"Air Over" Bag Placement

[Oonrahnjay]

    On my bus, I'm reworking the suspension.  The standard leaf springs are very sturdy but they're not really the best for a bus conversion.  What appears the best way to go is to do an air-over-leaf-spring conbination.  The springs can be modified so that they set just a little lower, then at normal height, the weight is carried by the spring and a "helper" air bag.  For parking, I'd have the air system to set the bus level.   (Due to the configuration of the frame, a "pure air" system would be impractical and expensive.)
   I'm working with a mechanic on this; he has lots of experience with air suspension.  The *perfect place* for the air bag on this setup is the top bracket to be next to the arch in the frame and the top plate of the spring attachment system.  On my bus, there is a mounting pad on the axle with four 7/8" holes each side.  These holes are two inside the leaf spring and two outside (between the spring and the steering arm, king pin structure etc.).  The top plate is about 5/8" thick.  
   The arrangement that I'm envisioning is with the helper air bag sitting right on top of that top plate.  My mechanic says that it's not possible to put the bag on that plate and push down on the already-clinched-down spring at that point.  For the life of me, I don't see why that would be a problem.  Any other system with outrigger brackets, etc. would be very tricky and difficult.
   Anybody have any thoughts on this???   Thanks,  BH

[Oonrahnjay]

[brmax]

Sounds fine to me, with a plate above the original leaf securement, but ok using the same rated leaf bolts just a bit longer. I cannot say this top mount and structure is ok with out understanding the build and attachment plan.
The leafs have dropped overtime in height, and gives standard reason, in that I strongly say not to use these in a continuous vehicle overweight capability use.
I'm not clear on your (spring modifications) and this may change my decision, some helper airbags for smaller trucks are available and I have been familiar with a couple installs a few moons ago.
The manufactures have been making them for quite awhile retrofits so take a look at the big names, I'm sure you have already.
I suppose the key point is the support above, and knowing these would be installed with a safety sensitive air valve on the line as well.
What clearance is there in compression with say one wheel at its maximum.
Is there a model or brand that has the choice at this time, if so whats the weight range specs. and what distance is this.

good day
Floyd

[Oonrahnjay]

    Thanks, Floyd.  This is complicated.  I calculated the spring rates and they're rated to take 21,000 pounds to depress the front suspension one inch.  The entire bus only weighed 17,300 after it was stripped out.  If you add in the rear spring rates, you get a total of almost 54,000 pounds for a 1" suspension deflection.   Here are some variables:

Empty weight, full fuel,       18,900 pounds
99 passengers, plus driver   16,000 pounds
     TOTAL:                        34,900 pounds Gross Weight

Stripped weight, full fuel     17,300 pounds
Conversion addition             5,000 pounds ( Guess - tanks, cabinets, plumbing, generator and house batteries, etc.)
Engine Change                  -1,500 pounds (and that's 5 feet to the rear of the rear axle)
Three people, groceries, etc.   800 pounds
     TOTAL:                        21,600 pounds

     New front axle, brakes, etc.  from Gillig donor bus  GAWR = 14,000; actual weight on front axle, approx 8,200 pounds,  Airbags to be used are also from Gillig donor bus, GVWR 36,000 pounds. 

     All this tells me that I'm not looking at a heavy load (that's not to say that I intend to have any flimsy or under-built parts in the front suspension); all should be well within the Gross Axle weight ratings and Gross Vehicle weight ratings; in fact, it looks as if I will not be loading them past about 60 -- 65% of those ratings.  Firestone rates the front airbags at about 5200 pounds/inch at 60 Psi so I'm guessing that I'll be running a good bit less than 60 Psi most of the time. 

    These are the numbers that i'm looking at.  I think I'd rather have a "pure air" system, but as I said, it's not practical with the frame configuration I have.  So, that's why I'm looking at the stock leaf springs (and I have no idea why the bus was designed with 54,000 pounds for 1 inch deflection when Gross Weight is 34,000 pounds.  But to get good control and a slightly gentler ride, this is what I'm looking at. 

     Thanks for you reply!  I'd appreciate any thoughts that you may have.  BH

[Jim Eh.]

Bruce, keep in mind these are static numbers. You don't drive much in Canada do you?

[chessie4905]

I don't see any improvement in ride unless you remove a leaf or two. If it is mainly for leveling when stopped, the current spring pack will resist raising much. I have seen semi suspension that had the rear spring hanger replaced with air bags. I think that it was to raise or lower back of tractor for some situations. Btw my ACF Brill had upper and lower spring leaf packs. Axle rode between the parallel upper and lower springs. Each spring was 5 or 6 inches wide with 3 leaves. Rode pretty decent for a spring suspension.

[Iceni John]

I heard that some older Mexican buses (maybe DINA or MASA?) had spring and air suspension.   I've never seen one to know if this is true, but it's plausible.   As Chessie says, you probably need to remove some leafs to soften the ride.   I can only think that your stiff springing was to control sway and pitch when fully loaded with passengers on both decks.

