Original Air Bags and the 4 bagger system

[steve clevenger]

>Date: 20 Jun 2000 23:01:30 -0000
>From: tom warner
>Subject: Re: GMC: Original Air Bags and the 4 bagger system
>At $1500 or more for the basic 4 bagger system I can buy an awful lot of
>$264 bags for my GMC. Considering that most of us are riding on bags that
>are

The Harrison 4 Bag System was $1240 delivered to my door.

>By the way how much is a single bag for the 4 bagger system and where can
>they be obtained? Can you readily get one if one blows out on the road?

They are standard Firestone part overnight anywhere in U.S., if
local Heavy Truck dealer doesn't happen to have one in stock.--Cost is about
$120 They are smaller, double bellows bags--much less stress on them.

>What do you accomplish with them?

It provides independent suspension, which eliminates transferring of
vibration from one wheel to the other-a major problem with the stock air
bag. It allows me to isolate any rear wheel & raise it & lock off the air
valve to hold it up
My coach now runs straight & true in the worst road ruts.
I have peace of mind--I don't give a thought to air bag failure. If it does
happen, it won't be catastrophic & won't shut me down.

>In addition the 4 bagger and the Caspro sway bars severely reduces the
leveling
>capability of the coach, one of the very things it is famous for.

I don't see any change in the leveling capabilities--does everything it did
before.

>I would think twice about adding either.

It was the best money I have spent on my coach & I've never regretted
spending it.

To each his own.

My comments are from someone who actually has the 4 Bagger System & I highly
recommend it.

Steve Clevenger
76 Transmode 230
Tulsa, OK.

 

[henry davis]

>The arguments you present may have some merit but not from either an
>economic of engineering perspective.

I believe that there are good engineering reasons to use a 4-bag system
now. There were many reasons why GM decided to go with the single bag
system when they originally designed the suspension. Most of those reasons
were in my opinion materials related - and those problems have been solved
since GM stopped manufacturing the GMC MH.

> The addition of this system is your
>personal choice but as before I would never recommend it to someone else.
>And for sure if someone did chose to buy it, I would recommend unless he has
>deep pockets it would be way down on the list of items to replace. I still
>contend in a properly functioning suspension system there is no improvement
>in the handling of the NORMAL coach.

It did make a tremendous difference in the handling of my 76 Palm Beach. As
I reported earlier when recommending the 4-bag system, I had restored the
entire coach suspension to original condition. I still was launched pretty
strongly on parts of Interstates crossing the Texas Panhandle at reasonable
highway speeds. The behavior was independent of coach loading. But, I got
launched even when the coach was empty.

>If you are hauling heavy loads of
>course that may change, because in that instance you are using the coach for
>something it was not designed for. I see far to many people spending money
>on the 4 bagger system and sway bars when they are still running D rated
>tires, and have not checked the other suspension components.

LREs on Alcoas. Bilstein shocks, reconditioned brakes, replaced worn front
end parts, R&R'd rear pins. And I still went to the 4-bagger.

>As for the idea that the 4 bagger system reduces vibration from one wheel to
>another I dont think it is true unless of course your wheels are out of
>balance, but that seems like a very expensive way to eliminate that problem.

The 4-bagger shifts the natural resonance frequency of the system pretty
substantially. It's entirely possible to get a resonant frequency just
driving down the road independent of what the rear air suspension is made
of. The 4-bagger bag attenuates vibration better (by about 5% which can be
significant in harmonic oscillation) and doubles the natural oscillation
frequency from around 1Hz to over 2 Hz.

>My coach with alcoa wheels and all steel tires is smooth and totally
>vibration free.

YMMV - much depends on the speed you are traveling and the road surface.

>The single original air bag costs $265 and the equivelent 2 bagger costs
>$240, not a reduction in cost that I can see. In addition there absolutely
>is a loss of leveling when you change to the 4 bagger system. You still have
>the same space between the vertical arms, and you are reducing that space by
>two extra end caps from the bags and the width of the center tower.

The end caps are much shorter on the Firestone 224 bag than on the
original. This is not the source of leveling problems. The original bag
has stroke of almost 6" beyond the design ride height. The 4-bagger gets
around 4" in Leigh's design. This translates to less absolute height. The
center tower also has nothing to do with leveling. Leveling in the 4-bagger
comes from the useful stroke - the tower simply fills the gap between the
design point for the original design and Leigh's design. FWIW, the original
design operated the bag substantially below the recommended design point
provided by Firestone. The GMC configuration was an exception to the static
data chart.

The original objection voiced by some about Leigh's design was that it
changed the dynamics of loading on the bogies. Draw a vector diagram of the
"tower" and airbags and you'll see that the same loads are carried through
the same members.

The next complaint was that the 4-bagger loses height. That's true.

The remainder of what I hear is subjective opinion. We all have our own
ideas of what kind of ride we like. I like the 4-bagger much better than
the original - and my coach was fully in spec.

Henry

 

[henry davis]

>By the way how much is a single bag for the 4 bagger system and where can
>they be obtained?

MANY truck stores carry them. The 224 is a commonly used item. The prices
I've seen are $120 to $140.

> Can you readily get one if one blows out on the road?

Yes.

Henry