FOUR AIR BAG SYSTEM
Now, to address your request, I have previously expounded a number of times
on the advantages of the four air-bag system.
First let me say that the only negative I know about it is a slightly
restricted maximum travel distance of the suspension, supposedly on the order
of an inch or less. That isn't anything I've found to be a problem, however.
Some of the noticeable benefits I found are:
a. A reduction of about 60% of the wiggle traversing rutted asphalt paving
that I previously experienced.
b. A much more stable ride in that the fore-to-aft and side-to-side movement
was dampened, which stabilized the effects of running off the edge of paved
roads and onto shoulders.
c. It eliminated a tendency for the coach to bottom out when going over
railroad track crossings.
d. Independent suspension for each rear wheel.
e. A doubling of the rear-rear axle spring rate from approximately 415 psi to
820 psi.
Paul Bartz
Date: Mon, 01 Jun 1998 23:05:43 -0700
From: Jim Bounds <jimbounds#sprintmail.com>
Subject: GMC: 4 Air Bag rear suspension
Fri. I installed another 4 air bag system and again made another GMC owner
pleased.
Other than a few testy bolts, the install went smoothly. I had the 2 pin
bolts on the driver side put up a valiant struggle. I had a 3/4" breaker bar
lifting the coach via a 2 1/2ton floor jack before it surrendered. I did not
want to heat the nut unless I had no other choice not wanting to risk the
temper of the pin.
The kit fits perfectly, install time was 4 hours. I added separate shut off
valves for each bag to give the system flexability. Paul Gibson, head of
Firestones Ride Rite division test drove the system and was completely
impressed. Paul was involved initially in the GMC rear suspension design
using the single 2 sided air bag. He was so impressed that Firestone
acknowledged the systems viability by designating it with its own Firestone
part #. This is not a gimmick or fad, it is truly a next step improvement in
the evolution of a better ride for our motorhome.
The customer's drive test was impressive as the installation fit. All the way
home to Venice Fla., the customer said the coach tracked like it was mounted
to rails. He was concerned that the unusual handling characteristics of the
rear suspension would prevent his wife from driving the coach. He stated that
he can now travel at speeds that would have concerned him before. He now has
the confidence to take it out on any 2 lane, grooved, multilevel road we
wants.
Although the air compressor went into thermal when we were resetting the tank
pressure, it is adequate for operating the system under normal conditions. I
would like to experiment adapting some of the technology I found when
outfitting the air dump systems on ambulances I worked with last year. They
use a higher CFM compressor and "ping" tanks tied to the air bags for more
volume of air. A properly installed air drop system on an ambulance can drop
the suspension to the ground in 4 sec. and reinflate the system to ride
height in no more than 10 sec., a pretty impressive control. I feel this
would be a worthwhile modification to the GMC air ride system and would like
to set one up, any takers out there?
The bottom line is, you really should investigate the 4 air bag system if you
are going to be in the market for new air bags any time soon. It costs a bit
more than replacing the OEM bags, but with 2 bags costing around $500.00, the
cost of this tremendous performance modification is not as far out of reach
as it seems.
If you want to look further into this system, give me a call (1-877-275-4462)
or find someone who has installed the system and they will surly support all
I tell you.
Input on this subject is most certainly welcome.
----------------------------Date: Tue, 2 Jun 1998 03:20:25 EDT
From: RickStapls#aol.com
Subject: Re: GMC: 4 Air Bag rear suspension
Jim et al,
I am intrigued by this system, especially with all the glowing testimonials.
Where can I find out more info, especially: a. Does the 4-bag system use the
stock suspension arms and shocks, and/or are they reinforced/replaced? b.
Does it afford the full range of wheel travel and levelling that the stock
system does? c. Does it provide as soft a ride as the stock system? d. Does
it consume more air than the stock system (as seems to be implied in your
post)? e. Does it leave at least as much clearance for oversize tires as does
the stock suspension? f. Are suspension loads transferred to the frame
approximately as in the stock system, and therefore is it able to withstand
at least as much impact load (potholes) as the stock system?
Most importantly, where can I find pictures, diagrams, and technical info on
the 4-bag system? My position is all modifications are like UFOs: I have an
open mind and am quite prepared to believe in them, but I need lots of hard
facts before I can. (And LOTS of facts/details before investing all-too-rare
dollars!)
