I doubt that would work all your doing in moving the weight to the rear if the trailer.So i think it will not work also if you raise the suspension on the unit the weight would then be on the front axle not the drive.
Captain Caveman 76:
OK, I think I have it.I was right about the angle of the trailer having an effect on the forces acting on the headboard.
Taking it as a torque problem since torque is simply a force acting about pivot.
Torque = distance from pivot x mass x sin angle of the trailer from ground.
Therefore, the more the trailer lifts, the more force (or the mass of the trailer) affects the headboard.
That’s the best I’ve got. I’m in the bath right now and have better things to do than remember my physics! !
Your in the bath!!
Talking of trailer rear axles, in a past life i was on artic bulkers, and was on them during the change from 32 to 38 ton.
Lots of conversions were done, some trailers stretched but some that were fairly big bodies simply had another axle plonked in front of the other two trailer axles, course we’re talking steel springs.
Anyway, one of the bulkers in particular never looked right, its seemed very low at the back and from what i could see of the springs and shackles they were all identical, and axle 3’s tyres always bulged like buggery, and basically it was too short and the axles too far forward, they should all have been moved back in hindsight.
One day fully loaded i goes into a grain site with an axle weigher, unusual in those days, was only normally the ministry had them.
Anyway, trailer axle weights approx from memory (i know number 3 is right cos me eyes nearly popped out me skull) 1-5tons, 2-7ish tons, 3-12tons 
That trailer went straight off the road and was refitted with fluidride, which whilst stable and evened the weights out, effectively made the trailer so heavy we were down to a 23 ton payload at 38ton.
UKtramp:
Captain Caveman 76:
OK, I think I have it.I was right about the angle of the trailer having an effect on the forces acting on the headboard.
Taking it as a torque problem since torque is simply a force acting about pivot.
Torque = distance from pivot x mass x sin angle of the trailer from ground.
Therefore, the more the trailer lifts, the more force (or the mass of the trailer) affects the headboard.
That’s the best I’ve got. I’m in the bath right now and have better things to do than remember my physics! !
You’re **(Sorry, couldn’t resist)**in the bath!!
I know! A truck driver having a bath!!
And it’s not even my birthday!!!
Yeah Franglais has it down. Normally the rearmost of the trailer aft of the centre wheels isn’t putting any weight on the unit, in fact it’s taking it off the unit as it’s the other side of the pivot (a negative moment arm). When you start lifting the trailer, the rears become the pivot and more of the trailer is on the unit side of “the seeswaw”.
There still would be a negative shift aft of c of g when you tilt the trailer but the change of pivot point overcomes this negative effect on the unit. If you carried on tilting the trailer to extremes (say nearly on its arse vertically), eventually the benefit would disappear again but you don’t get that far.
Colin_scottish:
I doubt that would work all your doing in moving the weight to the rear if the trailer.So i think it will not work also if you raise the suspension on the unit the weight would then be on the front axle not the drive.
Juddian did the weighing with a real vehicle.
So long as the 5th wheel is in the same place, having the unit’s suspension up or down won’t make a lot of difference to ratio of weight distribution tween front and rear axles.
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You blokes are losing me now discussing CoG’s and stuff.
Surreal having a virtual chat with lorryists when one of 'ems in the bath, far out man 
Juddian:
You blokes are losing me now discussing CoG’s and stuff.
Surreal having a virtual chat with lorryists when one of 'ems in the bath, far out man
Not actually suggesting doing this just to visualise 
. Get a 6 foot long, thin plank of wood or a fence post. Put a couple of books under it about a third away along. The fence post is lying across the books, one end touching the floor . Lift the end furthest away from the books, the end lying on the floor. Lift it up a bit and see how heavy it is. Keep lifting a few cm’s until the other end of the post touches the floor. Keep lifting past that point and it’ll suddenly feel heavier in your hand as it lifts away from the books. Keep lifting and it’ll start to feel lighter again. Keep lifting slowly and eventually it’ll feel the same as when you first lifted it when it was on the books. Lift further still and it’ll get even lighter, until it’s nearly standing upright and you could push it with your little finger.
This would be the same with the Empire State Building laying on its side in principle. There would be a point which you could lift the last little bit with your little finger before it toppled onto its base, but it would be so upright by that point it would appear as pushing it to most people
Juddian:
You blokes are losing me now discussing CoG’s and stuff.
Surreal having a virtual chat with lorryists when one of 'ems in the bath, far out man
Imagine you have a long beam secured in the middle to a pivot. On one end theres a weight, on the other, you. You’re holding the beam horizontally, then, you apply a force to push the weight up until the beam is at 10 degrees to the horizontal, and hold it there. You then push it again and again so the beam rises in 10 degree steps. Each push you make requires more force, even though the steps are the same. Effectively, the greater the angle on the pivot, the more force is acting on the weight.
Ps, I’m out the bath now. 
Juddian:
You blokes are losing me now discussing CoG’s and stuff.
Surreal having a virtual chat with lorryists when one of 'ems in the bath, far out man
I try not to get too many mental images of some members… I mean forum members of course.
