can anyone explian negative air. im happy with putting air in the top of the forks to the reccommended pressure, ish, and setting sag etc. then theres the negative air valve on the bottom of the forks.what is this for what does this do and how does it affect the suspension, bike geomotery suppleness etc

A positive spring is one that you compress a negative spring is one that wants to compress and pulls towards its centre. Hope that makes sense.

In suspension terms more negative pressure means a more supple ride and in some cases reduce the amount of travel the fork offers.

OK – I’ll try and keep this brief…

It’s basically to over come the friction in the seals.

Imagine you have a spring, one end against a wall, you’re pushing on the other end. It’s hard to get it to move at first, but once it’s compressed a little, it’s easier to shift…

/////////// < Push here

Now, imagine you have a smaller spring pushed up against the end of it, both springs are compressed against each other a little… now you try and compress the bigger spring… it’s easier because the smaller spring is helping.

Big spring = positive air chamber
Little spring = negative air chamber

/////////// | /////

| < Pushing here

You can also use this neg’ve air spring to tune your suspension. Slightly less air in the Neg’ve will act less on the Pos’ve air spring, so it will be harder to move it initially, for a firmer feel.

Slightly more air in the Neg’ve spring and the main (Pos’ve) spring will be easier to compress, making your suspension much more supple on the small stuff.

No more than 15psi difference between the 2 cambers. I usually run mine with pretty much the same pressure in both, sometimes a tiny bit (5psi) more in the Neg’ve.

Hope that helps.

For the record – it shouldn’t reduce the fork travel

Mainly used to counter “stiction” caused by the extra seals used in air forks.

stooo – Member
For the record – it shouldn’t reduce the fork travel

But for the record – it definately does with the RockShox ’08 Reba and ’10 Revelations I’ve got it. If you put more air pressure in the negative than you have in the positive, it will pull the fork down in its travel.

I can’t see how it does anything to reduce seal friction. This is purely mechanical.

An aior fork is progressive – the effective spring rate gets haerder the more the fork compresses. a negative air chamber gets bigger the more the fork compresses so pressure in it drops. altering the air pressure in the negative chamber allows you to tune the shape of the curve of the progression.

It ‘can’ have a tendency to ‘pull’ the fork travel down a bit if you have a bit more pressure in the negative, as mentioned by PeteG55 above… but shouldn’t. I have seen it start off fine and get gradually worse (no more than 10mm) over a few months. Resetting both pressures usually sorts it.

To be honest though – most folk will run the same pressure in both. In which case it’s just helping get over the seal friction… this is a good thing.

Stoo, that’s the best description of it I’ve ever read, love the little diagrams too! 🙂 Spot on mate.

I’d echo what Pete says though, don’t go much more than 10psi over on the -ve or the for drags down and the pressures will eqaulize anyway/

Also be careful how you st your pressures, it’s easy to end up with an imbalance without knowing. Let ot the -ve, set the +ve, then reset the -ve. 🙂

Didn’t some marzocchis use negative air as a method of travel adjust?

EDIT: And negative air is exactly the method Magura use for their flight control travel adjust system. You open a valve between the chambers compress the fork and close the valve. The nagative air pressure will have increased and the fork runs 40mm or so lower.

I can’t see how it does anything to reduce seal friction. This is purely mechanical.

That’s it’s main reason for existing Teej. 🙂 Stoos expanation covers why it works very well IMO.

PeterPoddy….. < Yep, nice addition… what he said.

(oh, and.. cheers, glad you liked the exp’)

PP & Stooo, I don’t think that’s the reason. It removes/allows tuning of the ‘preload’ effect caused by the nature of using air as a spring (even at full extension, the air is trying to force the fork to extend – a bit like a preloaded coil spring).

Matching the negative & positive pressures effectively removes this ‘preload’ and gives you a spring starting at 0, like an non-preloaded coil.

Conversely, you could also run with little negative pressure, and have a fork which feels like a preloaded coil fork – i.e. it requires more force to over come the starting point of the travel, the ride height is higher etc.

I’ve heard it explained two different ways which are both pretty easy to understand…
First – It’s like lifting a weight that is sat on a spring.
Second – It’s like lifting a weight that is under water.

Either way, it just makes stuff more supple and smooth. 🙂

http://www.sram.com/_media/techdocs/95-4015-022-000%20print.pdf

Positive air:

high pressure makes the fork harder to bottom out and reduces sag, low pressure=more sag, easier to bottom out. Set this based on your weight (table in link/on your fork)

Negative air:

works against the positive (which is why it shouldn’t be more than 15 psi more inflated). Higher negative air makes your fork easier to compress (good for small bumps) higher positive air makes it less prone to pedal bob but less sensitive to the small stuff.

