force, yes, but due to the high stiffness there is negligible asymmetry in movement.
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    Nice answer. It may be worth mentioning forks like the Cannondale Lefty, which show very clearly that asymmetry needn't be an issue. It also shows that it's possible to combine both the spring and the damper in one leg :-)
    – Will Vousden
    Commented Jun 3 at 15:04

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