Hi all hopefully I don’t sound too much of an idiot for asking these questions but I’m still new to it all so here goes.ive seen afew trucks while out and about that have axles on both the tractor and trailer where some are lowered while others I have seen are up and not touching the road.what is the purpose of these axles???i was thinking maybe even weight etc etc but could be soooo wrong. 
Is it the drivers responsibility for operating this axles or do they operate themselves when maybe the correct weight is on the trailer■■?
Sorry for asking maybe a silly question but my grandma used to say the only questions that are silly are the ones you don’t ask. 
Thanks,scottythegeordie
You thought correct. Axles which lift on units and trailers r fir weight purposes, some have different configurments I;e mid lift or tag axle, truck lift axles work on weight on the tractor unit, they will automatically drop when a heavy load is on board which helps with stopping over loading and stability. Once empty they should be lifted to help with traction and also saves unnessacery wear and tear once lifted off the ground, hope this helps
just to add a bit to what Mark said, they will automatically drop to the ground when the weight is too much for the others, but, they do not automatically lift when empty, the driver usually has to push a button on the truck/trailer to raise the axle
Do you mean on skeletal trailers where the axles appear to be in the middle of the trailer.
These are two 7 meter trailers coupled together.The front trailer has a sort of 5th wheel on the back which locks into the rear trailer and has guides to lock the whole assembly into one rigid unit.
It was originally a Dutch idea I think for carting 20 foot containers round Europoort so one tractor could deliver two box units.Drop one with its own trailer and the truck could then go and deliver the second box elsewhere.Both trailers can act as independent units.
Weird looking things but very clever as you can couple 3 or 4 together.(on the dock of course)
Just to add to the above posts, and not a daft question at all because all lorry makes seem to be different here, certainly they nearly all work differently and have their own peculiarities.
Have a play with them and learn how they work, keeping those axles not needed off the ground helps with economy and drag but also the lorry will be less skittish in the wet.
Righto, assuming a tractor unit doesn’t have a drivers handbook, (if its got one read it)…
There will usually be two switches on the dash (sometimes, but not always together)for tractor tag axle control, one is a switch for lifting the axle completely…this usually has a picture of two axles one of them up and an arrow pointing up…pressing this once lifts the axle after unloading, but give it a second or ten to operate because pressing it again extinguishes the light and drops the axle again, not all lorries operate the same way.
Some lorries the switch is two way and for example you can lower the tag before dropping the trailer with an empty Scania, otherwise the thoughtless driver who winds the legs fully down when dropping an empty leaves it so high that an equally thoughtless driver can miss the pin completely when backing under it next with another make of lorry.
The other tag axle switch will probably have the same two axles pictured, and a semi circular arrow pointing from one axle to the other.
This is an air dump valve for use when the vehicle is loaded and all axles are down, operating the switch dumps the air from the tag axle which puts more weight onto the drive axle, this is useful for getting better grip in adverse conditions, such as snow or mud, but can be useful once used to it to help in exiting junctions on slippery roads as it helps prevent wheelspin.
The air dump valve usually doesn’t work above about 15mph, so as you build up speed it automatically reverts to standard axle loading, you can of course touch the air dump valve again which repressurises the tag axle airbags.
Some lorries give no indication that the dump valve is operating, indeed you would have to be beside the tag to hear it working, others (Scania) show the pressures as approximate weights on the dash display while the system is operating.
Some lorries it should be noted, (Axor definately but may be others), do not automatically drop the tag axle when loaded, the driver has the unique position of total control over the tag axle…very useful this facility for driving in heavy snow etc when you can raise the tag axle fully before approaching a snowbound hill in order to maximise traction, but the drawback is that the driver has to remember to operate the system…personally i like Axors because they are a drivers lorry for this and many other reasons but i’m probably in a minority here.
Similarly a very light load may not trigger automatic lowering of tag axle on any vehicle, all lorries are different.
Stand to be corrected here, and i avoid them as much as possible, i don’t recall finding a dump valve for the tag axle on Iveco Stralis…though i might have missed it cos its got more bloody switches dotted will nilly than you could shake a stick at.
Modern trailer tag axles are usually automatic lowering/raising, those of ours are, as said above some older trailers will have a manual valve which you can depress to raise the tag axle, button usually but not always beside the parking brake button.
