The amount of toe-in dialed in depends on the diameter of the tire, as manual alignment gauges work by measuring the distance between the front of the tires across the car; and obviously larger tires will appear closer together than low-profile tires when angled slightly toward each other, despite the actual angle being the same. Therefore, around 30 minutes, or a half-degree, should be dialed in when using smaller-diameter tires, and 20 minutes with larger tires. The actual adjustment is made by turning the tie-rod ends or steering rack in or out until the gauges read correctly.
BumpsteerBumpsteer is closely associated with toe-in, and is common on cars that have been lowered, especially in situations where a steering rack has been added in a non-stock location. Imagine an independent front suspension moving up and down as it does every time it traverses a bump. It will move through an arc as the lower ball joint pivots around the lower control arm's inner mounting point on the chassis.
A farther arc is defined by the outer tie-rod end or spherical rod end on the steering arm, pivoting about the ball joint inside the steering rack. Where these arcs differ, the tie rod will either pull the wheel in or push the wheel out when the suspension moves up or down. Hitting a bump causes a change in the steering, hence the term bumpsteer, which can make for some unpleasant handling characteristics. The greater the variation in the two arcs , the more severe the problem, as toe-in changes during suspension travel, causing the car to wander violently with no input from the driver.
Factory suspensions and most aftermarket IFS systems will have all but the most mild bumpsteer engineered out from the outset, though lowering a car can increase the likelihood of its occurrence as the suspension travel arc is increased, or at least will start higher than it was originally designed to. If you're swapping front clips or adding a steering rack where previously there was a box, there are a number of points to observe that will help to eliminate bumpsteer.
First, ensure the lower control arms and steering arms are parallel when the car is at ride height. Horizontal would be preferable but not as important as them being parallel, as they cannot move in the same arc if they are not parallel to start with. Second, the pivot point of the control arm and the ball joint pivot inside the steering rack must be the same distance from the chassis centerline. Again, this ensures similar arcs will be attained.
One area we haven't mentioned is bumpsteer on rods with steering boxes and beam axles. Obviously such cars don't have control arms or steering racks, but the principle is the same. When such a car has a steering box mounted on the chassis with a Pitman arm and drag link, the latter should be parallel to, and the same length as, the axle locator, be it a split wishbone, radius rod, or hairpin. Any discrepancy in these lengths will cause bumpsteer, as the arcs in which the axle and drag link travel are different. If the steering box is mounted farther forward than the rear mount of the axle locator, you can avoid a lot of bumpsteer by ensuring the drag link is located at such an angle that an imaginary line along the drag link continues past it and intersects the axle locator at its pivot point. Of course this applies more to traditional rods, as those with cross steering, such as Vega systems, won't have the geometry that causes bumpsteer in the first place. Cowl-mounted steering boxes, however, should also be mentioned since the geometry can cause horrendous bumpsteer if there is a lot of suspension travel, though of course most rods have a minimum of travel anyway. Again, there's that form-over-function scenario. Of the four alignment issues so far discussed, bumpsteer is the hardest to rectify because it invariably will require some form of fabrication and movement of pivot points.