The IFP, or Internal Floating Piston, is found in most modern shocks. Usually, it’s located at the back of the damping circuits, where the oil flows as the suspension is compressed. For simplicity’s sake, we’ll talk about shocks, but it can work the same in a fork when used there (some forks do, but not all, which is another story).

As the shock compresses, oil is pushed through the compression damping circuit, and that oil needs to go somewhere. That “somewhere” is a reservoir, external or internal, which means there needs to be space to hold that fluid. Older designs, particularly forks, had an open space with air in the reservoir, and the oil would compress the air as it was squeezed into that space. The problem with that design is that air and oil could mix as things were sloshed around, and air bubbles could end up getting sucked back into the damping circuits and wreck the damping circuit’s effectiveness.

The solution is the IFP…

rockshox vivid air shock with countermeasure negative spring for ifp
The Rockshox Vivid IFP is the piston with the blue seal, separating the nitrogen that would be inside the larger, darker part of the piggy back reservoir.

An IFP is exactly what it sounds like, a piston floating inside the shock. Typically, it’s separating a high pressure nitrogen chamber on one side from the oil on the other. Mainly, this prevents cavitation by keeping pressure on the oil to help push it back through the rebound damping circuit, which is good. And, it keeps the oil in an airtight system, so bubbles can’t mix in. But, there are two downsides to the traditional IFP.

Typical pressures behind the IFP can reach 300psi, which puts extra pressure on the system, amplifying the positive spring pressure. To begin travel, you not only need to overcome the positive spring pressure, but also start pushing oil against the IFP. Several brands have developed different solutions to this over the years:

  • X-Fusion created a two-stage IFP, where the first stage had very low pressure, making it easy to get the oil moving, with a higher pressure second stage kicking in deeper into the stroke.
  • Rockshox has CounterMeasure, which is like a negative spring for the oil chamber that balances the IFP’s pressure. It’s the spring below the white piston shown in the photo above.

ALTERNATIVES TO A FLOATING PISTON

The other downside is that the piston needs a tight seal to separate the oil from the nitrogen. That tight seal creates friction, and that friction must be overcome to initiate travel. That friction also creates heat, which can pervade the system and affect suspension performance on long, fast, aggressive descents.

One solution is a bladder based design, like the nitrogen filled bladder like on the new Specialized BRAIN shock (shown above). The benefit of a bladder design is zero friction and heat, meaning less effort required to get things moving.

Why nitrogen? Because its molecules are larger than oxygen and more inert, so it doesn’t break down or leak out over time like air would. That’s why you virtually never need to recharge the IFP chamber on your shock.

To summarize, the IFP essentially creates a closed oil circuit inside the shock, preventing air from mixing with the oil so your suspension works the way it should.


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18 COMMENTS

  1. The DVO Jade uses a bladder instead of IFP, as does the Cane Creek DB Inline.

    An increasing number of modern forks use a bladder too: Fox FIT from ages ago, and more recently from RS (Charger), DVO, and Cane Creek, off the top of my head.

    Seen the same kind of bladder from the DVO Jade and Brain in a moto shock’s reservoir, and the oil was still foamy despite 250 psi.

    • RS’s countermeasure is merely a coil top-out bumper. It works similarly to a negative spring in an air spring, but it only works on a very limited range (few millimeters past top-out). It does an incredible job for coil shocks, but it’s not as effective on modern air shocks with well tuned negative air chambers.

      With lower breakaway force, you’ll notice less harshness whenever the bike happens to extend to full travel and touches back down onto the ground, such as the rolling over a wide rut perpendicularly, the wheel falling into a hole/gap, or rolling off any sort of platform/ledge. I’ve personally measured breakaway force to be as low as 25 lbs (BOS Deville 160), Pikes and Fox Float forks around 35 lbs, EVOL and Debonair inline shocks around 35 lbs, and older inline shocks with smaller negative air to be up to 45 lbs (Spec autosag).