I'm going to add small air springs to my leaf spring suspension, but only as helpers to level the bus side to side.   Crown put two more leafs in the left rear spring compared to the right rear because of the two 8D batteries and radiator on the left side, but I'll have about 800 pounds more weight on the right because I moved my start batteries there and I also have eight house batteries and my generator there  -  for this reason my bus now leans slightly to the right.   I plan on making brackets against the frame rails that will each hold two small-diameter air springs to bear down against the top of the spring mounts on the axle;  because I want to level the bus I need these air springs as far outboard as possible.   The right air springs will be inflated, but the left shouldn't need any air at all.

John

[lostagain]

This might not be a good comparison, but we have had several one ton pick ups with air spring bellows. The purpose is to level the truck with a heavy load, like a goose neck trailer. It works really well, but it does not soften the ride. As has been suggested, removing some leaf springs might be necessary to soften the ride.

JC

[Oonrahnjay]

I don't see any improvement in ride unless you remove a leaf or two. ...

   Thanks, Chessie.  That exactly my line of thinking.  Here's the static spring rate chart.



    It appears to me that the prime suspect for removal is the 32" long leaf, second from the bottom at approx 2300 lb/in each.   That's 4600 lbs from a pair of spring assemblies that are now rated at 20,400 pounds but are actually carrying approx 8,200 lb.  The top four leaves are in a "bundle" and they are integral to the cushioned mounting "eyes" at the end of each spring assembly so I wouldn't feel safe fooling around with them; the springs #5 - 9 appear to offer a good choice for "fine tuning" things, although they're only 1/3" each, and removing them won't change the spring pack thickness much.  Removing that 2300 lb. leaf and one of the .300-thick leaves will drop the entire pack thickness about an inch -- which will be an advantage for me.  

    I've been thinking about all this.  

    Thanks for the note about the double-spring ACF-Brill.  I have looked at options (including an all-air mod) but the layout of the chassis frame just makes big changes impractical.  But I'm glad to note that that bus rode well -- it's a good sign that it's possible.  One of the things that's brought all this on is that my bus, prior to any conversion items being added, rode like a buckboard.  I am going to put on new shocks and I realize that conversion parts will dampen things down a bit but I'm still going to be over 10,000 lbs below the design gross weight.  

... I can only think that your stiff springing was to control sway and pitch when fully loaded with passengers on both decks. ...
John ...

   Yes, that has been a factor in my thinking, too.  I believe that my conversion items upstairs will actually total about the weight of seats, hand rails (unnecessary ones - I'm keeping ones that I need), and other items in the upper saloon that I took out stripping the bus.  My tanks are all at chassis level and my generator (640 lb. Onan 10Kw) is just above downstairs saloon floor level.  I think that I'm going to be much less top-heavy than the original bus with a load of passengers up top, but it's an issue, as is my factor of side-wind load (although at 13'+, I'm not really going to be making much more side surface area than a 12 1/2' N. American coach).  

    Since I propose to keep most of the original spring items, and add a fairly low air weight carrying capacity, I think that it's going to be at least as stable in sway and pitch (and roll) as originally manufactured.  Also, the shock dampers are shot and I'll be replacing them with new.

Sounds fine to me, with a plate above the original leaf securement, but ok using the same rated leaf bolts just a bit longer. I cannot say this top mount and structure is ok with out understanding the build and attachment plan.
good day
Floyd

    Thanks, Floyd.  That top plate on the springs is a slight modification to the original; actually, the new plate is a fraction thicker.  The bottom of the air bag is secured with screws from below so there has to be a slight column bracket underneath to allow for the screw heads but it's mainly in compression so a properly braced and triangulated bracket is similar to other bag mounting systems.  Regarding the upper bracket, my bus is stored at a truck maintenance facility.  I've looked at the upper bracket layout on a number of heavy trucks and for both front and rear, sometimes up to many tons of axle carrying capacity, and the general layout that I've shown is consistent with what they are fitted with.  It's also similar to the air bag fitting system on the Gillig donor bus.  The brackets on many air bag upper mounts (the ones that are outrigger to the frame rails, which is frequent) are almost entirely made from 1/4" fabricated plate steel - even those for axles rated at 30 - 34,000 pounds.  The sketch that I've shown does not reflect the actual depth and angles of triangular gussets, but it's generally like a number of truck and bus brackets.  I'll arrange for a final design that's suitably sturdy.  One thing that I'd like -- it there's sufficient fitting space -- is to make the bracket with a U-bend at the bottom so that the bracket wraps around the frame rail.  But I see your point clearly and I'm taking all this into account.  

Bruce, keep in mind these are static numbers. You don't drive much in Canada do you?  

   No, I don't have much experience with Canadian roads .... except for a few winters in the 1990s in Timmins, doing cold-climate vehicle testing.  I think I see where you're going here, and I understand!  

