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Presumably the negative chamber has a much smaller piston area otherwise it would totally cancel out the support from the positive chamber.
It should be possible to approximate the resulting displacement/force curve with a relatively simple equation, ignoring stiction (which is horribly non-linear) and damping (likewise).
I wonder if negative air chambers are critical in keeping fork performance more consistent as the air in the fork gets hots through repeated compression cycles?
It doesn't reduce mechanical friction. But it DOES reduce the EFFECT of mechanical friction, by reducing the rate early on in the compression stroke.
That's pretty spot on. So, again, what negative air allows you to do is fine tune the fork's rate.
If you have sag, then the fork is in equilibrium (spring against rider weight) so any bump will cause the fork to move
Well, not entirely - that's where the stiction/seal drag comes in - and that's what's unaffected by all this - sensitivity will largely be determined by how much seal drag there is though the fork's spring rate at this point will also affect how soft it feels.
Molgrips - everyone including me.
I have some understanding of therm in theory which is more than many. Whaty it means in use to handling I am not clear on.
Nothing you do with air springs can possibly reduce mechanical friction. No one has given nay explanation of how this could occur
It doesn't reduce mechanical friction. But it DOES reduce the EFFECT of mechanical friction, by reducing the rate early on in the compression stroke.
How can it not reduce the friction but reduce the effect and rate of that friction
What yo say about preload is also rather muddled.
Reread what clubber says - he has a decent grasp of it.
Well, not entirely - that's where the stiction/seal drag comes in - and that's what's unaffected by all this - sensitivity will largely be determined by how much seal drag there is though the fork's spring rate at this point will also affect how soft it feel
Sensitivity is affected by negative air, for the reasons mentioned above. Drag is the same, but the initial rate (ie opposition from the spring) is less.
I find that in practise to be honest.
Reread what clubber says - he has a decent grasp of it.
Yes, and he agrees with me. And is being reasonable about it too and not insulting my intelligence all over the place.
TJ, I do understand it pretty well, so let's just accept a communication issue and leave it, shall we?
Let's face it 'sensitivity' is a pretty wooly phrase - I'm taking it to mean how small a bump the fork will react to and this is determined by the friction in the fork.
What Molgrips is saying is that by having a negative chamber, you can have the fork running softer at the sag point without it being too soft further into the travel which partially mitigates having fork drag in the first place.
But still, adjusting the negative pressure will only affect the sag point, not the overall sensitivity.
And TJ, you are being patronising as usual. Clearly you grasp the ideas but no better than many others and you're being pedantic and rude about it to some, molgrips in particular who I note that you seem to persue with unnecessary zeal across several thread... Not what I expect from someone who's so vehemently opposed to bullying.
Why are we not differentiating between static and dynamic friction here - theres a marked difference between the two in systems such as those that are being discussed here.
and why, if -ve pressure is to adjust sag, do Rockshox manuals tell you to set sag [i]prior[/i] to setting the -ve pressure?
RS manuals also tell you to put stupidly high pressures in the forks...
Static, dynamic, different but the difference isn't really relevant to the current discussion by my reckoning.
How so? Please explain. IME the 'breakaway point' - i.e the force required to get the fork moving from a static position, is noticeably reduced with greater, rather than lesser -ve pressure, and you appear to be suggesting that it doesn't make any difference to this and is just a way to adjust sag.
I get the rest.
IME the 'breakaway point' - i.e the force required to get the fork moving from a static position, is noticeably reduced with greater, rather than lesser -ve pressure
Not in my experience - how can it be? The seal friction is still there - you still have to load up the fork until the point where that's overcome.
Of course by increasing -ve pressure, you will be making the effective spring at that point in the travel softer so that once it does break away, it's easier (softer) to move.
Ah, I see now where I was confused.
In practice, one can't separate the resistance of the spring and the seal stiction easily.
True enough which is where I think a lot of the confusion comes from. In the real though, set them up with the same pressure and for the majority of people for the majority of riding, it's just fine. Princess and the pea types will need to fiddle more 🙂
Ie me 🙂
I fiddle a lot.
And my experiences tie in with SO's.
I did not mean to be rude or patronising but it is
clear to me that most bikers - and motorcyclists are the same - have very little idea about how suspension works. You see some very badly set up bikes out there and as on this thread you hear some very odd ideas from people as well as some misconceptions.
Such as mechanical static friction can be reduced by use of a negative air chamber.
It cannot.
it is clear to me that most bikers - and motorcyclists are the same - have very little idea about how suspension works.
True. However some on here DO know how it works 🙂
Such as mechanical static friction can be reduced by use of a negative air chamber.It cannot.
No, but its effects can be mitigated which is the point we are trying to make; and the end result is very similar.
All you are doing molgrips is to reduce the effective spring rate. NO effect on the stiction at all.
clubber - MemberNot in my experience - how can it be? The seal friction is still there - you still have to load up the fork until the point where that's overcome.
Of course by increasing -ve pressure, you will be making the effective spring at that point in the travel softer so that once it does break away, it's easier (softer) to move.
TJ:
Imagine an air spring as a piston i a cylinder (which it is).
