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  #1  
Old 07-25-2010, 07:01 PM
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Chris64 Chris64 is offline
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AirBag question

Hi there, it's been a long time since I've been here. Way too busy but I'm getting the building itch again.

Anyway, I'm designing suspension for a sand rail. Rather than going with coil-overs I really liked the flexability airbags offer on Funco rails. Now the way they do it is they use coil-over style on the front and a sort of linkage style plate lever for the rear that holds the mounting plates parallel through out the suspension stroke.

My question is this. If standard airbags are used (non-coil overs) mounted to swiveling plates, trying to keep the pivot point as close to the bag as possible, do you think that the bag could handle maintaining it's own shape and not deflect (twist and bulge out of the side instead of holding parallel)? I had a plan and that would be really helpful in my design, but I never see them this way and I'm sure there's a reason for that.

Or any other experinces with airbags would be helpful.
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Old 07-25-2010, 11:44 PM
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The Linkage you refer to on the Funcos used to be on the front and rear till they went with the "bag-over-shock" design they now use on the fronts. That linkage lever keeps the airspring plates parrellel. With out it the bags will bulge out the side and rupture from the plates pretty easy (depending on laods and angles). As you noted they still use it on the rear and for a reason.

GOTTA keep those airspring plates within manufactuers specs for misalignment! The flatter they stay to each other the more abuse they can take in stride!

Unless the parts you put the springs on are coming at each other flat-ly and nearly without side movement (something most pivoting parts do (The side movement)). You cannot just put a pivot point at each end of an airspring. If the bag goes even a little to one side as suspension arms cycle like a Funcos without that linkage and a large load hits it its gonna try to bulge sideways.

These pics below are some that Chain drive rails did. It keeps the airsprings plates nearly parrellel. That setup give that machine about 25 inches of travel. (Those are 35-36 inch tall front tires. The rears are 44 inchers. The rear ends have over 30 inches of ACTUAL WHEEL TRAVEL with airspring travel but are a bit different than the front on Chain Drive rails buggies)

If all three pics show up you can see the front end decent and that they are NOT afraid to push them some in the name of fun.

When I get to my mini buggy design it will be using airsrpings too and will be something like this.
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Old 07-26-2010, 11:46 PM
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Thanks a bunch for that. That's a great design. And good info.

That buggy is a buffet of out-of-the-box thinking. I dig it. I'm also still interested in the chain drive rear end. I was thinking with that you could maybe gear it down enough to use a standard frontwheel drive trans (it'd be cheap & easy to get). Topic for another thread probably.

Thanks
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Old 07-27-2010, 12:01 AM
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They run built v-6's and other lots bigger engines on propane thru VW trannies like Bus units and they survive due to the 3 to 1 geardown ratio of the chain drive setups. The torque is all AFTER the tranny. Plus wheel travel is able to be made huge.

This shot is not trick. They do this with airbags or hydraulics depending on what the driver wants. Remember those rear tires are 44 inches tall which if a testament on both the travel and the tranny surviving.
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  #5  
Old 07-27-2010, 12:27 AM
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I do agree if you can deal with any computer stuff (a lot of todays automatics use a bit of computer control to work) you could even do a rail with an automatic which would have a lot of advantages to some people. Using any front drive tranny. The setup can make it so the cv's at the front do not move much at all. The layout Chain drive uses the front drive cv's are actually stationary.
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Old 08-04-2010, 10:48 AM
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The suspension design pictured is a great way to mount the air springs. Free pivoting plates with air springs is a bad idea. As soon as it is forced to rotate about the pivot it will snap to the extreme of the travel of the pivot very quickly with quite a bit of force. They can take quite a bit of misalignment as they are, depending on the internal clearances of the spring. So long as the bellows is not rubbing against itself internally, it should live a pretty long life. Best bet is to solidly fix the cap and piston, it doesn't have to travel perfectly along it's axis to function properly.
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Old 08-05-2010, 04:30 AM
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Forgive my lack of knowledge but what (besides rubbing) would be wrong with putting a sliding cage (mounted to one of the plates and sliding on the other end) around the airbag to keep it from deflecting?
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Old 08-05-2010, 09:50 AM
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Something sliding against the bellows would create a fairly fast failure. I also don't quite understand how a cage sliding on the the spring would keep it in alignment. Air springs can take pretty significant offsets without problems. Some of the OEM applications I've worked with for independent suspensions go through crazy geometries. If there is no internal rubbing, and there is no pulling at the crimps/bellow to cap or piston interface, you should be fine.
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Old 08-07-2010, 01:55 AM
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I'm running that style bag on an awkward canti-lever suspension setp on the rear end of my truck and they do handle quite a bit of misalignment at the extreme ends of my setup, but not more than you see on nearly every semi you pass. I'm not at all worried about mine not staying perfectly parallel.
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Old 08-07-2010, 02:44 PM
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Please don't take this wrong- I'm just throwing out the thought. What is the reason you want to run air springs on a buggy? Only thing I can think of is saving some money over coil overs, but still having a fair amount of travel. Having adjustable suspension height is great for some types of vehicles and bags work well for that, but a buggy typically doesn't need to. Unless it's just something you want? On the down side, the bag's spring rate ramps up quickly as it compresses and they rebound harsher than metal springs so for typical off road use of a buggy the coil overs work better.
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  #11  
Old 08-07-2010, 07:51 PM
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Quote:
Originally Posted by 4x4 Nut View Post
Please don't take this wrong- I'm just throwing out the thought. What is the reason you want to run air springs on a buggy? Only thing I can think of is saving some money over coil overs, but still having a fair amount of travel. Having adjustable suspension height is great for some types of vehicles and bags work well for that, but a buggy typically doesn't need to. Unless it's just something you want? On the down side, the bag's spring rate ramps up quickly as it compresses and they rebound harsher than metal springs so for typical off road use of a buggy the coil overs work better.
Air springs can be quickly adjusted for more or less passengers even on the trail. Or a heavier or light driver in a one seater.

