Used to watch the artic with these on the trailers manouvering in the yard, fascinating to watch.
How the hell do they work?
There must be some kind of sensor on the unit so the axle knows which way to go?
There are several types, a simple castor steering axle which follows the road whichever way the trailer is going, these have a lock for reversing,
Some are operated hydraullically either manually (low loader) or by a rod connecting the fifth wheel to the trailer.
I’ve only driven one, and I can safely say, it seemed like you needed to be a Jedi Knight in full command of the force to be able to reverse it 
:mrgreen:.
Whilst there are ways of powering an axle, I would think most work along the lines of “following along the path of least resistance”. They do this by having built into the design a degree of Castor Angle.
This is where it gets technical. 
If you push anything on castors across a flat surafce the castors will swivel so that they ‘trail’ the pivot point.
Similarly, if you get a conventional bicycle, and push it along simply by holding the saddle then it will go in a straight line. This is because of the angle of the forks to the vertical. If you imagine a line extended down from the head stock of a bicycle to the ground you will see that it projects to a point forward of a vertical from the hub. The difference between these lines is the Castor Angle.
Castor Angle is also what gives you Self-Centreing Steering on a car (or any other vehicle). If you look at the King Pin on a steered wheel you with again see that a projected line down the length of the King Pin will strike the ground ahead of a vertical projected from the hub. Whilst King Pin’s are now rare in cars, the same principal applies with Swivel Arms and Mcpherson struts. This is why, when a car is being driven in reverse, the steering will not Self-Centre. If fact, the opposite applies. If left alone, more steering will be applied because the wheels are trying to react like a Castor.
Now, applying these principles to the trailing axle of a trailer, going into a turn, friction between the trye and the road surface will cause the axle to turn - because the tyre wants to roll in a straight line. Coming out of a turn, both friction and Castor Angle will force the wheels to align with the direction of the vehicle.
The other factor involved is the relationship between the two wheels of a steered axle. In any turn, the outer wheel will be following a greater radius than the inner wheel, and they must therefore be engineered to not always be parallel to each other. This is achieved by having Steering Arms (linked by a Tie Bar) configured at an angle to the centre axis of the vehicle. (Ackerman’s Angle)
Simple isn’t it. 
Krankee:
Whilst there are ways of powering an axle, I would think most work along the lines of “following along the path of least resistance”. They do this by having built into the design a degree of Castor Angle.This is where it gets technical.
If you push anything on castors across a flat surafce the castors will swivel so that they ‘trail’ the pivot point.
Similarly, if you get a conventional bicycle, and push it along simply by holding the saddle then it will go in a straight line. This is because of the angle of the forks to the vertical. If you imagine a line extended down from the head stock of a bicycle to the ground you will see that it projects to a point forward of a vertical from the hub. The difference between these lines is the Castor Angle.
Castor Angle is also what gives you Self-Centreing Steering on a car (or any other vehicle). If you look at the King Pin on a steered wheel you with again see that a projected line down the length of the King Pin will strike the ground ahead of a vertical projected from the hub. Whilst King Pin’s are now rare in cars, the same principal applies with Swivel Arms and Mcpherson struts. This is why, when a car is being driven in reverse, the steering will not Self-Centre. If fact, the opposite applies. If left alone, more steering will be applied because the wheels are trying to react like a Castor.
Now, applying these principles to the trailing axle of a trailer, going into a turn, friction between the trye and the road surface will cause the axle to turn - because the tyre wants to roll in a straight line. Coming out of a turn, both friction and Castor Angle will force the wheels to align with the direction of the vehicle.
The other factor involved is the relationship between the two wheels of a steered axle. In any turn, the outer wheel will be following a greater radius than the inner wheel, and they must therefore be engineered to not always be parallel to each other. This is achieved by having Steering Arms (linked by a Tie Bar) configured at an angle to the centre axis of the vehicle. (Ackerman’s Angle)
Simple isn’t it.
Thats what I said 
using a few less words 
For a newbie, i’d say the most important thing is to be sure you know how to ‘lock’ it before leaving the yard.
Trying to reverse a trailer without knowing how to lock the rear-streering axle can be… well… a right ZB!!!
Mmmm, been there, done that within my first few months of agency work (1990-ish) 
Sorry, but I must cue Jim here to give his input on how rear steering axles work on trailers. He has quite a bit of experience with them, especially reversing them - isn’t that right Jim 
i have seen a driver cursing the rear steer when he was trying to back it on to a bay. after reading this, my guess is he didnt know how to lock it and struggled. i have to admit, it was amusing to watch the trailer going where it wanted instead of where HE wanted it to go
Reversing them? That wasn’t part of the original question.
On selecting reverse an additional air chamber ejects a pin that locks the steering mechanism. Obviously for this to work, the axle needs to be in the ‘straight ahead’ position to begin with.
If the air chamber fails, then manually inserting a 12mm (1/2 inch) bolt through additional holes has the same effect.
A tip. Having achieved the desired trajectory, pull forward a couple of feet, remove the bolt, and then finish the reverse, otherwise you probably wont be able to pull it out, especially if ‘dropping’ the trailer.
When picking such a trailer up. always ensure that no-one has left a bolt locking the axle because, as tri-axle, rear steer, trailers have the bogies set further back than standard tri-axles, with the third axle locked, they will not comply with the turning radii defined by Con & Use Regs.
Another issue to consider with this type of trailer is that because the bogies are set further back. When at maximum weight, there is a greater likelihood of the Drive axle being overloaded.
Last resort, Air chamber fails to operate. No suitable bolt to hand. Simply dump all air from the trailer suspension and it will go in a straight line. Don’t ask me why. It just works.
Ahh so most are just simple pivoting axles then.
The ones i saw though (Tesco ones) seems to be powered in someway as they operated when reversing. It was like the the trailer was steering onto the bay and the unit was following.
So if they have a bar on the 5th wheel to operate that do you have to have a specially equiped unit to use it?
dennisw1:
Ahh so most are just simple pivoting axles then.The ones i saw though (Tesco ones) seems to be powered in someway as they operated when reversing. It was like the the trailer was steering onto the bay and the unit was following.
So if they have a bar on the 5th wheel to operate that do you have to have a specially equiped unit to use it?
The fifth wheels I have seen are standard, it is just a peg which drops between the V of the plate, like a multidrive system works