Max suspension fill for rolling lobe air bags

[H3Jim]

I have been procrastinating on setting up a manual leveling system for when I'm parked.   I want to understand the dangers / limitations before I commit to a system:

What's the maximum fill level for the rolling lobe suspension air bags.  Can I over fill them? What if any damage can I do by overfilling them?  I'm leaning (pun intended) toward using the Sean Welsh linear actuators to adjust the factory leveling valve. I'd like to know what happens if I push the limits of the valve for over filling as well as the other side, down on the stops.


Thanks,

Jim

1995 H3-41

[buswarrior]

Down on the stops is fine.
On the over-inflate, the bag is only capable of a certain travel, then any more air you try to put in won't accomplish anything other than promoting failure.

In setting up your controls, establish at what point that corner/circuit has no more lift, and don't inflate beyond that.

The MCI Rennaisance is able to both lift and lower it's suspension. When I fooled with one a few years back, the difference between all up and all down was maybe 6"? I can't recall the exact measurements.

happy coaching!
buswarrior

[JackConrad]

    When we replaced our OEM air bags with rolling Lobe air bags from Mohawk Industries, I called them to ask this question. A Tech called me back and said he recommended a maximim of 85 PSI. We have found that we reach maximum lift before we reach 85 PSI.  Jack

[Sean]

What's the maximum fill level for the rolling lobe suspension air bags.  Can I over fill them? What if any damage can I do by overfilling them?  ... I'd like to know what happens if I push the limits of the valve for over filling as well as the other side, down on the stops.

Jim,

I'm not completely familiar with the Prevost setup.  On my Neoplan, it's not possible to overfill the bellows with the factory valve, even if it is locked in the "raise" position.

Going down on the stops, though, can be a problem if you are not parked on flat ground.  (Not level -- flat.  There is a difference.)  I cracked a windshield that way.  The factory air suspension is designed as a three-point system.  As long as there is air in all the bellows, you are on a three-point plane, and there will be no flex in the chassis.  However, the stops are four-point, and if the ground inflects in certain ways, it is possible that when the bus settles, those four points will not constitute a plane.  Then the chassis will "twist" as the weight settles onto the stops.  The resulting stresses will be transmitted throughout the skeleton, skin, and glass.

Ever since cracking the windshield, I have been very careful to make sure that there is always enough air in the common axle (rear on mine -- front on yours) to allow however much tilt is being called for by the separately controlled axle.

If I'm laying the bus up for a while, and expect the air to leak down, I make sure that all the wheels are on the same plane.

FWIW.

-Sean
http://ourodyssey.blogspot.com

[prevost82]

Hi Jim ... I installed Pete's campground 3 point level kit in my Prevost and it works great. I always level down from travel height, then come up if I have too, never had a problem. As Sean say it's best if you keep it off the stops so you don't twist the body. I have had the front off the ground at max height and never had any problems with over pressure to the bags ... I think the shocks travel limits the upward movement of the airbag and the bags can handle system pressure.
Ron

[H3Jim]

My concerns with Pete's system are:
1.  overfill, maybe this is a no issue based on these posts, but something I didn't want to install blindly and damage something that works.

2.  I upgraded my leveling valves to be fast fill, so when going around a corner, they can fill the bags fast enough to make a difference to keep the bus level.  Pete's valves are all smaller than my air lines and valves.  Not only will I need to find the fittings to go to a samller diameter, but I suspect I will lose some if not all the fast fill feature while driving. 

Number 2 may be somewhat unique to my coach, or newer coaches.  Other than this, I think Pete's system is a great way to go.  He has put a lot of thought and effort into getting the best system for the $ spent.

Having said that, I think I will remove a rear fender, disconnect the leveleing arm, and measure the amount of travel I have in the arm.  I'll probaby have the bus aired up so I can see the amount of travel that actually affects bag movement.  Do the same for the front. Then I will know the travel I need for the linear actuators.  I suspect 6", but could be 8".

[prevost82]

Hi  Jim ... yes you have about 6" on the rear and about 8 to 9" on the front. If you talk to Pete about your travel level speed, he may have a solution for it. What valve did you used to increase the speed for cornering and did you have to increase you line size to 3/8 or is the 1/4 line OK
Ron

[Sean]

Jim,

You may recall that your issue #2 was one of the reasons that we ditched our HWH system (which, I think, you ended up with), so it is a valid concern.  Our bags are supplied with 3/8" lines, but the HWH valves were necking that down to just 1/8".

