Eric Worpe delivered a superb presentation at the MGCC ‘T’ Register’s ‘Rebuild’ seminar earlier this year. 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 dealt with the first ‘Deadly Sin’ i.e. CHASSIS – is it true? in Issue 19 (August). In this issue we’ll look in depth at the second ‘Deadly Sin’: FRONT AXLE GEOMETRY
• Kingpin inclination
• Camber angle
• Castor angle and self centring
• Front axle set-up
1. Kingpin inclination (see Figs 1 and 2)
The kingpin is inclined at some 7.5 degrees so that a line drawn through it will strike the road at the point of contact of the tyre with the road surface. Ideally this point of contact should be at the centre of the tyre, but this is difficult to achieve as the kingpin is forced away from the true centre of the wheel by the hub, spokes and brake drum, and increasing the kingpin inclination has some undesirable effects such as an increased self- centring action and a higher stress on the kingpin.
On the TA/B/Cs, positive offset is used, such that the centre line of the wheel meets the kingpin axis just below the road surface, the offset being given as X. This has disadvantages because the wheels tend to splay outwards as the car moves forward due to the centre line of the wheel being outboard of the kingpin axis. This would result in heavy steering but for the balancing out effect of both wheels.
A TA with side laced wheels has a reduced offset X which should improve its steering characteristics particularly in the case of unequal tyre pressures. As one front tyre deflates, the positive offset will increase causing the car to veer to that side especially when braking.
Modern cars have a negative offset which offers greater stability in the event of unequal tyre pressures or braking efforts.
2. Camber angle
The slight incline of the stub axle splays the wheel outwards at the top by 3 degrees to the vertical and this is called a positive Camber angle. Whilst Camber contributes to reducing the offset X (see kingpin inclination diagram – Fig. 2 – under ‘Kingpin inclination section), its main purpose seems rather intriguing and not a little obscure.
If the axis of the stub axle is projected to where it would contact the ground (Fig 3) and the two radii from this pivot point to the top and bottom of the tyre are drawn, a cone is formed.
This cone would tend to roll around its pivot point producing another splaying-out effect on the wheel as the car moves forward and the wheel tries to follow the circumference of the cone. Providing both wheels have the same camber angles, this action is balanced out resulting in no net effect on the steering.
When cornering, the outward weight shift of the car generates a greater splaying-out force (Fig 4) from the outer wheel, which steers the car away from the bend. This results in the driver having to exaggerate the steering effort. This is known as understeer and was considered a safety characteristic at the time.
The 3 degree camber angle is quite high by modern standards and probably originates from the time when most roads were “crowned” to aid drainage. The wheel’s camber would then realise an improved tyre contact footprint as the wheel is more likely to be at 90 degrees to the crown’s circumference.
A 3 degree camber would suit roads having a crown height of just 0.6 inches between the wheels (Fig 5). This suggests that the TABCs are more suited to the narrow, twisty country lanes that predominated in the 1930s. This is indeed fortunate as such roads are more fun to drive on.
Independent suspension is often designed to vary the camber angle to reduce tyre wear and aid adhesion.
TA/B/Cs set up for competition are sometimes de- cambered by bending the centre of the beam axle (Fig 6). This can be achieved by using an hydraulic press (Photo 1). A de-cambered axle will have inclined spring mounting pads and these must be compensated for by suitable wedges so that the spring’s eye is aligned with the front eye’s locating pin.
3. Castor angle and self centring
The castor angle is made up from two components. The beam axle has an inherent castor angle of 3 degrees (see Table 1) – this is augmented by the slope of the front springs, which for the TA and TB was also 3 degrees.
However, when the rear trunnions were exchanged for shackles on the TC it resulted in an increased spring slope of 5 degrees, giving a total of 8 degrees as opposed to 6 degrees. for the TA and TB. Subsequently wedges of 2.5 degrees were offered to reduce the total castor angle of the TC to 5.5 degrees (see Table 1 and the illustration at Figure 7).
Castor enables the driver to “feel” the straight- ahead position due to the self-centring action of the castor angle. Fig. 8 shows how the pivot centre line of the wheel intersects the tyre’s footprint ahead of the centre of contact.
Although the castor steering feature is similar to a castor wheel fitted to a trolley, where the wheel’s centre trails behind the pivot axis, an alternative explanation is more suited to the specific geometry of a car.
Turning the wheel about the pivot axis results in an edge of the tyre lifting up the wheel (see Figure 9); this can be illustrated by holding a tin can in the hand and holding one’s arm vertically with the can resting on a table. Swivelling the can about the centre axis of one’s arm produces no reactive effect. However, inclining one’s arm to the vertical and swivelling the can should cause one edge of the can to lift.
The weight of the car brings about a “reset” effect, forcing the wheel to return to its lowest (straight ahead) base level. Thus the castor return action is mainly a function of the castor angle, weight of car and width of tyre.
It’s essential that the front wheels posses some self-centring tendency to restore them to the straight-ahead position after deflection by any road undulations, otherwise wheel-wobble or shimmy could occur. Too much castor produces hard steering, whereas too little causes wander.
4. Front axle set-up (checking the beam axle)
Many TA/B/Cs will have had a colourful history, particularly around the 1960s. Some of the legacies from these wild times might well be a distorted chassis or bent front axle resulting from unsolicited encounters with substantial objects.
Checking the front axle for trueness can be accomplished using a rigid platform such as a RSJ or length of steel right angle section.
The following points should be checked:-
A, The spring mounting pads sit squarely on the flat plane of the platform.
B, The section of the beam axle between the mounting pads is straight.
C, The Castor angle is 3 degrees.
D, The kingpin inclination angle is 7.5 degrees.
C and D can be checked by inserting a length of 3⁄4 inch bar in the axle eyes and using either a special Dunlop gauge (photo 2) or protractor (photo 3).
Some distortions can be corrected by using an hydraulic pipe bender (photo 1) or clamping part of the axle to a substantial concrete base with anchor bolts and then using a long rigid lever clamped to the axle, to twist the axle section between the mounting pads back to alignment. These operations should be performed with the axle in its “cold“ state.
Ed’s Note: Much food for thought here from the very knowledgeable Eric Worpe. The next issue will cover the front springs.