    • The oil volume that the IFP is intended to compensates for is from the space the shaft is taking up, as it enters the oil chamber. If it were like the article implied, it would be like the entire column of oil was pushing on the IFP as if it were another air spring, rather than using oil flow to provide damping force. Generally, the oil will backfill behind the piston head that is pushing against it…

      The other purpose of the IFP is to help maintain pressure, to prevent cavitation. When there’s low pressure on the backside of the piston face, it can create a vacuum, which causes microbubbles to form, which then collapse and create a microjet when they pop that causes damage to inner surfaces. The IFP pressure prevents emulsion of air in the oil, which can make the damping feel inconsistent (spongier, as the air can be compressed). Dampers without oil pressurization use this fact to actually compensate for the shaft displacement.

      The problem with small shocks is that the IFP volume is quite small, and therefore there is a large variance in pressure as it gets compressed. An external reservoir is a solution to this problem, which the BRAIN sort of counts as. Also, I’ve seen patent drawings of a reservoir in a Cannondale Scapel SI, fitting in the top tube.

      • Article gave me an eye twitch. FFS, can we stop conflating emulsion & cavitation? emulsion bubbles are gas mixed into the liquid(almost always air for our purposes.) Cavitation is reducing the pressure on a liquid below it’s boiling point, causing THE LIQUID ITSELF to phase change into a gas. As in, the chemical makeup of those bubbles isn’t air, it’s oil. When the vacuum goes away, the bubbles collapse back into liquid oil.

        THIS IS AN IMPORTANT DISTINCTION. Because a sealed damper that has air emulsion going on is leaking, & need to be completely rebuilt & bled, but a sealed damper that’s cavitating either needs more pressure in the IFP, bladder, or whatever other volume compensator is being used, or has a design flaw.

        • Oh, that’s painful. Referring to emulsion as cavitation isn’t unclear or imprecise; it’s just wrong. They’re completely different phenomena. Real cavitation can eat away at metal when the gaseous oil bubbles collapse (though this is more of a problem with water…erosion due to cavitation is a real problem with ship propellers. The effects of emulsion and cavitation are completely different.

          • True, though propellers intentionally cavitate in order to boost thrust, so I assume it’s a combination of the water & a many times greater amount of cavitation.

            My understanding of the issues in dampers, is that cavitating damping fluid can cause flow rate through the damper to fluctuate.

            • Well, if the damage from cavitation alters the size of the porting in the piston, I guess it’s true that they affect flow rate.

              Cavitation in propellers isn’t intentional. After realizing it, and seeing the damage done, engineers went to lengths to minimize it (ex. bigger propellers spinning at lower RPM). Engineers also made use of the knowledge to invent supercavitating vessels, which take advantage of the lessened drag to travel faster underwater (ex. torpedos that generate gas that’s emitted through its nose, which surrounds the torpedo).

              I also take back what I said about the RS Countermeasure feature in air springs. “Modern” air springs, such as those with EVOL and Debonair, still have high breakaway. I was told by MRP that their forks, with their dual chamber air, is tuned well enough to get breakaway force down to 5 lbs. If that’s true, it should stick to the ground well on the “drop test”. Fox and RS air sprung suspension still fail at this test, but then again they do sell to an industry that generally thinks suspension shouldn’t move when pedaling.

            • I’m with Zooey on this: props in general are less efficient (and generate less peak thrust) when they start to cavitate. There’s such a thing as a supercavitating prop, but that’s a pretty special case and used only on racing boats and a few military ships. Submarine props, for example, avoid cavitation at all costs because cavitation is noisy, and it’s easy to detect a noisy submarine.

    • Right? “IFPs are the solution” No bro, they’re one of MANY solutions. C’mon Tyler Benedict, at least go read the Motorcycle Suspension Bible before you try to act like an authority on this stuff.

  2. “…air bubbles could end up getting sucked back into the damping circuits. That’s called cavitation…”

    I recently learned that this is not the correct definition of cavitation thanks to a Vorsprung Suspension post – https://www.facebook.com/VorsprungSuspension/posts/878724148810169

    Basically cavitation is the formation and collapse of vapour bubbles in a liquid caused by a change in pressure, not the introduction of air/nitrogen into the liquid.

  3. There are several factual errors in this article – You may want to go over it and fix the problems.

    As mentioned previously, you’re wrong on the cavitation/emulsion issue, as well as the “size” of nitrogen molecules relative to “air”. Air is comprised mainly of Nitrogen – By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. There is no real world difference in air’s ability to stay put relative to pure nitrogen.

