Understeer – what is it, has my car got it?

Books have been written on this, so I’ll try to be brief:

A car should track around a corner along an arc with its centre at the meeting point of the rear wheel and front wheel axes. The more the front wheels are turned, the closer to the car the intersection point and the tighter the arc the car should be following. (When the front wheels are straight ahead, these axes never meet; oddly enough the car isn’t turning a corner then!)

Understeer (simply put) occurs during cornering when a car tries to go wider round a corner that the line described above which the wheels should be theoretically tracking along. Oversteer is when a car goes tighter in the corner than the theoretical wheel tracking line. When pushed hard enough to reach the limit of grip, understeer results in the car ploughing straight ahead off the road, while oversteer results in a spin.

The failure of a car to follow the line its wheels are tracking along is due to distortion in the tyres. This in turn is caused by a variety of factors including:

  • weight carried by the wheels, in turn affected by the weight of the car, it’s front/rear distribution and the strength (or otherwise) of the springs and anti-roll bars,
  • the angle of the wheels to the road (“suspension geometry”) and
  • tyre pressures

The first two of these are either fixed by the car’s design or require a fair bit of work/modification to change. However tyre pressures are easy to change and can make major difference to the balance of a car in a corner.

Understeer is often confused with the self-centring action of the steering. While both sometimes feel the same, they are in fact quite different. Self-centre is a feature of castor action – a similar effect, though not as pronounced, to castor wheels on a supermarket trolley.  The greater the castor angle, the greater the amount of self-centring, the greater the feedback through the steering wheel – and also and the harder it is to turn the steering wheel.

Most cars without power steer had low castor angles (only a degree or two). Power steering masks self-centre so often power steered cars have quite a bit of castor introduced to restore some feel to the steering.

So is my MG’s heavy steering a sign of understeer?

Many MG’s were set up with high castor angles (eg 4 deg in the MGA, 7 deg in the MGB; though the Midget at 3 deg was almost in the normal range). The result is very good feedback through the steering (you can feel the tail go very early in the piece) but also a very heavy steering effort.

This strong self-centring action is often confused for understeer, especially as we are used to the low effort needed for the power steering in modern cars we drive.
The evidence that this isn’t understeer comes at the limit where standard MG’s tend to break away quite evenly, usually a bit more at the tail first.  The front/rear balance is well within the range where it can be fine tuned with small changes in tyre pressures to produce either understeer or oversteer according to personal preference.   Well balanced handling like has always been a strong point of MG’s compared to most other cars of the production period.

If your MG does have genuine understeer, the problem will most probably be in the tyres – a bad combination of tyres and or pressures. If the same tyres are used all round and the pressures are equal then the effect you are experiencing will most likely be self-centre action.

However, if there is still genuine understeer, it indicates a major suspension problem (for example, collapsed or broken spring). Get it checked over immediately.

Some people find the heavy steering disconcerting when swapping between older MG’s and modern cars. Kits (such as the Frontline wedges) are available which reduce the castor angle and lighten up the steering considerably. Care is needed not to take this too far, as too little castor will eliminate steering feel.

Another (more expensive) approach used by some people in the MGB is to adapt the power steering system from the MGF.

The best example of steering effort perceptions being misconstrued is the MGC. In producing the MGC, MG attempted to counter the effort caused by high castor action on the heavier car by lowering the gearing of the steering. This resulted in the steering wheel needing to be turned substantially more for any given corner, and in part generated a lot of the press claims of understeer at the time it was launched. (One even commented that the understeer was caused by the low geared steering!) The fact is that although the steering wheel turned further, the front wheels didn’t. A further point of interest in this respect is that the supposedly terminally nose-heavy MGC has less weight over the front wheels than that well-known tail-hanger the Ford Escort!

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