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Over steer and under steer

A tyre rolling freely in a direction will when subject to a sideways force follow a path at an angle to the original direction unless prevented from doing so by the driver. The angle between that the unaffected tyre would take and that the tyre subject to a force would take is known as ‘slip angle’.

The tyre is not slipping as the term suggests the word is used as a name not a description. To counter this feature of nature the driver simply turns the steering wheel to compensate for the differing direction of travel. As the driver does this the tyre distorts.

Newton’s Law’s on equal and opposite reaction mean that to counter these movements a sideways force is produced which is increased with the degree of the bend or the vehicles speed. Thus increasing the slip angle force increases the amount of steering required from the driver.

As a side note to this part of the text I would mention here that at the point at which the side ways forces are greater than the coefficient of friction the vehicle will start to yaw and enter into a critical speed situation, a principle covered later.

The amount of slip angle present at any given time will be dependent upon the following:

The degree of force developed in cornering to produce one degree of slip angle is known as the cornering power or cornering stiffness and is effected by the tyre design, the stiffer the tyre or the more resistant to distortion the tyre is the more force is required to produce one degree of slip.

The tread pattern of a tyre has several functions one of which is relevant here, the rubber will always be struggling against the sideways forces that distort it to get back to its original position in life. This feature of the tyre is known as its ‘self aligning torque’ and produces the self centering effect of a vehicles steering. If you turn the steering wheel to one side or the other by a few degrees and release your pressure on the steering wheel it will return to its straight ahead position as the vehicle travels forward, this is self centering and all vehicles should be subject to it dependent upon tyre condition.

Centripetal force which acts on the centre of gravity of any mass moving in a circular path is resisted by the tyres. As the speed of a vehicle increases the slip angle also increases, changes in the tyre pressures from those that are recommended by the manufactures to deal with the geometry of the vehicle by design will come into account when cornering and subject the vehicle to centripetal and sideways forces.

Neutral steering is obtained when centripetal force acts on the centre of gravity of a vehicle or object which is moving in a circular path. If the centre of gravity is to the centre of the vehicle and the tyres are all correctly inflated at say 24 pounds per sq inch, the slip angle will be the same both front and rear of the vehicle.

As the speed of the vehicle increases the slip angle will increase but will remain the same front and rear and the driver will need to make only minor adjustments to the course of the vehicle making the experience pleasant and easy to handle.

If for example the vehicle is designed to have an all round tyre pressure of 24 psi and the fronts have dropped to 20 psi but the rear remain correctly inflated the tyres will distort more allowing the slip angle to alter and the driver will have to apply more steering than would otherwise be required. The natural tendency of the vehicle is to travel in a straight line and on a bend in this condition it will want to leave the road. This is known as ‘understeer’.

Oversteer is based on the same principles as follows:

Reverse the tyre pressure difference so that the front tyres are at 24 psi and the rear 20 psi and of course the effect changes, if the rear tyres are underinflated and the fronts normal less steering would be required by the driver to negotiate the same bend. When more steering is required the term oversteer is used to describe the situation.