Under steer and over steer
Under steer is the condition where a car needs more than normal front –wheels steering angles to go around a corner; the front end of the vehicle tends to break loose and slide or push towards the outside of a turn.
Over steer is when a car needs less than normal front-wheels steering angles to go around a corner, the rear end of the vehicle tends to break loose and slide outwards
Adams herb (chassis engineering)
Under steer is caused by the front wheels of a car having less cornering power in relation to the weight they support than the rear ones.
If the front wheels have more cornering power than the rear ones then the tendency is towards over steer.
Michael Costin and David Phipps (chassis design)
While taking a turn centrifugal force acts on the wheels, this effects resulting in two cases: under steer and over steer.
Under steer occurs, when the slip angles (angle between the axis line of the tire and the direction of travel of the wheel) of front wheels are greater than those for the rear wheels. Thus radius of the turn increased, when the turning radius is increase the vehicle will turn more slowly in response to steering input.
When the slip angles of the front wheels are less than those of the rear wheels, the radius of the turn decrease.
The decrease in the turning radius will make the vehicle turn sharply in response to steering wheel input by the driver.
To keep the vehicle on the right path the driver need to steer less than the normal.
Dr.kirpal Singh (automobile engineering)
The above paragraphs are the direct sentences or paraphrase sentences from some of the well-known automobile books explaining under steer and over steer.
To simplify under steer occurs when the front wheels lose grip to road due to excessive speed while cornering or hard braking through a corner, which results in wheel locking. The driver wants to take the corner but the car continues on straight way, because the front wheels are sliding at the road surface.
Similarly there is over steer when the rear wheels lose grip to road during turning through a corner.
As the wheels lose grip the rear end of the car step out and rotates around it axis.
Why under steer and over steer?
These two effects are mainly due to several reasons, where the most commons are:
- Going into a corner with too much throttle (high speed)
- Lifting off the throttle while cornering at high speed
- Braking too hard through a corner ( causing wheel locking )
- Wheels Camber setting
- Lower inflation pressure at the front tire than at the rear
- Used of cross-ply tires at the front and radial tires at the rear
- Lower than desire gear is accidentally selected at high speed (rear wheels lock up)
Luckily they can be control or avoid by proper driving skills and by appropriate suspensions and steering step ups.
While both the effects are undesirable under steer is preferable than over steer because it is more predictable and easier to correct that is why new generation vehicle are designed basically to under steer than over steer.
The steering ratio is the ratio between the turn of the steering wheel (in degrees) and the turn of the wheels (in degrees). In passenger cars, it is between 12 and 20:1. Example: If one complete turn of the steering wheel (360 degrees) causes the wheels to turn 30 degrees, then the ratio is 12:1 (360/30=12). so if the steering wheel turns at 12° the vehicle wheels turn 1°.
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