Continuing Education

Porsche Technical related discussions.

Continuing Education

Postby David J Marguglio on Wed Sep 08, 2004 10:57 am

Perhaps it would be a cool feature to have a different topic or concept explained weekly (or monthly) in this section. We could begin to utilize the vast knowledge and resources of many of our membership to the unwashed masses such as myself.

I propose the first topic: scrub radius. The textbook definition is: "The distance between the extended centerline of the steering axis and the centerline of the tire where the tread contacts the road. If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative. Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher SAI angle. Using wheels with different offset than stock can alter the scrub radius."

Now what the hell does that mean??? :?
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Postby Kim Crosser on Wed Sep 08, 2004 11:35 am

Draw a line through the ball joints of the front suspension to where the line intersects the ground. If the line intersects the ground inside (inboard of) the point where the center of the tire rests on the ground, then you have a positive scrub radius. If the line hits the ground outside the center of the tire patch, you have a negative scrub radius. If the line and the center of the tire patch intersect at the ground, you have zero scrub radius.
Adding wheel spacers to push the tires out farther would increase (make more positive) the scrub radius.
At zero scrub radius, as you turn the steering, the wheel will pivot around the point where the steering line and the center of the tire patch intersect - if you put a dime under the tire there, turning the steering should simply rotate around the dime.
With + or - scrub radius, as you turn the steering, the wheel will roll forward or backward while turning. With positive scrub radius, as you turn left, the right front wheel will rotate around the steering axis in a counter-clockwise direction (it would roll forward off the dime).

Of course, the real question is what is desirable. For karting, a large positive scrub radius is good, but what is desirable for a (your type here) Porsche?
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Postby David J Marguglio on Wed Sep 08, 2004 11:45 am

So this is why old Mercedes look like the wheels move like 3-4 degrees of camber (neg) when you turn? Is SR unique to the front? My buddy, who I was having the discussion with, seemed to feel that it also incorporated the rear. Spacers aside, you would need to modify the geometry of your suspension through adjustable tie rods or trick A-arms to change your scrub radius?
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Postby Kim Crosser on Wed Sep 08, 2004 12:22 pm

I think SR would only apply to the front, except for those odd cars that have steerable rear wheels, since it relates to the steering axis.

By the way, my simplistic analysis above assumed you had a vertical kingpin/ball joint axis. As you adjust the caster (front-rear alignment of the kingpin/ball joint) that moves the steering axis intersection point fore and aft, so with a typical positive caster even with zero scrub radius you can get some tire rotation.

Changing the camber would change the scrub radius. If you increase the camber, the tire patch is moving inwards, while the steering axis is moving outwards, so the scrub radius is becoming more negative.
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