[Oonrahnjay]

   Thanks, everyone, for the good info and thoughts.  One more time before we let this thread slide -- does anyone see anything wrong with putting a bag directly on top of the leaf spring, as shown in the diagram above?
   Thanks,  BH

[Lee Bradley]

I would be very careful about putting a load bearing bracket off the side of the frame as drawn; the frames are not generally designed for twisting loads.  How about a heavy steel box beam in place of the flat spring with the front of the beam in the spring mount and a air spring mount on the rear of the beam and an air spring between there and the rear spring mount on the frame.

[Oonrahnjay]

I would be very careful about putting a load bearing bracket off the side of the frame as drawn; the frames are not generally designed for twisting loads.  How about a heavy steel box beam in place of the flat spring with the front of the beam in the spring mount and a air spring mount on the rear of the beam and an air spring between there and the rear spring mount on the frame. 

      Thanks Lee.  Yes, I see what you mean there.  You'll notice that my sketch does not show a shock -- that's because the stock shocks are lever-type shocks (which must have been designed by the same guy who designs gaskets for British engines - and the shocks leak about as much, too).  I have an idea in my mind for a shock mount that includes a brace that runs from one frame rail to the other (probably 2 x 1" rectangular steel) to adapt an ordinary plunger type shock.  If the brace is located in the right place, it should support both frame rails against each other; that would serve two purposes.
      I have looked at the "pure air bag suspension" that you describe.  There is "hump" in the frame rail, as is common over an axle, but the hump is small and there is no location that provides enough space.  Also, I'd have to do big fab work to provide the front pivots and there's no area on the frame to locate an A-frame pivot brace to locate the axle side to side.  I'm sure that it's *possible* but I can't see a simple, reasonably priced, and practical way to do all this.  This if the big reason that I'm going with the air-over springs leaf spring suspension. 
      Thanks for the thoughts and attention; I appreciate your help.   BH

[Lee Bradley]

Bruce if you replace the springs with steel box beam they would locate the axle just as the springs certainly locate it. Then replace the rear spring mount with the air spring mount, which should be several inches shorter and the beam will not have the arc that the springs do and end the beam just short of the rear mount so you can bring the lower air spring mount off the bottom of the beam you should have enough room for the air spring.  Plus being at the end the beam/lever the air spring would take half the load. 

[bevans6]

On leaf springs with progressively shorter leaves, the over-all spring pack is progressive - it is softer for the first amount of travel, when the full length leaves are doing more of the work, and gets stiffer as it bends more and the shorter springs start to do more of the work.  Each shorter spring is driven by the longer springs on top of it, and the really short and stiff springs on the bottom don't flex much at all, but really serve to limit the active length of the springs above them.  So your spring pack will be a lot softer in practice than the sum of the individual springs.  If you just take out the shortest springs you reduce the over all rate a bit, if you take out some of the longer springs you soften the initial rate more.  It's a complex equation to write, but you can judge what is happening intuitively.  That really short, very stiff spring is almost more of a spacer than a spring until the highest loads are realized.   With your light weight, taking it out might have almost no effect on ride.  It does serve to reduce the active length of the springs above it, though, it makes them stiffer even if it isn't acting as a spring itself.

This naturally progressive rate increase is why leaf springs are so widely used for vehicles that have widely varying loads, and air springs can have a similar characteristic.  Coil and torsion bar springs are not naturally progressive and you have to play serious tricks to get them to increase their rate with compression.

As far as the mount, as noted it kind of depends on how much of the over-all load you plan to share on the air bag.  If they are just helpers, your offset mount is what a lot of aftermarket kits do.  If you plan to have them take up a lot of load, which they can do if you use them to lift the coach a lot, then putting the load directly into the frame is a better approach.

The simplest way to judge this is to, after you take out the leaves you think will work for you, to measure the difference in height between zero load on the tire and full load as you drop the coach off the jack.  The axle will take up the load and settle down on the springs.  If you know how much the axle weight is, and you measure how far the coach drops as you let it down, then you know the spring rate of the axle spring packs at that ride height.  10,000 lb axle weight, it droops three inches, the overall axle spring rate is 3,333 lbs per inch, each spring pack is half that - at that ride height.  If you raise the coach one inch with the air bags, you are taking off 3,333 lbs of initial load plus some other amount since the leaf spring is acting less strongly due to it's progressive rate, so maybe you are putting closed to half the overall axle load on the air bag.  If you have load cells that can measure the weight on the jacks vs the weight on the tires as you drop the coach down you can track the transfer and get an understanding of your progressive rate, but that is kind of a specialized setup.  

I hope this helps a bit.

Brian

[Oonrahnjay]

... I hope this helps a bit.

Brian

    Yes, very much so, Brian!  Thank you for your helpful and informative reply.  I am traveling and I cannot give the full response your thoughts deserve but I'll be back in a few days.  Thanks,  BH