Keeping it on the straight and narrow – Aspects that affect TA/TB/TC steering (Part 7)

Eric Worpe delivered a superb presentation at the MGCC ‘T’ Register’s ‘Rebuild’ seminar in March 2013. Eric used flip charts to aid his presentation and I have been working with him to ‘flesh out’ the flip chart notes to produce a series of articles for inclusion in TTT 2.

Eric divided up his presentation into seven headings which he termed as “Seven Deadly Sins”. We have so far covered the first six ‘Deadly Sins’ i.e.

CHASSIS – is it true? – Issue 19 (August 2013).
FRONT AXLE GEOMETRY – Issue 20 (October 2013)
FRONT SPRINGS – Issue 21 (December 2013)
KING PINS – Issue 22 (February 2014)
TRACK ROD AND DRAG LINK ENDS – Issue 23 (April 2014)
TYRES AND TRACKING – Issue 25 (August 2014)

In this issue Eric covers the Bishop Cam steering box. Over to Eric:

I often find myself resisting the urge to waggle the steering wheel of any TA/B/C that I come across just to see how much “free play” exists. It was only after trying the waggle test on a WW2 Willys Jeep that had over 4 inches of play that I realised why there was such an acceptance of T-Types by American GIs.

Although free play is not the only component of a nomadic steering system, it certainly amplifies other faults and promotes considerable skill and concentration from the driver to avoid over-compensation in steering. The Bishops Cam (BC) box should be set up for minimum play (# pinch) when the sector arm’s peg is in the centre of the track formed by the worm’s groove (Fig.1 on the next page), corresponding to the straight ahead position of the front wheels. The peg in this position seemingly has an “optimum mesh” with the groove.

If only it were that simple!

The peg scribes an arc as it moves along the track, tending to disengage its mesh as it swings away from the centre, leading to increased free play. However, this characteristic is complicated by an unusual feature of the BC box. I’d assumed that when the peg was in the centre of the track, its position would be vertically above the centre line of the worm’s axis; this is not the case as it actually overhangs the centre by almost 1/8 inch (see photo 1).

Photo 1 – illustrating how the peg overhangs the centre line of the worm’s axis.

This leads me to believe that there’s an ingenious relationship between the peg’s “overhang” and the taper angle used on the groove and peg that sets up an extended “optimum mesh” in the centre of the worm’s track. This relationship was revealed by a “rebuilt” box that I’d bought from Toulmin Motors in the 1960s. The box had a replacement worm and peg machined by a local engineering workshop which used a different taper angle. This resulted in two “pinch” points either side of the centre where the gear’s mesh was optimised at the expense of some free play in the centre.

Further investigation of the relationship between the peg’s position along the track and the level of “free play” suggested that the groove is not parallel. So, our simple BC box has a few tricks up its sleeve that were not always appreciated by those re-manufacturing spare parts. Photo 2 shows a test rig that measures the displacement or “end float” of the sector shaft as its peg moves along the track.

Photo 2 – the test rig referred to in the text.

Having surprised ourselves with the discovery of some possibly ingenious design aspects of the BC box, we now find that some less than sound engineering practices take over. The oil-seal at the bottom of the sector shaft’s housing consists of an interference fit cork ring whose effectiveness is limited, especially when play develops between the sector shaft and its housing. The subsequent oil loss can accelerate wear considerably due to the high levels of friction within the box, unless the box is “topped up” regularly.

The BC box is made from a malleable grey cast-iron which is stronger than basic cast-iron, due to some of the free graphite being “burnt out” by keeping the box at red heat for a long period directly after the casting operation. This unfortunately leaves it a less than ideal bearing material for the sector shaft, which would have been heat treated for toughness at the expense of a surface hardness level that would have reduced wear rates.

On a more positive note, the worm gear is carried between two sets of ball bearings which are “pinched” together to eliminate play by an end plate clamped to the main body of the box by 4 BSF bolts. Judicious choice of shims sets up the right “pinch” effect (please refer to Fig. 1).

Fig. 1 – showing ‘cut-away’ illustration of the BC steering box.

Graph 1 (below left) shows the relationship between the “end float” and the position of the sector arm. The pink line is from an original BC box and the orange line is from a replacement part having a parallel groove, which progressively increases play as the peg moves towards the end stops.

Graph 2 (below right) shows some serious machining problems in a batch of worms that had to be rejected, re-machining spare parts to high levels of accuracy calls for considerable effort and skill in setting up production for quite small batch numbers.

