Musings on Morgan Steering Geometry

Since the very beginning the Morgan car has employed an ingenious and simple front suspension mechanism
which has proved its worth for almost one hundred years. The advantages of independent front suspension
are well known and documented but the system used on the Morgan is unlike most cars in that the vertical
movement is fixed in a straight line as the stub axle moves up and down on a fixed pivot pin. The camber
angle therefore is always constant relative to the chassis. This creates some interesting anomalies in
the way the steering characteristics are influenced.

As with the vast majority of motor vehicles over the years the Morgan makes use of the
Ackerman linkage principle which, by angling the steering arms relative to the fore and aft
axis, causes the inner steered wheel to describe a circle of smaller radius than the outer
steered wheel, thereby compensating for the difference in turning radius. Clearly this is
beneficial at low speeds as the tendency to scrub the tyres is greatly reduced or even
eliminated. However, at higher speeds and greater loadings this does not necessarily follow.
In a high speed corner, weight is transferred to the outer wheel thusa increasing the loading
and therefore requiring a greater slip angle. It is, therefore, not uncommon for competition
and sporting cars to employ a certain amount of "anti Ackerman".

In its original form the Morgan used a fixed length track rod in the same way as would be
used on the beam axle, however because the wheels are independently mounted the
geometry is a little more complicated. With the beam axle the relative positions of the stub
axles and track rod remain constant, however with the Morgan system the stub axles are not
fixed by the beam and are free to move independently. As the centre pins are inclined at 2°
the relative positions of the stub axles change according to the deflection of the suspension.
If both stub axles move up together, as in passing over a ridge, the stub axles move slightly
closer thus turning the wheels outwards and on rebound have the opposite effect. This can
give rise to instability usually known as bump steer as the most loaded wheel will tend to
deviate from straight ahead. In practice the effect is so small as to be negligible.
When cornering hard the outer stub axle will tend to climb the centre pin whilst the inner will
relax thus cancelling the effect of the centre pin inclination but as the track rod is now
effectively shortened because of its angle there is a slight anti-Ackerman effect. Again this
is very slight.

However when the cars started to use rack and pinion the effect upon the steering
characteristics changed noticeably. With fixed geometry suspension as employed on the
Morgan the ideal track rod length is infinite so as to avoid affecting the steering angles
during movement of the suspension. Obviously this is not possible so the track rods
should be as long a possible. In the case of the Morgan this is approximately 20" to 21"
depending on the model.

At first this may seem to be a problem because angular deflection of the track rod will alter
its effective length relative to the stub axle which is travelling in a straight line. As has been
shown this has the effect of changing the wheel angle when the suspension is working. In
fact it is possible to take advantage of this effect to create an anti-Ackerman effect when
cornering and increase the slip angle of the loaded wheel. Unfortunately this will also tend
to reduce the slip angle of the unloaded wheel but to a lesser extent and, as the unloaded
wheel is contributing considerably less to the cornering force, and nothing at all if it lifts off
the ground as used to happen to my supercharged +8 in the chicane at Croft circuit, this is
of little significance

How can the existing steering layout be used to best advantage? Like most things it is a
compromise and there are two main factors which should be considered.

1)Bump steer:

Unlike the early steering box arrangement the effect of the suspension movement
with current short track rods is greater and increases further if, in the normal running
position, the track rod is not at right angles to the centre pin.

2)Ackerman effect:

As has already been discussed it can be advantageous in a fast corner to increase
the slip angle of the loaded wheel. If the track rod is normally at right angles to
the centre pin any suspension travel up or down will have the effect of
shortening the track rod length thus pointing the wheel inwards. When the car
rolls in a corner the outer, loaded, wheel will climb the centre pin and the slip
angle will be increased whilst the inner wheel will travel down the centre pin
which will also turn the wheel inwards but reduce the slip angle. The change in
angle is only a few degrees and can, with advantage, be increased on the loaded
wheel by angling the track rods upwards by a few degrees. This will also
decrease the reduction of slip angle on the inside wheel.

The compromise is to improve the cornering power without creating too much bump steer
and the more the track rods are angled up at the stub axle the more bump steer will be
introduced. There is also the danger of increasing the slip angle too much.

The ideal setting is dependent upon a number a factors such as tyre construction, camber,
spring rates, weights etc and of course personal preference.