Armchair engineer mode – engaged.

First things first:
Let the air out of the negative side.
Set to the positive pressure to whatever you want. In this example, the positive pressure value is ‘a’. I imagine this is around 90psi for most people.

Now, as you add air to the negative spring (x axis), the pressure in the spring chamber increases proportionally (y axis) for the first (straight) part of the blue curve – until you reach pressure a, at inflexion point b. At this point, the negative and positive pressure chambers are equal.

At b, if you continue to add air to the negative chamber, the forks will lower down into their travel – I call this the ‘suckdown’ phenomenon – demonstrated by the red line on the graph.

As the suckdown continues and the fork lowers itself, there is a change in the volume of both of air chambers. The negative air chamber enlarges and the positive chamber decreases in size. Because the negative chamber is enlarging as you’re adding more air, there is less effect on the pressure for each bit of air you add. The curve flattens off. If you’re simply looking at your shock pump’s pressure gauge at this stage you won’t see much difference in pressure between point b and point c. Maybe 1 or 2 psi, not enough to really notice. However as you can see from the graph, at point c you’ve already lost a significant proportion of your shock’s travel to suckdown. This will result in more sag and potentially more bob. At point c, the pressure starts to ramp up again as the positive chamber pressure increases.

My interpretation of all of this is that point b is roughly where you want to be. Equal pressure but no suckdown. Getting there involves carefully recording your positive pressure, then adding air slowly to the negative side only until you reach that recorded value. If you add air too quickly, you can end up at point c before the shock pump registers significantly greater pressure. According to my theory, running the negative pressure only 1 or 2 psi above the positive pressure can have profound effects on how the fork sits.

Additionally, I suspect a lot of people put too much -ve air in. It’s very easy to make the fork feel ‘car park plush’ by adding extra -ve air. With less -ve pressure, when you’re actually riding you’ll naturally sag into the travel and the springs will reach a different equilibrium – you should still get good small-bump sensitivity, but you’ll be sat higher in the travel, meaning you’ve got more in reserve for bigger hits, as well as the beneficial effect that has on head angle etc. Of course this is all very personal, but try I’d recommend at least trying less negative pressure.

Essay over 😀

summary?

Very insightful and makes perfect sense.

I will take more time and add the air slower into the negative when I set up tonight, see if I can reach point B with no suck down!

Cheers.

Its pretty good advice.

Its very easy to suck down Dual Air RS forks.

I might check mine again after reading that

Nice graph. Don’t completely agree with your “point b” conclusion. The optimum amount of suckdown is bike specific IMO. Some suckdown gives:

1. Faster response to compression (just increase the damping if too bobby)
2. Lower front and BB – better cornering, and less waggy when fully extended under climbing

For example: my TranceX, a 120mm bike, rides better overall with the 150mm Revs “sucked down” to ~130mm.

Frame geometry, fork length, suspension action and handling are intrinsically linked, which means for a given frame, there is a practical limit on the length of travel. For the correct length of fork, suspension action is key, IMO.

I haven’t got my head around Threshold yet!

I’m not sure about point 1 – I don’t really understand why more sag would equal faster response to compression. My thoughts are that as soon as you’re riding and therefore forks are sagged, you’re already in an equilibrium between +ve and -ve, so the initial resistance to movement is just a function of the stiction in the seals? (Provided that you’re anywhere to the right of point b on my graph – otherwise there will be the effect of an unopposed +ve pressure to consider).

As for point 2 – you’re quite right in that forks with too much travel (or axle-to-crown) can adversely affect handling. Was there a reason you didn’t just lower them ‘properly’ using the internal spacers? I can’t really work out the pros/cons of either approach.

buzz-lightyear – Member
Nice graph. Don’t completely agree with your “point b” conclusion. The optimum amount of suckdown is bike specific IMO. Some suckdown gives:

1. Faster response to compression (just increase the damping if too bobby)
2. Lower front and BB – better cornering, and less waggy when fully extended under climbing

For example: my TranceX, a 120mm bike, rides better overall with the 150mm Revs “sucked down” to ~130mm.

Frame geometry, fork length, suspension action and handling are intrinsically linked, which means for a given frame, there is a practical limit on the length of travel. For the correct length of fork, suspension action is key, IMO.

Sorry Buzz but that is stupid. Space them down to the travel you want.

Can this not be simply summed up by “Dual Air is a shit idea, hence the ‘new’ Solo Air”?

Can this not be simply summed up by “Dual Air is a shit idea, hence the ‘new’ Solo Air”?

I think if you understand it AND you’re a tweaker, there’s an extra level of tuning available. I think for most people it’s more hassle than it’s worth, so I can completely see why RS have done away with it.

I can’t fault the logic but it doesn’t match the real world experience- mine suck down way before equal pressure, no matter how slowly you add air.

So following the advice from yourself and the other thread, I have managed to get my forks to 50psi with 3mm of “suckdown”.

I have 80 psi in the positive, with me in the attack position it sags 40mm (just under 30%).

The fork feels fantastic once again, nice smooth stroke, minimal bob and good small bump.

As to why I can’t get them balanced – who knows.

But theyre working again and feeling good, so I’m gonna run with it.

I had a problem with ‘suckdown’ to the point that even if i fettled before every ride, the forks (Pike U-turn 426) would ‘pack down’ and over the course of the ride i reckon i’d lose about 30mm of travel.

Sent them off to J-Tech and the chap on the other end of the phone explained that a delron ( i think) bushing had swollen ever-so-slightly due to water ingress and was creating enough friction to over ride the +ve air pressure. I found i was running less than 2/3rds of the +ve in the -ve chamber and was still having problems.
He resized the bushing by turning it down slightly, since then i’ve been able to run equal pressures without any ‘suckdown’ and have some very supple forks.

Thats interesting. If I find my forks sucking down more and more I will consider bushing resizing. Cheers

Neg air makes air shocks feel like coil shocks

Making the fork plusher and reducing travel a tad without dismantling it to fit spacers is a win win for me.