Thread: Rear airsprings - inflation/control options

Originally Posted by Bush65

With a Land Rover, I doubt you would see 150 psi very often in an air spring that uses a Firestone 1T14C-7 bellows.

Near fully compressed with 150 psi the air spring would support up around the 6000 + lbf mark (this was just a very quick check). That is a lot of energy absorption to compress like that and would need a heavy loaded Land Rover.

A 250 lbf/in coil spring would have to be compressed 24" to reach 6000 lbf.

Thanks John, that's a good sanity check. I've got the spec sheet for the spring here but it never occurred to me to use it in that way.

Steve

Originally Posted by steveG

Originally Posted by Bush65

With a Land Rover, I doubt you would see 150 psi very often in an air spring that uses a Firestone 1T14C-7 bellows.

Near fully compressed with 150 psi the air spring would support up around the 6000 + lbf mark (this was just a very quick check). That is a lot of energy absorption to compress like that and would need a heavy loaded Land Rover.

A 250 lbf/in coil spring would have to be compressed 24" to reach 6000 lbf.

Thanks John, that's a good sanity check. I've got the spec sheet for the spring here but it never occurred to me to use it in that way.

Steve

I was hasty writing my last post.

When I compared the 250 lbf/in coil spring needing to be compressed 24" that was to simply support 6000 lbf.

I had mentioned energy absorption in respect to the air spring, and IMHO is a valid physical response to evaluate for suspension behavior in rough terrain. What springs do is absorb and recover energy.

The energy absorbed/recovered by the spring is:

Energy = Force x Distance

The force to compress the spring changes with deflection so we need the average force x deflection. If it were linear from initial to final force, then the energy would be:

E = 0.5 x (Finitial + Ffinal) x deflection.

The energy to compress the air spring to 150psi (if it is actually 6000 lbf) will be a lot less than the energy to compress the 250 lf/in coil spring 24", because of the much smaller deflection with the air spring.

But the above doesn't happen in practice, because our bump travel is so limited.

With coils, you either have stiff springs, that are harsh on small bumps, or softer springs that can't absorb the energy resulting from large bumps and the axle hits the bump stop where the remaining energy is absorbed by a large force x a short distance. Progressive springs are in between.

Air springs are extremely progressive and comfortably absorb a larger amount of energy in the available suspension travel than a coil spring. The comfort depends a lot on the ride height and the piston shape for reversible sleeve types. The ride height governs the volume of air in the spring. Inflate the air spring to raise the ride height thus increasing the volume of air in the spring and decreasing the spring rate.

BTW many people incorrectly think you change the air pressure to change the ride height. What you do is change the volume of air in the spring. The pressure may change a little as the bellows changes shape on the piston.

However if you change the load, then you change the pressure.

Further to my previous posts, these pics captured from a spreadsheet compare 3 rear springs for a 110.
1.) NRC 6389 (stock), rate 330.04 lbf/in, free length 16.02 in
2.) LRA 110 rear spring, rate 250 lbf/in, free length 16.8 in
3.) Firestone W01-358-5712 air spring

At stock ride height a 110 rear spring is about 12 in high, so that is what I have made the air spring for these calculations. At bump stop contact, the spring height is about 7.5 in high. So the calculations were only concerned with that range of spring height.

Two loads were used, 1200 lbf and 1700 lbf per spring.

For the coils springs the ride heights were calculated from those loads and the spring details.

For the NRC 6389 the ride heights were calculated at 12.38 in and 10.87 in.

For LRA 110 they were 12 in and 10 in.

For the air spring the height was maintained at 12 in and the air pressure adjusted, as that is what would happen in practice. The calculated air pressure was 38.4 psi for 1200 lbf, and 53.1 psi for 1700 lbf.

Then each spring was compressed in increments of 1 in, to some height beyond 7.5 in (bump stop contact) and the spring spring force and absorbed energy calculated at each interval.

The results tabulated here:


The following charts give a graphic comparison of the three springs:





Note: in the charts for height vs force, the slope of the curve is the spring rate [force / deflection] or lbf/in. The area below the curve (to the X-axis is the absorbed energy [force x distance] or lbf.in

The following charts compare the absorbed energy:





Note: in the case of hitting a large bump, if the spring can't absorb the energy the bump stop has to absorb the excess and it is not so flexible so the impact force becomes very high.

The 1700 lbf cases show a large advantage for the air springs. The energy that the coils can absorb is about half what the can absorb with the 1200 lbf load, but the energy that the air spring can absorb increases with load (due to higher air pressure and the greater static ride height.

