self steering trailer

No fancy hydraulics,electronics or doodads.This is early 60's technology at,in my opinion,
it's best.
W.P.(Bill)Walker,very clever engineer and trailer builder of Hamilton invented this
type of self steering unit.



NZ Arc Welding (Tui) and Domett Truck and Trailer probably built the most units.
In those days a single,dual tyred, axle could carry 8 tons as long as it was more than
8 foot distant from the next axle.A closed tandem(bogie)could only carry 13t.
It's the same thinking that brought Australians the 8ft 1in fixed widespread.
NZ had their share of these,too.
Walker created the self steer to reduce the horrendous tyre drag/stress on the main frame/road damage that the fixed axle units caused.
In the beginning these units were only two axled but as time wore on and regulation permitted the tri-axle version appeared,probably rated for 19 tons.
The front axle is controlled by two springloaded "drawbars" which operate as a parallelogram between the front axle and an anchor to the main trailer frame.Walker's
original design had these drawbars in front of the steering unit hanging down under the trailer in plain view and this was how you could identify a genuine Walker unit from everyone elses.All the other builders have the drawbars virtually hidden under the sub frame between the front axle and the central turntable.Thus,the front axle moves sideways but always remains square ( at 90 degrees ) to the main frame but "steers"
the sub frame around corners.
On Pete's pic you can just see the bars under the subframe at about 2 o'clock on the front tyre.By the suspension and the coaming rail,I'd call this particular unit as Domett-Fruehauf.
You with me so far ?...........
Situated on the central turntable,sometimes on the outside,sometimes on the inside depending on manufacturer,is a spring loaded roller running on a track with a detent
set in the straight ahead position so the steering unit doesn't wander all over the show
on the road camber,etc while at highway speeds.On any turn the trailer frame angle
(with the sub frame) changes,overcoming the detent whereby the "self steering" takes over.Note that these are called Self Steerers not Self Trackers.(it's a K1w1 thing,like
Eskys and Chilli Bins)
There is an air operated lock on the central turntable,controlled from the cab,for reversing because they are nearly impossible to reverse without one.

In the pic is an RFL pan,that I'm pretty sure is one of the ones Motu refered to,
shown at full turn position.As I recall these were Arc Welders s/s units and chassis with Fairfax boxes.Fairfax Fibreglass was NZ's equivelent of Reinforced Plastics in Aus. in the development of GRP pans,tankers,mudguards,etc.Good gear.
The KWs are approx 1970 (possibly '69) K121s originally Cummins 250 powered and running as 4x2 "tractor units" pulling single axle axle A train fridges.Mid 70's they were
stretched and a lazy axle added to appear as in this pic dated 1976.



Again as Motu said,they've been banned just as the Aus Widespread has been.In Aus
we are able to register a W/s on club/vintage plates and so you get to see the occassional one around.ErgoMike tells me NZ doesn't have a club plate scheme as such and all their old trucks have to have full rego but there must be some dipensations made
(i.e. to "year of manufacture" regulations) that allow this unit to get out and about.

There is still a few around down here in the SouthIsland, one tri in Dunedin and I think there is a short two axle one with a water carrier down near Invercargill.

Thanks for that Rufo and Mike, i can just about understand this. It would seem essential that the pivot point of the subframe be forward of centre to make it come back to the straight ahead position after a turn.

I think these made it to Europe as well. Was there ever a version that had a drawbar attached to the rear of the prime mover so it was like towing a dog trailer which helped the steering so the body of the trailer was like logs on a log jinker attached to the truck and trailer and pivoting?

Yes the Kiwis used these as well, there is a drawing of one type on page 32 of my Atkinson book.
This Bill Walker sounds like a very clever engineer.

this is one setup see www.flickr.com/photos/46046969@N03/5142018243/