Air Springs Revisited

[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.

[Jon Wehrenberg]

There has been confusion in the past on air bags, pressures, ride height valves, etc.

When in the road or driving position there is no way to regulate air pressure in a set of Prevost air bags to adjust the quality of the ride. The air pressure in our air bags is controlled solely by the ride height valves, and each valve controls its respective set of air bags.

All ride height valves function exactly the same. A lever arm from the valve (which is fixed to the bus body) is attached to a portion of the suspension, such as the front axle or the rear drive axle carrier. When the bus is sitting in its lowest position and the suspension system control is placed in the road or drive position, the arm which has been pivoted up because the bus body is down relative to the normal ride height opens the ride height valve to admit air to the air bags.

As air enters the air bags the pressure inside the air bags increases until the air bags begin to lift the bus body. When the bus body has risen to the normal driving height, the lever arm on the valve body is centered in its range of travel and the flow of air into the air bags stops. Whatever the resultant pressure inside the air bags is what it takes to support the bus body at the normal driven height. That pressure varies with circumstances. An empty bus will have less pressure inside the air bags at a given height. A bus with 45 fat people will require a greater pressure to place the bus at its driven height. And when those fat people get off the bus, the pressure in the air bags is in excess of what is needed and the body will rise as a result. But as the lever arm on the ride height valve is lowered because the increased air pressure is lifting the bus relative to the suspension, the ride height valve begins to bleed off excess air pressure until the bus is restored to the correct driving height.

Our buses will not have 45 fat passengers. our weights are somewhat constant if you ignore our fuel and water consumption. But our ride height valves are still working as we drive. Going around a wide sweeping curve will make the bus start to tip due to centrifugal force. As the bus begins to lean the outside (of the turn) ride height valve admits air into the air bags on that side and the inside ride height valve exhausts air keeping the bus distance from the road equal side to side.

The only time we have the ability to exercise any control over the air pressure in out air bags is when we level our buses at a campsite. Even then we are more concerned with how high or low we need to make a certain point on the bus as opposed to how much pressure do we need in each air bag.

We have air regulators on the bus for various purposes such as belt tensioning, but our suspension system itself has no pressure regulators, only valves which open to admit air or exhaust air to the air bags as required by circumstances.

Think of the ride height valves as a three way switch. In one position it opens the flow of air into the air bags. In the center position it is the neutral position and air is neither flowing in or out of the air bags. This is the position the valve arm is in almost all the time we are driving. The remaining position allows air in the air bag to be exhausted from the air bags.

[phorner]

So, what's the rationale for an IFS chassis, that is still a MH chassis, to have the larger front air bags than a non-IFS bus that's only a couple of years older?

I can understand the difference when applied to entertainer vs motorhome applications, but apparently there is some reason that Prevost went to the 12" front bags for the use in motorhomes?

The more I think I understand, the more confused I get

[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.

[Jon Wehrenberg]

So, what's the rationale for an IFS chassis, that is still a MH chassis, to have the larger front air bags than a non-IFS bus that's only a couple of years older?

I can understand the difference when applied to entertainer vs motorhome applications, but apparently there is some reason that Prevost went to the 12" front bags for the use in motorhomes?

The more I think I understand, the more confused I get

To rephrase it another way Paul, why would Prevost want to carry extra inventory? If the 151 is deemed by the Prevost engineers to work in all applications, why bother with the 259?

There must be a reason why that is listed as the standard, but as Joe learned trying to get information is not easy.

[lonesome george]

Jon,
From your explanation it seems to me all I did with the Beaver is slow down the reaction time, which may have been why it had a regulator in the circuit in the first place.

[Joe Cannarozzi]

Brian did he say the 151's could be used on a straight axle non-entertainer, specifically?

The bases are slightly different.

I agree with Jon. Every straight axle I put the 159's on easily brings the front all the way up at idle, compressor not running.

[Jon Wehrenberg]

George,

Without looking at the Beaver system it would not be possible to determine the function of the pressure regulator.

Slowing or speeding the process of adding or exhausting air from the air bags is something Prevost does on older coaches using a time delay ride height valve, but they have gotten away from that and now use instant reaction valves. They will also supply the instant acting valves as replacements for the time delay ride height valves.

If Beaver used a regulator to slow down the reaction time they could have saved lots of money by just using a valve in line to function like a faucet, opening or closing it as required to regulate the flow of air.