Not sure why you say a buggy typically does not need to have adjustable suspension height. That sounds more like an opinion than a fact. Being able to raise or lower a vehicle can have many advantgaes.

As for the bags spring rate ramping up quickly if thats what you want great but all you need to do do is have remote reservoirs (EASY to do) and by varying the reservoir size you can tailor an airbag so much finer than any metal coil spring. The larger the reservoir the softer it "ramps up" I call this a tuning aid.

As for the rebounding harsher I am not sure where you got that from but saying they rebound harsher than metal springs? Not sure why you think that at all.

But its just not the case.

MANY people go to airsprings as they will give you a better ride. Not to many I have ever heard go to air springs ever go back to steel springs. I have heard of a few but they refused to learn how to deal with the airsprings. Which are easy to deal with and often easier to choose and you can tune them with any air compressor.

You can even make it so the outside airbags in a turn are filled with more air or have thier reservoirs cut off so that they give a firmer outside suspesnion in a turn. Try to do that with metal coils.

The ONLY advantage I see to metal coils is they take up a tad less room. But other than that the airsprings are hands and feet above them.
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Old 08-09-2010, 05:15 PM
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^^^^^^ this is all true

Most of the airbags I have seen used are pretty large as it is, meaning a less progressive spring rate as mentioned. People generally choose the big heavy duty semi truck style in my experience.

If you use an external res. make sure you use a decent sized hose to minimize restriction of some relatively fast moving air.

"rebound harsher than metal springs"
Coil springs always have a higher transmissibilty than air springs of similar capacity...and why particularly in rebound?

Anybody ever see a rolling lobe sleeve style air spring used offroad?
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Old 08-10-2010, 10:44 AM
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Sorry I didn't make myself clearer. What I meant was having a cage (think ultra heavy duty birdcage, minus the top and bottom. Just a cage cylinder really) attached around the periphery <sp> of the lower plate. Perhaps around maybe 340 degrees of the plate. Have the airbag inside this cage. The top of the cage is free to slide up and down over the edge of the top plate. Thus enclosing the airbag but allowing it freedom of movement.
I hope that's a little clearer. Sorry but I don't have a scanner at the moment or I could attempt to draw a (bad) picture.
Though you guys say that the bag is rather forgiving vis a vis misalignment issues so this may be a moot point.
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Old 08-10-2010, 11:10 AM
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You could try a "paint" drawing if you have "Paint" on the computer you use.

I think I understand what you are trying to say but one thing needs to be made clear. NOTHING should touch the rubber bellows of an airbag but its top and bottom plates!!!

I too have seen some gosh aweful misaligned airbags/airsprings and if not overloaded you can get away with it for a good while. But I see misaligned airsprings as just kinda poor engineering in my book. No reason to not keep them close to parrellel or at least within the makers specs in my book.

A misalgined airbag thats loaded high (or not) will fial sooner and usually with little to no warning. The damage can be big. Again I feel this alone is worth getting them right.
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Old 08-24-2010, 12:53 PM
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Sorry for not replying sooner, but time hasn't allowed me to be on lately. I agree that bags are great for when height adjustability is desired or when weight is changed. If the vehicle is going to be a trail machine bags can work good there too, but I'm not a fan of bags on a vehicle that is jumped such as a sand rail which is what was pictured and what I typically think for a buggy.

