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Bladders, Sliding Gas Pistons, and Assumption

Tech Note 003

In the third Tech Notes article, we are looking at the question of the best way to pressurise shocks, given that recently it's become quite topical again.

So for all of you looking for instant enlightenment, I don’t have a definitive answer for you, but I do hope to reply to some of the more emphatic claims made by some in a slightly more balanced manner.  Before we get into the business of which is better or worse and why, I need to state that I am not a chemist. Some of the ideas I am presenting here are based on discussions with industrial chemists and I may have inadvertently managed to get the detail slightly wrong, but for the purposes of this discussion my limited knowledge should be enough to enlighten in most instances.  Of course, if any suitably qualified chemist can correct me or provide the theory behind the various chemical reactions at play I would be most grateful for your input into this document.

It has taken quite a while to arrive at my current thinking.  I have because of the opinions of others taken a few detours down dead ends, and I confess that my own thinking was often based on assumption rather than fact.   It’s the problem with not applying proper tests to our observations that lead to ill founded assumptions.  When suspension techs get together and talk shop, the question of bladders versus sliding gas pistons often occupies those discussions.  The major factor that fuels this debate is, that when you strip a shock with a bladder separating the pressurised nitrogen from the suspension fluid, if the shock has been assembled for any length of time the oil will ‘fizz' and often quite dramatically so often resembling the head on a beer.  In contrast, when working on a shock with a sliding gas piston, this normally doesn’t happen, even if the shock has 'been to the moon and back'.  This easily observable phenomenon supports the observation that gas is getting through the bladder and getting in to the fluid.  The matching assumption that initially seems quite fair is, that bladders are bad because this happens and sliding gas pistons are good because it does not.  It also supports the further assumption that because there is nitrogen gas on the fluid side of the bladder it degrades the performance of the shock, right?  Well maybe its not quite that simple….

To be honest given what I had seen and the comments of others I for a long time thought this was how it was as well, ignoring the not so obvious facts, and not really thinking it through.  I remember discussing bladders some years back with a very competent suspension technician who strongly disagreed with bladders.  To prove his point he took a shock that had been sitting on his shelf unused for three or so months after being rebuilt, depressurised the shock and opened it up, the fluid foamed up as proof of the poor performance of a bladder in service. 

So how can you argue with that solid observable fact?  Simply you can’t, but the untested assumptions that follow on from the observation don’t stand scrutiny.  Let's throw a few more points of information into the mix.  For the purposes of this discussion, let's accept that after a period of time (read a few months) some gas dissolves in to the shock fluid.  Obviously the only significant source of gas is on the other side of the flexible rubber bladder.  Let's agree given the lack of another competing source of gas that the gas that drives the foaming in the fluid of a freshly dismantled shock is from the other side of the bladder.  Given that the gas is readily mixing with the suspension fluid as soon as it's assembled, if you buy a bike thats just a few months old the shock is already compromised and filling up with gas, right?  Well not exactly.  This is one of those assumptions we made before.  The fact is that even quite old shocks off bikes that have done an enormous amount of work over 10 or more years don’t foam up much if any worse than a shock just a few months old that has never been used.  The important but easy to overlook point is that at some point the process that drives the gas getting across the bladder slows down considerably or stops altogether.  We will come back to this soon.

When you buy a bottle of carbonated beverage from your local supermarket or similar, you can shake it and it doesn’t foam up.  However when you release the pressure by opening the container the beverage starts to bubble away, as the gas dissolved into the fluid under pressure is released over time due to the reduction in pressure caused by opening the container and the gas in suspension needing to assume the same pressure as the surrounding atmosphere, the gas falls out of suspension.

So, thinking about our suspension fluid, to cause the observable foaming the gas must be dissolved into the fluid. Quite similar to our above bottle of our favourite carbonated beverage.  This also means that for as long as the shock is assembled and pressurised the gas is dissolved in to the suspension fluid, and critically not a bubble of gas impeding the performance of our shock.  What drives the migration of the gas through the bladder is far less obvious.  In discussions with very qualified industrial chemists the consensus amongst them is that it is likely the process of osmosis that is drawing the nitrogen gas across the bladder, hence as the fluid approaches saturation the process slows significantly and in most cases stops.  Lets keep in mind that we are talking about a very modest amount t of gas dissolved in to the suspension fluid.  The reason I say most cases is that there are rare instances of the bladder all scrunched up and the shock body has pockets of gas in it, not desirable at all.  Having said that, almost all of the shocks found in this condition are leaking fluid and are low on pressure so the ‘evil' bladder can’t be blamed completely.  Another often talked about trait of the bladder equipped shock versus the gas piston shock is that the gas dissolved in to the suspension fluid means the fluid in the bladder equipped shock has much less cavitation resistance.  I have not been able to verify this in real world testing, I think it’s another one of those untested assumptions.   In a modern shock absorber cavitation is effectively suppressed by maintaining favourable pressure balance in the shock.   If your design or spec is so close to the onset of cavitation in normal operation that you need minute improvements in fluid quality to keep things in balance I think there are greater problems to be solved.

Because the sliding gas piston does not let gas into the fluid its still better, right?  In isolation this might be a fair statement, but the sliding gas piston is not without it's little secrets either.  Sliding gas pistons are for the most part a plastic material to keep weight, and importantly inertia low, some are made from aluminium wich is dimensionally more stable, but are undesirably high in weight.  Given a shock that is changing direction often many times a second the mass of the gas piston is quite undesirable.  Worse is the method of sealing the piston in its bore. Almost universally it is an O Ring that is squashed between the piston and the body of the reservoir to make a gas tight seal.  While the piston is moving, it is in all likelihood similar to the bladder in performance, when it stops however things are different.  The O Ring grabs the body of the reservoir and does not want to change direction again, or for that matter start moving at all.  Clearly the bladder is vastly superior in this instance.  But what does that mean in the real world?  In the case of a motocross machine landing a jump, the forces at play effortlessly overcome any stiction that the sliding piston arrangement has, I would think the differences  between the two solutions in this instance would be nigh on unmeasurable in otherwise identical shocks.  However consider a motorcycle braking hard trying to keep a very lightly loaded rear wheel in contact with the ground, here the bladder has a clear advantage allowing more freedom of subtle movement.

So which is best, I will leave that up to you the reader to decide.   Next time you see a sensationalistic report of suspension fluid all frothy in a freshly dismantled shock, keep in mind how it got there, questions if it is actually doing anything measurably bad, think about what you might be giving up just to have better looking fluid when the shock is pulled apart, and enjoy the show.  Don’t forget assumption is the mother of many an error!  For my money the simple construction and freedom of movement of the bladder gets my vote.

Kerry Dukie

Dukic Performance

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