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Old 03-06-2009, 09:24 AM   #1 (permalink)
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Variable air suspension

I've been thinking about lowering my suspension a few cm, to get less drag at highway speeds, but I'm afraid of being too low. The roads here have potholes deep enough to swallow your wheel, plus parking in the city usually involves climbing high curbs. Some cars have a hydraulic suspension system (many old Citroens, some SUV's), but those are heavy and tend to leak. While reading Mike's Insight I saw he added air springs and a small compressor to handle the weight of extra batteries. I wonder how hard and how expensive would it be to install air suspension? I'm thinking about a setup for all 4 wheels, which will let me lower the car 5-10cm on the highway and jack it up an extra 5cm when maneuvering in the city.
Would a project like this be worth the trouble? What kind of parts would I need, would I have to modify the suspension I have? How heavy would everything be?
I've seen jumping ghetto-mobile low-riders

but let's just say I don't want to go that far...

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Old 03-06-2009, 12:31 PM   #2 (permalink)
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I suppose it would depend on the vehicle but you should be mindful of suspension and steering geometry, wheels don't raise up and down on suspension in a straight line typically.

I had an Isuzu Trooper on which I had installed air shocks on all 4 corners, all of which were cross-linked (shared common air) so that on highly uneven terrain the contact pressure to the ground would be more equal - a dramatic increase in available traction for an independent front suspension vehicle. Because of the air pressure, I had to lower the vehicle from stock height so that when the shocks were pressurized, normal ride height was restored. I had an engineer friend build me a 4-way ball valve so I could simply reach down and isolate all 4 shocks for road travel. The ball valve leaked.... so after 2 or so days the 4x4 looked like a lowrider - but this had a frustrating side effect of dramatically changing the front wheels' camber since Troopers use unequal-length control arms. The top of the tires stuck way out, the bottom sucked in, and the edges of the tires saw most of the vehicle's weight. Obviously not a good thing.

So... lowering your vehicle for aero has merit, but be careful that you've thought it all the way through to make sure it's the right mod for your vehicle.
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Old 03-06-2009, 03:08 PM   #3 (permalink)
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Piwoslaw -

I've heard you can do it, but it's a lot of $$$. I definitely think it's the best of all worlds if you can afford it. Doesn't a model of Lexus "auto lower" at freeway speeds? Is that an air-suspension-tech solution?

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Old 03-06-2009, 03:12 PM   #4 (permalink)
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Me -

Here's one, but I can't find the price :

AccuAir - Air Suspension

Here's one for Pontiac Firebirds, $1100 (US) front and rear :

Firebird Air Suspension Kits to customize your Pontiac

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Old 03-06-2009, 04:00 PM   #5 (permalink)
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My mark VIII did this, "for highway fuel economy" it was a big thing apparently in '93

BUt the effect was never quantified. I know a lot of people end up ditching the air compressor and going to regular springs because of the hassle. I wonder if they notice the difference?
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Old 03-06-2009, 07:20 PM   #6 (permalink)
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That's the thing, I think. Lowering at speed is likely to help fe... but is your fe savings going to pay for your fancy, complicated suspension adjustment system? Probably not.
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Old 03-06-2009, 07:44 PM   #7 (permalink)
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Coming from someone who has an air suspension for a totally different reason, they are a pain in the arse. Expensive, especially when you consider that in order to control it you will need some sort of valves and switches with air pressure gauges.

Many high end set ups have memory settings where its a simple function of pressing a button to reach a pre determined pressure. But over time the maintenance becomes the hassle. The bags rarely break, but the valves freeze, it is nearly impossible to get a leak proof system and of course the air compressor is noisy, not to mention you need space to store it plus the air tank.

Some have done dive tanks which have 2000 psi and it lasts quite some time with minimal hassle except for the ocassional trip to the dive shop.

So unless it is an oem application that is currently trouble free, all you are asking for is something really cool that requires constant vigilance.
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Old 03-06-2009, 08:44 PM   #8 (permalink)
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Cost Depends HEAVILY on the vehicle.

I got a used airbag setup for my dad's E-350 for $150 it was ok, ended up spending another $100 on a replacement bag.

You would be amazed how low you can get away with; all of my cars are "slammed" and I go off road more than most SUV's do.
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Old 03-06-2009, 08:55 PM   #9 (permalink)
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Piwoslaw ,

Google "air bags" for a heap of sites and prices.

Most are Firestone (the largest maker by far) air springs and still require separate dampers.

Basically you will need to re engineer most of the suspension of the car if you go this way.
Expect to provide space for the air tank reservior as well as the compressor. Then the lines and gauges and finally the sir springs and fittings them selves.

Many large trucks and buses use air springs so check out a few of them as well.

The air adjustable dampers available as an accessory for many cars here to give a temporary change in ride height typically of around the + / - 50 mm range may be easier to do.

You will still need a four way adjuster to lower the front at freeway speeds and raise all corners for curbs.

Good luck , Pete.
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Old 03-06-2009, 11:20 PM   #10 (permalink)
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Most luxury sedans have this, most are automatic, those who don't have one
the owners might install it.

Why, if you don't mind not being so fancy, I used to have regular air shocks in
the back of a Thunderbird, however these are used more to adjust for stiffness
of ride than anything else, such as when carrying a load...

The point I am making, however, is these air shocks are inflated and deflated
like a tire, via a tire valve that the installer conveniently placed inside the trunk.

All it takes is a little planning, and adjust the pressure before going on a trip,
also it has to be checked about once a week or so, like tires.

How hard they are to install, I don't know, spending this kind of money I had it done.
The rear I hear say isn't so bad, which also that's the only place I had them...
Most folks wouldn't dream of putting air shocks in the front, I am not even sure
it can be done.

But I am doing front struts on another car right now, and I can honestly not
recommend this as a DIY procedure... It is somewhat technical but there is a
considerable danger factor involved, and you need special tools.
Also the amount of labor, specifically some parts have been together so long
they are real tough to get apart, even with pneumatic tools this has been an experience.

Several things to consider:
The price, I think I paid $400 installed and all...
But will the meek gains in FE outweigh even that?

Also, like I said, they're mostly used to adjust for stiffness, not height.
So I suspect the reason is hardly FE, why would someone owning a Lincoln
Continental Mark VIII care, there's a dog gone 5.0 V-8 under the hood...

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