Juddian:
snipSome lorries it should be noted, (Axor definately but may be others), do not automatically drop the tag axle when loaded, the driver has the unique position of total control over the tag axle…very useful this facility for driving in heavy snow etc when you can raise the tag axle fully before approaching a snowbound hill in order to maximise traction, but the drawback is that the driver has to remember to operate the system…personally i like Axors because they are a drivers lorry for this and many other reasons but i’m probably in a minority here.
For me, not as " unique " as Juddian say’s
With previous employer 100% common, on there tri’axles rigids 
.
As most were Flatbed,
leaving the 3rd (undriven) axle down unnecessary’ly could be extremely embarising 
In fact, darn right dangerious 
There is a very simple answer to the main part of the question … When empty it saves tyre wear.
Pat Hasler:
There is a very simple answer to the main part of the question … When empty it saves tyre wear.
And the reason most artics are on six axles is because it means they pay less tax (VED).
Santa:
Pat Hasler:
There is a very simple answer to the main part of the question … When empty it saves tyre wear.And the reason most artics are on six axles is because it means they pay less tax (VED).
So technicaly it is illegal to run a 6 axle outfit with a raised axle because you are fiddling the tax man.Even when empty.
Bking:
Santa:
Pat Hasler:
There is a very simple answer to the main part of the question … When empty it saves tyre wear.And the reason most artics are on six axles is because it means they pay less tax (VED).
So technicaly it is illegal to run a 6 axle outfit with a raised axle because you are fiddling the tax man.Even when empty.
No that’s just a myth! It’s the sort of story that intellectually challenged drivers tell each other in RDC waiting rooms
Santa:
Pat Hasler:
There is a very simple answer to the main part of the question … When empty it saves tyre wear.And the reason most artics are on six axles is because it means they pay less tax (VED).
Thought it meant you paid more tax as most six axle combinations are plated at 44t otherwise why would you buy a twin axle unit and plate it at 38 or in some cases 30t depending on what work you are doing
If you got two lift axle switches in the cab one of them is a traction switch.
This reduces lift axle air bag pressure to so that the drive axle gets more grip on a slippy surface but only for ten seconds.
The signal from the drive axle airbag pressure sensor tells the suspension ecu when to lower the second axle as more load is put onto the truck.This cannot be overidden by the lift axle switch but the traction switch allows a temperory overide to stop the drive axle spinning.
On some newer trucks the ABS system will sense the difference in rotation speeds of the axles and do this automaticaly through the traction control programme.
@mark h :thank you mate very very helpful post.well you do learn something new everyday as they say.
@bking:just any axle really mate because I’m relatively new and thought the extremely knowledgable guys on here could answer my questions.
@juddian:thank you very much mate another very helpful post. 
Denis F:
Bking:
Santa:
Pat Hasler:
There is a very simple answer to the main part of the question … When empty it saves tyre wear.And the reason most artics are on six axles is because it means they pay less tax (VED).
So technicaly it is illegal to run a 6 axle outfit with a raised axle because you are fiddling the tax man.Even when empty.
No that’s just a myth! It’s the sort of story that intellectually challenged drivers tell each other in RDC waiting rooms
@denis f:so is there plenty of stories to tell at RDC waiting rooms then mate.
The '55 '56 and '07 plate MANs I’m driving have two main settings for the lifting rear axle, aside from the regular air suspension setup.
- Raise/Lower
- Raise to put more weight on mid axle but not completely raising it. This is a kind of traction assist mode without a difflock.
Having not had the pleasure of driving it on icy roads I don’t know how good it is versus a regular diff lock but I would prefer one of those or an limited slip differential in the mid axle…
Driven artics with a mid lift but not a rear lift yet and no lifting axles on the trailer, but only because I’m bumbling around in rigids on class 1 rate right now
scottythegeordie:
@denis f:so is there plenty of stories to tell at RDC waiting rooms then mate.
So I’m told, but fortunately I’ve never been in one
There are so many configurations of lift axles it would take all day to play with them all. I have had trailers that lift axle 1 and 2 when empty, those that axle 1 lifts as soon as it gets below a certain loading, one had an automatic rear axle that lifted off the ground as soon as the weight came off, some have buttons, some have levers, some just do as they want.
A three axle Scania unit is still a three axle Scania unit whether the mid lift or tag axle are on the ground or not!
Denis F:
scottythegeordie:
@denis f:so is there plenty of stories to tell at RDC waiting rooms then mate.So I’m told, but fortunately I’ve never been in one
i wouldn’t be surprised if they dealt with more BULL than you did though Denis