TIA
Rick Staples
Date: Tue, 2 Jun 1998 10:33:39 -0400
From: "Bartz, Paul" <s9d3452#mail.drms.dla.mil>
Subject: RE: GMC: 4 Air Bag rear suspension
Rick:
I installed the four air bag system in Mar 1995. Can't say enough about it.
You can't go wrong with it.
To install the four air bag kit, you remove the original air bag and cones,
install a steel "cage" on top of the center casting, an air bag on each side
of the cage, securing the opposite end of the air bags to the swing arm, 'a
la' the original air bag and reconnect the air lines. The kit comes complete
with nothing else to purchase.
There is a slight reduction (approximately 1") in the suspension height
travel, but you probably wouldn't notice the difference and therefore, to me,
it's a non-issue.
The ride is significantly better and smoother. After all, now you have
independent suspension at each rear wheel. The rear-rear wheel spring rate is
doubled from approximately 415 psi to 820 psi, which results in a much more
stable coach ride and more effective braking (i. e. less likelihood that the
rear-rear tires will skid under severe braking conditions). The fore-to-aft
and side-to-side movement of the coach driving down the road is dampened,
normally without bottoming out the shocks, particularly the front ones.
I don't notice any difference in air consumption.
There is no intrusion on tire clearance vs. the original bag.
The suspension loading remains essentially the same except for the spring
rate on the rear-rear axle, as noted above.
The kit originator is Leigh Harrison. Call him: 703-494-9914 for info and
literature. Tell Leigh I said Hi!
Paul Bartz
Rick,
I am pleased that you are interested and willing to investigate further.
First, let me try to answer the questions in your Email.
A.
Yes, the system connects to the stock arms. It is a bolt up kit,
replacing the 4 nuts & bolts on the vertical surface of the "C" channel and
new nuts replacing the original pin nuts.
B.
As fa as travel, down motion is of course, the same as original, the up
motion is reduced by only one inch.
C.
Yes, the ride is very soft. Unlike stiffer shocks and sway bars that
have been used in the past to bandaid handling problems, this system does not
rely on opposing or impeding swing arm travel. The suspension is free to
react to road conditions without the "push pull" motion of the original
system. Standard shocks should do a good job of doing what a shock is
designed to do, absorb small vibrations and average out the performance of
the suspension.
D.
Yes, it is recommended to readjust the air pressure in the tank up to
130 PSI. The Thomas single cylinder compressor seemed to keep up with the
pressure requirements, but did heat up a bit while fiddling and cycling the
system repeatedly. The V twin Hadley or the larger system I'm working on
would be a more dependable compliment to this system.
E.
The system actually takes up less room between the wheels and there are
no large, wide cones at the end that have problems clearing over sized tires.
I have a full restoration in house right now with Alcoas and 235/75 tires. I
am going to install a system soon and will be able to give you a more
accurate clearance # then.
F.
As far as load points and info on such as that, I would defer your
questions to Lee Harrison, the developer of the system. He has a small
classified ad in MHMP under suspensions, be sure you told him we had
discussed this on the net, he will be pleased this is happening and will be
able to answer those questions fully. He can also furnish you with pics and
install info, I think you will be surprised on how easy it is to install.
If you don't have a copy of the mag., call me tomorrow toll free at 1-877275-4462 and I'll give you his #.
I am sure that if you look into this system, you will pick one up soon.
Let me know if you have any other questions.
Jim Bounds www.gmccoop.com
Date: Wed, 3 Jun 1998 10:57:13, -0500
From: JDDP32B#prodigy.com (MR EUGENE R FISHER)
Subject: GMC: 4 bag
- -- [ From: Eugene Fisher * EMC.Ver #2.5.3 ] -I may be a novice at this, but it seems to me that with the 4 bag system you
loose the advantage of the boggie (sp) system. With the two bag system, the
front and rear wheels linked you cut the road bumps in half.
