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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 where you will see the castor angle on

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 - 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.


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.
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