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Replacement Air Bag - Printable Version

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Replacement Air Bag - MPerry - 09-17-2017

I recently replaced one of the right front air bags on my classic. Now in travel mode that corner sits a little low, about an inch. I have only been able to come up with 2 possible reasons. 1) Different air pressure side to side that is unrelated to the bag replacement. My air valves do leak and could be causing a pressure difference left to right 2) New air bag does not produce as much height for given pressure. I would check the pressures but currently do not have the fittings to tie in a gauge.

edit.. The front uses one HCV to supply both sides and was also recently replaced


RE: Replacement Air Bag - whatsnewell - 09-17-2017

I am not an expert in air bag replacement and the related issues resulting from same, but I am suspecting that point #2 may be the culprit. I've been fortunate that none of the airbags on my '82 have bitten the dust, but if one needed replacing I would just replace both.......it is a good rule of thumb to replace both airbags at the same time. Are you losing air pressure on your dash air pressure gauges?


RE: Replacement Air Bag - MPerry - 09-17-2017

I am losing air pressure at the dash, on the secondary not brakes, which I am sure is part of my problem. Given that left and right seem to be tied together I suspect its from having different bags left vs right. I cant say that I fully understand how the 6 pack air valves work. I do have several rebuild kits and will install them in the travel valves to see if that helps with air problems.


RE: Replacement Air Bag - Richard - 09-17-2017

Ok if in travel mode, all of the theories about six packs and different pressure side to side do not apply. Very simply for whatever reason the airbags do not match is diameter. When in travel mode the front airbags are tied together via a tee at the hcv. It is highly unlikely that you have a pressure difference cause pressure doesn't behave that way in a closed system.

The ability of an airbag to lift is this equation. Weight = psi (Diameter of bag/2) (diameter of bag/2)

It doesn't take very much difference in diameter to change the lifting characteristics


RE: Replacement Air Bag - Chester Stone - 09-17-2017

I am curious as to how you determined the number for the air bag you replaced. Was the new the same as the old?


RE: Replacement Air Bag - bikestuff - 09-17-2017

Does an '84 coach have three HCVs like the new ones? If that is the case, it might be that the rear heights (controlled by the HCVs) are not perfect and that show up as a difference in the front. I have also seen variability of around 1" in repeated measurements.

If you drop coach all the way down and then go to travel height, wait for it to settle and then measure the frame rail to ground distance at all four corners and then repeat several times, you will probably see some variability.

In other words, with only an inch out, you may not have a problem.


RE: Replacement Air Bag - MPerry - 09-17-2017

Great points, thank you all. I only replaced 1 air bag because it was leaking, very noticeable with bubble soap. Also before replacement the coach would not stay up after a few hours. The bag numbers are molded into the bags (firestone) so the same model number was used as replacement. (i think it was print on the metal plate too but only visible after removing it.) The coach will hold level now so I dont suspect leaking air bags at the moment.

I was thinking that maybe the old bags were lifting higher because they are worn (softer), but with considering the psi vs diameter I would expect the new bag to lift higher, so my idea there was wrong.

My thought on the air valve is that the one which lets the coach lower (while leveling), is also the one that prevents the air from coming out when in travel mode? In which case I think a leak on the right side down valve might cause lower pressure (and lower stance). I admit I have not crawled under to trace all the air lines back.

The coach does have 3 HCV. 2 on rear and 1 up front. I will have to check the 2 on the rear and see if they may be bad.


RE: Replacement Air Bag - 77newell - 09-17-2017

I found the same pattern that Bill is reporting in terms of variability. I'm also inclined to focus on the new air bag being somehow different now than the one on the other side (it could be the same part number and yet somehow stiffer of slightly different in shape).

If the when parked with the leveling off and the coach stays at the same height as when turned off then the air loss is not due to the new bag since it is holding air when isolated by the 6-pack valves. Likewise , if that corner of the coach drops when all the 6-pack valves are not energized, then the new bag installation probably is related to the air loss.

There is an instance in which in the travel mode you could have the front corner running low. If there was a leak in the air bag installation and there was a restriction in the flow of air between the HCV and the air bag via the TRAVEL solenoid valve then you could have the HCV unable to keep up until slightly low. This assumes the HCV progressively opens as it senses a low position - I do not know for sure if this is the case but I suspect it is. A quick test to see if a part of this scenario is happening is to do the leak test I mentioned above.

I have pondered long and hard regarding the weight carrying capability of an air bag and its relationship to ride height. What I've concluded is that weight on the individual front air bags has little influence on their respective heights at the shared air pressure coming from the HCV. The front air bags do not carry the same weight, there is a side to side disparity. For some coaches this difference is small and for others it is considerable and, yet, our coaches ride level in the travel mode. It seems to me that in the TRAVEL mode the rear ride height has a strong influence on the respective front heights, side to side. Given this, it also seems to me that if the bag can achieve close to the ride height of its opposite bag then it is unlikely to be the wrong size. I believe that raising that last inch is more a matter of increasing air volume than increasing air pressure. I admit that I've never tested this perspective with pressure gauges. I think in this case it's more likely that upon changing the air bag you started making measurements that you caused you to notice a situation you were previously unaware of and Bill's suggestion above could give you a more reliable understanding of what the case is. I'm convinced that something I said here is true, the problem is I'm not sure which part Smile


RE: Replacement Air Bag - kyleb - 09-17-2017

In travel mode, the rear HCVs control the side to side lean ( or lack thereof ).
The front HCV only sets the average height of the front end. If the front bags made that much difference in front side to side travel height, that would mean your coach was twisting & likely popping rivets like crazy.

The ride height on mine varies about an inch too.

Measure the height in front of the drive & behind the steer.


RE: Replacement Air Bag - MPerry - 09-17-2017

As said above, the coach may have always been an inch off and changing the bag just made me aware of it. I am going to let it settle to travel height and measure the new air bag to compare it with the others.