Which weight oil should I use in my fork?

Hey Lee,
I just recently serviced my Fox TALAS fork and i put in fox’s 7wt susspension fluid. My question is what is the difference in the weight of the oil? And what is best for XC and DH racing through full on extream Freeride.

Hey Chris.

7wt is stock for FOX forks, and I would use stock oil.

OK, let’s step through this:

Damping is the amount of force resisting the movement of your suspension. Compression damping helps your spring resist compression. Rebound damping resists the spring’s tendency to rebound. The ideal amount of damping would give you a ride that is both plush and controlled. Not stiff. Not a pogo stick.

The ideal damping depends on the terrain, the machine and your riding style. There are so many variables here, it’s crazy. Stock forks and shocks come with an adjustment range that suits “typical” riders. Companies like PUSH make a good living at custom-tuning the damping for riders who are exceptionally big, small, fast, slow, aggressive, timid or just picky. I’ve had some shocks PUSHed, and they made my Enduro and SX — both great bikes — phenomenal.

The amount of damping is the product of your oil weight and your valving. If you have an opening of X size and oil of Y weight, it takes Z amount of force to push the oil through the opening. Modern suspension uses a series of ports and shims to provide different levels of damping in different parts of the suspension stroke and even at different speeds. It’s really complicated!

You can change your damping by changing your valving or your oil weight.

Lighter oil reduces your damping. Might be a good idea for a lighter or slower rider.

Heavier oil increases your damping. Might be a good idea for a heavier or faster rider.

But realize that changing your oil weight is a blunt instrument. It changes your damping en masse: low speed, high speed, beginning of stroke, end of stroke, rebound, everything*. Changing your oil is quick, cheap and easy, but you’ll get a more precise tune by changing your valving.

If you’re sensitive enough about your suspension to want to change oil, I think you’ll appreciate a custom tune.


— Lee

*On a FOX. Some forks use separate compression and rebound oil.

5 replies
  1. martin says:

    i have one question, which fork uses separate compresion and expansion oils? can you please give an example? thanx. martin

  2. Trevor Flint says:

    really old boxxers, the new motions control ones use the same oil for both comp and rebound, but a seperate oil for dampening, which you should change really really regulary same with fox 36 and 40 forks, makes them work soo much nicer,

    as for changing oil weight to change a forks feel, dont!!!! every single person who works with mtb suspension industry says its a really bad idea to change the weight for the fork oil, the forks are designed to work with a certain weight oil, changing it can cause lots of issues, too thicker oil can bend the shims as it forces it way through the holes, to light and you wont be able to get enough rebound dampening.

    Most people who want to change the oil weight need the check they are using the right weight spring first, most people arn’t!! then think about getting a custom tune,

    Rant over

  3. Tom Basic says:

    Bruthas and Sistuhs,

    Concerning suspension fluid weights, the following is exceedingly helpful:


    The gist is that the stated SAE weights are meaningless when comparing different brands of fluids, e.g., “Castrol Fork Oil (10 wt)” is actually lighter than “Spectro Fork Oil (5 wt).” Centistokes (cSt)is the accurate unit of viscosity measurement, and the link, above, has an awesome chart displaying cSt’s for quite a few brands.

    I have a Marzocchi EXR with non-adjustable rebound, so I went to a heavier oil to slow my rebound down.

    Trevor, I hear what you’re saying about changing oil, and I agree. But, as the article shows, you may very well be changing your oil weight if you rely on the stated SAE weight (if you don’t use the exact brand and weight recommended)!

    Concerning my fork, there are no damping shims, only flow holes in the damping rods, so I’m O.K. And, the only reason I changed my cSt level is because it was the only way I could modify my damping (I only have adjustable spring preload).

    Also, the above article only confirmed my usage of ATF in my Minute 1:00, which works great and costs a lot less than the repackaged bike fluids.


  4. John Sullivan says:

    Mixing oils: I’ve done quite a bit of research on motor oil and apparently it’s a big no-no to mix synthetic and non-synthetic (and in most cases brands). A component of motor oils is various detergents, conditioners, and viscous additives. Apparently manufactures have radically different approaches to these and they most often do not play well together. The result is significantly detrimental to seals (not performance). This is pointedly a problem between synthetic and non – even by the same company – a big no-no to the savvy. Of note, between synthetic/non there is insignificant initial performance differences, but longevity for particulate retention is the key differentiator/advantage of synthetic (NOT reduced oil breakdown, which is apparently not a factor at all when changing motor oil. Synthetic simply has more uniform molecules – and can deal with particulates better over time). Now to bicycle suspension: my first thought (being practical minded) would be to mix 1/3 of the total with heavier oil to slow down a fork (I weigh 260+ and stock springs often do not accommodate my level of funness). So as long as I stay with the same brand/type (given the detailed caveats above), I still suspect this might be a viable idea – and rebuttal to the false dilemma of either/or oil weights – and its blanket denial. (Bu’ my fork oil goes to eleven…)

    Also, I’ve been told recently by a solid bicycle industry mechanic that although Fox fork oil is bicycle specific it “foams easily”, whereas Marzocchi uses standard motorcycle grade fork oil (Spectro) and it apparently does not foam – or at least no where near as easily. This would only be applicable in the extreme case of long DH runs, but I wonder if there is any general support for this notion?

    One more final share for the Joe reader: “dampening” is a term very often used by the suspension newbie – including myself – and I’ve been keenly reminded that water has nothing to do with most suspension design: “damping” is the correct term.

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