The air suspension trailers I have used has had 5 positions Raise, Hold, Travel, Hold, Lower. Leave it in travel when on the move (if you have had it in raise dump some air so the bags go to just about touching the lips. When unloading/loading adjust as needed- get the trailer level by raising/lowering then putting it in hold (sometimes you can play with the tractor units air bags too). Just be careful when on cages as these only take a bit of lean to the back for them to roll off & remember that a tail lift may alter the level of the trailer with weight on. Play with it & get used to it. No one can say its wrong else it wouldnt be there. Just remember that ist is in travel when you pick up air suspension trailers 
As coffee said, most trailers will now re-set themselves once you get above a certain speed if the ride height has been adjusted manually. Most trailer manufactures state that the air should be dumped out of the suspension prior to dropping the trailer. This prevents damage to the landing legs, as if the air drains out then extra strain is placed on the legs. The air should also be dumped if you are on a bay or loading, this is to stop the bags going “pop” if they get overloaded by forktrucks/pallettrucks. It also makes them more stable during loading and unloading as they are not trying to adjust height all the time. Hope this helps.
smcaul:
As coffee said, most trailers will now re-set themselves once you get above a certain speed if the ride height has been adjusted manually.
I think thats a little ambitious smcaul . Maybe most newtrailers.
I haven’t pulled a new trailer for a long time. The company I work for keeps there trailers in good nick. Even on our 10+ year old trailers, the curtains slide open with a tug. Our newer trailers have the 3 position handles on them, push it in to adjust height, centre holds the setting. Pull it out to return to running height.
I’ve pulled newish & oldish (10 yr+) air ride trailers around & not yet come accross one that won’t self-level once on the move, providing the handle has been returned to the correct position. (Usually pointing down).
It is also a good idea to check the raise/lower valve works before going to road as if you are stopped in a VOSA check point & it don’t work, its a PG9. - as is not having an operational rear fog lamp. (in the middle of summer) 
If I were unloading a trailer, then I’d use the suspension on the front, not the leaver on the back as you have more adjustment and put a bigger slant on it (not too much or you could ahem scramble a pallett of eggs ). The only time I’ve really used the suspension on the back of the trailer on UK work - I wouldn’t have thought you’d travel far with it in a different position - I think it may damage the suspension is if someone (not me I hasten to add), is lifting something off the back of the trailer or you’r too high for a loading bay (or you’r a bit worried about a bridge your trying to clear and really shouldn’t have put yourself in that position in the first place but that’s another story 
).
im in the process of buying a load ride tractor unit at the moment, and unless its a very specialized piece of kit, it wont actually have smaller wheels, the wheels are the same size as normal, they are fitted with 60 series tyres.
the only trailers which have a dual ride height are the mega trailers which are designed to run on a fifth wheel height of 950mm on the continent, they have a special connector which you plug the airlines into which drops the suspension to normal continental running height, when in the uk you put your normal airlines on and the trailer rises up on its suspension to counteract the obviously sharp rearward sloping of the trailer.
paul
Simon:
smcaul:
As coffee said, most trailers will now re-set themselves once you get above a certain speed if the ride height has been adjusted manually.I think thats a little ambitious smcaul
I haven’t pulled a new trailer for a long time. The company I work for keeps there trailers in good nick. Even on our 10+ year old trailers, the curtains slide open with a tug. Our newer trailers have the 3 position handles on them, push it in to adjust height, centre holds the setting. Pull it out to return to running height.
I apologise to everyone 
.
I had to drive under a low (13’3") bridge last week. Its on a route we use quite often and I fell into the trap of becoming blase(?) about it because we use euro height trailers . Last week I had a rental trailer which is a little taller than our own, though not by much. After dragging it around for a few days I forgot and when I was sent to load at this place I arrived at the village then remembered that a had the higher trailer on 
. It might have fitted, but I wasn’t sure so decided to check. I measured the height and decided that it would be safer to lower the suspension before going through the bridge. This trailer has the newer height adjustment lever, the one you push in and turn to adjust the height, centre holds the position and pull it out to reset to road ride height.
I lowered the suspension on the trailer, then set off for the bridge. My unit suspension has memory buttons for height settings. As I built up speed the trailer suddenly started making a hissing noise and when I stopped just before the bridge, the lever had popped back out returning the trailer to road ride height.
That would suggest to me, that trailers fitted with this type of height adjustment will automatically return to road ride height when you reach a certain set speed. I will check on a few of our older trailers with the same type, to see if this works on them. In the past, so far , I have always remembered to reset these trailers to road ride height. I’ll report back on my results asap 
I am often asked by fellow drivers here “Why don’t they invent a system whereby you can adjust the level up or down on tractors and trailers ?”
When I tell these guys that we have had such a system in Europe for years my buddies find it hard to believe 
Next thing you’ll be telling them the Europeans have synchromesh gearboxes in trucks
allikat:
Next thing you’ll be telling them the Europeans have synchromesh gearboxes in trucks
PMSL
allikat:
Next thing you’ll be telling them the Europeans have synchromesh gearboxes in trucks
They already don’t believe me on that one, and EPS is pure fantasy
What’d they do with a Volvo FH12 with I shift then ■■?
simon:
I lowered the suspension on the trailer, then set off for the bridge. My unit suspension has memory buttons for height settings. As I built up speed the trailer suddenly started making a hissing noise and when I stopped just before the bridge, the lever had popped back out returning the trailer to road ride height.
At least it happened before, rather than leaving you wedged under the bridge…
I had one of our own trailers, with the push-in and-turn type lever for adjusting the height of the trailer. I tried adjusting the the height and running with it like that, tro see if it would automatically reset to normal drive height, it didn’t.
That seems to suggest that you shouldn’t rely on any trailer automatically resetting to drive height. Always check to make sure that its set to normal drive height before driving off. Yet another thing to check during your pre-drive off check.