Set the positive relative to your weight, then the negative relative to that value with your terrain in mind. I think I’ve got my negative air 5psi higher than my positive.

Happy pumping!

First – It’s like lifting a weight that is sat on a spring.

That’s keeping it brief!
Nice description… I’ll remember that one in future.

Also be careful how you st your pressures, it’s easy to end up with an imbalance without knowing. Let ot the -ve, set the +ve, then reset the -ve.

I was very sceptical of this, but did notice my Pikes were losing a few mm of travel, let the -ve air out before setting the +ve and all good. It’s more faff but now converted to the above

With the +ive & -ive pressures about the same in my SIDs they sit about 10% into their travel without me being on the bike. This increases to 20% when I’m on it which is the kind of sag I like to run anyway.

Reading the comments so far, have I done something wrong or is this normal? I know others who have exactly the same with theirs so have not been worried by it so far.

Sounds like there is a thousand ways to get it wrong.

I’m with TJ on this (god help me…)

What the negative spring does is adjust the sag by reducing the spring rate of the fork at full extension (and then a reducing amount as it compresses).

Unless you ride your forks topped out (eg no sag) then the negative pressure doesn’t affect ‘sensitivity’, only how much sag you’re getting – the stiction is still there regardless.

Let’s say you set the fork up as most people do – eg same pressure in both (set up correctly of course – +ve then -ve). Because the pressure is equal, when you’re not sat on the bike, the fork will sit at full extension. Add some weight to the bike (enough to overcome the stiction) and the fork will sag.

If you set the same fork up with no negative pressure chamber then the added the weight to the bike, the sag would be less because you’d first have to overcome the preload of the +ve air chamber.

So having the negative chamber allows you to run more sag without having a spring that’s too soft for the rest of the travel but it doesn’t actually make the fork more ‘sensitive’. (at least by my definition of ‘sensitive’ which would be that there’s basically less stiction so it responds better to small bumps).

#remains confused# 😐

My understanding was that it makes the spring rate more linear. A normal coil spring will have a fixed compliance of x N/mm whilst an equal stiffness air spring will have a compliance of roughly x N/mm for the first part of its travel, ramping up to 2x N/mm by the time it’s half compressed, ramping to 4x N/mm by the time it’s 3/4 compressed and so on.

The negative air spring ramps up in the opposite direction, thus cancelling out some of that increase.

Nobby – are you pumping up the +ve THEN the negative (with both empty to start with?

My understanding was that it makes the spring rate more linear

Quite the opposite – the negative pressure makes the fork more rising rate since it reduces the overall effective spring rate at the start of the travel.

My understanding was that it makes the spring rate more linear.

That sounds much more likely.

I can’t see how it has anything to do with reducing friction. In Stooo’s original example of a spring against a wall, I don’t see what friction you’d be overcoming. Adding a second spring pushing the first is equivalent to just using a less stiff spring in the first place. If the springs are linear (force proportional to distance moved) this is true across the entire range of travel.

Air springs are not linear (they’re proportional to 1/(spring length – displacement) ), so I think opposing springs are used to reduce this non-linearity.

In Stooo’s original example of a spring against a wall, I don’t see what friction you’d be overcoming

It’s really a bad example. Air springs are not like coil springs at all.

Because of the seals pressing on the sides of the chamber, air springs have a lot of initial resistance aka stiction – coil springs do not have this. The idea behind the negative spring PARTLY is to provide a little kick start to get the piston moving.

My understanding was that it makes the spring rate more linear.

Again, no. The negative chamber will only have a negative effect until it’s extended to the point where the pressure inside is equal to atmospheric pressure. Further compressing the fork results in negative pressure adding to the spring rate. And as negative air chambers tend to be quite small, this happens early on in the stroke.

Pace forks used a coil negative spring which helped make the fork seem really linear, because when fully compressed the coil spring had no effect; an air negative spring would have had a great deal of negative pressure in it at full compression increasing the overall effective spring rate.

The idea behind the negative spring is to provide a little kick start to get the piston moving.

It doesn’t though – the drag is still there.. All it does it increase the sag for a given positive pressure meaning that you don’t have to run the positive too soft (or too rising rate) to get a decent amount of sag.