As for travel, it's trickier to measure than you might think.  Ours turned out to be 6", 3" in either direction from "normal" ride height.  What you'll need to do is hold the actuator arm down until the bag just hits the stop, then bring it back to the "neutral" position and measure from the linkage attachment to the end of the actuator.  Compare that to the normal length of the linkage rod, and that's your down travel.  Then bring the actuator to the raise position, and let the bag fill until either (1) you hit the up travel limit of some other part of the suspension, such as the shock absorber or (2) the "lobe" of the bag just barely disappears -- you can see past it to the lower bag seat.  Again measure to the linkage attachment and compare to the normal length of the linkage rod, and this is your up travel.

Depending on the geometry of the system, an adjustment to the length of the linkage can create more, less, or the same amount of adjustment to the ride height.  Ours is pretty close to the same, but it actually takes just a tad less movement of the linkage than we get in ride height change.

In order to get an actuator with 6" of travel, in its "centered" position, to fit where the old control linkage rods were, we had to relocate the fixed linkage mount in the rear, and actually modify the actuator arms in the front.  I no longer remember the actual numbers, but the 6" actuator was something like 9" long closed (so 12" in the "centered" position), and the fixed rods were perhaps 9" or 10".  So we were motivated to get actuators that had the least amount of travel that was sufficient for our purpose.  If you measure out your travel at, say, 7", you might think about getting a 6"-stroke unit rather than, say, an 8"-stroke unit if 7" isn't available.  In other words, go one size down rather than one size up, and position your mounts accordingly so that your travel stays in the "sweet spot" of actual bag travel.

-Sean
http://ourodyssey.blogspot.com

[H3Jim]

Ron,

My lines are 3/8" to start with.  Pete's valves are 1/4".  He had no solution as he had gone to great lengths to find the least expensive, 100% duty cycle valves available. I think to get larger valves would increase the cost considerably.  The fast fill valves I put in were just stock Prevost valves, installed by the dealer.  The old ones were leaking and it was not much more if any to upgrade to the fast fill.

The linear actuators are less expensive, as long as I don't make an engineering error along the way.  Conceptually very appealling too.  and somewhat simpler (maybe this depends on your point of view) - but does not require cutting any air lines.

Sean,

Thanks for your detailed and thorough response.  I had figured I'd have to measure as you indicated, not just the swing of the leveling valve arm.  Good point about using an actuator with less travel rather than more travel if I end up with 7" or something.  Of all the linear actuators I've seen, they all seem come come in 2" increments.  The FA actuators I was looking at are 12" closed, so 15" in the middle, that's pretty long.  I may need to keep looking for some shorter ones.  The force required is next to nothing, so hopfully with that tradeoff in mind I can find a shorter length.

Perhaps another option is the geared cable retractors, but they don't offer positional information / control.

[Tim Strommen]

A bit off-topic, but I have an idea I was kicking around for leveling in my rig:

All electronic autoleveling.  Replacing the leveling air valves with hall-effect rotary sensors (>10million cycle life-span) in the same location as the original air valves (to retain the factory sensing geometry).  Then, with the radiometric output of those, and a microprocesor, control the fill of the air bags.  3-Way (spring return to center) air valves would control the addition and subtraction of air pressure - one set at 3/8", and another at 1/8" would allow rate control (these would not need to be continuous-duty since they only need to be active when a change is "requested").

For actual In-Station leveling a dash-mounted button would ideally need to be pressed to change the "mode" of the microprocessor, a MEMS inclinometer mounted over the rear axle would provide one tilt reading (for side to side), while another mounted in the middle of the bus would provide the other reading (for front to back).  A simple algorithm would lower the bus as it levels until it hits the first lower-limit (read by the rotary sensor) - at which time it would add air to the remaining bags until it was level - or hit the extension limit of the position sensor.  Level could easily be automatically re-adjusted every 10seconds to 640minutes (depending on an interval setting).

When the engine is turned over, the "starter" signal can automatically kick out the "In-Station" mode and resume auto ride height based on the position information provided by the hall-effect sensors.  A common PID algorithm can use a combination of the two valve size sets to operate the add/subtract valves when in-motion (to allow for fast-fill when cornering).

By centralizing the valves (i.e. near an accessory tank), this would reduce the wiring a great deal (only requiring the power, ground and signal wires from each height-sense position (one "signal wire for the radiometric output of the hall-effect sensor).  If the In-Station mode was only achievable when the dash button was pushed (and the bus was not in gear) - and the In-Motion mode was only active when the engine was running after a start (so the microprocessor would automatically enter a sleep state if the engine was killed) - this should reduce the risk of running the batteries down, while also eliminate the hunting that can drain accessory tanks when the auto ride height type systems are parked on un-level ground (settling should be greatly reduced, of course subject to the quality of the 3-way valves).

Not sure anyone else would want to do it this way - but there it is.

-Tim

[H3Jim]

Well Tim, you "leveled" me with that one.  On a good day, I might be able to understand this, but I'd never be able to implement on my own.  Not sure about trouble shooting either.  Actually, I am sure, I couldn't do it.  Interesting though.