Since the front and rear wheel are no longer linked in the 4 bag system, it
looks like the front wheel will fall into a hole and all of the weight will
be on the rear wheel. when the rear wheel goes into the hole the coach drops
the whole distance.
gene
Date: Wed, 3 Jun 1998 12:31:53 -0400
From: "Bartz, Paul" <s9d3452#mail.drms.dla.mil>
Subject: RE: GMC: 4 bag
Eugene:
In the first place, I sure wouldn't be driving my coach over any such surface
that was holey enough to dangle an unsupported wheel???? Don't believe it's
feasible to drive on such a surface?? Did you have something else in mind
that I don't understand?????
On the contrary, the shocks, regardless of which suspension system you have,
restrain the swing arm travel and not the air bags.
Paul Bartz
Date: Tue, 9 Jun 1998 22:24:33 -0700
From: "Heinz Wittenbecher" <heinz#bytedesigns.com>
Subject: Fw: 4-Air Bag system
Don,
Thanks for your note. I'll cc this to the gmcnet in case someone is wondering
the same.
The 4bag system came from Leigh Harrison, 703-494-9914
Cost I can't find at the moment. (I'm in the wrong place). I did get the
optional air manifolds and after installing them and realizing their
potential I highly recommend them.
I assisted in the installation :-) Unfortunately my free time is somewhat
limited so in order to get something done in reasonable calendar time I use
part-time help. However, time is the only thing that kept me from doing it
myself. Installation was straight forward but some diligence and patience is
required with some of the bolts due to confined space.
It is said to give a straighter ride. I have yet to confirm that as I'm still
in drydock working on my GenSet. With each wheel now independently suspended
and the manifold allowing me full control with shut-off valves to trap air in
an individual bag I would expect that I could limp off the road safely in
case of a blowout and probably even make it to the next service
establishment.
I've only had one blowout in my 200k+ miles of travel, but is was not fun and
resulted in minor door damage. With the single bag each side you're stopped
dead in your tracks with a tire or airbag failure. I've always carried a
spare bag but was always hoping that I'd never have to use it. As soon as I
became aware of an alternative (4bagger), thanks to GMCnet, I went for it.
Thanks to folks like Leigh and GMCnet, our coaches are getting to be better
than new :-)
Heinz
'76 Transmode
- -----Original Message----From: DVM22727#aol.com <DVM22727#aol.com>
To: heinz#bytedesigns.com <heinz#bytedesigns.com>
Date: Tuesday, June 09, 1998 9:49 AM
Subject: 4-Air Bag system
>Heinz:
>
>I found your web page and looked in awe at the 4-bag air suspension system.
Now some questions (from me) and answers (from you) if you will please:
>
>1) Where did you get the system?
>2) What was the cost?
>3) Did you do the work your self?
>4) Why did you install this system in place of the 2-bag system?
>
>
>If you have already addressed these questions on the "net" and I >missed the
words, please excuse. If you didn't, others on the GMCMailing list might be
interested also in your answers.
>Thanks,
>Don Martin
>dvm22727#aol.com
Date: Wed, 9 Dec 1998 08:06:06 -0600 (CST)
From: LA-HOGS@webtv.net (Terry Krauskopf)
Subject: Re: GMC: Air bag Lift Kit
- --WebTV-Mail-396636319-88
Content-Type: Text/Plain; Charset=US-ASCII
Content-Transfer-Encoding: 7Bit
So how would you rate the importance of this upgrade.... (a) nice to
have but really doesn't improve that much... (b) very important ride and
suspension improvement that all wise and informed GMC folk would
make.... or (c) this is a crtitical upgrade that should (must) be made
on all coaches from a safety aspect.
Are there any problems caused by adding the lift kits that must be
overcome by adding something like a "lift kit stabilizer" and then a
"lift kit stabilizer equalizer"? Just trying to do my research so as to
make the best informed decision for upgrading/repairing my coach.
Next, If I want 'em......where do I get 'em.
Thank you Dave and all you other informed people who seem so willing to share
their experiences and knowledge with some of us trying to make daily
decisions on our coaches. While this info is not always easy to sort out and
sometimes even adds to some of the myth and fact confusion, the info is
always welcome.
Later,
Terry
75 GMC Avion
Date: Sun, 27 Dec 1998 03:16:14 -0600
From: Jon Bradford Peterson <mun01671@centuryinter.net>
Subject: GMC: air bags how much are stock ones / How much is a 4 bag system
Price wise what is the difference between the stock are bags and the new
four air bag retro fit.