Having thought about it a bit more.. it increases small bump sensitivity NOT by reducing stiction, but by reducing the rate right at the start of compression…

Resistance to initial movement = stiction + initial rate

So minimise the initial rate, and get better small bump sensitivity 🙂

It works too.

…or you could fit them, ride a bit, fettle with the recommended settings, ride a bit more etc. When you’re happy with the way the forks rides, come back on here and tell us all what you found the negative air spring did for you 😉

A lot of waffle on this thread.

It’s simple really:
F=force
T=travel

A coil spring (red) starts compressing with only a small amount of force.
An air spring (green) needs lots of force to get it to start compressing (it’s ‘pre-loaded’).
Add an negative spring (blue) and you can counter act the pre-load of teh positive spring.

Several mistakes in that glen

A coil spring usually is preloaded and a air spring does not have to be

Usually the initial spring rate of the air spring is lower than the coilkd thus its actually easier to get it moving.

all this thread shows is that as usual bikers don’t understand how the suspension works.

How a negative air chamber can reduce mechanical friction in the fork is beyond me.

Glen – Only if the fork is topped out which isn’t the case since everyone rides forks sagged, even if they don’t have a negative chamber.

TandemJeremy – Member

Several mistakes in that glen

A coil spring usually is preloaded and a air spring does not have to be

Usually the initial spring rate of the air spring is lower than the coilkd thus its actually easier to get it moving.

all this thread shows is that as usual bikers don’t understand how the suspension works.

How a negative air chamber can reduce mechanical friction in the fork is beyond me.

How do you pressurise an air chamber without preloading it?

IMVHO Glen is pretty much spot on there, and that is was I was trying to explain in my post:

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retro83 – Member

PP & Stooo, I don’t think that’s the reason. It removes/allows tuning of the ‘preload’ effect caused by the nature of using air as a spring (even at full extension, the air is trying to force the fork to extend – a bit like a preloaded coil spring).

Matching the negative & positive pressures effectively removes this ‘preload’ and gives you a spring starting at 0, like an non-preloaded coil.

Conversely, you could also run with little negative pressure, and have a fork which feels like a preloaded coil fork – i.e. it requires more force to over come the starting point of the travel, the ride height is higher etc.
Posted 5 hours ago #

TJ – please explain how an air spring (in a fork or shock) can not be preloaded (and not be far too low a spring rate).
Also, none of the coil forks I have used have been preloaded (although it has it’s uses).

Clubber is right of course, the body weight on the bike might be enough to overcome an air spring pre-load, but you wouldn’t get much sag for the correct spring rate.

can not be preloaded (and not be far to low a spring rate).

I think he meant basically that – eg no pressure above atmosphere which admittedly would be pointless as it’d be too soft.

but you wouldn’t get much sag for the correct spring rate.

Exactly – sort of (see the last para below). But what the negative chamber doesn’t do is adjust sensitivity though more -ve pressure may make it feel like the fork is softer to some extent (particularly to people who run their forks too hard otherwise) as they have more sag and as a result you can feel the fork moving about more when you monkey about on the bike.

The problem with no negative spring is that often the fork is too linear if you want to design it so that it sags a reasonable amount or it’s too rising rate and you get little sag. That’s why the negative chamber is a good thing as it allows you to tune the spring curve.

Simple glen – start it at atmospheric pressure. Then its not preloaded.

When you have correctly set sag both types of fork have an equal preload.

Every coil spring I have ever seen is preloaded sometimes not by mucH.

Right, so you run your forks /shock at atmospheric pressure then and let me know how you get on TJ. 🙂

The point is, for that to work you’d need a completely unfeasible compression ratio.

Once again you are talking BS.

Don’t worry, Glen, he’s just being a pedant though of course technically correct.

How does Solo Air work? Does that mean that both coil springs are set at atmospheric pressure so the preload of one is equal to the other but slightly different to overcome the initial stiction?

so should positive pressure be set to get the correct sag and then add the negative pressure? if i did that with my revs the positive pressure would eb about 70 pis and i am 12.5 stone!!

Yes Glen being pedantic – but a coil spring needs preload as well.

Clubber is nearest right in this

The problem with no negative spring is that often the fork is too linear if you want to design it so that it sags a reasonable amount or it’s too rising rate and you get little sag. That’s why the negative chamber is a good thing as it allows you to tune the spring curve.

Only If its too rapid rising rate then you get too much sag and too linear you get too little

Amount of rising rate is related to the volume. Using a negative air chamber allows you to refine the spring curve further than by merely altering volumes as you have two different curves combining to give a final effect spring rate.