Using a laser system, you...
Using a laser system, you need to place targets in front of and to the rear of the wheel at equal spacing. Preferably, measure the tire diameter and multiply by 3, 4, or 5 so your multiplier used to divide into the readings is an even number. Example: for a 28 inch diameter tire, go 84, 112, or 140 inches out to each target. The farther you go, the more accurate the readings will be.
The single dial indicator gauge is a little different and one I personally like. Using a swing stand, it has one dial that rides on one side of the plate and a roller that rides on the other side. As the wheel moves vertically, the stand follows it in and out. If the wheel has zero B/S, then the roller and the dial shaft will move together the same amount and the dial will not change its reading.
If the dial does move, it's recording the total amount of bumpsteer, unlike some who say it only records half the bump. Since the roller is stationary, the dial records the movement between itself and the dial, or the total distance between them, whereas with the double dial, each dial records half the B/S and you subtract to find the total.
With the laser systems, the laser is mounted on the hub or wheel and we use targets placed ahead and behind the wheel center the same distance. This way, any difference in movement of the laser on the two targets, as the wheel moves vertically, will be divided by the distance to the target from the center of the wheel divided by the diameter of the tire. So, if a tire where 28 inches in diameter (88 inch tire), and the targets were 112 inches away, we would divide the difference in movement of the laser front-to-rear by four. If the wheel were bumping 0.030 inch, the differential readings on the targets would be 0.120 inch or about an eighth of an inch.
We can see how the width of...
We can see how the width of the dial indicators is much less than the tire diameter we use for measuring toe. To translate the numbers for bump into toe, we need to multiply the results by the ratio of the gauge to tire diameter. For example, 0.030 inch of bump could be as much as 0.050 inch of toe.
It's important to consider that most bumpsteer gauges are not the width of the tire diameter for practical purposes. So, we must translate the readings from the width of the tool to the tire diameter if we want to have the bump equal toe. This is easy, just divide the width between the dials, or the dial and the roller depending on the type, into the tire diameter and use that number to multiply times the bump reading to see what the bump is in toe equivalent, which is something we all readily understand.
Conclusion Once we understand all of the things that affect bumpsteer, we will know when we need to re-measure the car so we can maintain near zero bumpsteer. If you make moment center changes, antis changes or spindle changes, or even change setups, re-measure your B/S. If you have only measured bump at static ride height with the wheel pointed straight ahead, maybe it's time to re-measure the bump at mid-turn configuration. It could make a difference in how the car feels to the driver as the car moves vertically on corner entry and when going over those ruts on a rough dirt track. CT