Try this image. Same trailer and load. Put the trailer axle just behind the mid point. Half the trailer weight is on the unit. Slide the trailer axle out to the rear of the trailer. See how much more weight is on the unit?
Tomorrow dump the air out of trailer again, the rear tyres will be bulging. That shows the rear trailer axle has most load. So effectively dumping air moves the centre of load from the middle axle (normally all 3 are equal) to the rear.
Does that sound better?
I’m in my bunk with a small phone so it’s a bit awkward having no blackboard n chalk.
That’s another horrible image for TNUK to deal with. Sorry.
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Franglais:
Punchy Dan:
Just wondering what the weight will be on the trailer rear axle ?A lot. Look at the tyres when you dump air out the trailer next time. As Juddian described it with the unit jacked up you’ll almost be able to change the wheel on the lead axle! Probably a greater effect on a shorter trailer (tanker?) than a longer one too I’d think.
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I don’t think the weight on the trailer rear axle will have gone up massively, even at full tractor height we’ve shifted 3 tons, if i had to guess i would say the two rear axles have gained maybe a ton each, maybe the rearmost slightly more than the middle, which leaves a ton less on the front axle, there’s your 3 tons.
Good example Freight Dog, funny you should mention 6ft planks, just seems apt
I’m glad CC is out of the bath, you did remember to wash behind your ears i hope
Be off for a shower in a minute mesen, see if i can tempt the good lady in to wash me back 
Strewth, now Franglais on the bunk with a small phone, is that a euphemism i haven’t heard before
Captain Caveman 76:
Juddian:
You blokes are losing me now discussing CoG’s and stuff.
Surreal having a virtual chat with lorryists when one of 'ems in the bath, far out manImagine you have a long beam secured in the middle to a pivot. On one end theres a weight, on the other, you. You’re holding the beam horizontally, then, you apply a force to push the weight up until the beam is at 10 degrees to the horizontal, and hold it there. You then push it again and again so the beam rises in 10 degree steps. Each push you make requires more force, even though the steps are the same. Effectively, the greater the angle on the pivot, the more force is acting on the weight.
Ps, I’m out * the bath now.
*of
Juddian, at play time today, … sorry, when you’re on duty at the TNUK Research Establishment, will you be able to do another weigh?
After loading put just the rear trailer axle on the bridge. Weigh that axle in normsl mode and then as before in air-dumped high unit mode.
I’ll predict that they’ll be a substantial change. Guessing 50% increase?
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UKtramp:
Captain Caveman 76:
Juddian:
You blokes are losing me now discussing CoG’s and stuff.
Surreal having a virtual chat with lorryists when one of 'ems in the bath, far out manImagine you have a long beam secured in the middle to a pivot. On one end theres a weight, on the other, you. You’re holding the beam horizontally, then, you apply a force to push the weight up until the beam is at 10 degrees to the horizontal, and hold it there. You then push it again and again so the beam rises in 10 degree steps. Each push you make requires more force, even though the steps are the same. Effectively, the greater the angle on the pivot, the more force is acting on the weight.
Ps, I’m out * the bath now.
*of
Ah, the problem of typing the way one speaks. I take it such ghetto colloquialisms haven’t reached the heady heights of Kirk Ella. However, even here in the ghetto, there’s no forgIving a homophone error.
This is without doubt the most grown up and sensible post i.ve ever read on TN, some of you even sound like you know what your talking about
chaversdad:
This is without doubt the most grown up and sensible post i.ve ever read on TN, some of you even sound like you know what your talking about [emoji38]
Nah, you’ve been here so long now, your standards have slipped !
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Top tip indeed!
The first thing I thought of to explain the weight transfer was the sliding tandem axles that Americans use. Slide towards the unit to take weight off the pin, slide away from the unit to put weight onto the pin.
I did all that, raised the suspension fully on the tractor unit (couldn’t get the mid axle to lift), diff lock in , still needed a mk1 bloody land rover to pull the wretched empty scania up the slough nr Redditch, the traffic stopped and I just couldn’t get that cow to move more than about 15 foot before it started spinning again, one of those CAG gearboxes, it was a tow or the clutch. I’d never been stuck like that before.
Franglais:
chaversdad:
This is without doubt the most grown up and sensible post i.ve ever read on TN, some of you even sound like you know what your talking about [emoji38]Nah, you’ve been here so long now, your standards have slipped !
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Chuffin hell 2005 ! Wheres the time gone
Franglais:
Juddian, at play time today, … sorry, when you’re on duty at the TNUK Research Establishment, will you be able to do another weigh?
After loading put just the rear trailer axle on the bridge. Weigh that axle in normsl mode and then as before in air-dumped high unit mode.
I’ll predict that they’ll be a substantial change. Guessing 50% increase?Sent from my GT-S7275R using Tapatalk
Will do when i get the w’bridge to meself for a few minutes, which might not be this week.
I reckon 1500kg increase, however as i’m poor and you’re bleedin loaded i aint betting with real money, i can’t afford to lose any