Clearly for carrying heavy loads on rough tracks the coil springs need to be stiffer, which compromises ride with low loads, or they need assistance from air bags, or be driven slower (reduce energy from bumps).

Edit: With the air spring in this example, the calculated air pressure when the spring is compressed to 7 in high is:

87.9 psi at 7 in high for the 1200 lbf example (starting from 38.4 psi at 12 in)

110.3 psi at 7 in high for the 1700 lbf example (starting from 53.1 psi at 12 in)

Great explanation as always John. Thanks.

A few people here are running the same 1T14C-7 bellows as the air spring in your example, but with the longer 5" "cone" (apologies for terminology).
Spring P/N is W01-358-5426

I haven't been able to find a Firestone graph of the extension/pressures for that combination and have been wondering how it relates to the shorter cone.
Is it as simple as the longer one having a wider constant spring rate range as it moves down the parallel part of the cone before it becomes progressive rolling on the lower shoulder?

Steve

Originally Posted by steveG

Great explanation as always John. Thanks.

A few people here are running the same 1T14C-7 bellows as the air spring in your example, but with the longer 5" "cone" (apologies for terminology).
Spring P/N is W01-358-5426

I haven't been able to find a Firestone graph of the extension/pressures for that combination and have been wondering how it relates to the shorter cone.
Is it as simple as the longer one having a wider constant spring rate range as it moves down the parallel part of the cone before it becomes progressive rolling on the lower shoulder?

Steve

Same bellows, but with an internal bump stop that takes away a little volume.

I haven't been able to get data sheets for the air spring assembly with 5 inch high piston.

I have four of the same air springs with 5" pistons, one for each corner of my 120 (the air springs I kept from my old rangie are smaller diameter). Some months ago I bought a couple of the assemblies with the 3.6" high piston with the intention of seeing how they perform in comparison, but have not had a chance yet. I also hope to test a custom piston made from 100 DN pipe. Whatever pistons I end up using the unused bellows will become spares, even though the top mounts are slightly different (that simply means extra holes in the mounting plates).

The shorter piston should avoid any need to pack the bump stop down, but because the stroke is the same I am unsure what happens to the rolling lobe at maximum compression. I'm thinking that it will only be a matter of providing a larger base plate that it can spread onto and move any edge that might abrade the bellows away from any contact point.

I believe the taller piston will allow more scope for change in ride height, because like you said it should, extend the range where the the effective diameter stays nearly constant when the lobe is on that nearly parallel section of piston.

On that section as you adjust the static ride height, the rate will reduce as the volume of trapped air increases, and vice versa, but the air pressure will be nearly constant for the same load, only changing slightly because of the small taper in the piston for manufacturing purposes.

As the the suspension travels and the trapped air expands (rebound) or is compressed (bump), the rate will reduce or increase respectively.

I bought an airock system from Off Road Only, because for my use I am particularly interested in different static ride heights, normal stock height (or a little lower) for on road and relatively smooth tracks, and about 3" higher for better ride and clearance on low speed, rough tracks.

With this system, temporary changes off road are easy. For difficult off camber situations the ride height is best lowered as far as practical, while the front or rear height can be increased for obstacles needing better approach or departure, or both raised for better belly clearance. It can also make diagonal changes for cross axle articulation. When stopped it can lower to enable better access to gear, etc.

When provided with a speed signal it automatically reverts to the on road mode (and height) above a set speed of approximately 40 kph. In on road mode it has active cornering to counter body roll, or dive under heavy braking.

John, if looking for travel and offroad capability, is a linear coil a better proposition than air, and an airbag with a greater volume better than one with less volume.

Originally Posted by Slunnie

John, if looking for travel and offroad capability, is a linear coil a better proposition than air, and an airbag with a greater volume better than one with less volume.

Just my opinion.

If off road capability means short and long distances, over a wide variety of terrain including some difficult obstacles, and with variable loads that may exceed the GVM, and not punishing the vehicle (air springs have replaced steel springs in other markets due to reduced wear and tear, damage to goods, etc.), then IMHO that is where air springs shine.

If off road capability means negotiating difficult obstacles, with a more or less light load, then linear coils would give the best results. Simply in terms of travel, air springs can't match what can be achieved with coils. Tall springs with the right spring rate rule.

But as you well know, getting the best coil for the purpose is not simply a matter of buying any replacement springs that some aftermarket business promotes.

For example I have 16" swayaway racerunners with eibach coils for my bushie. They have two 16" coils in series (32" total free height). No way would I compare a set of dislocating coils and 18" travel shockies to them, even though they can travel more, or ramp further.