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Discussion Starter · #1 ·
A bit of a technical question but i'd like to learn a bit more about the technical side of suspensions (air in this particular post). Because i'm quite light, and i set relatively low pressure values, does this influence the performance of the fork for a light person compared to a heavier person? Because like, it makes sense that the heavier you are, the more pressure you need, vice versa, that is relative, but then of course some things in suspension is fixed, like the air chamber size. I sometimes hear people talking about wanting a bigger or smaller air chamber for performance reasons. So like does the spring rate or something have a different curve for a lighter person than a heavier person? Sorry, you'll have to bare with me, as you can see i don't know so much about the technical/mathematical side of suspensions. Thanks.
 

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Discussion Starter · #3 ·
Sorry, i'm not sure if there was some underlying message there but that thread does not seem to mention what i'm asking. Just wanted to know if a suspension (fork suspension or rear shock) would perform the same for a light and heavy person given that they are running the pressures relative to their weight (and of course both have the same fork/shock).

If i put it another way, say i'm light now (which i am), then some how i gain another 60lb, and add more air as required for the new weight, would i still be getting the same performance and characteristics?
 

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As far as the math and physics goes, there shouldn't be a big difference on the spring side of an air fork, as long as the pressure change is ~proportional to your weight change, which it should be. That's not to say that the damping won't be different, which it most likely will be, but I'll leave that to someone else to explain for now.

You say you're not good with the math for this stuff, so I'll make an attempt to explain the basic principles of air springs.

The math behind an air spring is based on Boyle's law, which states that the initial pressure times the initial volume of a closed container is equal to the final pressure times the final volume, or P1 * V1 = P2 * V2.

An example calculation goes as such: I start with a piston/cylinder assembly that is 6" long with a 1" diameter piston, pressurized to 100 psi. If I compress the cylinder to 3", the pressure goes to 200 psi. Compressing to 1.5 inches makes the pressure 400 psi. If the chamber is compressed to "0" length, theoretically the pressure goes to infinity.

So essentially, the pressure doubles every time the volume of the chamber is halved. This makes a force curve that is "progressive." The force from the air spring is not directly proportional to the compression of the spring.

This is different from a coil spring, which is governed by Hooke's Law, which states that the force from a spring is equal to the "spring rate (lbs/in)" times the distance the spring is compressed from its free length, or F = k * x. This gives a "linear" spring curve, because plotting force vs. displacement gives a straight line, just like y = m*x + b.

Back to air springs, the reason people want larger or smaller air chamber sizes is to change how progressive the spring curve is. For a set amount of travel, say 130 mm, the air chamber volume is reduced by a set amount when going from full extension to full compression, equal to the travel (130 mm or whatever) multiplied by the area of the air piston, I'll call this the used volume. This used volume is subtracted from the initial volume to get the final volume used in Boyle's law.

If the initial volume is equal to the used volume (which it never really is), then the final volume at full compression equals zero, leading to a theoretically infinite final pressure. This is an exaggeration of a "small air chamber volume." You can see that the spring curve ramps up a lot as travel is used. This is very progressive. A setup with a small air chamber will ramp up more at the end of the travel. One characteristic of a small air chamber is that it is harder to bottom.

The other end of the spectrum is to use a very large initial volume. Say the initial volume is ten times the used volume. The final volume will be 90% of the initial volume, which is not a big change. Because the volume doesn't change very much, the pressure doesn't change very much from full extension to full compression. This makes the curve much less progressive. One characteristic of a large air chamber is that it will bottom easier in comparison to a small air chamber.

That's the basic theoretical background for air springs. I'm sure I wasn't clear on everything, and probably forgot to mention some things, but that should get you started on understanding the math behind air forks. I've barely scratched the surface on explaining how the theoretical stuff relates to real world applications, and honestly, I don't think I could do a good job of that anyways. Feel free to ask for clarification or further explanation, and please feel free to correct me if I made a mistake, I'm just trying to help :thumbsup:.
 

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I believe the stiction forces, being more or less fixed, will be more noticeable for a lighter rider.
That said, the weight of spring is more or less fixed (marginally less for a lighter spring but not by much) and will therefore be a bigger percentage of all up weight for a lighter rider.

Everything is a tradeoff...

Back to you original question, yes, for a given frame & shock the heavier the rider the more pressure you will need just like a heavier rider will need a heavier spring or more preload.
Air chamber size makes a difference in how the spring rate of an air shock will change as it goes through it's travel. A bigger air chamber will ramp up more slowly and a smaller chamber will will ramp up more quickly. Choice of air chamber size is more a riding style or tuning thing than a rider size thing.

You would want a bigger air chamber on an air shock that is tuned correctly as far as preload and damping go but isn't using all it's travel. Schoolie's explanation of why how this works is really good.
 

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Discussion Starter · #6 ·
Thanks guys, that was well said, both in detail and in summary. It does make sense that stiction maybe more noticeable for a lighter rider, thats unfortunate though, for me haha. Yeah i wanted to learn a bit more in detail about suspensions, i think it'll definitely help in the future when looking for the right product, but i'm sure theres still much to learn.
 
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