Air Springs Revisited

[gmcbuffalo]

With the extra 1" in diameter will we have any rubbing issues?
GregM

[Joe Cannarozzi]

I have never been given that # from anyone at Elgin nor have they ever sent that bag to any motor home owners that have had parts ordered ahead of time and Mr. Jenson never mentioned that bag when he informed me they were not responsible for sending bags that wouldn't raise the front on a motor home.

Brian do you have the name of the fellow you spoke with?

[Jon Wehrenberg]

Brian has just revealed the truth..........

Lets assume the 259 is indeed a direct replacement for the 126. Assuming their diameter and height dimensions are identical they should lift the front end of the bus easily. Using the 11" diameter as the effective lifting area the air bags have a 95 square inch lifting surface. The lifting surface area determines how capable an air bag is at raising or supporting the weight placed upon it. In the case of the 259 and 126 this translates into an ability to support 9500 pounds per air bag at 100 PSI. This is theoretical because we do not know the actual lifting area, but is good for analysis.

The entertainer air bag 630151 has a 12 inch diameter. That additional 1" of diameter does not seem like a lot, but it calculates to 113" of lifting surface, or one that is capable of supporting 11,300 pounds at 100 PSI.

Stated another way, the 259 requires 73 PSI to support my coaches front end weight and the 151 requires 62 PSI to accomplish the same thing.

Again, these numbers are bogus, but serve to illustrate how increasing the diameter has a huge impact on the pressure required to lift and support our coaches. Before the mad rush to replace front axle air bags however there is a need to consider the impact of using a larger air bag. If the air bags now in use work, that is they support the coach and allow you to raise it to the maximum height you may not want to make changes. Like a lot of things on our coach the sizes of components has been decided after a lot of factors have been taken into consideration. Air bags are one of those things that require a lot of consideration.

Apart from space considerations, making sure a larger diameter will not rub on anything, a larger diameter should only be used if the axle weight is substantially greater than normal. Entertainer coaches certainly meet that criteria. The longer wheelbase and the substantially reduced rear overhang places a greater load on the front axle than the typical motorhome conversion. That increased front axle load will require a greater internal pressure for an air bag that has a 100 PSI limitation branded right into the sidewall of the air bag. Entertainer coaches need the greater diameter to keep the pressure within an acceptable range.

So what happens if you place bigger diameter air bags in your coach? Depending on the front axle weight, you could end up with a coach that mushes down the road, that dips dangerously on hard stops, and that leans excessively in turns. With a substantially lower internal pressure the ride will go from firm to very soft. Does anyone remember how poor the classic cars used to handle with their soft ride? The cars of today have a much more tightly controlled ride and as a result their handling is very good. The same is true of our coaches.

I'm all in favor of changing air bags on a coach that struggles to reach ride height or which has a hard time extending to full height. That tells me the air bags are likely overworked and may benefit from a change in size. But if the coach does not struggle and you do not have to do special things like lifting a tag axle to raise the front, I would suggest staying with the 126 or 259.

As far as extra height, as long as an air bag has greater lift than 4" available at its installed height of around 11" it is more than a handful and is not necessary.

[Kevin Erion]

I guess I got lucky, maybe not if you go with Jon's statement about if you need the extra capacity to lift. I replaced all my air bags about 3 weeks ago and the ones I received from Mira Loma for the front are the 630151 bags. I will take the bus for it's first trip to Havasu this weekend after the replacement and report if there is any difference in ride quality. I am afraid that because it has been about 4 months from the last use I won't notice anything, I am hoping that's the case anyway!

[phorner]

For what it's worth, when Prevost replaced my front air bags (1999 IFS shell) they used the 630151 12" air bags.

[Jon Wehrenberg]

A little bit of apples and oranges. The geometry of the IFS which Paul and Kevin have cannot be compared to the issues that Eric and Deb had for example unless the IFS coaches have no more or less mechanical advantage in supporting or lifting the coach. This issue makes me want to put gauges on my suspension system just so I know the values of the pressures required to go from full down, to ride height to full up.

In Kevin's case I'd bet his coach will ride and handle closer to what the engineer's designed because Marathon conversions tend to be a little (or a lot) front heavy. Paul likely has a softer ride. I don't think either is dangerous as long as the shocks are in good shape because they tend to dampen the mushiness.

[Denny]

Ditto to Paul's post. Joe recently replaced my air bags etc, and Prevost sent 630151 12" air springs

[lonesome george]

Here is a foot note to Jon's reply.
Our old coach was a Beaver Patriot with a Maginum chassis and the front suspension was harsh on small amounts of suspension travel, it had a regulator mounted under the body near the height control valve, not user friendly and not ment to be and no gauge. We installed a gauge to check the pressure and it was about 60 psi. I removed the whole thing and ordered a non-relieving regualtor and gauge, mounted it in the storage bay so it could be adjusted easily. Turned out it took about 35-36 psi to lift the chassis at all, at 40 psi it would come up about as fast as it did at 60 psi. The improvement in ride qaulity at 40 psi was remarkable and I did not find any change in body roll or increased bump travel on hard braking. Did the same thing on the rear but could only drop the pressure about 5 psi from the OEM set-up.
This may not work on a Prevost, for one thing it takes our's several minutes to lift it's self up after being lowered, so I think the pressures on a Prevost maybe very close to the minium pressure to lift the coach, the Maginum has 4 air spring on each axle so it is a different beast..
It was a fun project that had a good result.

[BrianE]

Joe, As you can see from Kevin and Paul, the 151 bags are being used as replacements. I initially spoke to Donavan Pelky, a parts team leader in Mira Loma and he told me about the 151. I then looked in my own Catbase Prevost Parts program and confirmed they are an option so consequently should fit without rubbing issues. I also spoke to Charlie an old time parts pro in Chicago because I wanted to get specs on the 151. As far as I know the actual specs are not available from Prevost. The diameter is of course 1" larger, the relaxed height is about 1" higher, the only unknown is the fully extended height. It can be assumed that height is somewhat more than the 259. As to ride height concerns, the ride height valve doesn't care what bags are installed, the ride height won't change as long as any airbag is capable of reaching ride height.

[PartsSurfer]

Joe,
If you would like some help with the airbag situation, please feel free to give me a call. I actually had a really good time trying to help Mr. Esler figure it all out. In fact, I would love to hear from any of you who have motorhome specific questions. I spent some time with Marathon Coach,Country Coach, and Monaco.
We are also lucky enough to have a considerable amount of experience in the shop right out my window. If I don't know the answer, I'll find someone who does!