|Caster, Camber, Toe, Part III: TOE
This alignment characteristic refers to the parallelism of the tires. Are the tire patches parallel, keeping the car straight as it rolls forward, or are they piegon-toed -- "toe-in" or splayed -- "toe-out"? Considered independently (that is, separate from camber and caster), toe-out makes a car "quicker" in cornering because it reinforces the turning of the tire. When the left tire is already "turned" left and the right tire is turned right, when you turn the car left, you're continuing in the direction the tire already wants to go, and you're moving the right tire away from opposition and toward the corner. Same thing when you turn right. When the tires are toe-in, you're scrubbing the tire to which weight has transferred against the direction it is headed. That's the basic situation. What happens relative to steering angles and whether the suspension is front steer (the steering arm PUSHES the front of the wheel out on the side of your turn and pulls the front of the other wheel toward the turn) or rear steer (suspension pulls the back of the wheel in and pushes the back of the opposing wheel out to cause the turn) is where the effect of toe must be considered. "Bump steer" is a function of toe. Toe is adjusted at the center tie-rod, acting as a turnbuckle on the left and right tie rods, the arc of the steering arm becomes shorter than the arc of the A-arm. So when you adjust to toe-out, the rolling of the car in a corner or over a bump will cause the car to steer more. Alternatively, if you're finding the car understeers, your problem may be too much toe-in.
So far this discussion has focused on the front suspension. If you have too much toe-in in the rear, you'll get terrific understeer. Because the geometry produces a zero vector -- the tires are literally working at cross-purposes -- the car just will not turn. Toe-out in the rear makes for terrific oversteer. You only put toe-out in the rear if you're trying to correct a monster understeer problem. Lastly, we find people don't think enough about roll toe -- ie what happens when the weight transfers as the car rolls into a corner. Roll steer isn't bump steer; it's created by the Ackerman. Ackerman, named for the man who defined it, considers the fact that the radius of the inner edges of the wheels/tires is different than the outside edges. Ackerman describes the fact that the inside turns more because it's traveling a shorter distance and the short side of the steering arm shortens up before the longer side. Ackerman is defined by the angle from the steering ball joint back to the differential. It's a concept that falls in and out of fashion in street car suspension set-up. Race cars don't consider Ackerman simply because it adds another zero before the decimal point in terms of the complexity. Ackerman really doesn't make any difference in how well the steering works -- it's just a different way to look at suspension set-up and to recognize that you must look at suspension as a dynamic system, not just what it looks like on the garage floor.
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