| Understeer |
| |
| In technical terms understeer is the condition when the slip angles of the front tyres are greater than the slip angles of the rear tyres. In other words the front wheel angle is greater than the angle normally required for the turn. The slip angle is the angle of the tyre in relation to it's direction of travel. |
| |
| Oversteer |
| |
| Oversteer is the reverse of understeer, that is where the slip angles of the rear tyres are greater than the front tyres. In cases of oversteer the rear tyres tend to describe a larger cornering radius than the front tyres. |
| |
| Darting |
| |
Darting is an over response to steering input. It is commonly caused by a partially seized component within the steering linkage system or steering gear. Generally this fault will be evident when making small changes in vehicle direction, when changing lanes or passing, for example. It can be more pronounced in power steered vehicles than in manually steered vehicles. This is because small increases in steering effort will result in significant increases in steering gear output. System pressure gradually rises until the break loose point is reached. And as the pressure required to maintain steering movement is usually less than the pressure required to break the system loose, the system over reacts.
|
| |
| Self Centering |
| |
Self centering is the automatic return to the straight ahead position of the steering wheel after a corner has been negotiated. Previously mentioned was the fact that almost all power steering systems rely on front end caster for the force required to self centering the steering. Self centering problems can be caused by the following: |
| |
A. Insufficient Caster - Check vehicle manufacturer's specifications. A quick check for insufficient caster is to drop tyre front tyre pressures by 20%. This increases the rolling resistance and tyre trail and consequently alignment torque. If there is nothing binding within the system then the need for more caster is indicated.
B. Partially seized steering linkage.
C. Excessive column torque. Column friction should not exceed 2 Nm (15 in/lbs.).
D. Lost tension or incorrectly set steering gear valve spring or springs. Column drag may be displacing the valve and the resulting pressure will oppose the alignment torque generated by caster.
|
|
| |
Not many steering gears have adjustable valve springs. Many have torsion bars which can not be adjusted. Torsion bars can loose tension if subjected to excessive heat. This can occur if the valve becomes blocked. Pieces of the rubber liner from pressure hoses are commonly found jammed in valves, this restriction will cause very high temperatures, high enough to anneal springs. |
| |
| Steering bias or pull left or right |
| |
Generally. steering gears will not make the vehicle pull left or right.although many wheel aligners would like to think they do! Most steering gear designs used on passenger cars use a torsion bar valve spring. The torsion bar is pinned both ends and is therefore locked in it's central or unbiased position. There are some steering gears with adjustable spring loading on the valve and it is possible that these can be out of adjustment. There is a simple way to prove if the steering gear is at fault or not. Position the steering wheel in it's straight ahead position, disconnect the steering column, the pitman arm or links from the end of the rack. Start the engine, if the steering gear does not go to one lock then it is not at fault. |
| |
| Steering wheel kick back |
| |
When the front wheel of a vehicle strikes a pothole or severe bump reverse torque is applied to the steering gear or a force is applied to one end of the steering rack. This torque or force is normally absorbed hydraulically. That is as the piston within the steering gear is displaced the elements of the valve are also displaced causing pressure to be applied to the appropriate end of the piston thereby opposing the reverse force. Excessive kick back at the wheel can be due to the following factors: |
| |
| 1. Low pump relief pressure. There is insufficient pressure to oppose feed back forces.
2. Air in the steering gear allows excessive piston movement as the air must be compressed before hydraulic pressure can build up.
3. Wide, incorrectly offset wheels/tyres result in a larger offset from the centre of the tyre contact patch to the steering axis centre line. This results in greater torque about the steering axis which may overload the steering gear.
|
|
| |
| Wandering |
| |
A wandering condition is one where the driver is constantly correcting the steering. The vehicle is very difficult to hold on line. This condition can be caused by any on of the following: |
| |
A. Worn steering links resulting in excessive free play.
B. Insufficient caster. For a simple check refer to the chapter on Self Centering.
C. Binding column - steering feel is masked by excessive column friction.
D. Unevenly worn tyres due to wheel misalignment used on the front end after it has been adjusted. |
|
| |
| Vague or Mushy Steering |
| |
| Mushy or vague steering is where the steering feels "dead", there is little or no road feel and the vehicle is unresponsive, particularly in it's straight ahead position. Common causes of this are: |
| |
A. Low pump flow rates.
B. Air in the system.
C. Incorrect front end toe in/out adjustment.
D. Excessive steering column drag or friction. |
|
