That's to prevent bump steering.
Coming from 1:1 racing I only had the text for a full size car available:
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Definition
Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages.
Most car builders design their cars so that the effects of bump steer are minimal. However, you must still take care to bolt on your suspension carefully so as not to create unwanted bump steer. Make sure that you are always using the correct components for a particular car. Bump steer must be designed into the car and cannot be adjusted out if improper parts are used or if pivot points are moved without considering bump steer design principles.
In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm (See diagram below).
The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.
Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.
To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.
Bump Simplified - When designing a car, if the centerline of the outer tie rod lines up with the centerline of the lower ball joint, and the inter tie rod lines up with the lower pivot point then the length and angle of the tie rod and suspension will be the same resulting in zero bump. Most car builders design their cars in this fashion.
Corrections
Symptom Cure
Symptom 1. Toes out in compression and in on rebound all in one direction.
Cure 1. Decrease shim on outer tie rod or lower the inner tie rod.
Symptom 2. Toes in on compression and out in rebound all in one direction.
Cure 2. More shim at outer tie rod or raise the inner tie rod.
Symptom 3. Always toes in both compression and rebound. Cure 3. Lengthen the tie rod as it is too short.
Symptom 4. Always toes out on compression and rebound.
Cure 4. Shorten tie rod as it is too long.
Symptom 5. Toes out on compression, then in on rebound and then starts back towards out with more rebound travel.
Cure 5. Less shim at outer tie rod and shorten tie rod.
Symptom 6. Toes in on compression, then moves out on rebound and then starts back towards in with more rebound travel.
Cure 6. More shim at outer tie rod and lengthen tie rod.
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This is the 1:1 theory
