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Dual spring rate K5....Coilovers AND airbags??

Greg72

Greg72

@MIGHTASWELLK5
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I'll admit, I don't know enough about airbags and that's probably why I can't come up with the answer to this one yet.

I have been mulling over the idea of building a softly-sprung vehicle (kind of like I already have!) maybe something with around 250Lb-in springs for offroad use....then use an airbag setup to increase the spring rate for street driving...up to maybe 500Lb-in (or whatever I choose)

What I'm not clear on is how to make sure that the increased "rate" added by the airbag, doesn't also give the vehicle more lift at the same time. If anything, I'd like an inverse-effect where the stiffer spring rate would make it sit LOWER for street use. Most airbags I've seen will gain vertical height as they are inflated since they aren't constrained in any way. Mounting a bag inside a coil might work, (maybe) or somehow using a limiting strap to prevent it from gaining height. Unfortunately, that implies that the suspension could only compress, and never rebound beyond it's static ride height. That could make for a REALLY uncomfortable street ride, since presumably every time you hit a bump you'd probably get airborne....! /forums/images/graemlins/tongue.gif

Any ideas, BTDT source photos or sketches of a way this might be accomplished?
 
The only thing I can think of would be opposing air bags. Not sure how you could realistically implement this. Maybe with a long arm torsion bar with a 3rd arm in the center? Anyway, imagine the aforementioned 3 arm sway bar, and put an airbag above and below the 3rd arm. Equal pressure increases would effectively stiffen the ride both up and down. Pressure differentials would cause change in ride height while also stiffening. So, you could add more air to the "squatting bag" (top or bottom depending on arm orientation) to lower the ride height by sort of "preloading" the springs AND give higher rate. Make any sense?
 
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Make any sense?

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No.....but I'll just keep reading it until it does! /forums/images/graemlins/grin.gif
 
What if you used the limiting strap idea but used a type of high tension bungee cord? Now the tire shouldn't lift up as easily but it could still be enough to compress the airbag /forums/images/graemlins/dunno.gif.
 
/forums/images/graemlins/thinking.gif actually if you just limit the height of the airbag to just below ride height and dial it it so that as the body rolls it will gently absorb the roll just firmly enough to control it you could keep your soft ride and maintain a reasonable amount of control. Kind of a dual rate coil feel to it /forums/images/graemlins/grin.gif

It wouldn't be as firm as you were shooting for because you don't want it to feel like its bottoming out on a bumpstop or something but it would be an improvement.
 
Did it ever makes sense?

Imagine one of those long arm torsion bar sway bars attached in the normal way. Now imagine a 3rd arm in the center, identical to the outer 2 arms. Now, put a fixed airbag above and below the end of that arm. Valve open, you use the normal springs and sway bar behavior. Add air and you make the center resist rotation, effectively shortening the torsion bar and raising it's rate. At the extreme, you nearly lock the center and roughly double the sway bar rate. If you add more pressure to the bottom bag than the top, you use the sway bar to pre-load the springs, lowering ride height AND raising rate for roll. If using multi-rate (stacked coil) coil-overs, this would also raise compression rate at some point, same with leafs which would get stiffer with more compression. How all this plays in together depends on the characteristics of each piece...
 
After about 4 re-reads and seeing your second post, it DOES makes sense now! /forums/images/graemlins/thumb.gif

Don't take it personally....as I've mentioned before, I do eventually understand your posts, but it takes a while for me to put your descriptions into a "context" for myself. Once I get my mind wrapped around a new way of looking at something, it makes a lot more sense.

I've probably gone back a dozen times and re-read the old 3-link & 4-link posts I started something like a year ago. It amazes me how much new info I get each time I read responses from you and others about things to watch out for, etc. I think it was ntsqd that gave a nice description of caster change in a front suspension design as a criteria....something I'd never even worried about.

Usually, I'm just happy to see that you responded. I know that EVENTUALLY what you've written will sink in and give me a new avenue to my thinking and creativity. /forums/images/graemlins/peace.gif
 
One thing about air bags is that the spring rate increases exponentially as the bag compresses. I have them on my tow rig. I removed some factory leaves to soften the ride unloaded but then to restore load carrying and adjust ride height the bags can be filled. It works very nicely for regular driving and when loaded heavy, it's really a quite awesome ride (very secure feeling and comfortable).

