340
Review of the B&G Castor Reduction Kit
An improved castor angle reduction kit has been produced for chrome and rubber bumpered MGBGTV8 and MGB models by Brown & Gammons, the MG specialists at Baldock. This note from Chris Hunt Cooke and Victor Smith describes how fitting the kit rotates the crossmember forwards thereby reducing the castor angle, reviews the engineering improvements, provides some useful background information on the need for positive castor to self-centre the steering at speed and give good steering response, and describes how reduced castor with modern radial tyres gives a reduced steering load. (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.

The B&G Castor Reduction Kit is designed to do two things - first to reduce the castor angle by 3 degrees from the original 7 to 4 degrees and second to maintain the integrity of the mounting of the crossmember to the chassis leg. It is worthwhile understanding how this new kit achieves that with well thought through engineering details which ensure the mounting studs (or bolts) continue to be positively located in taper seats in the chassis legs and the rubber mounting pads are not crushed to achieve an accurate castor angle setting. This is seen as an improvement on another kit currently available, which when fitted results in the taper of the stud being held away from its seating and the rubber pad being crushed when the assembly is torqued up.

How is the crossmember mounted to the chassis leg before the castor modification?
The MGB front cross member is fabricated out of pressed and welded steel sheet and is mounted on the underside of the chassis legs (which are box sections extending forwards from the monocoque) with four high tensile steel mounting studs which are positively located on taper seats into the chassis leg. In the attached drawing of the crossmember you can see on either side that the topmost part is the platform with four holes on which the lever arm shock absorbers are mounted. Just inboard from those platforms are the two large holes on either side through which the crossmember is bolted to the chassis legs by the mounting studs.



Front crossmember is fabricated out of pressed and welded steel sheet and is mounted on the underside of the chassis legs (which are box sections extending forwards from the monocoque) with four high tensile steel mounting studs (or bolts) which are positively located on taper seats into the chassis leg. (Drawings: Parts Manual and B&G)

Those mounting studs have screw threads at the top and bottom and a thicker plain section in the middle, with a taper at the top. The intention of the design is that the taper locates to a corresponding taper seating in the bottom of the chassis leg. Hence the mounting bolt is positively located in the centre of the hole in the chassis leg when it is bolted up with a torque of 56 lbft.

Mounting stud (or bolt) with upper pad, lower pad and steel plate with the locking nut under. Note the mounting bolts have screw threads at the top and bottom and a thicker plain section in the middle, with a taper at the top. (Diagram: V8 Parts Manual)

This leaves the bottom part of the mounting stud protruding below the chassis leg with a plain section, and beneath that a narrower threaded section forming a shoulder at the end of the plain section. Over the plain section of the stud is fitted a rubber pad which acts as a packing piece between the chassis leg and the mount on top of the fabricated crossmember. This is held up by a rectangular washer with a smaller diameter hole so that the washer sits on the shoulder of the plain section of the mounting stud but is held in place by the bottom locking nut. The pressure on the rubber pad between the chassis leg and the crossmember is therefore limited so crushing is avoided.

What is castor?

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 known 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.
How does the kit reduce the castor angle?
The method used to reduce the castor angle is to rotate the crossmember towards the front of the vehicle by providing a precisely engineered stainless steel packing piece between the front crossmember fixing points and the underside of the chassis leg. Since the steel packing has used some of the length of the plain portion of the mounting stud, a steel collar is supplied with the kit which has to be fitted. In effect it extends the plain portion of the mounting bolt back to its original length. Without this collar the rubber mounting pads would be compressed too much thereby ruining the mounts and the ride quality - and of course the crushing would give rise to variances in the castor angle, even between each side of the vehicle. New slightly shallower high tensile steel locking nuts are provided in the kit to fit the reduction in useable thread length of the mounting studs.

Steering rack packing pieces
Because the angle of the crossmember upon which the steering rack is mounted will have changed slightly in relation to the chassis legs, the body of the steering rack mast will quite probably no longer align with the steering universal joint. The steering rack mounts will therefore have to be packed at the front in order to realign the rack with the universal joint. Six packing shims are included in the kit.

Can a V8 enthusiast fit the kit?
The B&G Castor Reduction Kit can be fitted by a competent DIY mechanic but as with most matters relating to vehicles, the modification does need to be carried out with the right equipment and conditions and sufficient knowledge, mechanical skill and aptitude. If you have any doubts whatsoever, the kit should be fitted by a professional mechanic. B&G estimate that fitting the castor reduction kit requires approximately three hours work.

Now a few cautionary notes regarding RV8s and Heritage shells and crossmembers
There is a mistaken belief about that the castor angle on the RV8 is the same as 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 is 3 degrees 48 min +/- 54 mins (see the RV8 Repair Manual AKM7153ENG) so using a castor reduction kit that would remove 3 degrees of castor would leave only 0 degrees 48 mins +/- 54 mins which is not sufficient. In addition the crossmembers supplied by the British Motor Heritage plant at Witney, whether supplied individually or incorporated in new BMH shells, already have a reduced castor angle. BMH have confirmed that the castor angle reduction was incorporated on the crossmembers they supply. Therefore any MGB or MGBGTV8 fitted with a Heritage crossmember, or even in some rare cases fitted with an RV8 crossmember, should NOT be fitted with a castor reduction kit.

Prudent check for cracked steering rack mounts While you are working in this area on fitting the castor reduction kit, it is well worth checking the condition of the steering rack mounts for any hairline cracks or more serious fractures. These have been reported in detail together with the information on the new strengthening gusset supplied by B&G. See V8NOTE338. A routine check on the condition of the mounts should be included in your annual servicing checklist.

The B&G Castor Reduction Kit (AHH6195 CASTOR) is available now from stock at £29.95 including VAT. The kit includes comprehensive fitting instructions and detailed diagrams. The Steering Rack Mount Strengthening Gusset (AHH6195 BRACKET) is also available from B&G. They can carry out the inspection for you and if cracks are discovered, supply the gusset and MIG weld it to the mount and crossmember at their Baldock workshops. For details of the new strengthening gusset see V8NOTE339.