[MNBoxster]

@SoCal,

Hi,

It's not unusual for someone to have never heard of the scrub radius because it is essentially a fixed (not readily adjustable) imaginary point. This point is fixed by the configuration of the suspension geometry and will not change unless a piece of the suspension breaks and/or is replaced by an aftermarket piece, or by a collision or by altering the Wheel Offset.

The point at which the SAI contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. A true plumb line, placed at the center of the wheel at the point of road contact, and the projected line created by the strut, or the upper and lower ball joints, determine SAI (Steering Axis Inclination). The two lines will intersect at a point just below the road surface on most vehicles. On some front-wheel-drive vehicles the point of intersection will be just above the road surface (this helps to reduce 'Torque Steer'). The distance between the projected line and the vertical line at the road surface is called the scrub radius.

If the scrub radius is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the pivot point. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. You must also take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero, or a positive one to become larger.

Ideally, the scrub radius should be as small as possible. Normally, the SAI angle, the centerline of the tire and the wheel intersect below the road surface, causing a positive scrub radius.

With struts, the SAI angle is much larger than the long-arm/short-arm type of suspension (unequal length). This allows the SAI angle to intersect the camber angle above the road surface, forming a negative scrub radius.

The smaller the scrub radius, the better the directional stability. Installing aftermarket wheels that have additional offset will dramatically increase the scrub radius. The newly installed wheels may cause the centerline of the tires to move further away from the spindle. This will increase the scrub radius. A large amount of scrub radius can cause severe shimmy after hitting a bump and/or impede the return-to-center action of the steering after a turn.

Four-wheel-drive vehicles with large tires typically use a steering damper to compensate for an increased scrub radius. Scrub radius is not directly measurable by the conventional methods and so care must be taken when changing any of the things which affect Scrub Radius.

Whether a 7mm increase to Offset will be unmanagable I cannot say. But, if you get any shimmying or twitchiness, this is most likely the cause. I'd say to proceed, but not to sell off the Factory Wheels until you're satisfied that the increase in Scrub Radius by the '02 Wheels (and it will increase) is to your liking. Hope this helps...

Happy Motoring!...Jim'99