Understanding Suspension: Cavitation

CAVITATION: What It Is, And What It Isn't.

You will commonly hear people in the bike world refer to cavitation in the context of suspension. The simplest possible technical explanation: cavitation is when a small amount of your damper oil temporarily turns from liquid into vapour because of very low pressure (aka vacuum).

This occurs when the pressure drop over a damper piston equals or exceeds the gauge pressure in front of the piston. This is caused by any combination of high damper shaft velocities, insufficient gas charge pressure or insufficient pressure drop across base valve relative to the piston/midvalve pressure drop. To translate that technical description to something a bit simpler: imagine your shock shaft/piston moving through the oil in your shock as it compresses. If you were able to compress that shock fast enough that the pressure forcing the oil through your piston/shim stack couldn't keep up, you would draw a vacuum behind the piston, at which point a small amount of your oil would vaporise in order to fill the gap. This is cavitation: the point at which oil pressure behind the piston hits the fluid's vapor pressure (essentially once a vacuum is drawn). The typical symptom of this is a sharp compression spike, followed by a knock as the damper changes direction between compression and rebound, at which point the shock will extend, moving your main piston back and compressing the vapour until the pressure builds sufficiently for it to return to its liquid state, at which point the piston "slaps" the oil as rebound damping kicks in sharply, since you're now forcing oil through the rebound valving rather than simply using the piston to compress a gas. Cavitation is STRICTLY a dynamic phenomenon - that means it only happens under set circumstances, whilst things are moving.

What cavitation is NOT: when air gets mixed into your oil, the oil becomes aerated and looks foamy. This isn't cavitation (or "cavitated oil"), this is the result of air and oil emulsifying (an emulsion is the mixing of two different fluids - yes, gases are fluids - that aren't soluble in one another). Air and oil mixing can occur for a few reasons:
1. Because either your damper is an emulsion damper (such as Rockshox's non-Charger dampers), where there is no physical separation of air and oil
2. Because your damper is an open bath damper (most Marzocchi forks for example), where once again oil is not sealed off from air
3. Because your sealed damper (all rear shocks, Fox FIT cartridge forks, Charger dampers) has either been bled imperfectly or ingested air (this happens over time in certain dampers).

In sealed cartridge dampers, air in the damper means service time, but in emulsion and open bath dampers, aerated oil is not a factor you can easily control. Most oils have anti-foaming agents in them, however these simply break down bubbles more easily rather than preventing your oil from getting sloshed around with air in the first place. In open bath forks with high oil volumes, it's less of a concern because the most heavily aerated part of the oil typically sits well above the inlet/outlet ports of the damper.

If cavitation is truly occurring (it's a lot more rare than the use of the word would imply), it is absolutely nothing to do with the "quality" of the oil you're using, it is a function of the damper configuration (including valving, gas charge, design etc). In other words, changing your oil won't fix it.

So next time somebody tells you that you've got "cavitated oil" or that your suspension problems are caused by cavitating, or that a particular type of oil will prevent cavitation, please be aware that what they're actually telling you is that they don't fully understand what they're talking about.

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?

I concur, much appreciated!

Excellent. More of these, please.

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.

Awesome explanations Steve. Maybe do one of these posts every other week or once a month? :banana:

you should write some articles for the front page

Excellent. More of these, please.

Yes,keep it up(if you have time)much better than the some suspension guru rubbed my shock/fork the right way,and now all is awesome but I don't why threads.:???:

thanks for the great information, very interesting to read!

please could you give a layman's explanation of the difference between 'de carbon' type rear shocks (i.e. most of the shocks on the market) and the Ohlins technology 'double barrel' used in the Cane Creek DB mountain bike shocks?

thanks for the great information, very interesting to read!

please could you give a layman's explanation of the difference between 'de carbon' type rear shocks (i.e. most of the shocks on the market) and the Ohlins technology 'double barrel' used in the Cane Creek DB mountain bike shocks?

Are you looking to understand the physical differences in layout/function, the conceptual differences, or how the two affect real world performance? Because the first one is simple, the second one is a longer explanation, the third is a long, long discussion with a lot of background knowledge required to fully understand.

Are you looking to understand the physical differences in layout/function, the conceptual differences, or how the two affect real world performance?

I've read Cane Creek's pamphlet on the "Double Barrel" which had some graphics and kind of understand the idea, but not enough to understand how it would actually effect a real world riding scenario

It seems the DB setup allows more control of the oil flow using the adjusters, whereas 'de carbon' shocks have a very limited adjustment range as the adjusters account for a very small amount of the oil flow going through the piston. I also read something about DB shocks recirculating oil.

What I am having problems understanding is if the DB shock just allows a better initial setup for a specific frame(which could be achieved on de carbon shocks using a shim-stack rebuild or whatever other trickery is involved)

Or is the DB just physically a superior shock because of the way it works when dealing with the real terrain?

Any information you could provide without getting too technical (I don't mind reading tech stuff..I enjoy it! but don't want to waste your time) would be appreciated

many thanks!

Rob C

Are you looking to understand the physical differences in layout/function, the conceptual differences, or how the two affect real world performance?

I've read Cane Creek's pamphlet on the "Double Barrel" which had some graphics and kind of understand the idea, but not enough to understand how it would actually effect a real world riding scenario

It seems the DB setup allows more control of the oil flow using the adjusters, whereas 'de carbon' shocks have a very limited adjustment range as the adjusters account for a very small amount of the oil flow going through the piston. I also read something about DB shocks recirculating oil.

