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, Sat, 2 Dec 2000 13:18:01
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Without arguing too much about terminology, let us call any change in steering wheel position as a result of an accelerating wheel torque "Torque steer".
A wheel is steered about the "kingpin axis" The kingpin axis is defined on a strut by the line that intersects both the lower ball joint, and the upper strut mount. The "spindle length" is defined as the horizontal distance from the center of the tire to the kingpin axis. (On most struts the spindle length is about 60 mm, SLA type suspensions can be much lower ~20mm) There is one more definition and that is the scrub radius. the scrub radius is defined as the horizontal distance from the kingpin axisa to the center of the tire at ground level.
Let us assume that the bushings are infinitely stiff (revoulte joints). This will aid in understanding and we will go to the case of compliant bushings next. Also lets assume that the fore/aft tire force distribution can be replaced by a single force acting at the center of the tire. (For smooth roads this is a decent assumption) Since there is a horizontal span between the place where the tire force is acting, and the axis that the wheel is steered about there will be a steering moment induced on the wheel equla to the force at the tire times the spindle length.
If the forces acting on the left and right tires are equal than there will be a balance and there will be no net steering torque and the user will not feel any torque steer. If the force on one wheel is greater than that on the other, their will be a net steering moment. The wheels will steer and the driver will feel the steering wheel tug in the direction that the wheels are steering.
So how do we eliminate torque steer? The best thing to do is ensure that the force acting on each tire is as close to equal as possible, and to minimize the spindle length (the lever arm that generates the steering torque).
Unfortunately there are practical packaging limits as to what the spindle length of a strut type suspension can be. The top mount is usually already moved as far out as the package will allow. The lower ball joint is also about as far into the wheel as the package will allow.
There is the posability of using a multilink lower control arm. This type of arm forms a "virtual" ball joint at the intersction of the two links. This could allow a very short spindle length by moving the virtual ball joint way out. The dproble is that as the ball joint is moved out the kingpin inclination as well as the scrub are increased beyond their acceptable values. In order to get everything, the upper mount must move outboard. with a SLA type suspension, the upper ball join can be above the wheel and far enough outboard to maintain an acceptable spindle length, scrub, and kingpin inclination.
Back to the force times distance concept. Uneven forces come from things such as road irregularities and unequal torque input. As someone previously mentioned use of unequal length halfshafts can lead to a torque imbalace due to the fact that the front view angles of the shafts are not the same. Use of equal length shafts reduces this possability, but it is still very possible when the body rolls relative to the ground. So the use of equal length halfshafts and short spindle lengths help to minimize the torque steer.
Steer induced by toe changes with vertical wheel movement is known as "bump steer". Modern cars have very linear toe curves with very little change in toe -vs- wheel position. Bump steer is usually a mimal affect.
As we take into account bushings we will find yet another source of torque steer. If the movement of the suspension relative to the rack due to bushing compliance is great, it could cause unwanted steer affects as well.
All that said I have driven a 9-3 SE 5spd and it most definately suffers from torque steer. My guess is the cause is a combination of the long spindle length, and the chassis/bushing compliance.
-Joe
, Sat, 2 Dec 2000 13:18:01
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