As for why there should be a toe-in, we should start from the beginning.
1. First of all, in order to automatically keep the steering wheel held by the driver in the center position and reduce the driver's fatigue, it is necessary to tilt the main shaft backwards. It's like the front fork of a bicycle leaning backwards, which can relax your hands.
2. However, after the main shaft leans backward, the front wheel will swing at high speed, which will lead to the instability of the car body. In order to overcome the small swing of the front wheel, it is necessary to turn the kingpin outward. The slower the vehicle speed, the more uneven the road surface, and the greater the outward angle of the kingpin, such as a four-wheeled tractor. Because the camber directions of the two front wheels are opposite, the small swing of the front wheels cancels each other, and the car body is more stable.
3. However, due to the outward inclination of the kingpin and the outward inclination of the front wheel, the front wheel tends to turn outwards, that is, the left front wheel turns left and the right front wheel turns right, and the front wheel is installed on the vehicle and cannot turn, resulting in serious tire wear. Toe is to solve this wear problem. The wheels on both sides leave an angle inward, which just offsets the tire deviation caused by the camber of the kingpin, so that the tire can go straight ahead without dry grinding, thus achieving the purpose of reducing tire wear.
This is the origin of toe-in, which started from reducing the driver's labor intensity and developed from one ring to another. The spindle tilts backward-the kingpin tilts outward-toe-in, and a ring is buckled.