starfighter:
The only time I have known a trailer to settle is when you run a fridge in electric mode on the bay. We have a paddle you must move to plug it in, this applies the trailer brakes. When you unplug it and move the paddle back, the now loaded trailer drops a good few inches.
But that’s because the suspension has lost some pressure or more weight has been added, therefore drops a bit, but as it drops with the brakes on the action of the suspension rolls the trailer forward angainst the unit or the legs if uncoupled. What ever forward force is applied to the unit or legs is equalled by the brakes, therefore when releasing the brakes the force is transfered the the suspension and the trailer will drop
Maybe I’m using the wrong term. You go down a ramp with a close-coupled fridge, and the very bottom of the ramp, the trailer tips forward and either scrapes the legs (despite them being wound up) or tips the top of the fridge compressor into the aerofoil on the cab… I dunno the term for the latter event, so I’ll just refer to the scraping the legs bit that I reckon is called “Grounding”. Both events can occur at the same venue though. Pumping up the rear tractor axel prevents both at least.
“Settle” - If you have a fully pumped up trailer, and you are dropping it on a high bay (hence leaving it pumped up in the first place!) then after taking the red line off, there is “give” when taking off the yellow one. It occurs to me that taking off the red one CAN’T be purging all the air from the trailer, otherwise it would “settle” when taking the RED line off!. Air is left in the trailer (keeping it pumped up), and some I think escapes when taking the yellow line off that’s all I think I’ve been observing here.
I pull fridge trailers up to 13.6M and I can see your logic but suggest that there is a flaw in the application of that logic. If you traverse an acute angle, that is one that tips the top of the trailer towards the wind deflector, pumping up the trailer suspension will compound the problem as the gap between trailer and cab is reduced before the ground affect takes place. If you traverse an obtuse angle, one that increases the gap between the top of the trailer and cab, the legs and side under run bars are what ground. In that instance lifting the unit suspension will help, certainly would see altering trailer suspension as a last resort.
I’ve only ever been to 1 place where I’ve had to contend with an obtuse angle at the top of a ramp followed by an acute angle at the bottom, before levelling out to sit on a bay. I lift the unit suspension to get over the obtuse angle then restore to normal once the legs are clear, reversing the process on the way out.
I have had trailers settle when disconnecting suzies so I agree that it happens, don’t know the explanation, but it does happen and so unhook from the side or lower the legs and then drop the unit suspension before disconnecting the lines.
Wiretwister:
I have had trailers settle when disconnecting suzies so I agree that it happens,
Red air line deffo, but the suspension drop when pulling the yellow?
I’ve thought about this some more and I think you can get some movement in the same way if you pull up to a junction in a car and come to a halt and yank the handbrake on whilst the footbrake is still depressed. When you release the footbrake the car settles backwards - I’m not in any way suggesting that a load of BS hasn’t been posted by others describing this.
I think, in some units, you would see similar if you stopped with the footbrake and clicked on the parking whilst the footbrake was still depressed and then dropped the red and yellow lines there would be some movement in the trailer which might ‘seem’ like it was the suspension. If after pulling up you released the footbrake before clicking the parking brake on I think you would not see the effect.
It does depend on the trailer, we run several different types, when I was in an XF (catwalk steps o/s) I did the yellow last (furthest away) and some trailers were pressurised (hit the back of the cab first time I did it ) and there was some settle but not suspension, what caused it I have know idea, maybe akin to parking car, pulling handbrake on whilst footbrake still down then releasing footbrake. In my FH (access from n/s) red comes off last, yellow first (no pressure) and no unsettling settling (see what I did there ). As for the trailer brake, it’s easier to make sure your handbrake is on .
As for grounding a fridge, kinda seen it a few times, trailer catching deflector or tops of wheels at the pizza factory Finch Close, Lenton Lane, Notts.
MADBAZ:
It does depend on the trailer, we run several different types, when I was in an XF I did the yellow last (furthest away) and some trailers were pressurised
In my FH red comes off last, yellow first (no pressure) and no unsettling settling.
What you have posted illustrates the difference between tractor units, not trailers.
The DAF does apply the trailer brakes with the handbrake and does this by pressurising the yellow line.
If you have set the unit handbrake there will be a release of pressure whenever you remove the yellow line,
regardless of the type of trailer.
The Volvo does not and therefore there is no pressure in the yellow air line when the handbrake is on;
as there is no pressure, there is therefore no effect on the trailer when the yellow line is removed.
) and there was some settle but not suspension, what caused it I have know idea, maybe akin to parking car, pulling handbrake on whilst footbrake still down then releasing footbrake. In my FH (access from n/s) red comes off last, yellow first (no pressure) and no unsettling settling (see what I did there 
). As for the trailer brake, it’s easier to make sure your handbrake is on 
.