FRONT AIRBAGS

[rahangman]

Several years ago the King and I decided to change all the air bags on our coaches. The incentive was that Roger wanted to change them, but Prevost had shipped the wrong front air bags.


Our motorhome shells are designed to have about 4" of both up and down travel from ride height. The purpose is to allow leveling on uneven campsites. A seated coach is designed to go down (kneel), but not be raised above normal ride height. It is likely that anyone that orders air bags for their coach or has them replaced by Prevost is not going to get airbags the equivalent of what was on the coach when it was built. Prevost continues to ship and install the wrong airbags on motorhome coaches.

This is a critical piece of information that everyone should know and understand. If you are going to have air bags for your coach replaced there are some things you should do. First, measure the ride height, the lowered height and the maximum raised height of your coach. The proper way is to measure the distance between the upper and lower air bag mounting plates, but it is just as easy and understandable to measure from the bottom of the bay door just behind the front wheel to the floor surface. When having the air bags replaced be certain that the installer understands you need to retain the amount of travel you have, and that if you do not get that travel with new air bags they need to be replaced with those that will provide the travel.

There are other issues related to this that all of you should know. First, the current air bags being supplied have larger fittings for the air supply, and the top mounting plates on some coaches do not have a hole large enough to accomodate the fitting. The hole in the top mounting plate in that case may need to be bored or torched out to a larger diameter to accomodate the fitting in the top of the air bag. Another issue is travel. While it is important to retain the amount of vertical rise above ride height, it is equally important to not have an air bag that will provide more travel. It is possible to break the top mounting ring from a shock absorber if there is too much air bag travel. A broken shock absorber can do some damage so it is important to not install airbags that can break the shocks.


If you allow yourselves to become a victim of reduced height due to the wrong airbags remember Prevost will not correct the problem. To their way of thinking the air bags they list and install are what is called for in their parts lists and they will not do anything to correct this so it is up to you to get an understanding from Prevost up front as to what you expect after air bags are replaced. It may not seem like a big deal now, but if you damage the lower front of your coach or wipe out a generator baffle because you cannot raise your coach up this issue will take on a greater significance.

OK, so if I have this right, (I fall into the late 80's early 90's category) If I go to Prevost they will put in 2 new airbags (after enlarging the mentioned access hole) that will not raise my bus over its required ride height. I will be in a pickle for some manuvering which could cause damage. If I put the other air bags in that could possibly raise my coach beyond the 4" excess height, I could pull apart a shock and also cause damage. SO if I then decide I should change my shocks to prevent an overheight inflation situation, can I find shocks that will be long enough when extended to avoid this? Conversely as mentioned by others, maybe a different airbag other than Goodyear 259 or 260 can be found? I must say this is a stimulating subject and I thank the MPD Gods for guiding me to POG to provide this assistance.
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[Jon Wehrenberg]

I do not know of any shockes other than those supplied for our coach. There might be some out there, but my guess is if the shocks extend further so breaking the rings (or as Kevin points out ruining thE bushings) they will not collapse enough so the lowering travel may be compromised.

That is why I suggested if anyone is going to replace front air bags, just prior to having it done measure the lowest height and the highest height and the ride height, and let the service people know those are the range of dimensions you expect and require.

If they cannot assure you they can do that, I suggest you not get the job done.

[Orren Zook]

Increasing the port size on a bag will do nothing to increase air volume unless all associated air lines, fittings and valving size (inside diameter) is also increased. I would guess that the increased inlet size on Prevost air bags is to speed up the kneeling process and recovery to ride height for transit coaches and would have little benefit for any motor coach conversions with smaller diameter air lines. Sounds like the old adage: "sell 'em what you've got, not what they want".

[Jon Wehrenberg]

Orren,

Your analysis is correct, however Prevost may be attempting to reduce line items in inventory. The current coaches use an accumulator tank pneumatically tied into each air bag. However, while my coach has a larger diameter fitting and air line going into the air bag in the front, the controlling orfice is either in the ride height valve or Norgren 5 port valve which are in line, or the air hoses to or from them, which in all cases is smaller.

I do know from conversations with parts personnel that there are or has been a lot of different air bags listed for 40 footers, 45 footers, motorhome shells, seated coaches, etc. This entire issue may be a result of someone's attempt to reduce the number of variations they stock.

[tdelorme]

I know we're beating this horse to death, but just for the record, do we know who made the original front bags for motorhome shells?? And, do we know a part number other than the original Prevost part number? I've got a NAPA parts guy that can cross reference up a storm and I sure would like to give him a shot. In fact, if someone has the original Prevost part number, he can usually use that.

[bluevost]

Jon, Truk, and other POGgers

The following comments apply to my solid axle coach, chasis vintage 1994:

On March 3, 2008 I had Bluevost in the Prevost Mira Loma service center for service. Using the "Werenberg 10 year rule", I requested to have the front bags removed and replaced with new ones. The part number on the new bags is 630259. This is not the same part number as the originals. I still have the original bags, but they are in California and I am in Connecticut, so I can not include that part number here.

I have not been able to satisfactorally raise the front of my bus since. I can't tell you how many times we have camped with the front of the bus low. Level low will not do it, and the automatic level will not do it.

I planned on talking privately with Bill J at OKC. This was not of my doing. Shouldn't Prevost change the bags to the correct bag, at their expense?? Even though it has been 1 1/2 years.

Makin' our way (round about) to OKC,

Ken

[phorner]

Ken, I agree with you. There is more to an air bag than simply being able to hold air without leaking.

You have lost an important part of the performance of your front air spring suspension system, which rightfully ought to be restored at the expense of the service center that provided the less than satisfactory installation.

[grantracy]

Just replaced mine as well I guess I will dig out the work order to see what was installed. Had to have it back after the installation to repair air leaks and install missing hardware. Did not leave me feeling too confident about their abilities.

[Jon Wehrenberg]

I know we're beating this horse to death, but just for the record, do we know who made the original front bags for motorhome shells?? And, do we know a part number other than the original Prevost part number? I've got a NAPA parts guy that can cross reference up a storm and I sure would like to give him a shot. In fact, if someone has the original Prevost part number, he can usually use that.

The problem Ted is what number do you want to cross reference? My bus has two different part number air bags. Those air bags are not the same as for a 40 foot bus. A seated bus has different numbers. An older coach has different numbers and fitting sizes whether it is a motohome or a seated bus. We don't even know the extent of this issue yet. Does it apply to H3 owners? IFS owners? If the Prevost part numbers we have are converted to industry numbers (my now obsolete Prevost 630126, industry part number 1R11 296) is a proprietary air bag specifically for Prevost. I am not willing to risk using a Firestone apparent duplicate even if it can be cross referenced unless I can be assured it has all the characteristics including diameter, height, internal bump stop, etc.

We can use the internet to cross reference until the bovines arrive back at the ranch, but first we have to get at the truth as to what air bag is supposed to be on each coach.

As we hear from folks like Ken and maybe Granville (who has an IFS) we are going to learn more. I do know this, Folks like Dale F and myself absolutely require the capability to raise the front of our coaches as high as they are designed to go because if they do not raise sufficiently both of us are going to do some damage under the coach. Others are likely to be equally affected and may not know it yet because they don't routinely have to have full up travel in the front.

[truk4u]

Ken,

Mine is a 94 (93 chassis) and I have 259's on the front and 630104's on the rear. I have full travel both front and rear.