Recommended rear bags for light car

[richardd07]

Hi guys currently have UAS air house 2 bags in the rear of my peugeot 306 and I find its very bouncy. I run around 20psi at drive height and I find if I have a full load of people in the car the ride massively improves.

Can anyone recommend a more suitable bag for a smaller car to improve comfort?

Thanks

[Oh Damn, it's Sam]

What shock absorbers are you running? I suspect you're under-damped.

[richardd07]

What shock absorbers are you running? I suspect you're under-damped.

Gaz adjustables. Have them set fairly soft just through trial and error. I suspect the actual rear beam setup could play a set up too as its a torsion bar axle originally.

[DriftGirl]

Use a double convoluted air bag

[richardd07]

Use a double convoluted air bag

You think this would do the trick? Any specific recommendations for this application?

[KyleAnderson]

20 PSI at ride height is extremely low. It doesn't take much force to compress them at that pressure. As Sam said, you're also likely under-dampened. The first thing I would do is get a bag with a shorter length in order to get a higher driving pressure. Easiest drop-in replacement for you will be the Air House I. It's 7" extended compared to 8.5" for the AH2. With a higher pressure you may be able to adjust your current shocks to be able to control them.

[Oh Damn, it's Sam]

Following Kyle's line of reasoning, another approach is to go with a bag with a smaller diameter, which will also increase the pressure inside of the bag. Unfortunately, there aren't many bellows bags out there smaller than Air House 2s. Slam Specialties SS-5s are a hair thinner (5.5" vs 5.875" in diameter), but that small of a change would only bring up your pressure by 7%. I would look seriously at some sleeve-style bags and see if there isn't anything you could adapt to.

[richardd07]

Am I being silly here in thinking more pressure in the bag will make the underdamped situation worse??

My first though was to simply replace it for a double bellow

[Oh Damn, it's Sam]

The instantaneous spring rate of a bag is strongly correlated to the ratio of the volume of the bag at ride height and the volume fully-compressed. I suspect, as I believe KyleAnderson does, that with such low pressure in the bag you're getting very little resistance from the bag. As the suspension cycles, I'd guess you're bouncing off the bumpstops, either the ones on your shocks or the ones inside the bags, if so equipped. The effective spring rate of something like a poly bumpstop is far greater than what the bag is providing, so at that point, you're effectively severely underdamped. It's also possible that with such a low spring rate (I get between 50 lbs/in and 100 lbs/in for your rear instantaneous spring rate at rest) your bags are compressing a lot on bumps, which is causing your spring rate to spike wildly, again, leaving you under-damped.

While I prefer double-bellows bags, it has not been my experience that they perform dramatically differently in terms of progressiveness than single-bellows.

[richardd07]

as standard its a torsion bar setup with a approx spring rate of 135lb/inch

[Oh Damn, it's Sam]

So, knowing balls about the 306, it looks from photos of the rear suspension that your motion ratio back there is about 0.7 or so, just taking a wild-ass guess. That'll put you in the 80 lb/in to 150 lb/in range depending on how far inflated your bags are at ride height. But, with such low instantaneous spring rates, you're going to compress the bags a lot over a small bump, which is going to push those spring rates up into the stratosphere. Assuming your bags are filled to 6" at ride height, 2" of bag compression will double your spring rate, and 3" will quadruple it. By 3.5" of compression, your spring rate is eight times what it is at rest. That's a pretty dramatic spike for any shock absorber to try and control.

The first step I'd take is to ramp up the damping factor of your rear shocks and see how it behaves.

[richardd07]

will do sam. Is there any point in changing bags then?

[Oh Damn, it's Sam]

It's a little hard to say, because the simplified model I use to calculate against doesn't cover every little variable that goes into how bags perform. I do think that at how low of pressure you're running at that you're going to have a lot of the energy inside the bag as it compresses transformed into ballooning rather than pushing back against the suspension. This is then going to cause the bag to compress even more, spiking the spring rate further, and so forth. Moving to double-bellows will help control ballooning, but there are other issues at play here as well.

I suspect that KyleAnderson is onto something here with wanting to get those bag pressures up, but the only bags I can think of that would really help get those pressures up are going to be sleeve-style. That isn't a bad thing, 'cause sleeve-style can work great, but it might require some refiguring of your bag plates.

EDIT: You could also consider mounting your bags so that they don't have as much compressive travel at ride height, which will give you a higher instantaneous spring rate, but also a higher rate of spring rate increase. Inversely, giving them more compressive travel will put you in a flatter portion of the spring rate curve, at the cost of bringing the instantaneous spring rate down. Without riding in the car and observing it myself, it's hard to know what the right action is here, but I suspect the results of increasing the damping factor might give us some useful data.

EDIT 2: How tall are your bags at ride height currently? I'm working off a lot of guesses here, and it would help to have some real numbers.

[KyleAnderson]

We're on the same page. My thinking is that the bag provides so little resistance during movement that it doesn't want to get back to static height, and the shocks just can't keep up (because they're on the lowest setting). Higher pressure (and turning up the shocks) would provide some resistance and allow the suspension to return to static height in fewer cycles.

Shorter height or smaller diameter will get a higher pressure. Like I said, AH1 is the easiest drop-in replacement, but Sam's suggestion of sleeve bags might be the only way to get the pressures high enough. A reduction in diameter changes the volume more than the same reduction in height.

[richardd07]

Okay so with 20psi in the rear the bag is 6" tall, although it sits at a slight angle so one side is slightly shorter.

I have adjusted my shocks fully hard and ran 40 psi in for a short drive, which seemed to improve the bouncyness somewhat but still pretty bad