Air Suspension - Why are there two air lines?

[AlastairH]

I have been researching semi air suspension and notice that one manufacturer offers a unit with one air line that splits into the two bellows. There is also one pressure gauge. It makes me wonder why most manufacturers offer two gauges and two separate air lines to the bellows. I would have thought that both bellows should be at the same pressure anyway. Am I missing something?

 

[nabsim]

Mine has two lines and two gauges, I can lift either side independently. Came in useful for the tyre guy to get hit jack in when I had a blowout, also useful for a bit of levelling up

 

[Two on Tour]

Am I missing something?

Not all motorhomes are of equal weight on each side of the van, I know mines not.

 

[Happy1]

Handbook says you can alter pressures to level van on road to allow for uneven weight distribution

 

[BwB]

As well as the above, I would think with just one line split between the two air bags that all (or a lot of) the air would rush from one bag to the other when cornering? Not a good thing. Closing off the valves after setting the air pressure prevents this from happening on a twin line system.

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[Deneb]

As well as the above, I would think with just one line split between the two air bags that all (or a lot of) the air would rush from one back to the other when cornering? Not a good thing. Closing off the valves after setting the air pressure prevents this from happening on a twin line system.

Yes. Early systems were like this and whilst they are OK to restore ride height with heavy loads they can actually increase corner roll as the air from the bellows under compression is pushed into the opposite bellows helping to push the vehicle further over, whilst the loss of air from the bellows under compression allows it to compress further.

Split circuits mean that the air in the bellows under compression does what it is meant to do and restricts the compression of the suspension, whilst the opposite bellows has no effect on it at all.

 

[AlastairH]

Understood. I see that two lines are better. Thank you.

 

[TerryL]

Although I've got two lines, there's only one gauge. But each line has it's own tap to isolate it and the gauge then reads the pressure of whichever line is "open". Generally I travel with both lines at the same pressure so whilst I always run with both taps closed, periodically I open both to equalise/check the system pressure.

Some systems have a pump incorporated to better allow adjustments, e.g. for adjusting ride height. I don't but carry a tyre compressor which I can use to top up or adjust as necessary.

Just a note - my gauge is in the drivers door footwell so 2 gauges would be intrusive. I've seen where they've been fitted into a small panel low down on the right of the dash which looks a nicer option.

 

[ShaKen]

Has anyone fitted the gauges in the garage?

I was thinking of fitting semi air without the onboard compressor and putting the garages in the gauges; my thoughts being that there was no need to put the gauges in the cab as I wouldn’t be adjusting whilst driving.

Does that make any sense?

 

[Minxy]

Has anyone fitted the gauges in the garage?

I was thinking of fitting semi air without the onboard compressor and putting the garages in the gauges; my thoughts being that there was no need to put the gauges in the cab as I wouldn’t be adjusting whilst driving.

Does that make any sense?

Yes makes perfect sense, if you're not going to need to keep altering the pressure the gauges can go as near to the rear as you wish.

We have a version (a lot more expensive for an Alko chassis as we have) which came as standard with a compressor so it's mounted next to the driver so we can alter it as required but if we didn't have a compressor it could have gone anywhere.

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[Minxy]

I have been researching semi air suspension and notice that one manufacturer offers a unit with one air line that splits into the two bellows. There is also one pressure gauge. It makes me wonder why most manufacturers offer two gauges and two separate air lines to the bellows. I would have thought that both bellows should be at the same pressure anyway. Am I missing something?

I wouldn't go anywhere near one that has only a single line, in fact I'm surprised they are still being sold as I would think they are potentially dangerous if they allow a vehicle to be 'pushed' over more to the 'wrong' side when cornering etc.

 

[Deneb]

Has anyone fitted the gauges in the garage?

I was thinking of fitting semi air without the onboard compressor and putting the garages in the gauges; my thoughts being that there was no need to put the gauges in the cab as I wouldn’t be adjusting whilst driving.

Does that make any sense?

Totally. They were always in the boot on our police Range Rovers and I fitted them in the boot on all my tow cars. You only need to see them either to check or adjust the pressure.

 

[Two on Tour]

Our gauges and controls are mounted in a panel on the lower right of the steering wheel and on occasions it's been very useful to be able to adjust the air bag pressures on the move when the wind starts getting up on your journey and you start getting hit by gusty crosswinds. I can simple increase the pressure in the bags until the van feels more stable in the gusty crosswinds and is a more comfortable ride.

 

[Mr mrs tupcox]

Ours has 2 line to clocks.
It's great for helping with levelling out..drop one side lift other never need to use ramps on rear now.

 

[tacr2man]

Two lines are better as these systems don't have any ride height sensors .

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[icantremember]

We have the ASairsuspension which has the on board compressor and gauges within easy reach of the driver.
With this setup it's easy to raise the rear end when boarding ferries etc. and also to stabilise the moho in the event of cross winds.

 

[Reallyretired]

I can level across the rear of mine using the airbags, usually lower the pressure on the high side and raise it on the low side by about the same amount so that when I move off I just need to open both valves for a couple of seconds to equalise the pressures and then shut them.

 

[BusyBuilder]

I put my twin gauges in side locker, I use a foot pump to top them up, on my B544 (4250kg MAXI chassis) I tend to keep them at 2.5 bar. I have tried 3bar but think the suspension is too solid at that, but I'm open to what others do.

 

[AlastairH]

I ended up fitting two separate air lines and two gauges. The gauges are mounted in a shallow cupboard where I can see them easily and where I have easy access to pump them up.
The fitters only put minimum pressure in them and it only took me a few strokes on the foot pump to inflate them to 2 Bar.