MG TC Front Axle Castor Angle

This summer, my TC was displayed along with other vintage, historic & classic cars at our local village fete. I got talking to someone who was staying with relatives in the area and who’d recently purchased a TC (although not available on the day). On discovering my car still retained the Bishop Cam steering box, he said he found his car had a tendency to wander on straight roads. I advised him to check the castor angle as well as checking the steering box was correctly adjusted (i.e. correctly shimmed – unless he had the aftermarket Tompkins screw adjustment cover plate), check the ‘toe in’ was correctly set and no significant ‘play’ in the king pins, drag link and track rod ends.

Since he was interested in understanding the castor angle, I recounted my own experience when purchasing my 1946 TC in 2003. I bought my car from a classic car dealer (in fairness to him not specifically an MG dealer) who simply commented “like many older cars with worm & peg steering boxes, don’t expect the steering to be pin sharp”.

As purchased, the steering was very sensitive with very little self-centring action. The steering box did need slight adjustment by removing a 0.005″ shim and with the top plate off I took the opportunity to fill with ‘Steering lube’ which is a liquid semi- grease (as thick oil tends to drain out of the box through the drop arm seal and grease is so thick the action of the follower peg in the worm simply deposits it on the sides of the box, depriving the moving parts of essential lubrication).

However the real culprit was a lack of castor angle caused by the axle being fitted back to front during some previous rebuild – and this is not an uncommon occurrence! Mike Sherrell’s book TCs Forever! and a number of other publications explain the castor angle set up. Castor is achieved when a projected line drawn though the centre of the king pin touches the road ahead of the tyre contact point and provides a self-centring action (think of the castors on a supermarket trolley).

The castor angle is simply the angle that the king pin leans backwards from the vertical to enable the projected line to be ahead of the centre of the tyre contact point. The greater the castor angle the more the self-centring effect (but also the harder it is to turn the steering wheel at low parking speeds), so MG designed the steering geometry to achieve a balance between straight line stability and ease of low speed manoeuvring.

When correctly set up and taking the weight of the car (providing they’re not worn or sagging), the front springs of a TC rise up forward by 5 degrees from the horizontal. In comparison with the king pin axis the underside of the front axle mounting pads are set 3 degrees from horizontal which means if a loose axle is placed on a flat surface the king pin axis will lean back by 3 degrees. When correctly installed on the car the total castor angle is 8 degrees (5 degrees on the spring and 3 degrees cast into the axle). But of course, if the axle is reversed by being incorrectly assembled off the car during a rebuild with the LH and RH stub axles fitted to the wrong side of the beam axle then when fitted to the car the castor angle is actually reduced to only 2 degrees (5deg minus 3 deg).

Mike Sherrell’s book states a first indication as to whether a TC front axle has been reversed is that the cast in lettering on the axle beam should be to the rear of the car (i.e. not readily obvious from the front). A number of MG dealers I talked to were slightly sceptical as to whether this generalisation applied to all TAs, TBs & TCs (to be fair Mike only refers to this with TCs) but certainly a sure fire way of confirming if the front axle is the correct way around is that the round heads of the king pin retaining cotters should be at the rear and the cotter nuts to the front of the axle. (i.e. the head of the king pin cotter acts as a steering stop with the bolt at the back of the stub axle).

On TCs built from late 1947 onwards, MG installed 2.5 degree wedge plates between the axle and the spring. Fitted from the rear of the axle this had the effect of rotating the axle forward reducing the castor by 2.5 degrees to 5.5 degrees. MG did this in response to complaints from the USA that American women were finding the TC steering heavy when parking. I and many others would advise removing these later wedge plates to regain the earlier cars’ better straight line stability with 8 degrees castor.

However, these 2.5 degree wedge plates (available from Moss and others) do have one useful function. If like mine when purchased, your car has a reversed front axle you can’t just turn the axle around since this would cause a major safety issue regards reversing the left hand and right hand stub axles & nuts even when you change over the hubs and back plates to the correct side etc. Therefore until I removed and fully rebuilt my front axle correctly, as a temporary measure I fitted these 2.5 degree wedges between the axle and the springs but from the front so rotating the axle backwards and improving the castor from 2 degrees to 4.5 degrees.

I’ll admit that whilst I was planning improvements to enable my TC to keep up with modern traffic I considered fitting the Datsun or VW steering box conversion as well as the 5 speed HI-Gear Ford gearbox etc. But in truth, since I’ve got a tubular extractor manifold on my stage 2 engine the modern steering box wouldn’t fit. Therefore I’ve kept a correctly adjusted standard Bishop Cam steering box, which together with NDT crack detected drop arm, reconditioned front springs, John James’ polyurethane shackle bushes, new stub axles and Roger Furneaux’s taper roller front bearing conversion in new hubs, the steering is precise with good straight line stability.

A final modification I’ve carried out recently – more for fun than any real need on a road car (and a possible thought of club hill climbing at some future date as it’s normally a track conversion) – is to fit a new rose jointed drag link and track rod and a front Panhard Rod which better locates the front axle and takes out the sideways lateral movement inherent with the bushed shackle spring retainers.

This makes no – or only very marginal – difference under normal driving conditions but the car is better poised under hard cornering. The TA and TB’s front springs were retained at the rear by sliding trunnions which gave a better lateral location (being developed from MG’s racing experience through the 1930s), but as these tended to wear quite badly on higher mileage road cars they were changed on the post war TC to the harder wearing bushed shackles.

Mike Harvey

Ed’s note: The 2.5 degree packing pieces were introduced at chassis number TC4251.

The castor angle at the axle base is 3 degrees for the TA, TB and TC; this seems to indicate to me that the front axles were the same for the TA/TB/TC.

The (total) castor angle on the TA and TB is 6 degrees, 3 degrees at the axle base and 3 degrees on the chassis. So the difference in (total) castor angle (6 degrees, compared with 8 degrees) must be due the difference in spring mounting at the rear of the spring from a trunnion arrangement to one of shackles.


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3 thoughts on “MG TC Front Axle Castor Angle

  1. Benny Grumer says:

    Hi Mike
    Thank you for the very detailed article.
    On my 1947 TC the lettering on the front beam axle is facing the rear of the car but the head of the cotter pin is to the front of the car.
    I assume that the lettering on the beam is not a the indication whether the axle is installed backward. from investigating other cars it seems the head of the cotter pin is always facing backwards. My car is off-road for restoration but with the axle still on it. I will invert the front axle at the chassis restoration.
    Thanks and Regards
    Benny Grumer

  2. Peter Jones says:

    I have always wanted to measure my castor angle but never managed it to any degree of accuracy.

    Looking into axle straightness is the first essential. It was suggested to me (by Morgan Marshall) that laying it on a kerb stone was the easiest way of finding a flat surface of the right length – if it wobbles then it probably needs straightening.

    If OK then looking along the axle to see if the ends are leaning the same way can be interesting.

    Removing and inspecting the front retaining pins in the chassis can also surprise many owners of high milage cars.

  3. Eric Worpe says:

    Your suggestion that the orientation of the cotter pin will confirm the axle is the correct way round does not take into account the possibility that the cotter pins can be put in the wrong way round.
    The wedges restore the TC caster angle to that of the TA and TB, the initially increased TC caster angle probably being an oversight by MG.
    Is there really documentation that “American women” complained about the TCs steering? Americans driving anything from Willys jeeps to saloon cars would only have known multi-turn steering, consequently the Tabcs would have seemed heavier to everyone.

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