not sure whether this was a psa or a rant… either way i appreciated it!
so vs, i have a question about emulsified oil - what's the symptom/how would you know if this is happening? i've been riding a shock recently that starts making a "frothing" sound towards the end of hard and fast descents, which seems to be accompanied by a decrease in control.
would it make sense that the oil has emulsified/is emulsifying under these conditions, and this is affecting damper performance?
Half/half PSA/rant - mostly just an explanation to clear up a common misconception.
Emulsified oil requires that you have air in your damper that is not dissolved in the oil. All oils can dissolve a certain amount of air; the amount of air (or other gas) that can dissolve in the oil is linearly proportional to the pressure the oil is under (this is called Boyle's Law). At atmospheric pressure (roughly 13.5psi absolute pressure depending where you are), your oil does contain a tiny amount of air dissolved in it: THIS IS NOT A PROBLEM. Air that is fully dissolved in your oil becomes "part" of the oil, and your oil viscosities are measured with air dissolved in the oil.
Let's consider the difference between a solution (as in, when air is dissolved in the oil) and an emulsion (when air is mixed in the oil). When any substance is properly dissolved in another substance, it is broken down to a molecular level, whereby the solute (substance that is dissolved, in this case air) is evenly distributed throughout the solvent (substance in which other things are dissolving, in this case oil), and due to the tiny mass of air that can be dissolved in oil at atmospheric pressure, has negligible effect on the properties of the oil. That is to say, dissolved air doesn't pose a problem.
An emulsion, however, results from the mixing of two substances that are unable to dissolve in each other. This results in bubbles/droplets of one forming in the other - for example, if you mix oil and water, you will see that they distinctly do not dissolve in each other but rather the oil sticks together and the water sticks together. If you forcibly mix them around, you will break up the oil and water into smaller droplets but they will still be emulsified - consequentially they will separate easily. The same thing happens with air - after the oil has dissolved however much air it can dissolve at the pressure it's under (which is very, very little at the pressures we're talking about), all you can do is mix large bubbles of air in with the oil. If you keep on mixing (ie your suspension keeps moving), the bubbles will stay somewhat mixed in with your oil, but if you let it sit for a while, the bubbles will all rise to the top and your oil will de-emulsify itself for the time being.
When such an air/oil emulsion is forced through a damper's valving, you will get very rapid (hundreds or thousands of times per second) changes in the damping force being generated, as air and oil have massively different densities and viscosities. What you feel this as, is commonly perceived as a slightly "rough" damping feel when you bounce on the bike, as though your oil has tiny grains of sand in it. This is most noticeable in emulsion dampers such as the Mission Control and Motion Control systems used by Rockshox.
Now, to answer your question directly: if your shock doesn't sound airy or make any noises when it's cold, emulsification of your oil probably isn't the issue. If it has air in it, you may be correct, in which case it's time for a service. For emulsification to occur during a run, your damper needs to have air in it (the amount that dampers can suck in over time is quite small and takes a relatively long time, air is not coming and going every run). If there's no air in your damper, what you may be noticing is the way the oil noises change due to the change in viscosity as the oil heats up - between 40C and 100C (very much a realistic operating temperature range for rear shocks), even the best oils will drop to between 25-40% of their original viscosity. With shimmed dampers this actually doesn't affect high speed damping very much (long story worthy of its own discussion) but it has noticeable effects on your low speed damping - most noticeable in that your rebound speeds up and that your low speed compression damping can feel like it's more open than when you started your run.
Ultimately though, the easiest way to tell whether your oil has become aerated is to open the damper up. When we service shocks, it's common to see anything from zero bubbles, to literally an explosion of foam as soon as you crack the damper open. For things like Boxxers, take a 24mm flat wrench with you on a ride one day, and pull your compression cartridge out as soon as you stop riding (even after 20 seconds of riding). You'll probably be surprised by how foamy the oil is.