Thread: Can it be done?

Originally Posted by lyonsy

the amount of volume does set the height, but if you want the same amount of travel in both compression and expansion as it is designed for then when you set the heights it will be 2in higher as the rear springs are longer then the fronts by around 2in.
yes you can just adjust with volume but you are ether loosing compression or expansion at that point

So, do you mean that by setting my ride height 40mm higher with the computer that I'm not getting proper wheel travel?

I haven't done the math on this and I could be doing bad physics... but...

For a given height and weight you get a spring rate from the air bag. Increase the pressure to increase the height with the same weight and you get a different spring rate, i.e. stiffer.
Increase the weight and no pressure then you reduce the height and I guess maybe the spring rate goes down.

So, if you merely increase the pressure to effect a ride height increase then you have increased the stiffness of the spring and changed the handling. Better to get spacer blocks and keep the same rate and upwards travel (if articulation is your thing).


I do wonder if I've got a false physics assumption though, I'm trying to figure out balanced forces, comprehensibility of gas things in my brain as I read this and type and without actually getting a pen and paper along with google for gas laws beyond PVT I can't be sure.

Originally Posted by John_D2

So, do you mean that by setting my ride height 40mm higher with the computer that I'm not getting proper wheel travel?

yes and no by raising it 40mm with no spacers you have just moved the airbag higher on the base it rolls over,
So you have 40mm less expansion movement and if you dont have limiting straps or std height shocks a much greater chance of poping an airbag off its base
but you do get the 40mm back in extra compression but realisticly only if you have good cross over pipes and height control sensors on each wheel to fee more air once the other side has bottomed out.

steve_a

I haven't done the math on this and I could be doing bad physics... but...

For a given height and weight you get a spring rate from the air bag. Increase the pressure to increase the height with the same weight and you get a different spring rate, i.e. stiffer.
Increase the weight and no pressure then you reduce the height and I guess maybe the spring rate goes down.

So, if you merely increase the pressure to effect a ride height increase then you have increased the stiffness of the spring and changed the handling. Better to get spacer blocks and keep the same rate and upwards travel (if articulation is your thing).


I do wonder if I've got a false physics assumption though, I'm trying to figure out balanced forces, comprehensibility of gas things in my brain as I read this and type and without actually getting a pen and paper along with google for gas laws beyond PVT I can't be sure.

spring rate on popper airbags not load assist bags is mostly determined by the how tight roll over is from memory the tighter the lip the harder the spring rate but that could be the other way, that's why you can buy after market spring bases that have 2 or 3 steps as this gives a multiple rate air spring.

the pressure inside the bag more effects the weight carrying of the bag then spring rate in a well designed system where the increase in pressure when required can escape/vent or be pumped up quickly, if they are not designed well it will effect spring rate as the air can not get out or in fast enough to allow proper travel so instead of compressing it will try to blow the side of the bag out, which is how load assist bags work

Hmm, I'm pretty sure I was right.... Intro - AccuAir Suspension not exactly an authoritative source, but does say as I thought that pressure equates to spring rate.

If you assume the air bag is basically a fixed volume then the air inside acts like a spring. More air (i.e. more pressure) means stiffer spring.

Originally Posted by garrycol

So by going D3/4 you wanting air suspension on the front or independent suspension or both?

Still not sure what the ultimate aim is here. Air adjustable suspension, independent suspension or both.

Based on the discussion seems to be air adjustable suspension which doesn't need D3/4 suspension.

Maybe the OP should explain what he wants to achieve - the end result, so innovative solutions can be offered.

Originally Posted by lyonsy

the amount of volume does set the height, but if you want the same amount of travel in both compression and expansion as it is designed for then when you set the heights it will be 2in higher as the rear springs are longer then the fronts by around 2in.
yes you can just adjust with volume but you are ether loosing compression or expansion at that point

Thats a completely different issue to what I was talking about. To put it back into context, my comment was in relation to putting a D2 rear bag in the front and saying that doesn't cause a 2" lift.

Originally Posted by steve_a

Hmm, I'm pretty sure I was right.... Intro - AccuAir Suspension not exactly an authoritative source, but does say as I thought that pressure equates to spring rate.

If you assume the air bag is basically a fixed volume then the air inside acts like a spring. More air (i.e. more pressure) means stiffer spring.

in a fixed volume air bag it would be, these are mostly used as assist bags not actually proper air suspension bags as their travel is very limited and their ride is horrible
with air bag that has a roll lip that allows much more travel its that lip that gives the spring rate basically working on pressure vs area the tighter the lip the stiffer the spring.
so if you have a std air spring and have vehicle at std weight then adding volume wont effect pressure and wont change spring rate unless the piston becomes larger in the bag,
if you then add 1000kg but don't change the volume of air the pressure will increase and the vehicle will drop lower until the pressure on the lip overcomes the load and becomes stiff enough (same as any spring the more compressed the more spring pressure their is for example 10kg for every 1mm of compression)
if you then add more volume of air the vehicle will raise but the pressure will be the same as before unless the spring bottom out that's the advantage of lip type air springs they stay at the same height so you still get all that suspension travel with out the ride deteriorating so the spring rate has increased
but wont give harsher ride like a conventional spring or non lip load assit type bag as the rate has only increased for the load and should feel as if you have not put a load on

to actually change the spring rate of an air spring you need a larger piston for a tighter lip so less area + same pressure will give a stiffer spring rate and smaller piston will do the opposite


here is some much better information for ya
Arnott Generation III Air Springs

Originally Posted by Slunnie

Thats a completely different issue to what I was talking about. To put it back into context, my comment was in relation to putting a D2 rear bag in the front and saying that doesn't cause a 2" lift.

i can put rear springs in and cut 2in out and wont cause a 2in lift ether but with out cutting 2in out it will