What I am having problems understanding is if the DB shock just allows a better initial setup for a specific frame(which could be achieved on de carbon shocks using a shim-stack rebuild or whatever other trickery is involved)

Or is the DB just physically a superior shock because of the way it works when dealing with the real terrain?

Any information you could provide without getting too technical (I don't mind reading tech stuff..I enjoy it! but don't want to waste your time) would be appreciated

many thanks!

Rob C

The CCDB has 3 main selling points:
1. It's very adjustable - with separate (I avoid the term "independent" because it just isn't accurate, all HS/LS adjusters have overlap) high speed and low speed adjusters for both compression and rebound, all of which have a fairly wide range, it does work well off the shelf on a wide variety of bikes. Cane Creek also give pretty good starting points for their damper settings for almost any bike on the market, and the adjustments are consistent and precise.
2. Cavitation is almost impossible within this damper. Because oil reaches the back side of the piston both through the main piston's ports/shim stack, and simultaneously through the adjusters, cavitation can't really occur if the shock is functioning correctly (correct nitrogen pressure, standard valving etc). However, this is a fairly theoretical advantage since cavitation is actually very rare in the other high end De Carbon dampers on the market.
3. Because it has no Boost Valve or any kind of platform setting (bar the CCDBA with the Climb Switch), and it has a very small chrome plated shaft, friction is very low.

However, there are a few things worth noting:
1. The CCDB also has highly digressive compression and rebound curves. This means lots of low speed damping with proportionally less high speed damping. Regardless of what you do, this is what you'll have to some degree. For compression it is pretty well unanimously agreed upon that a digressive damper curve is a good thing, otherwise you end up with nowhere near enough LSC and way too much HSC. However, the rebound curve involved much slower LSR than most other shocks on the market - Fox for example are pretty well dead linear across the board, Elka/MRP use a very progressive rebound curve (which I personally am quite a fan of on a DH bike), which means lots of high-speed rebound damping with fairly open and free moving low speed rebound, and BOS use various curve shapes depending on application.
2. The claim that you can't get as wide an adjustment range out of a De Carbon shock is a fallacy - the adjusters simply run smaller ports and/or heavier valving if less oil is being displaced through them. However, no other shocks on the market attempt to be the one-size-fits-all shock that the CCDB tries to be, every other shock I can think of is available with multiple tunes, and as a result the range of adjustment usually doesn't need to be as wide. Keep in mind that a wide range of adjustment typically also means a lot of the range is not usable for you, and that a narrower, entirely usable range of adjustment can be more useful.
3. The CCDB is a strictly speed-sensitive shock. On certain frames, it's absolutely brilliant, but on any frame that lacks sufficient ramp up you'll be making a lot of use of the bottom out bumper unless you spring it quite stiff. Singlepivots, Demos etc aren't an ideal match for the CCDB coil for that reason, at least in this part of the world where seriously big hits are so frequent.
4. The layout configuration of your shock (twin tube, De Carbon, inline, whatever) does not dictate the characteristics of the damping curves (and thus on-trail performance) that you get from your shock. Within reason, pretty well any curve can be generated by any style of shock - the parameters that directly control the generation of pressure and thus force within the shock (valving, port dimensions, shaft diameter, bore diameters etc) are what dictate the way the damper behaves.

What this basically brings us to is that the CCDB is a very good damper, in the right frame, but that the twin-tube design, whilst technically excellent, doesn't offer the inherent and noticeable benefits that marketing departments would have you believe. Real world performance in modern DH shocks is primarily determined by the actual damping curve that the damper is generating, and as mentioned before, that isn't directly related to the layout of the damper. As with all things suspension, the devil is in the details - but details are very, very hard to market!

no other shocks on the market attempt to be the one-size-fits-all shock that the CCDB tries to be

how would you compare it to a manitou isx-6?

but the twin-tube design, whilst technically excellent, doesn't offer the inherent and noticeable benefits that marketing departments would have you believe.

but, but, whatabout all of the guys who buy one and swear that it totally transformed their bike (with the caveat that they're still working on getting it properly set up)?

how would you compare it to a manitou isx-6?

but, but, whatabout all of the guys who buy one and swear that it totally transformed their bike (with the caveat that they're still working on getting it properly set up)?

I have never had the opportunity to do anything with an ISX-6 so I'm not the best person to tell you.

The CCDB is a very low-friction shock with good baseline settings provided by Cane Creek for most people. If you run appropriate sag (the ideal amount for each person/frame varies quite a bit) and set it to the baseline settings, chances are you've got a shock that rides quite decently. What it's very good at is not sucking in any particular way - there are a few occasions (ie frame/rider/terrain combinations) where I would say that there are other shocks that outperform it, but on the whole it's a very solid unit. If you've got a shock that performs poorly, isn't well suited to your frame, isn't valved appropriately, is not well maintained, and/or is not well set up (this is a big one), then installing a super smooth new shock with good baseline settings probably IS a big improvement. In that respect it's a bit like buying a Trek or a Specialized or a Norco or a Giant - you may not be buying THE best bike on the market but you can be pretty sure that what you're getting is at least very good and won't have lots of characteristic foibles that just annoy you.

Is it the be-all, end-all of suspension? No - nothing is. Is it a good solid shock with performance ranging from good to excellent depending on application? Yes.

For the record, my personal bike has a CCDB Air on it. It's good. :)