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Old 05-01-2003, 10:41 PM
Duke2.6 Duke2.6 is offline
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Join Date: Mar 2003
Location: Southern California
Posts: 2,055
Tire size has no effect on alignment measurements other than toe-in if measured between between the OD of the tires, but most toe alignment now is done in degrees, which is independent of tire OD. The OD of the tire in combination with alignment settings establish the "scrub radius" and "trail" both of which can affect steering feel.

Draw an imaginary line extending the steering axis to the ground. The scrub radius is the horizontal distance between the point the steering axis intersects the ground and the center of the tire contact patch. The longitudinal distance between these two points is the trail.

Caster has a big impact on trail. The more caster the more trail, and the more trail the more the wheels will want to stay on center. Lots of caster and the resulting trail is what gives Mercs their good on-center steering feel. The inclination of the steering axis when viewed from the front is called "steering axis inclination" and it affects scrub radius. The steering axis inclination on most cars is not adjustable, but changing camber has minor affects on the steering axis inclination.

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