As Steve says, you have to consider the net force of an air spring. The positive pressure chamber will generate a resultant force on the air shaft (going downwards) equal to the instantaneous pressure times the piston area and the negative pressure will generate another force but pointing upwards (unless the instantaneous pressure is negative, in that case it goes downwards). Net force is the difference.
There are SO many parameters one can change on an air spring. Positive and negative pressure, piston diameter, non-compressed volume, initial negative chamber volume etc.
Here is a 210mm tall air chamber with 150mm of travel, 1.5 cubic inches negative volume, with 65 psi in the positive chamber and 70 psi in the negative chamber, 1.1 inch piston diameter, adiabatic compression of air:
And after adding 10 cc of oil in the air chamber:
People doing the air shaft cutting mod were doing the reverse process.
Just curious, has anyone here actually looked at the air spring assembly of any of the longer travel Fox forks? The negative coil spring is drastically different than older Fox forks. I have pictures of a 2013 Fox 36 Float RLC Factory if anyone wants to see the guts. It is quite interesting that Fox went to so much expense to create a more linear spring rate,
I saw FOX posted the 2013 air spring curve on their website using a longer negative coil spring. Sounds like a good improvement over the older float models. I would like to try one. looks like the weight went up a little as a result of the longer negative spring.
Then maybe you can explain how. Just denying it with no explanation does nothing add credibility to your argument. The guys at Fox and SRAM are no dummies you know.
Manipulation of the compression/expansion ratios of the positive and negative chambers alone is all that need to be done, as tacubaya has demonstrated. How that is achieved mechanically is up to the creativity of the designer.
Manipulation of the compression/expansion ratios of the positive and negative chambers alone is all that need to be done, as tacubaya has demonstrated. .
To back that up, see below, using adiabatic compression and expansion assumptions. The fitted trendline is linear, through the origin, with an R^2 fit value of over 0.999 - in other words, it's pretty damn accurate.
Most coil sprung shocks (exception being the CCDB with its tiny shaft) have a comparable variation from dead linear based purely on the gas charge effect on the shaft, and the difference between a 300lbs/in and a 350lbs/in spring is about 17%, vs the ~10% maximum differences in rate in the curve shown below (more playing around with the geometry and the variation in rate could quite literally be reduced to <1%, but that geometry is getting less realistic from a design point of view). The point with that comparison being that people select the nearest coil rate possible, so even with the most appropriate spring possible being chosen, the rider is potentially compromising on his ideal rate by as much or more than an air spring rate can be made to vary throughout its stroke, which then implies that the required variation tolerance for a given spring rate allows for more variation than an air spring can be made to provide.
On top of that, there is no reason to assume that any particular mathematical convenience (such as a completely linear spring rate) is inherently "ideal". Anything that is being ridden hard tends to work best with some degree of spring rate progression - how much progression is required is the real question. I think air spring development will render coils completely obsolete sooner or later. FOX and SRAM may be no dummies, but they aren't there yet either.
Edit: I should also mention these curves are achieved using only a single positive/negative chamber, no DRCV trickery or coil negative springs or whatever. Using more complex systems can allow for even closer to linear rates within certain design constraints (size and operating pressure are the big ones).
You have an interesting way of taking the derivative where a force can increase 50 lbs in .8 inches, yet the rate is less than 50. And where the force takes 2 inches to make 50 lbs yet the rate is above 25.
Steve: That is totally cheating LOL where you are taking only the small portion of a ridiculously large air chamber that won't fit in a fork. At least Tacubaya is using realistic volumes and still getting >2x variation in spring rate.
Steve: That is totally cheating LOL where you are taking only the small portion of a ridiculously large air chamber that won't fit in a fork. At least Tacubaya is using realistic volumes and still getting >2x variation in spring rate.
Not at all. It has nothing to do with absolute size and everything to do with ratios. Play around with it for yourself, the point has been proven. Guaranteed, the characteristic shown in my graph could be generated by a fork.
As I've also pointed out, arguing about this is irrelevant unless you actually want a completely linear spring rate - which in my opinion, is less than optimal unless you have a linkage (ie it's on a rear shock) to manipulate the wheel rate to become progressive.
When people start asking "what kind of spring characteristic should we be looking for" rather than "how do I make my spring rate match a simple but irrelevant mathematical formula" these discussions will become a lot more relevant and useful.
Not at all. It has nothing to do with absolute size and everything to do with ratios. Play around with it for yourself, the point has been proven. Guaranteed, the characteristic shown in my graph could be generated by a fork.
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why don't you share the numbers you used to make those graphs?
why don't you share the numbers you used to make those graphs?
Sorry, but if you're not able to perform the calculations yourself then I'm not about to publish it for free - this stuff is my actual job, and this discussion is not actually producing anything worth knowing. As I have said several times, there are far more useful characteristics you should be concerned with.
Well if you aren't going to share the numbers then mr bog and myself will remain skeptical that you can do much better than a 2x variation in spring rate like tacubaya has shown. And by sharing numbers, I just means air volumes and etc. I can do the very basic algebra to come up with the curve, thanks.
As far as I can tell, if you want to make the fork's displacement a small ratio of the total (limited) air volume, you have to use a really small piston, and to get the right forces, you have to use really high pressures. Unless there is some other trick I am missing. But it looks like you don't want to say.
You are correct that to increase linearity given design constraints of a fork, the operating pressure needs to increase. Once that pressure hits the ceiling of what you are willing to work with, you have hit the limit of how linear you can make a conventional pos/neg air spring. I'm not trying to hide stuff here as I have already stated that I don't believe a dead linear spring is anything special anyway. However I don't ever post up usable numbers online sorry, it's not in my interests to do other companies' research and development for them.
You have an interesting way of taking the derivative where a force can increase 50 lbs in .8 inches, yet the rate is less than 50. And where the force takes 2 inches to make 50 lbs yet the rate is above 25.
Steve: That is totally cheating LOL where you are taking only the small portion of a ridiculously large air chamber that won't fit in a fork. At least Tacubaya is using realistic volumes and still getting >2x variation in spring rate.
Yea that image was the first one I uploaded, which I immediately noticed was incorrect. It was replaced with the correct graph but photobucket is updating their system so I guess it will take a while for the image to update.
As Steve says, do not concentrate on the linearity of the springs, concentrate on what spring rate fits your terrain and riding style best.
I was set to install a '13 RC2 cartridge on my Fox Float 36 RLC '12 precisely to avoid the fork being too progressive but reading this thread is making me think it's not going to help???
^^^
Do you really feel your fork is too progressive? At the risk of being cliche, for me "that's not a bug, it's a feature."
If you go all the way back to the original post, you can see that the thread starter is happy with his 2013 Fox fork, and was wondering what all the beef is about. 3 pages later, we've aired a lot of beef and learned a few things.
One thing I learned here and in another older thread is that some folks have unreasonable expectations of balancing suspension travel, plushness, brake dive and bottoming out. If your trail has a wide of conditions, you are going to have to pick what you want most. Having highly adjustable components is a huge advantage, though, because you can quickly make changes to suit each trail.
In my case, I'd like to be able to use all my fork travel with the SAG I just need for my weight and my riding preferences, that's all, and I thought having the newer cartridge I could make the fork use all the travel playing with the HS adjuster...