Is castor reduction
a wise modification for an RV8?
the last 12 months a castor correction kit has become available which offers lighter
steering for the MGB Roadster, MGBGT, MGBGTV8 and RV8 models. Essentially the
kit uses wedges which look like tuning forks as a packing under the front mountings
between the crossmember and chassis legs which rotates the crossmember forwards,
thereby reducing the castor angle. This note highlights several concerns over
the kit and its use on the RV8 model. (Jan 06)
Heavy steering has been a noticeable feature of the MGB and the MGBGTV8
It is caused by the 7 degrees of positive castor needed to produce
the self centring steering action with the cross ply tyres available in 1962 when
the model was first produced. Since that era radial tyres have been developed
along with improved rubber compounds with greater grip which have the effect of
increasing the steering load, particularly with tight cornering or cornering at
speed. As modern tyres are far more directional, less self-centring force is necessary
and so less castor is required. Consequently these tyre changes provide scope
for reducing the castor angle and thereby obtaining the welcome benefit of lighter
steering with the MGB and MGBGTV8 models.
Was the castor angle reduction
addressed when the RV8 was introduced?
RV8 was designed in the early 1990s on the basis that radial tyres with improved
grip characteristics and a wider profile would be used but also that the additional
equipment in the engine bay would increase the weight of the car compared with
the MGBGTV8. Without a reduction in the castor angle, changing the gearing of
the steering rack or fitting power steering, the steering outcome would be heavy.
There is a mistaken belief about that the castor angle on the
RV8 is the same as that of the MGB and V8 and so the castor reduction kit can
also be fitted to the RV8. This is incorrect as the castor angle on the
RV8 was reduced by just over 3 degrees when the model
was developed and introduced. You can easily
check this in the RV8 Repair Manual AKM7153ENG
where you will see the castor angle on
castor angle is the angle, measured in degrees, formed between the axis of the
kingpin and the perpendicular to the ground looking at the vehicle from the side.
As the angle is formed longitudinally relative to the vehicle, it is more exact
definition is longitudinal castor angle. In practical terms it is know more simply
as castor angle. The castor angle given to the kingpin
two important phenomena for the ride and handling of the vehicle - first stability
in terms of maintaining the straight line of travel of the vehicle and the extent
to which the steering self centres after turning and second the tilt of the wheel
which occurs during turning. |
stability phenomenon is created on the basis of the distance between the point
at which the kingpin axis extension falls (in relation to the direction of travel)
and the point of contact between the tyre and the ground. In the case of positive
caster angle (where the kingpin extension falls ahead of the point of contact
between the tyres and the ground), the wheel is pulled, as it is the line of application
of the force applied to the axis that passes in front of wheels mid point without
taking the direction of travel into account, and each attempt made by the wheel
to deviate from straight line travel will be counteracted by the straightening
couple generated by the force and by the rolling resistance of the wheel. With
negative castor the wheel is pushed as it is the line of application of the force
applied to the axis passes behind the mid point of the wheel. Consequently the
best stability condition for straight line travel is obtained with a positive
castor angle. In this case the phenomenon of "wheel wobble" and the
consequent effects on steering are avoided. The different behaviour of the wheels
can be verified practically by driving the same vehicle in forward gear and then
the RV8 is in fact 3 degrees 48 min +/- 54 mins. So using a castor reduction
kit which would remove 3 degrees of castor, would leave only 0 degrees 48 mins
+/- 54 mins of positive castor which is not sufficient.
castor angle settings
Fitting castor correction wedges can compress the
rubber pads to such an extent that they are crushed, in some cases seriously so.
This can contribute to the variability in the castor angle setting and differences
between the castor setting on each wheel. The fitting instructions with the castor
correction wedges recommend tightening the mounting bolts "to approximately
75 lbft" when the recommended torque setting for that bolt is understood
to be only 56 lbft.
So our conclusion is there are serious concerns over
castor reduction of 3 degrees on an RV8 and that where fitting the castor correction
wedges leads to crushing of the rubber mounts, there are concerns that the castor
angle settings may be variable as a consequence. So on balance fitting a castor
correction or reduction kit on an RV8 is not recommended.
castor reduction kit
See a comprehensive workshop note on a new castor
reduction kit incorporating useful engineering improvements which is available
for chrome and rubber bumpered MGBGTV8 and MGB models. The note describes how
fitting the kit rotates the crossmember forwards thereby reducing the castor angle,
reviews the engineering improvements, and provides some useful background information
on the need for positive castor to self-centre the steering at speed and give
a good steering response.
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