[anden]

From the Reba manual: "More negative air pressure results in a suspension set-up that is more active, especially to small bump input."

As far as I understand, that would be wrong.

What the negative pressure does, is that it strives to compress the fork. With higher negative pressure, the spring curve will start at a smaller force, and then go up to about the same bottom-out point as with a lower pressure, but through a steeper curve. A smaller force will be required to start compressing it from unloaded. And the fork will get more sag.

Now, when you sit on the bike and get that bigger sag, you will be at a steeper point on the spring curve. Therefore, when you hit a bump the fork will not compress as easily. It will be less active. Not more active.

So, if you want a "plusher" ride, you want a smaller negative pressure. Not a bigger. If you then want to maintain 20% sag, you need to decrease the positive pressure as well. Because it's not possible to maintain both the sag and the bottom-out force, and get a more active set-up. You have to compromise at least one of them.

Currently riding my own Reba at 100 pos, 80 neg (160 lbs), and still experimenting.

Should I contact RockShox for an explanation, or did I get something wrong?

 

[tscheezy]

I run 10 psi more negative pressure than positive and do find RS's recommendation does improve plushness, and here's why:

When climbing or any time that the sag weight on the fork is reduced, the fork tops out when you are running low negative pressures. To reestablish sag, you have to overcome the "preload" that the positive spring now creates. With a high negative pressure, the fork is always held in a near-sagged state and there is no sensation of topout thumps even when climbing with the fork extended or when rolling through whoops.

Otherwise I agree with your assessment.

 

[anden]

If the fork is topping out, then indeed there may be too little sag. Like in the red curve in my first pic. But like I mentioned, you will want to decrease positive pressure too. Like in the pic below.

Either way, that's not what RockShox is talking about. They talk about the small bump input and "bobbing", which is about spring curve steepness, and not about avoiding top-out. They go on in the manual: "Less negative air pressure results in a suspension setup that does not move or 'bob' under rider input or small bumps".

 

[tscheezy]

I was adressing your plots above. Now you are showing a drop in the positive pressure too. I can't do this because the fork rides too low under heavy braking, etc. Too divey, bottoms too easily. I like a bit of mid- to late-stroke ramp up which the higher pressure provides. I also use quite a bit of negative pressure to still get it to sag nicely. It is the ability to control late-travel behavior (seemingly) independently of sag which is so cool about pos/neg chambers (though not truly independently as you have pointed out: I use the steeper overall curve). It does sound like the marketing arm at RS went to town on the tech descriptions though.

How do you generate your plots?

 

[marco]

hi i'm not tech minded and i'm 220 lbs but.....

if i put RS reco pressures in my reba race non u-turn i find that i cant get full travel ie the 100 mm.
so i have 125 psi in pos and 130 psi neg with a 1/4 turn from full clockwise on the rebound compression.
these settings are a good compromise but if i'm landing from a 2 foot drop off i find the fork really compresses which is slightly scary!
my priority is to have the fork plush out continous bumps. help!

 

[anden]

I was adressing your plots above. Now you are showing a drop in the positive pressure too. I can't do this because the fork rides too low under heavy braking, etc. Too divey, bottoms too easily. I like a bit of mid- to late-stroke ramp up which the higher pressure provides. I also use quite a bit of negative pressure to still get it to sag nicely. It is the ability to control late-travel behavior (seemingly) independently of sag which is so cool about pos/neg chambers (though not truly independently as you have pointed out: I use the steeper overall curve). It does sound like the marketing arm at RS went to town on the tech descriptions though.

How do you generate your plots?

I might send the question to them then.

It seems that the spring curve can be put pretty much anywhere on the chart, and be tilted to your preference. What you can't do, is to change the curve shape from that slight S-shape - you can't make it straighter or curvier. So if you want high bottom-out force and 20% sag, then you are going to have a steeper curve. Perhaps quite naturally.

About the plots:

I noted the pressures at unloaded and at sag by having the pump connected, measured the sag, estimated the relative piston area difference from the pics in the service manual, and entered it all to the relationships for gas pressure (p*V=n*R*T), force, and area. That way I got the effective chamber lengths and volumes, and the following formula, where P is unloaded pressure in psi, V is total chamber volume, L is unloaded chamber length, and X is travel:

F = ((Ppos*Vpos)/(Lpos-X) - (Pneg*Vneg)/(Lneg+X))/181 [N]
Vpos = 77,500 mm3
Vneg = 15,500 mm3
Lpos = 129 mm
Lneg = 30.4 mm

Which I put into Excel, and let it make smoothed plots.