Make it well sealed, and so that it doesn't leak it needs to tighten up as the spring compresses and air pressure increaces, you don't notice this in use as the stictions already overcome and the increaced force on the 'spring' makes the added friction negligable, in some ways its a good thing as it by default becomes a progressive damper.
Now say you put 100psi on the +ve side, the seal is now quite well jammed in and thus going to provide a lot of stiction/friction. But put 100psi on the other side and all is good again as the seal is no longer under any load. Thus you've made an air spring with no stiction!
The reason they feel best with slightly more in the -ve is that at the sag point the -ve chamber is larger so at a lower pressure, more -ve pressure to start with counters this.
Fox rear shocks do this by a gap in the wall of the piston that allows air past at a point but not anywhere else in the travel so the -ve side is charged automaticaly. Thats why when the air seal fails the shock tends to bottom out/lock down as theres still loads of air sealed in the -ve chamber.
Look, you're all wrong.
When you're whizzing downhill and hit a jump, you sometimes take off, and fly for a few yards. This is often referred to as [b]getting air[/b].
When I'm whizzing down the hill, hit the same jump point, and crash, (falling into the ground rather than rising above it), this is [b]negative air[/b].
Dunno what all this spring malarkey's all about.
Now say you put 100psi on the +ve side, the seal is now quite well jammed in and thus going to provide a lot of stiction/friction. But put 100psi on the other side and all is good again as the seal is no longer under any load. Thus you've made an air spring with no stiction!
It will still have stiction - and it will be no spring it there is no air pressure differential.
As soon as the fork gets into its working range - ie with the weight of the person on it it must have the same pressure difference across the piston ( as a fork wiothout negaticve air)- the pressure required to support the person - hence the same loadings on the seal and the same stiction
N
TJ - you're missing the point completely.
Molgrips explained it in a simplified way, in that the friction is indeed a constant, but that not all of the force required to overcome it needs to come from a single downward pushing action. If some of it came from a pulling action from the other side of the seal, the force required to push down would be lessened. It's just a case of re-distributing the required force differently.
Nope - I understand the point molgrips is trying to make
I deny that is what is happening. I think he is confusing reducing the spring rate for reducing the friction.
Take an average sized rider on the bike. single chamber air fork with [u]the rider sat on the bike[/u] the pressure in the fork is say 100 psi
if that was a fork with a negative air Chamber with say 20 psi in it { [u]with the riders weight on the fork[/u]) then the positive side would need 120 psi in it ( assuming the piston area is the same in both forks)
This with the rider on the bike the stiction is the same as the pressure differential is the same
You cannot gain a pulling action from an air spring - its always a push
I'm getting the pen and paper out to show what I mean.
You cannot gain a pulling action from an air spring - its always a push
Unless it's got negative pressure in it, like it has at the end of the compression stroke.
Its still not a pull - its a push from the side with higher pressure.
No such thing as negative pressure either. It cannot go below zero.
If one side of a piston is at zero pressure - total vacuum then it is pushed by the air pressure on the other side. Low pressure does not pull. High pressure pushes
Jesus Christ.
I gotta agree with TJ on the pressure thing. Basic physics.
LOL @ BWD though. I know how you feel.... 😉
Guys, you do know what 'stiction' is don't you? What it actually menas? 😈
No such thing as negative pressure either. It cannot go below zero.
Surely all this is gauge pressure, not absolute?? So you can have a -ve pressure. It's all relative to atmosphere not an absolute vacuum.
If people are going to be pedantic and we are talking absolute, then those people need to revise the psi values they have been using to describe the set-up of any fork using pressurised air.
Good grief TJ.. I've come to expect pedantry as an art form from you but on this thread you are being incredibly obtuse 🙂
You are correct with your exact definitions of the physical processes that are taking place and your definitions of them but you are totally missing the point of the 'perceived effect' from the pov of the user when all these forces are combined in the fork.
It's true you can't alter the co. of static friction between two materials - it's as you say a fundamental property of the matrerials... But you can create the illusion to the user that you have in effect reduced it by 'preloading' the perceived 'pulling' action of the negative spring (And yes I know that an airspring doesn't pull but the effect to the user of the mechanical implementation of it can make it seem that it is). I think most people understand the abstraction required to make that work as well as understanding the fact that the physical processes involved are adding to give an overall 'effect'.
In fact I think you do too, but you are just loving the pedantry of your argument far too much to admit that 😉
TJ, earlier:
[img] 
[/img]
We all know that 'pressure' in common terms is about pressure differential with respect to atmospheric pressure. We also know that a vacuum doesn't suck, but the atmosphere sucks.
But really, what ****in difference does it make in this argument? None at all, that's what.
Proper scientists and engineers (which you're not) know that you can combine factors to produce an effective result and treat it as such. So if you have a closed piston, and you pull it - it pulls back. Why this happens when you are designing a mechanical item like a fork is not at all important, so there's no point worrying about it.
World: 1 + 1 = 2
TJ: 1+ 1 = 4 - 2
glad i asked now.....
^^^^ +1
The fact that it's not even correct in the terms being discussed just makes it more ridiculous.
jonny m - Member
glad i asked now.....
😀 At least it should be clear to you now??!!
An amazing amount of garbage posted in this thread. Glenh and retro83 are the only two that understand the purpose of a negative spring.