Now the trouble.... The bags have about 6-7 inches of travel between fully extended and fully compressed. But you will never see fully compressed as the air inside will not allow this. Instead when you get the bag squished to 3" the spring rate will be too high. This translates into terrible off road performance. Any big bumps ( even train tracks on the road) will cause tremendous shock inside the vehicle.

The best riding off road trucks (in my opinion) have coil springs, so that will be a good idea. Especially if you get a dual rate version. You would have all the good stuff then - lots of travel, and a smooth ride over small and very large bumps.
 
what about stacking bags end to end? Say 2 or 3. Different air pressures, when one starts to "bottom out" another is there to take the load. or would they all just compress the same no matter the pressure in each?
 
I've been thinking on this for a couple of days now. The best I've come with so far is also complicated. Mount the fixed ends of the coil-overs on rocker arms. Attach an air or hyd ram to the opposite side of the rocker arm. Now you have variable ride height. Mount the air springs so that they hang from the frame and sit down onto some sort of indexer (tapered pin, etc.) on the axle housing. When you lower the ride height down enough the air springs will come into play.

Second thot: Anyone remember "Cadzilla", specifically how they varied the rear ride height on it? They used electric trailer tongue jacks. Could use a pair of those instead of the air or hyd cylinders to move the rocker about. Might even be able to tie the rockers together in pairs with linkage so that only one jack per axle is needed. Baddog's lever/bar solution could be adapted for this. Make the bar a large enough tube so that it doesn't have much torsional spring rate. A mixture of spring types makes getting the damping rates close REALLY difficult and Torsion bars are nearly the worst. Middle lever works against the tongue jack, end levers either are the upper c/o mounts or act on the rockers via toggle links.

Further thot: Dump the air springs altogether and use hydro bumps in their stead. They are designed and intended to be 'secondary suspension.'
 
My $.02---

Properly tuned coil overs ride like a caddy on and off road. Now, if you had some air bags and coils and links just laying around, it might be worth a try. Every rig I have rode in that has a tuned suspension is bitchin on the street, trail, rocks, and haulin arse over dunes...

Edit: If you want adjustable height, front and rear winches work awesome.
 
this may be a little off topic, but what about torsion bars for the rear suspension? Namely, mount one end up towards the middle of the rig. Spline the other end and mount an arm to it like you would if using it in a sway bar application. Then support that splined end with a bearing mounted on the frame(so you use the twisting force and not the bending for your suspension forces). Use a link with a heim down to the axle. Link it and the torsion bars act as your springs. mod a factory chevy or other crossmember for the front end mount so you have the adjustment bolts. /forums/images/graemlins/dunno.gif just a thought of something different.

I seem to remember Willyswanter talking about adding airbags to the rear of his crew cab to help with its towing duties after he went coilover. But if memory serves he got another tow rig so he probably dropped the plans.
 
My stupid idea

I have been reading through the posts on this, and have been contemplating something similar for some time. I think I might have a good way figured out. Instead of a bunch of complicated linkages, etc, why not use 2 different airbags at each corner? One would be a much larger, taller, larger volume bag with a low effective spring rate for offroad use, and the "street" bag would be a small, short, low volume bag. Set them up so that they are fixed on the upper end, and the small bag free on the lower end, except that it settles into a pocket on the axle end when it is in use. Set them up with separate control, and then deflate the larger bag so the rig settles onto the smaller, higher rate bag for the street. The weight of the rig should keep it sitting on the bag, and allow for normal uptravel in the system after compression. Travel would obvioudly be much shorter, but you shouldnt need much on the street. As long as you dont go launching the rig over curbs, etc, and you make sure of all of your clearances, you should be ok. When you hit the trail, you air up the large bag, and go for it, the high rate one would just be hanging there, ready to act as a pneumatic bumpstop for when you get really stupid.

Obviously, this will only work if you can find the right size/type bags, which I think should be possible.

Yes, this is kinda similar to the previous post by (sorry, i wasnt paying attention who) suggesting using a Pneumatic bumpstop, but this might be cheaper, and allows you to vary the springrate of either running condition.

Sounds good in theory.....:confused:
Someone build it for me and let me know how it works. :grin:
 
but then you'd be riding around with your tail end sitting lower than the front...
 
Re: My Stupid Idea

What makes you say that it would sit lower in the rear? as long as you have separate control for each end, then you should be able to set the ride height at whatever you want. Set it up with an in-cab control, which should be relatively easy with a few solenoids, and your off.

Unless I have blatantly overlooked something....:confused:
 
I second sticking with coilovers and airbumps. I'll let you know how mine handles on the street when done.
 
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