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Just got my 66 aligned and that got me to thinking (always dangerous).
Why do alignment specs require toe in? You would think they should be pointed straight down the road.
Also if someone could explain why radials require less toe in that the old bias ply that would be great.
Toe in takes the inherent slack or slop out of the linkage. It causes slight pressure on all joints. A very tight suspension will take very little toe to "feel ok", but one that is worn will be a "b....h" to drive without any. I assume that radials need less because of the movement of the sidewalls somewhat negate the effect of toe.
The drag force on the front wheel tends to push them to the rear, which alters toe, and static toe-in is usually set up so toe approaches zero with increasing drag force.
Radials need less toe-in that bias plys because they have longer (lengthwise along the car) contact patches than similar dimension bias ply tires, and radials have inherently more self-aligning torque. This combines with toe to place the tie rods in tension or compression, which removes jount clearances and makes the steering feel snug.
FWD cars typically require a bit of toe out because the drive thrust forces the tire forward instead of backward. Whether the wheel toes in or out when forced forward or backward with the prevailing normal drive or drag force is also a function of the layout of the suspension arms and steering linkage, which in plan view is a parallelogram.
Toe-in, like ignition timing is a chassis tuning parameter, and it affects transient response. Toe in usually promotes transient understeer and toe-out transient oversteer, so, for example, if you autocross a car with OE specified toe-in, it will probably turn-in better is you run a little toe-out.
Read an article on it...also dangerous. The article stated a couple of things, one being that under load the front suspension start to pull back a bit and some of the toe-in gets negated. It also said it provided straight line stability or tracking. I believe the example they gave was that if one of the front tires hits something and deflects right or left, at least one of the tires would be pointing forward, ie crack in the cement. Makes it less likely to grab the crack. It also takes the slack out of the steering linkage when driving in a straight line. I believe radial tires have less toe in because they have a smaller foot print as opposed to a bias ply tires that stood up and had a smaller foot print. That is sort of how I recall it. There are people on the board that know this stuff hands down though. I am not the expert and I easily stand to be corrected.
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