Brad Peterson
73 GMC
-----------------------------Date: Sun, 27 Dec 1998 02:01:55 -0800 (PST)
From: herm beeck <hbeeck@yahoo.com>
Subject: Re: GMC: air bags how much are stock ones / How much is a 4 bag
system
Aprox. $1000.00
- ---Jon Bradford Peterson <mun01671@centuryinter.net> wrote:
>
> Price wise what is the difference between the stock are bags and the
new four air bag retro fit.
>
> Brad Peterson
> 73 GMC
Date: Sun, 27 Dec 1998 08:38:58 -0500
From: "Bartz, Paul" <s9d3452@mail.drms.dla.mil>
Subject: RE: GMC: air bags how much are stock ones / How much is a 4 bag
system
Not a good answer.
As I recall, a new bag is approximately $180, and the four-air bag system is
approximately $1000.
Paul Bartz
Date: Sun, 27 Dec 1998 01:50:44 -0500
From: "Bartz, Paul" <s9d3452@mail.drms.dla.mil>
Subject: RE: GMC: air bags again
Richard:
I'd like to learn what you mean by "overloading the rear tires".
How do you see that happening??
The four-air bag suspension in effect gives independent acting suspension in
that each wheel is free to move up and down on its own. However, it takes
both rear wheels on each side to support the coach. Except for one occasion,
I've never see any of my four rear wheels not have weight on them. That one
occasion was when the right front-rear tire blew out. After I raised the
coach and took off that wheel, I let the jack down thinking the one wheel
would support the coach. Wrong. The axle/hub (the one without the wheel) went
nearly down to the ground. So I quickly jacked it back up and set a jack
stand under the frame.
Paul Bartz
From: Richard Guthart
Sent: 12/26/98 9:30 AM
Subject: Re: GMC: air bags again
Tom,
I, too, opted against the 4-bag system out of concern over overloading the
rear tires. My "light weight" 23' tips the scales at 5800 lbs on the rear
axle with all tanks empty. I'm guessing the left tandems carry about 3200 of
that (genset and kitchen on that side).That's 1600 lbs per left rear tire
with O.E. suspension, which distributes the weight more or less equally
within each tandem. But it looked to me that the 4-bag conversion would
result in the momentary transfer of the entire weight on one side back and
forth between the tires in that tandem, and our load range E tires are rated
to only 2680 lbs.
Richard Guthart
-----------------------------Date: Sun, 27 Dec 1998 19:10:09 -0700
From: "Richard Guthart" <rguthart@frii.com>
Subject: Re: GMC: air bags again
Paul, does it still take both rear wheels on each side to support a coach
with the 4-bag conversion? If this is true then I misunderstand how this
conversion works.
I thought the conversion involved the installation of a rigid metal bracket
(bolted to the frame or rear control arm mount) which serves as the support
for one end of each bag. Each bag would then press independently against
this bracket and you could then completely deflate one bag on a side and the
other would then be supporting the entire weight on that side of the coach
by pressing against the bracket (pictured here:
http://www.bytedesigns.com/gmc/4bag01.jpg )
That was the source of my concern; that whenever the front wheel hits a bump,
for example, it will momentarily become loaded with the entire weight on that
side, rather than transfer some of the jolt to the rear wheel in the pair as
in the O.E. setup. Then the rear wheel will go over the same bump and become
the loaded wheel in turn.
Let me add that I don't profess to be an expert in this area, having never
seen a four-bag conversion up close and personal and having been a GMC owner
for a grand total of only 6 months!
Richard Guthart
----------------------------Date: Sun, 27 Dec 1998 22:11:44 EST
From: RickStapls@aol.com
Subject: Re: GMC: air bags again
In a message dated 12/27/98 9:11:54 PM EST, rguthart@frii.com writes:
> Paul, does it still take both rear wheels on each side to support a coach
> with the 4-bag conversion? If this is true then I misunderstand how this
> conversion works.
