Negative Air Chamber Getting PSI right...??

KonaKurt

I am finding something very very fiddly to do!!

I have 2009 RS Revelation U-Turn 426 Air Forks, which of course, have a + and a - chamber. Acheiving the desired PSI in the + chamber is relatively easy, since my RS fork pump always loses about 15 PSI when I quickly detach it (so I just over pump that amount then detach.

But my - chamber seems to vary the PSI it loses when I detech as quickly as I can, between 20 - 40 PSI, making it near impossible to get a desired PSI spot on. This is made worse of course, by the fact that once I detach the pump, I cannot even make a good guess of what PSI is left in the chamber. When TFI serviced my forks recently, they said there was nothing wrong with my chambers, when I mentioned this problem.

Got any ideas?

KK.

KonaKurt

I'm not sure if this is normal, but I have also noticed that a fair amount of oil gets blown out of the fork valves when I detach. Maybe slightly more from the + valve.

I'm not sure if this is related, or normal...??

KK

supersonic

The pump loses that amount - not the fork! No need to overinflate. Pump to the amount that gives you the sag you desire.

There is a guide in the FAQ on dual air chamber and set up. Invert the fork to stop oil being blown out of the negative valve.

KonaKurt

The pump loses it...? Yes I know this.

But my point is that the amount of PSI that is lost when I detatch the pump from the negative valve VARIES, making it impossible to guess the PSI that I have left in the fork.

I know the exact PSI settings for my chambers for my 'perfect ride', but just cannot get the negative spot on, which makes a big difference in the fork response.

For example, suppose I want 110 PSI in the negative. I pump to 100PSI then detatch, then reconnect pump (as quickly as I can - I am good at doing this consistantly each time) and see a 20 PSI drop to 80 PSI.
So... I then I pump in 130PSI, and then detatch pump (expecting another 20 PSI drop to get 110 PSI)

But when I quickly reconnect the pump, I see that it actually lost 35 PSI, leaving just 95 PSI.

I could repeat again, and find that the pump then lost 15 PSI, or the next time 25 PSI... etc etc. I can't understadn how the same pump, the same chamber, the same connection times etc, can VARY the pressure lost, because as your FAQ's correctly suggest, any one pump should have a virtually constant PSI loss per valve connection.

It's logical to me that such small negative chambers will be more tricky to set correctly, but does anyone have ways around this problem?? Or are negative chambers purely just guess work (in which case, why do RS bother stating PSI ratings)?

KK.

PS: I thought it must be the pressure dial itself on the pump that was inaccurate, so I took it to a garage to be tested. It's virtually spot on each time to within a few PSI. So I do not have a fualty pump dial/gauge.

nicklouse

no, the air is not lost from the chamber it is what ever you set it at.

the reason you get different readings when you reattach the pump can be manifold. from the position of the pump handle to how much oil get s into the pump or even how much is still in there.. also as it is a dual air chamber every time you play with one chamber the other chamber can move effecting the volumes

so add air read the setting job done. why re attach the pump there are too many variables.

KonaKurt

OK Nick, so you are saying that to achieve 110 PSI in the negative, just pump in 110 PSI, and when the pump is disconnected, absolutely nothing will escape from the fork chamber (because the air I hear escaping as I disconnect the pump is coming from the pump, and not the fork)??

KK.

ma9mwah

that is correct

diy

Even more so if you are loosing oil when you disconnect - which is actually evidence that everything is correct. Its possible that you just need to wear the new seals in, by running more -iv for a bit. They are quit stiff for a while until you get everything sloshed about inside.

KonaKurt

Thanks everyone, understood!

DIY, thank you for that, that makes alot of sense to me. My forks have been a bit on the stiff side for a while, and until today I haven't used them much. I set my pressures, hung the bike upsidedown for an hour, then rode the forks pretty hard.

I noticed a big improvement. Travel is fuller and smoother. The 'feel' is plusher. Rebound is still a bit off, though I just keep mine set exactly halfway between the 'turtle' and 'hare'!

KK.

diy

be careful storing upside down, you can lose oil out of your dampener. When I went from solo air to dual air (tora to reba) I increased my rebound damping (more tortoise), as you can make the air spring much more progressive using -ive pressure.

Even though I have now got my rebas working properly after a full air, wiper seals and moco service (costing 60 quid or so in parts and oils), I think on balance I prefer the simplicity of the solo air set up.

The problem with the dual air setup is that you can achieve very similar results with vastly different settings, so its quite hard to work out what to adjust to make changes.

Northwind

Yeah, it's a seriously powerful adjustment tool which unfortunately makes it quite hard to get the best out of. Solo air is great provided the characteristics of the fork suit you as standard, dual air lets you change that around at the cost of, well, being able to change it