The spring rate for bags ramps up quickly because you are compressing air. As the bag is pushed down further the pressure increases quickly causing the spring rate to ramp up more quickly than a properly sized metal coil spring. This in turn causes the air spring to rebound harder too.

If air bags are used for a suspension, external reservoirs can make a big difference in the spring rate increase of the bags. They allow for a much greater volume in which to absorb the air pressure increase during bag compression. This allows some of the air to transfer to the reservoir instead of being compressed to a higher pressure in the bag itself. On the other hand, depending on the size of the reservoir, you can have almost no increase in spring rate as the suspension compresses. This can allow for bottoming out easier too. Solenoid valves between the bag and reservoirs would allow a person to isolate the bags and it would firm them up for cornering as you said. Set up right it can work nicely, but it's also some additional parts and timing on the solenoid valves has to be either done manually or some type of control would need built for it. Manually could cause problems if abrupt maneuvers were required and the driver couldn't flip the switch quickly enough.

Proper sizing is important for spring rate and ride quality. A properly sized bag with 60 PSI will ride better than a large bag with 15- 20 PSI. To show some numbers, a 5" diameter bag has 19.6 sq. in. of surface area and a 9" diameter bag has 63.6. With 1000 lbs of sprung weight the 5" bag will need 51 PSI and the 9" will need 16 PSI. This doesn't factor in ride height, bag design, or stroke, just simple numbers. As the suspension compresses, the vehicle will be able to compress the smaller diameter easier and will therefore ride better. The vehicle only needs to compress 19.6 sq in of bag area instead of 63.6 and because of that will be able to do so more easily with increase suspension travel. More travel means a better ride. There is much, much more involved with bag design and spring rates beyond this simple example and I don't claim to know it all by ANY means. I have tried and experimented with this though. I think the reason many guys will use large bags is they are more common. Probably because of semi usage.

I'm not against bags at all. We've built a few truck with them and I'm very please with the results. Like I said earlier though, I just don't care for them for a vehicle that gets jumped.

YJTypeR, most of the trucks we've built on bags have a rolling sleeve design. They are available with more travel and they have slightly more weight capacity than a convoluted of the same diameter.

I agree with what has been said about making sure nothing contacts the bellow- whether it's suspension components or sticks, rocks, etc.. from the trail. A cage for protection wouldn't be a bad idea in a off road environment. If the cage is designed for top and bottom plate alignment there will have to be some sort of slider built into it to maintain alignment, but allow for travel.
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Old 08-24-2010, 01:36 PM
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Not to sure I agree with your saying the different bags will ride potentially better with a smaller bag with higher pressure VS a largerbag/spring with less.

I will reread what you typed and see if I can see the light but it seem to go against what we have discovered in our testing.

The smaller bag with higher pressure may not compress any easier than the larger bag with lower pressure. They could easily be made almost equal. The larger bag having more volume inside itself (saying NO reservoir) will be a bit less progressive but thats potentially better (depending on the situation. Nothing is PERFECT for all off road situations (or on road for that matter) I have not done the math on your examples yet but will try to play with it later.

For jumping is just why I desire the bags. Easy to slow tailor the flow rate out of the bags to the reservoir as you build more and more speed into the jumps as you go so you do not "bottom out".

Other than outer diameter (as in taking up space size wise) to me airsprings have evey advantage there is over a metal spring.

Still I do like your opinions and input and if I can learn something from it even better.
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Old 02-11-2013, 08:46 PM
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Sorry to bump an old thread, but I just saw it today. The pics in post #2 shows me and my awesome Chain Drive Rail.

There is some good info on air bags in this thread, and some not so good conjecture.

It is true that if you have one or both ends of an air bag that can pivot, the air bag must be guided somehow to keep the ends close to parallel. Solid mounted ends can take up to 30* of angular misalignment.

As far as jumping goes, as you can see in post #2, I jumped it up to 95 feet, and each time was like landing on cloud nine.

This buggy does not use reservoirs for the air bags, and I feel it would be detrimental to do so.

It is currently for sale at a low price if anyone is interested. scott_freeman@att.net
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Old 02-11-2013, 10:27 PM
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That is killer design work! Great job.
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Old 02-12-2013, 12:58 PM
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I only designed the shock mounts on that buggy.

I am currently designing and building a much more advanced version of the concept.
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Old 02-12-2013, 07:05 PM
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Quote:
Originally Posted by Scott_F View Post
I only designed the shock mounts on that buggy.

I am currently designing and building a much more advanced version of the concept.
Which we would all love to see and learn more about
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