Richard,
Theoretically, one wheel could support that side of the coach in a 4-bag
system (if the aftermarket brackets etc are strong enough), however the
running air pressure in that one bag is not sufficient to support the entire
weight. Don't know if your air system would eventually develop enough
pressure to carry the load on one wheel and bag, or whether the one bag could
take such pressure.
Of course, when you hit a bump, the weight and pressure on each wheel and
airbag will be somewhat higher than in the original 2 - bag system. Myself, I
rather like the "step - over" effect of the original rear suspension which
gently lifts the coach over obstacles, while maintaining equivalent roll
stiffness when BOTH wheels move upwards.
BTW, I find it curious that the 4 - bag system does just the OPPOSITE of
what air bag extenders do. ie: the first stiffens the suspension, whilst the
latter softens it. Just my .02.
Rick Staples
'75 Eleganza
Louisville, CO
Date: Mon, 28 Dec 1998 23:29:25 -0600 (CST)
From: hdavis@ix.netcom.com
Subject: Re: GMC: air bags again
On 12/28/98 19:38:02 you wrote:
Keep in mind that the normal action of the 4-bag system is one of transitory
loads - just like the factory original configuration. The only question is
when do the loads occur in time and what is the limit of their magnitude.
Henry Davis Consulting, Inc / new product consulting
PO Box 1270
Date: Tue, 29 Dec 1998 01:14:55 -0600 (CST)
From: hdavis@ix.netcom.com
Subject: Re: GMC: air bags again
Well, that was interesting! Here's the rest of what I was writing
On 12/28/98 23:29:25 you wrote:
>
>On 12/28/98 19:38:02 you wrote:
>>
>>Paul,
>>
>>Thanks for the thoughtful reply.
>>
>>I'd love to take a demo ride in a 4-bag coach but I don't think I can make
>>it to the next Myrtle Beach Rally.
>>
>>Meanwhile, candidly, I'd still be concerned about overloading the rear
>>tires. If I removed the "front" rear wheels like Ed Edwards did at a
>>Sunshine Statesman Rally then my remaining rear wheels would each be
>>carrying about 2900 lbs. or at least the left one would. The load rating of
>>my load range E tires is 2660 when inflated to their maximum of 80 psi.
>>
>
NOTE: I do not have a 4-bag system. YMMV - it's a bit late, although I've
been thinking about this for a while. Corrections welcomed. If anybody REALLY
wants to do the math to calculate max bump/hole size let me know.
Keep in mind that the normal action of the 4-bag system is one of transitory
loads - just like the factory original configuration. The only question is
when do the loads occur in time and what is the limit of their magnitude.
For a level surface, a properly balanced coach will have all tires within
load limit specs at rest. Tilt the vehicle to one side and the weight on a
single wheel becomes the vector sum of weights. If we treat the weight
distribution as two masses, one over each of the rear wheels, we have the
"best case" of least weight transfer as the coach :wallows" when encountering
a side-to-side dip in the road. This means that if the coach leans to the
curb side at 45 degrees, the curb side wheels will carry all of the weight
originally on the curb plus half of what was on the street side. At smaller
angles the weight transfer is less. Conversely, the weight of the coach is
probably better modelled as a single mass centered on the midline of the
coach. In this case, all of the weight is transfered to the curb side tires
at 45 degrees. Even on the steepest hillside, it's pretty unlikely that the
coach will tranfer more than 50% of one side's weight to the other side.
If you could park the coach with the bogies on a really massive speed bump
and get the bogies at 45 degrees, the stock configuration will result in
weight transfer from front to rear or vice versa. But, you won't have all of
the weight transfer to one wheel because the coach weight is modelled in this
case by at least three masses - one towards the front of the coach between
the front bogies and the front wheels, one half way between the two bogies,
and the last somewhere to the rear of the rear bogies. In this case, the
highest loaded rear tire will be at most 2175 lbs - the original weight on
the tire plus half of the weight from the other bogie. Of course the bogies
can't travel to 45 degrees so the weight transfer is a bit more complicated.
Still, the practical angles keep the weight per tire down reasonably. I don't
know what the worst case scenario for weight distribution of the original
configuration, but we could figure it out.
