I would like to reopen this topic with a few suggestions on how to overcome
trailer instability (fishtailing).
My comments are most applicable to single axle trailers using standard leaf
spring suspension. From my experience, this is the main type of suspension
to suffer from fishtail instability. Rather than just moving the axles
rearward and suffering an excessively high tow ball load, the situation can
be fixed quite dramatically as follows.
Method 1. Fit shock absorbers, mounted more horizintally than vertically.
First weld a suitably strong steel plate to the lower U bolt attachment
plate and fix another bracket to the centre line of the trailer immediately
below the floor line. You may need to fit a steel tube or angle section
between the outer frames if the centre floor section is not sufficiently
rigid. The setup may need to be slightly canted forward or rearward to
clear the axle.
Mount two standard (Commodore, Falcon or similar) telescopic shock absorbers
between these brackets. The shockers will be about 30 degrees from
horizontal. I had a previous trailer that had the axle moved too far
rearward by the previous owners, to overcome instability. To the extent
that it was a two man effort to lift it on and off the tow ball. Clearly
this was not acceptable. After moving the axle forward to gain an
acceptable tow ball load, the trailer was dangerous above 80 kph before
fitting the shockers. Howerver, after fitting the shockers it was a totally
different proposition that could safely negotiate sharp bitumen shoulders at
up to 100 kph (in a couple of tests). I still applied the commonsense rule
of not exceeding 80 kph but instability was never again encountered.
Method 2. This was advocated many years ago by Tjaco Boersma of VMFG. I
have not used it but Tjaco did, and claimed that it was most effective.
First the theory. When the trailer starts to sway, it also rolls. With
standard leaf springs fixed at the front and either sliding or shackled at
the rear, this causes the axle to move rearward slightly on the side towards
which the trailer is rolling. Result is the sway is amplified by the
"turning" of the axle. Tjaco reversed the spring attachments so that the
rear point was fixed and the forward point used the shackles or sliding
mechanism. Turning of the axle against the direction of roll dampened the
sway.
Method 3. This method is used by Coromal caravans. The axle assembly is a
"Y" shape with the stub axle and springs at the tail of the "Y" and the open
end of the "Y" fixed at or near the centre line of the caravan. The stub
axle has to be set at a bit of an angle to the "Y" frame to be horizontal at
normal load. The two pivot points ensure that the axle can only move up and
down with no fore or aft movement. Springs used by Coromal slide at both
front and rear. Other advantage of this system is greater ground clearance
under the centre of the caravan as opposed to a more conventional bar axle
configuration. Disadvantage is that if the axles are not correctly aligned
(or suffer a heavy knock, sufficient to take them out of alignment) it is a
fairly major job to realign them.
And of course, with all the above, tandem suspension is always more stable
than single axle.
Trust this is of use to other owners of unstable trailers.
Graeme Rickert
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