Keeping it on the straight and narrow – Aspects that affect MG TA/TB/TC steering (Part 3)

17 Nov

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 have so far covered the first two ‘Deadly Sins’ i.e.

CHASSIS – is it true? – Issue 19 (August).
FRONT AXLE GEOMETRYIssue 20 (October).
In this issue we’ll look in depth at the third ‘Deadly Sin’ :

FRONT SPRINGS

Springs absorb road shocks and allow the wheels to follow the irregular contour of the road whilst controlling the degree of deflection of the chassis. However, a balance needs to be considered between a highly absorbent suspension system that allows considerable deflection of the springs and a minimal absorbent system that transfers most of the road undulations to the chassis. The latter case might just seem familiar to anyone with a TA/B/C.

A soft or absorbent suspension system gives a poor “roll stiffness” prompting instability and oversteer. Hard suspensions compromise road holding by upsetting the even distribution of the vehicle’s weight amongst all four wheels.

The TA/B/C models have quite hard suspension systems but the chassis is able to flex, thus helping to compensate for the limitations of the suspension.

Springs normally have a “constant rate” (deflection v force) according to Hook’s Law. However, with multiple leaf springs with leaves of various lengths, an increase in the spring rate occurs as the deflection intensifies. This is quite a desirable feature as it allows the spring to offer a soft ride to small deflections whilst avoiding being over- stretched by large deflection forces.

As the laminated spring set deflects, the leaves try to slide over each other but are subjected to friction. Although this has a beneficial damping effect, it does result in a harder ride and increased wear. Unlike some modern leaf springs, which have anti-friction features, the TA/B/C springs rely on occasional lubrication. The life and effectiveness of the lubrication can be improved by using a mixture of graphite and silicon grease. Silicon grease is immiscible with water and thus does not emulsify but it’s a poor lubricant. However, by suspending graphite in the silicon grease, good lubrication is achieved that’s resistant to being washed away. Graphite is very messy so wear old clothes and gloves when assembling the spring leaves.

Most new springs have the ends of the leaves chopped off square. Original spring leaves had their ends tapered and chamfered to prevent the ends digging in to adjacent leaves. Judicious use of an angle grinder could help form an original profile after the springs have been dismantled.

Ed’s Note: Please see Issue 1 (August 2010) for a comprehensive article on front springs which covers spring lubrication and tapering/chamfering.

Photo 1 – an after market PA/PB spring with its leaves chopped off square.

Photo 2 – an original TC front spring with its leaves tapered and chamfered.

Note: The clips are formed from the bottom upwards (and bent over the top leaf) on original TC springs. They are formed from the top on the PA/PB springs and ‘closed’ at the bottom with tubular rollers and nuts/bolts.

New springs are made from EN45, a low to medium carbon–silicon-manganese alloy, (C = 0.5%, Si = 2%, Mn = 0.9%). The chosen heat treatment renders the steel somewhat softer that the usual spring steel to help reduce breakages. This can result in the springs “settling” resulting in the “bump stops” limiting the spring’s excursion and producing an even harsher ride.

Old springs can be reset if not too badly worn, but this is not inexpensive as the springs, after dismantling are taken to red heat at about 800° C, re-arced and then quenched in oil. The springs then have to be tempered by heating to around 300 C to stress relieve and then quenched again.

Ed’s Note: An article in the most recent MG Octagon Car Club Bulletin casts doubts on the effectiveness of re-tempering and re-setting springs. According to the specialist (presumably a spring maker), who gave the advice contained in a letter on which the article was based, those who re-temper springs will not give a guarantee as to the expected life of the springs. There are some facts in support of the advice which need to be validated and we are in the process of doing this.

Modern front springs seem to suffer from poor quality control, the front eye being badly formed and consequently a loose fit on the pin.

Photo 3 – comparison of ‘eyes’ at the front of the spring; the spring on the left is an after- market PA/PB front spring; the one on the right is an original TC front spring. Although perhaps difficult to spot, the ‘eye’ of the spring on the left is slightly out of round, whereas the one on the right is perfectly round.

A rolled up strip of 7 thou. bronze shim can be used to “pack out” the eye using between 2 to 4 turns. Suitable shim is available from Noggin End Metals, www.nogginend.com at approx. £2-50 a pack. (see item under ‘Bits and Pieces’).

Fortunately the larger eyes for the shackle rubbers on the front springs of the TC are more reliably formed. The rubber bushes soon seem to deteriorate, worsening an already spongy feel to the steering when compared with the TA and TB.

Polybushes, available from the MG Octagon Car Club and NTG Services as well as from John James (Editor) jj(at)octagon.fsbusiness.co.uk {substitute @ for (at)} can make a real improvement without introducing any significant harshness to the suspension thanks to the ability of the shackles to swing whilst accommodating horizontal displacements of the leaf springs.

The rear location of the front springs needs to be robust, so check for side play in the bronze trunnions (TA/TB) or lozenging of the shackles (TC) when the steering wheel is forcefully oscillated.

If you are ordering new springs it might be possible to specify the front eye to be made 5/8 inches diameter. Bronze bushes (SAE 660) need to be made up to give an interference fit of some 20 thou. in the front eyes. The centre bore should be partially drilled out on a lathe to say 12mm dia. before the bush is pressed in to the spring’s eye. The spring should then be set up on a milling table so that the final drilling/reaming out to 1⁄2 inch can be achieved such that the bush’s bore is truly at right angles to the spring’s body. This may seem a bit OTT, but many spring eyes are “out of true” by several degrees.

Each spring leaf has a dimple and corresponding nodule on the reverse side. These are used to interlock the leaves together and fix the location of the spring set to the front axle. Check that the top leafs’ nodules are the same distance from the front eye and have not been worn down. Clamping the spring’s leaves firmly together is essential to allow the spring’s dimples and nodules to lock together and in the correct position to the front axle. The forces on the locating mechanism are considerable, especially when braking.

It’s prudent to check the tightness of the clamping bolts securing the front axle to the spring set and clamp plates occasionally. The weight of the car should be kept on the front axle as jacking up the chassis and allowing the wheel to hang would increase the spring’s curvature. Compressing the spring’s curvature between the clamp plates by tightening individual clamp bolts is not advisable.

Figure 1 – text as indicated on drawing.

Far better to allow the weight of the car to reduce the spring’s curvature between the clamp plates before tightening the clamp bolts. Any looseness of the spring to axle clamp mechanism would allow the forces on the front suspension to wear down the locating mechanism, don’t leave it to chance that someone else has got it right.

Ed’s Note: Thank you Eric for a most interesting and thought provoking article; the next issue will feature Kingpins (worn bushes, axle eyes & loose cotter pins) – is there no end to this misery????!!!!!


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