Leigh's 4-bag system is more effective at weight transfer and more complex to
model. Part of what he accomplishes is not terribly different from using
torsion bars etc - except is is done front-to-back. We already know that the
limit to weight transfer of the 4-bag system is a doubling of the tire load
(front + back bogies) when the tire is gone or there is very deep hole. In
between there is a continuum of weight transfers. We can model the bogies in
a simple way. Assume that all weight transfer is front to back and there is a
single mass mid way between the two bogies. This dramatically overstates the
loading issue, but it's easier to understand. Maximum weight transfer occurs
when the bag pressure in the bag being compressed times the piston face area
of the bag equals the total weight normally handled by both bogies. (ie. When
bag PSI times bag end face area = both bogie's level loading) You can
calculate the height of a speed bump that causes total weight transfer using
Leigh's bag size, the arc described by the bogie movement between the center
pin and the individual wheel, the distance between the bogie center pin and
rigid bag attachment point, and the length of the lever pressing on the bag.
Assuming that the weight is evenly distributed between front and rear bogies
when sitting on a flat surface, the weight transfer is maximized when the bag
has been compressed to half it's original length (more or less since the bags
do deform a bit as they compress). I don't have the exact dimensions for the
4-bag system, so I can't calculate the actual weight transfer. Eyeballing the
pictures on Heinz's site I'd guess that it will take at least a 10" speed
bump or hole to load the opposite bogie to maximum tire pressure. Or, a 20"
different in height between front and rear tires when on an incline. Bottom
line for me without doing all the math - if the frame doesn't hit the ground
there won't be enough weight transfer going over bumps to overload the tire
rating. A big hole is a different issue at highway sppeds. But then if you
hit a foot deep hole with the bogies you're in a lot worse trouble than just
overloading a tire rating. At low speed inside a camp ground, the momentary
loads approaching the tire rating should be no problem since the tires are
rated at the max for high speed and the high speed dynamic loads
Lastly, the highway travel issue is one of dynamic forces. Interstates don't
often have really big holes or bumps. Truck ruts a few inches deep are the
usual problem, and these won't overload your tires using the 4-bag system.
For more information on suspension and chassis design issues take a look at:
http://www2.netquest.com/gaffney/saeppr/paper.htm
Henry
Henry Davis Consulting, Inc / new product consulting
PO Box 1270 / product readiness reviews
Soquel, Ca 95073 / IP reviews
ph: (408) 462-5199 / full service marketing
fax: (408) 462-5198
http://www.henry-davis.com
-----------------------------Date: Tue, 29 Dec 1998 06:46:26 -0500
From: davegreenberg1@juno.com
Subject: GMC: air bags again
>If I removed the "front" rear wheels like Ed Edwards did at a Sunshine
>Statesman Rally then my remaining rear wheels would each be carrying about
>2900 lbs. or at least the left one would. The load rating of my load range E
>tires is 2660 when inflated to their maximum of 80 psi.
>
>Richard Guthart
>
Perhaps a minor point but Ed Edwards has a 23' coach.
Dave Greenberg
Date: Tue, 29 Dec 1998 12:28:47 -0500
From: "Balcom, Robin S" <BalcomRS@ccpci.com>
Subject: Re: GMC: air bags again(War and Peace Topic)
<Just a note. the 4 bag system does not work the same way the original
system did. With the 4 bag system the middle tower restricts any force on
the front bogie to only that wheel, in other words the 4 bogie wheels are
entirely independent. in the original sytem, hit a bump with the front
bogie and a force is immediately transferred to the rear one on the same
side. Much much different mechanics and an entirely different dynamic model.>
Tom:
I've never seen a 4 bag system in person, but from the pictures I've seen,
it appears that the front and rear bags are "connected" through the air
tubing. It appears that they can be made independent by closing the two
valves, but with the valves open, air can be transferred from one bag to the
other. This, in effect, duplicates the original system.
- - On the original system, the force is transferred instantly, as you
describe.
- - On the 4 bag system, if you have the valves open, the force is still
transferred, but by the force on the air going through the tubing.
- - On the 4 bag system, if you have the valves closed, each bag is
independent, and no force is transferred.
