air bags again

T

thomas g. warner

New member
Mar 24, 1998
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Everyone has to make their own decision relative to the modifications they
want to make to their coaches. The GM engineers put a lot of time and
effort into the design and execution of this coach as attested to its
increased popularity and use 20 years after its introduction.

I had already decided after talking to others that the 4 bagger system was
not for me. The air bag extenders were also decided against since I was
unable to find anyone that had ran the engineering numbers on them to see
what the negatives and postives were.

During my younger days I was only to familiar with the suspension
modifications that other rodders came up with only to have them fall apart
or catastrophicaly fail at speed. Sometimes its better to leave well enough
alone.

I do need another set of air bags with aluminum cones however. Any good
deals out there?

>-- [ From: Eugene Fisher * EMC.Ver #2.5.3 ] --
>
>There is a new article in the Cinnabar GMC news (Dec98) that says:
>
>Air bag extenders:
> do not extend bag life
> make the ride softer but increases the wallow side-to-side
> move more toward problems than improvements
>
>Good article we should all read it.
>
>Four baggers:
> stiffen the ride (reduce the wallow)
> eliminate the prefered bogie action
> reduce the leveling action
> raise the pressure required
> increase the bending load on the center arm
>
>I will find the reference and post it on my web page.
>
>Owners love all of these for different reasons. Be sure to read the
>literature to make your own decision
>
>gene
>http://www.california.com/~eagle/
>
>
>
>
>--
>Gene 76Palm Beach /Or/CA
>
>GMC Technical Information
>http://www.california.com/~eagle/
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
 
Gene,

While there is a lot of very good information in the Cinnabar news letter,
we have to remember that it may be pretty biased. I have actually been a
little disappointed with these news letters lately because of that. We pay
for the news letters, but lately they have been sounding more and more like
ads and less and less like articles based on objective studies.

I think the shock articles were a perfect example of this. A totally
unbiased article in the GMCMM mag favored the Caspro shocks because of
their strong design and the writer's good experience with them. That
article also indicated the Bilsteins were prone to and ocassionally had
catastrophic failure because of a weak design. The writer had tried
several different types of shocks. He had cutaway views showing the
dimensions and construction of each shock. Most importantly, he had no
stake in endorsing any particular shock. The next Cinnabar news letter
coincidentally(?) also had an article on shocks. It coincidentally(?)
blasted the Caspro shocks calling them "overpriced truck shocks". It also
praised the Bilsteins as the best shocks ever made. Guess which shocks
Cinnabar sells. There were no cutaway views of the other shocks showing
why they weren't as good as the Bilsteins. There were no shock dyno
results that showed how the Bilsteins were better at dampening the loads of
a GMC. IIRC, there was even somewhat of an admission that the Bilsteins
might not be as strong as some of the others. I don't recall seeing any
objective information in the article as to why Cinnabar considered the
Bilsteins better than any of the other shocks.

I am not saying that there isn't good information in these newsletters.
Most of the concerns raised in the news letter are based on fact. I do,
however, think that the news letter should be read as an ad first and then
a source of information.

I know I've said all this before. If anyone is getting tired of hearing
it, just let me know, and I won't say any more about this type of issue.

Zak

>-- [ From: Eugene Fisher * EMC.Ver #2.5.3 ] --
>
>There is a new article in the Cinnabar GMC news (Dec98) that says:

>Gene 76Palm Beach /Or/CA
>
>GMC Technical Information
>http://www.california.com/~eagle/
>
 
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

- -----Original Message-----
From: Thomas G. Warner
To: gmcmotorhome
Date: Friday, December 25, 1998 4:39 PM
Subject: Re: GMC: air bags again

>Everyone has to make their own decision relative to the modifications they
>want to make to their coaches. The GM engineers put a lot of time and
>effort into the design and execution of this coach as attested to its
>increased popularity and use 20 years after its introduction.
>
>I had already decided after talking to others that the 4 bagger system was
>not for me. The air bag extenders were also decided against since I was
>unable to find anyone that had ran the engineering numbers on them to see
>what the negatives and postives were.
 
Keep it coming, Zak. Your opinion is as valuable as anyone's and we need
to hear all sides of an argument.

We can't expect Cinnabar to promote something they don't sell so we
necessarily must use judgement when we read any comparisons whether it is
shocks or air bag lift kits.

David Lee Greenberg
GMC Motorhome Registry
200 MacFarlane Drive
Delray Beach, FL 33483-6829
 
>

>
>> here is a new article in the Cinnabar GMC news (Dec98) that says:
>>
>> Air bag extenders:
>> do not extend bag life
>>
>When has Wes endorsed anything Cinnabar doesn't sell??
>Lanier
>

Cinnabar is in a funny position - they are the GM licensee, so they do have restrictions on what they can endorse. It is my
understanding that Cinnabar does do other installations of aftermarket eqipment, including some of what we discuss here that
they can't publically endorse.

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
 
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

- -----Original Message-----
From: Thomas G. Warner
To: gmcmotorhome
Date: Friday, December 25, 1998 4:39 PM
Subject: Re: GMC: air bags again

>Everyone has to make their own decision relative to the modifications
they
>want to make to their coaches. The GM engineers put a lot of time and
>effort into the design and execution of this coach as attested to its
>increased popularity and use 20 years after its introduction.
>
>I had already decided after talking to others that the 4 bagger system
was
>not for me. The air bag extenders were also decided against since I
was
>unable to find anyone that had ran the engineering numbers on them to
see
>what the negatives and postives were.
 
>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 )

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

>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
 
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]
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.
 
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
To: 'gmcmotorhome'
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.
 
Richard,
I do understand what you are saying, but I tell you it sure rides nice.
I'll ask Leigh Harrison about that concept next time I talk to him. There
may be a good answer to that question.
See Ya,

Jim Bounds
- --------------------

>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
>To: 'gmcmotorhome'
>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.
>
>
>
 
>
>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.
>

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

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
 
Well, that was interesting! Here's the rest of what I was writing

>

>>
>>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
attachemetn 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
 
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
 
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