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Is castor reduction a wise modification for an RV8?
Over
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 models
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?
The 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
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The 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 creates
two important phenomena for the ride and handling of the vehicle
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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.
The
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 in reverse.
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where
you will see the castor angle on 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.
Variable 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.
Caution
So our conclusion is there are serious concerns over a
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.
Improved 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.
RV8NOTE232 |
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