Tri-Axle Performance
Investigations into the performance of trailers fitted
with tri-axle suspensions have shown that
installation is more critical than for Tandem
suspensions. The platform slope in the laden
condition, if above the recommended maximum
quoted, has apart from reducing equaliser travel, a
critical effect on axle loadings.
High axle loadings can cause excessive tyre wear
or even failure, particularly when cornering.
Therefore it is imperative when installing a tri-axle
suspension that particular attention should be paid
to the laden trailer’s slope. If the slope is greater
than one in two hundred, the packing pieces must
be welded to the spring seats on the front and
centre axles; the deepest being fitted to the front
axle.
The depth of packing can only be determined by
considering each installation individually after
judging fifth wheel height, wheel base etc. When a
steering axle is required MERITOR Engineering
department should be consulted.
The laden ride height of fifth wheel must also be
taken into consideration when determining trailer
slope. If a variation greater than plus or minus
15mm is expected from the specified slope, a tri-
axle suspension will not properly compensate.
The preceding criteria are not applicable to Multi-
axle air suspension configurations. For details of
mount heights, weights etc please send for
MERITOR’s air suspension literature and
specification.
Trailer Slope
20
Tandem
1
Corrected
1A
CORRECTED –
Higher Spring Seat
Widespread
2
Corrected
2A
CORRECTED –
“Alternative”: Packing Piece
welded to spring seat
Trailer Slope Relative to
Spring Heights
When considering the axle and suspension to be
specified for a new trailer, always take into account
the angle of slope. If this angle is greater than 1 in
100 (1° 35') for tandems, 1 in 150 (0° 23') for
wide spread, 1 in 200 (0° 18') tri-axle in the laden
condition, the equaliser will have excessive tilt. This
tilt will have the effect of reducing the equaliser
movement, causing the equaliser to strike the
frame under uneven road conditions. This can be
overcome by specifying spring seats 25mm (1.0
inch) higher on the forward axle than the seats at
the rear or alternatively, by welding packing pieces
25mm thick to the existing spring seats.
Also shown is the same malfunction on a
widespread suspension. A packing of appropriate
thickness welded in position will correct this.
Alternatively the addition of a suitably tapered
spacer welded between the hanger bracket and
the main frame will achieve the same result.
Angle of Slope