As I think about "transferred instantly" (on the original system), I can see
that not all of the force is transferred (some is absorbed by the action of
the bag expanding) and it's not necessarily instantaneous (it's slowed to an
extent by the same expansion of the bag). If there were such a thing as a
"stiffer" bag (thicker sidewall construction), more of the force would be
transferred, and it would be transferred quicker. Plus, a "stiffer" bag would
be more resistant to allowing the GMC to roll as it turns. I wonder if the
bags used in the 4 bag system are of a thicker construction? If so, that
would explain (to me at least) the improvement in ride that some have
reported.
Well, that's all, that's what I think. If we were talking face to face, I'd
say I was just thinking off the top of my head. Instead, though, I'll have to
say I was thinking off the ends of my fingers (two, actually).
Robin
'73 Sequoia
Corning, NY
Date: Mon, 28 Dec 1998 14:38:13 -0500
From: "Bartz, Paul" <s9d3452@mail.drms.dla.mil>
Subject: RE: GMC: air bags again
Before answering your question, I contacted Leigh Harrison to discuss your
concerns.
His explanation to my tire blow out condition and what happened after I took
that wheel off and then let the jack down (remember I said the coach wouldn't
support itself), is that the swing arm once I removed the wheel extended to
its maximum restrained (by the shock) position. In doing so, the air bag
expands in size, thus increasing it's interior volume. Under that condition,
interior air (rushing to take up the increase space) is reduced in pressure
in both bags (mine are T'd together). Consequently, the coach can no longer
be supported due to the reduced pressure. Leigh now has a manifold for the
air pressure system, whereby you can isolated each bag independently if you
want.
As to your concerns about running on one wheel and overloading the tires, he
tells me that he has done that when he got a flat and that Ed Edwards drove
into a Sunshine Statesman Rally with his front rear wheels completely
removed from the coach as an attention getter.
The pictures you viewed on Heinz's web site are the correct configuration of
the mechanical and structural components of the system.
Are you going to the GMCMI Myrtle Beach Rally next March? Leigh tells me he
will be there. I highly recommend that you look him up and see his system
and take a demonstration ride in his coach and experience both his brakes
and air bags.
Paul
From: Richard Guthart [mailto:rguthart@frii.com]
Sent: Sunday, December 27, 1998 9:10 PM
Subject:
Re: GMC: air bags again
Richard:
I'd like to learn what you mean by "overloading the rear tires". How do you
see that happening??
The four-air bag suspension in effect gives independent acting suspension in
that each wheel is free to move up and down on its own. However, it takes
both rear wheels on each side to support the coach.
Paul Bartz
Paul, does it still take both rear wheels on each side to support a coach
with the 4-bag conversion? If this is true then I misunderstand how this
conversion works. I thought the conversion involved the installation of a
rigid metal bracket (bolted to the frame or rear control arm mount) which
serves as the support for one end of each bag. Each bag would then press
independently against this bracket and you could then completely deflate one
bag on a side and the other would then be supporting the entire weight on
that side of the coach by pressing against the bracket (pictured here:
http://www.bytedesigns.com/gmc/4bag01.jpg
<http://www.bytedesigns.com/gmc/4bag01.jpg> )
That was the source of my concern; that whenever the front wheel hits a
bump, for example, it will momentarily become loaded with the entire weight
on that side, rather than transfer some of the jolt to the rear wheel in the
pair as in the O.E. setup. Then the rear wheel will go over the same bump
and become the loaded wheel in turn.
Let me add that I don't profess to be an expert in this area, having never
seen a four-bag conversion up close and personal and having been a GMC owner
for a grand total of only 6 months!
From: Richard Guthart
Sent: 12/26/98 9:30 AM
Subject:
Re: GMC: air bags again
Tom,
I, too, opted against the 4-bag system out of concern over overloading the
rear tires. My "light weight" 23' tips the scales at 5800 lbs on the rear
axle with all tanks empty. I'm guessing the left tandems carry about 3200 of
that (genset and kitchen on that side).That's 1600 lbs per left rear tire
with O.E. suspension, which distributes the weight more or less equally
within each tandem. But it looked to me that the 4-bag conversion would
result in the momentary transfer of the entire weight on one side back and
forth between the tires in that tandem, and our load range E tires are rated
to only 2680 lbs.