    It’s about the inert nature of Nitrogen, and the fact that it is a “dry”, more ideal gas that is less affected my temperature change due to not having any water vapor present..

    I have such high hopes for you, BRumor – but sometimes, your articles that stray from re-published press releases are disappointing.

  4. Negative springs don’t reduce “breakaway force”, the force needed to overcome stiction. Negative springs, as used in air spring systems, are used to overcome the inherent PRELOAD of an air spring. Coil springs can be run with little to no preload. Air springs, without another spring countering it, don’t have that luxury. Air springs have higher breakaway force because of the extra seals inherent in air springing. No amount of negative spring wizardry will change that.

    • The oil in the shock is under pressure, which adds to breakaway force. That’s one thing that the negative spring can counter.

      Regarding seals and bushings, there’s other wizardry going on like special formulated lubrication with strong film properties, that minimizes stiction. Having bushings reamed is another way to minimize stiction. Kashima coat is like a permanent lubricating film.

      The breakaway force I’m concerned with is breakaway at full extension, not drag in the shock and even the pivots at any other point in the stroke. 5 lbs seems plausible if you only consider the drag, but 35 lbs is something else. Rockshox stated that they’ve measured up to 60 lbs…

  5. All, thank you for the in depth comments. Good stuff. I’ve made a few clarifications to the article to reduce confusion about cavitation versus air bubbles getting tossed into the oil. Couple of points I’ll address:

    – Nitrogen’s ability to permeate small holes (like leaking through a rubber bladder or past a seal) is less than that of oxygen. Remember the fad of filling your car tires with nitrogen so they’d stay inflated longer and improve gas mileage back when gas was $4/gal?

    – Regarding oil backfill, yes, some oil is recirculating behind the damper, which is a common design. But in high speed impacts, the smaller ports and shafts can’t move the oil through fast enough, and the overflow is pushed into the IFP-backed reservoir (particularly on shocks with a piggy back reservoir, which serves multiple purposes beyond just being a place for more oil to go). So, Zooey, you’re technically correct in saying that not all of the shaft volume it being pushed against the IFP, but some is, and for the purposes of explaining what an IFP does I didn’t think it necessary to dive into that…trying to keep the focus on one small part of suspension each week so things don’t get overwhelming. Maybe I’ll dive into the recirculating paths in a future story. I’m open to suggestions for more topics, just use that link at the bottom of the post.

    – There may be a ton of other solutions used in in motocross, but the bicycle industry tends to use an IFP quite frequently, and we’re a cycling tech site, so we’ll focus on what’s used on the things we ride.

    – Next week’s article covers the bladder designs and other solutions used on forks, which is why I didn’t mention them here.

  6. Tyler, as several people have pointed out, the big deal with nitrogen is that it’s dry. Motorsports competitors fill their tires with nitrogen because it’s dry, which means the tire pressure changes less as the tire heats up than would an air-filled tire.

    Yes, that got pushed to consumers with all sorts of snake-oil justifications, but they were all snake oil, including claims that your tires would stay inflated longer. Air is 80% nitrogen. An air-filled tire is essentially a nitrogen filter, so as you refill it, its nitrogen content begins to approach 99%. High-pressure nitrogen chambers are hard for consumers to refill, so using nitrogen there extends the service life of the damper. But it’s air for the main shock chamber because after 2-3 top-ups, it’s nearly all nitrogen anyway.

    And, respectfully, I think you’ve misunderstood GrogHunter’s point about the Motocross Suspension Bible. If I understand him correctly, he’s not just pointing out that there are other options used in motocross but not in cycling. He’s suggesting that your knowledge of the subject is narrow and that you’d make fewer mistakes (e.g., cavitation vs. emulsion, misunderstanding the relationship between air chamber volumes and spring rates, etc.) if you read up on the subject in general.

    The Motocross Suspension Bible would be a good place to start that reading. In many ways, bicycle suspension is a subfield of motorcycle suspension. That’s why Mert Lawwill, Horst Leitner and others were able to waltz into the bicycle industry and come up with pretty great designs. A broader background in suspension design would help quite a bit, I think.

What do you think?