The sector shaft’s arm carries a hardened fixed peg which runs in a case-hardened groove machined in the worm gear. The peg’s taper creates an upward force which is countered by a slightly hardened top plate secured by 3 BSF bolts to the body of the box. One of the bolts should be drilled to allow oil (EP 140) to be pumped through a grease nipple into the box. Once again shims are used to set up the mesh of the peg in the groove so as to eliminate play.

Some modified top-plates are available that set the peg’s mesh by use of a thrust bearing that’s positioned using a threaded adjuster in line with the sector shaft. This produces a bending moment on the sector arm which has resulted in fractured arms (Photo 3).

Photo 3 – the fractured sector shaft referred to in the text (photo by kind permission of Werner Hofstetter).

If you have such a modified top-plate and find that the box has started to need frequent adjustment to reduce play, DO NOT delay replacing the sector shaft with a modern steel alloy replacement. Even if there are no signs of a fracture and you chose to continue using a modified top-plate, then at least consider replacing the old sector shaft before any signs of a fracture or of twisted splines occurs.

As part of the peg wears, two shoulders are formed such that when top-plate shims are removed to reduce play, the shoulders are thrust into contact with the edges of the worm’s groove. These localised pressure points can then break up the case hardened skin of the groove’s edges. This problem was eventually diminished by chamfering the worm’s edges, an operation not carried out on modern replacement worms unfortunately.

Considerable force is exerted by the box on its fixture to the chassis. Waggling the steering wheel whilst observing the box may well reveal a lack of rigidity, especially between the steering column’s outer tube and the box. These were originally locked together by an interference fit, but most have probably been braised together for robustness, which can be compromised if the bracket securing the outer tube to the scuttle is not secure.

The box pivots around a 3/8” bolt supported by two lugs on the bracket attached to the chassis. The holes in the lugs tend to elongate, but these can be drilled out to 10mm as well as the box’s support section, to take an HT 10mm bolt which may help reduce play. Choose a bolt length whose unthreaded shank just passes through both lugs.

It’s possible to overhaul the BC box by boring out the sector shaft’s housing to take either an oversized shaft or bronze bushes and a spring loaded lip seal. However, as clearance for the worm gear encroaches on an area of the housing that should be used to support the sector shaft, boring out the housing any further weakens support in a crucial area.

The VW Conversion

This seems to be a contentious issue so perhaps it should be left to the individual to decide after trying out a VW conversion on, say, a friend’s TA/B/C.

The VW box follows a similar design to the Marles-Weller box where the fixed peg of the BC box is replaced by a roller disk. This then follows a complex groove machined in the worm gear which is waisted in the middle and is supported by two sets of ball bearings (see Fig.2).

Fig. 2 – showing ‘cut away’ illustration of the VW steering box.

Instead of engaging the worm’s groove from the top, the VW box meshes its rotating disk slightly above the side of the worm, allowing the sector shaft to be supported by a top bearing (possibly a ball race) as well as a bronze sleeve bearing in its lower section. Such an arrangement greatly reduces friction between the worm’s groove and “follower”. Side thrusts experienced by the sector shaft are amply supported by bearings either side of the thrust force vector. An adjustment screw alters the mesh between the roller disk and worm gear allowing play to be minimised in the straight ahead position.

Gains and losses of the VW conversion

1. No permanent modification to the TA/B/C.
2. Reduced friction, wear and play.
3. Robust, well engineered box.
4. Less effort to steer.
5. Wedges can be removed, increasing stability.
6. Does not look too “out of place”.

On the other hand:

1. More turns lock to lock.
2. Slightly increased turning circle.
3. Likely to perturb some enthusiasts.

One criticism often put forward is the loss of the “classic car” feel, but the steering box is only one part of a suspension system based on cart springs, adjustable track rod ends and solid front axles all forming a rather basic steering geometry. With a VW conversion, steering is not only easier, but a greater sense of confidence in coping with the unexpected can be felt.

Numerous suppliers offer the VW conversion, but it’s not beyond the abilities of a competent small workshop to undertake the modifications to the VW box.

Some suppliers of parts for conversion:

VW box 111-415-061/A: vwheritage.com
Adaptor bush and splined shaft section: Roger Furneaux roger.46tc(at)virgin.net
Drop arm: mgsparesandrestorations.com (Andy King)

Farewell to ‘The Seven Deadly Sins!’

Well that’s the end of this series which I hope has helped fellow TA/B/C owners to sort out some of the beachcomber steering tendencies.

Eric Worpe

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