------------------------------
Date: Mon, 28 Dec 1998 19:38:02 -0700
From: "Richard Guthart" <rguthart@frii.com>
Subject: Re: GMC: air bags again
Paul,
Thanks for the thoughtful reply.
I'd love to take a demo ride in a 4-bag coach but I don't think I can make
it to the next Myrtle Beach Rally.
Meanwhile, candidly, I'd still be concerned about overloading the rear tires.
If I removed the "front" rear wheels like Ed Edwards did at a Sunshine
Statesman Rally then my remaining rear wheels would each be carrying about
2900 lbs. or at least the left one would. The load rating of my load range E
tires is 2660 when inflated to their maximum of 80 psi.
Richard Guthart
- -----Original Message----From: Bartz, Paul <s9d3452@mail.drms.dla.mil>
To: 'gmcmotorhome@mailinglists.org' <gmcmotorhome@mailinglists.org>
Date: Monday, December 28, 1998 12:38 PM
Subject: RE: GMC: air bags again
>Before answering your question, I contacted Leigh Harrison to discuss your
>concerns.
>
>His explanation to my tire blow out condition and what happened after I
took that wheel off and then let the jack down (remember I said the coach
>wouldn't support itself), is that the swing arm once I removed the wheel
>extended to its maximum restrained (by the shock) position. In doing so,
>the air bag expands in size, thus increasing it's interior volume. Under
>that condition, interior air (rushing to take up the increase space) is
>reduced in pressure in both bags (mine are T'd together). Consequently,
the coach can no longer be supported due to the reduced pressure.
>
>Leigh now has a manifold for the air pressure system, whereby you can
>isolated each bag independently if you want.
>
>As to your concerns about running on one wheel and overloading the tires,
he tells me that he has done that when he got a flat and that Ed Edwards
drove into a Sunshine Statesman Rally with his front rear wheels completely
>removed from the coach as an attention getter.
>
>The pictures you viewed on Heinz's web site are the correct configuration
of the mechanical and structural components of the system.
>
>Are you going to the GMCMI Myrtle Beach Rally next March? Leigh tells me
he will be there. I highly recommend that you look him up and see his system
>and take a demonstration ride in his coach and experience both his brakes
>and air bags.
Date: Fri, 29 Jan 1999 19:43:55 -0800
From: "Heinz Wittenbecher" <heinz@bytedesigns.com>
Subject: Re: GMC: 4 bagger suspension system
*This message was transferred with a trial version of CommuniGate(tm) Pro*
>
>I installed the four-air bag suspension system in Mar 1995, and have over
>40,000 trouble-free miles on it now.
>
>Go ahead and install it. You'll never been disappointed. Now isn't that
an opinion???
>
I'll second Paul's "opinion".
I only have 10,000 miles or so on mine but equally happy.
Re the fittings: Have a close look when you get them to be sure they're a
good seal.
I've since replaced some of the quick-fit convenience (push-in) fittings
with real compression fittings. Had I to do the install over again I'd
consider going compression from the start as they are a bear to get to after
installation.
I don't mean this as a negative reflection on the 4-bagger it's just that
sometimes timesaving devices (push-on fittings) only save time during
initial assembly and not for the long term.
Are you planning to get the manifold assembly as well?
Heinz
'76 Transmode with 4bagger
www.bytedesigns.com/gmc
Date: Sat, 30 Jan 1999 08:25:38 -0800
From: "Heinz Wittenbecher" <heinz@bytedesigns.com>
Subject: Re: GMC: 4 bagger suspension system
*This message was transferred with a trial version of CommuniGate(tm) Pro*
The bags are current Firestone. Saw then at the SEMA Show in 'Vegas last
fall. Firestone actually had quite a display re various air suspension
bags/pumps.
If the brackets failed and none are available I'm confident that a local
shop could repair and/or remanufacture.
As with most improvents, the initial 'value' is in thinking it up in the
first place and then designing/producing same.
And if all else fails one puts the 2 bags back on. Installing the 4bagger
does no modifications to the original that cannot be undone easily.
FWIW -- Heinz
>A question that no one has asked yet. Are all of the parts going to be
>available in the long term for replacement due to wear or failure? If
>Harrison goes out of business for some reason what happens? Are the bags
>readily available?
>