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If you look at a rear shock which uses a metal coil, you'll always see a large visible coil sitting outside of the shock:

Coil shock

However, front suspension forks that use coil are visibly almost indistinguishable from forks with air springs because they hide the coil inside the stanchions. On the next picture the internal coil is partially visible; usually it is fully hidden inside:

Coil fork

Why don't we see forks with coils placed outside, the same way it always is for the rear shocks?

I can imagine that there may be some advantages of doing so, such as the coil protecting the shaft from rock strikes and scratches. Larger diameter coils may also open some design alternatives, such as thinner (lighter?) shafts. Clearly some disadvantages seems to outweigh them, but what are the disadvantages?

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    Not an answer, as I am guessing. Forks have to handle torsional loads and require stiffness and light weight - tubes are the best way to achieve this with space for coils inside (additional benefits of aerodynamics and cosmetics come to mind). Shocks do not have these requirements (except maybe cosmetics), so putting the coil outside produces a lighter and probably cheaper solution.
    – mattnz
    Sep 18, 2021 at 21:39
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    My guess, the stanchions and CSU contribute to the fore-aft stiffness of a fork. To have external springs, you'd have to either narrow the stanchions at the expense of stiffness, or widen the spring at the expense of tire clearance. Then there's the question of aesthetics and symmetry.
    – Paul H
    Sep 18, 2021 at 21:40
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    Remember, answers are not comments. Even guesses are not comments.
    – Criggie
    Sep 18, 2021 at 22:21
  • Mainly aerodynamics and aesthetics. And ease of cleaning. Motorbikes used to have front springs on the outside.
    – Carel
    Sep 19, 2021 at 7:26

2 Answers 2

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Attributes of telescopic forks:

  • A fork is a structural part that takes all of the front wheel’s loads.
  • Its stiffness is greatly linked to its stanchion diameter. (Second moment of area vs stiffness relationship is not linear) [please expand on this for me if you understand the physics better]
  • An external coil combined with adequately stiff stanchion diameters (basically, the diameters currently in use now, as a coil spring contributes only minimally to a fork’s stiffness) would likely interfere with the tire.
  • The damper is in one stanchion, separate from the spring in the other stanchion. Each side has plenty of room to perform its independent function.
  • For forks normally sold with air springs installed from the factory, coil springs are often only installed as an aftermarket modification, and only by a small subset of riders too.
  • Forks sold with coil springs from the factory are generally low-end forks. Their stanchion diameters are already very small, so external coils would be even worse (likely infeasible) from a structural engineering perspective.
  • The preload adjuster is practical to use when it is located at the left stanchion top cap.

Attributes of rear shocks:

  • A rear shock is a non-structural part. The frame handles strength and stiffness; the shock just controls suspension movement.
  • A little flexibility (as in low stiffness) is honestly a good thing as it can compensate for frame misalignment.
  • Shocks are not as restricted in diameter as fork stanchions are due to their placement, allowing for larger coils to be used. What benefit this may have, I don't know.
  • The damper is coaxial with the spring.
  • The preload adjuster is practical to use when it is a threaded ring going around the outside of the shock.

What all that means is:

  • For a fork, maximizing stanchion diameter is mandatory for increasing stiffness, especially on heavier-duty forks where coil springs would usually be used anyways. For a shock, stiffness is a minor concern because the frame should be handling all loads.
  • A fork has the entire right stanchion to contain the damper, while a shock has to fit the damper inside the spring (well, at least part of the damper; note that piggyback shocks exist to overcome the dimensional restrictions placed by a traditional coaxial shock). Attempting to fit the damper around the spring or even placing the spring inside the damper just makes little sense.
  • An external spring on a fork would require threads somewhere on the outer diameter of the fork leg in order to have a preload adjuster. This would require a radically different design that may not play well with an air-sprung design, and certainly increases complexity unnecessarily. I don’t see it being worth it to make this change for the (currently) few people actually riding coil-retrofitted forks. An internal preload adjuster on a shock would require somehow gaining access to the ends of the coil, which would be hidden by the mounting eyelets/trunnion holes.
  • Smaller diameter shafts also have lower seal friction, so the small damper shafts found on traditional coil shock designs helps contribute to their notoriously high sensitivity. The larger seals found on air suspension cause more friction.

Changing spring geometry and/or metallurgy can compensate for suboptimal spring diameter, but the same cannot be as easily said for fork structural characteristics. The spring parameters found on bicycle suspension (at least for steel, fixed-rate springs) are nothing extraordinary.

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    "Most forks are sold with air springs installed. Coil springs are often only installed as an aftermarket modification." - cheap forks as found on BSO and entry level MTBs are almost always coil.
    – mattnz
    Sep 19, 2021 at 6:28
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    @mattnz Poor wording on my part. Will fix.
    – MaplePanda
    Sep 19, 2021 at 7:39
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    “The damper is in the left stanchion, separate from the spring in the right stanchion” – in my forks it's the other way around, damper in the right stanchion. I'd think that's actually the standard, and also seems to be the case in the OP's picture. Sep 19, 2021 at 21:06
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    @GrigoryRechistov FOX (Marzocchi), Formula and Cane Creek also do sell coil variants of their single crown forks.
    – airace3
    Sep 20, 2021 at 7:25
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    @airace3 So the coil forks are not dead after all. Sep 20, 2021 at 7:28
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GUESSING that a wider spring diameter has less stiffness than a smaller spring using the same wire, so its more "progressive"

Note that the rear of the bike carries far more than half the weight of the rider, presuming steady state.

However the front takes more sharp impacts when ridden off-road, so a narrower spring is more resistant to hard impacts - you don't want the front to compress a lot, quickly because that may cause steering/handling problems.

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    Spring rate can be changed through other characteristics other than just diameter. “ you don't want the front to compress a lot” is something that should be fixed with the damper, not the spring. A “progressive” spring’s spring rate increases through its travel; your use of the word may cause confusion. You are just referring to a spring with a higher constant spring rate in the answer.
    – MaplePanda
    Sep 19, 2021 at 6:04
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    @MaplePanda yeah - there's a deceptive simplicity in the engineering of a spring. I tried to simplfy.
    – Criggie
    Sep 19, 2021 at 14:45

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