Vehicle Steering Systems

When we talk about Vehicle Steering Systems we’re discussing the collection of components, linkages, etc. which will allow for a vehicle to follow the desired course. This is all part of the vehicle dynamics.

The way in which a vehicle steers can be described:

  1. Neutral Steering – handling characteristic between understeer and oversteer.
  2. Oversteer – the tendency of a vehicle to turn more than the apparent steering angle.
  3. Understeer – a handling condition in which the slip angle of the front tyres is greater than the slip angle of the rears.

The basic type of vehicle steering system found in most road vehicles is the Ackermann Steering system:

A double-pivoting steering system where the outer ends of the steering arms are bent slightly inward so that when the vehicle is making a turn, the inside wheel will turn more sharply than the outer wheel. This is done to compensate for the greater distance the outside wheel must travel.

As a rule of thumb the links connecting the track rod ends to the wheel steering pivots should be angled inwards such that with the wheels straight ahead the track rod ends lie on a line joining the wheel steering pivot and the centre of a line drawn between the rear wheel centres (as shown in the diagrams).

Bump Steer – A generally undesirable condition in which a wheel steers slightly as its suspension compresses or extends.

drag link steering system

Drag Link Steering System – Steering system used with solid axles.

Note that wheels are held relative to each other by the beam axle and drag link.

Four Wheel Steering – A mechanism that allows the rear wheels to steer along with the front wheels.

lever arm steering system

Lever Arm Steering System – This steering system allows large steering angles to be achieved at the wheels.

A rotation of the steering box output moves the steering levers through an arc that results in near linear motion of the drag links. The result is a rotation of the wheel.

Oversteer – The tendency of a vehicle to turn more than the apparent steering angle. Technically, it is where the rear wheel slip angles are greater than the front. Moderate oversteer may be advantageous on slow, tight corners, but any oversteer is usually difficult to manage on high speed corners. Causes the rear of the car to take a wider apex, causing the car to spin in extreme conditions. Also called loose in the US.

rack and pinion steering system

Rack and Pinion Steering – The most common steering system on passenger cars.

Plan view of a steering system. A rotational input at the pinion is turned into a translation by the rack and pinion that moves the drag link and this results in a rotation of the tyre about the kingpin.

Steering Angle – The angle the wheels are turned at any instance relative to straight ahead.

Steering Offset or Scrub Radius – The distance from the point where the steering axis intersects the ground to the longitudinal line that runs through the centre of the tyre’s contact patch.

Steering Shimmy – A steering wheel oscillation that often occurs at a certain road speed.

Turning Circle – The diameter of the circular path created by the wheel furthest from the center of the turn.

Understeer – A handling condition in which the slip angle of the front tyres is greater than the slip angle of the rears. An understeering car is sometimes said to push, because it resists turning and tends to go straight. Understeer is more common on slow, tight corners. Understeer is sometimes desired for high speed corners because it is safer.

Possible ways to correct understeer:

  • add more front downforce
  • soften springs and rollbar
  • reduce front tyre pressure

Zero-Offset Steering – A steering system whose geometry has a scrub radius of zero. This configuration minimizes the steering effects produced during acceleration (with front drive) or braking on varying traction surfaces.

There are many different vehicle steering systems and it would be great if you dropped me a line with ideas or examples that you have used in your own model cars.

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