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Old 08-19-1999, 09:20 PM
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Join Date: Aug 1999
Location: ajax, ontario, canada
Posts: 773
i've read a long time ago about the differences in philosophy of 3 different german car manufacturers with respect to steering offset. As i remember, this was the offset between the centerpoint of the tire patch and the point of steering. This was at a time when ABS was not yet popular and/or affordable. BMW went for positive offset, Audi for negative offset (probably because of front-drive) and benz for zero-offset steering.

the differences in behavior of these approaches become apparent on split-coefficient braking, where the left- and right-hand front tires have different grip levels on braking (like gravel on the right and asphalt on the left, a realistic situation during evasive maneuvers). Since the left tire has more grip, its behavior will overrule that of the right tire, which has less grip. If the steering point and and tire contact patch center do not coincide, this will impart a steering torque on that tire, which will be more that the steering torque on the other tire, which has less grip, resulting in a net steering torque on both tires.

bmw went for positive offset so that the tires will steer in the direction away from the impending spin. Though this would make the spin worse, it gave feedback to the driver through the steering wheel, who would (theoretically) instinctively apply opposite corrective lock. This reflected the bmw philosphy that the driver was part of the loop.

audi/vw left the driver out of the loop: the wheels tended to steer into the impending skid and would self-correct. You could theoretically let go of the steering wheel and the car would skid in the same forward direction, albeit diagonally. (audi now has a complex multitink steering mechanism, which could have a different behavior under these conditions)

benz took the zero-offset route, which gave no driver feedback with respect to steering. So you could imagine a benz driver locking the tires with no steering wheel feedback to indicate that the car was about to spin. This seemed strange to me UNTIL i realized that benz was a forerunner in ABS technology, which obviated the need for such feedback/response. However, this needs to be complemented with yaw control.

my question is this: does benz still adopt this steering geometry, in light of the different traction control mechanisms it uses for braking and yaw control?

The advances in traction control would normally change a manufacturer's approach to steering offset. On (impending) oversteer, the outside front tire is braked to reduce the oversteer. Positive steering offset would steer the tire in the direction of the skid, helping correct the skid; negative offset would exacerbate the skid. Zero-offset steering would result in zero steering torque on the braked tire, and everything is transparent. If modern benz cars still use zero-offset steering, then benz must have been right all along going for zero offset.

knowledge about this would give people hints about the steering effects of the inevitable offset change when one upgrades to wider wheels.

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