If the tie rod is aligned...
If the tie rod is aligned where it points below the instant center, then the wheel will bump in when traveling up and bump out when traveling down due to the difference in arc path to the ball joints.
So, the inner tie-rod intersection point is where the tie-rod line intersects the plane of the inner mounts and the outer line intersection point is where it intersects the ball joint line. A three dimensional geometry program can simulate this very well, but most of us don't own or know how to operate one of those or even have the patience to learn all of that. So, we must go through the process of physically measuring the BS in our cars.
When the tie rod isn't aligned with the IC and/or the length is wrong for the system, we have BS. As the wheel moves vertically, the wheel will either steer left or right. We will refer to the direction from a driver's perspective only in this discussion.
If the tie rod was pointed so the tie-rod line passes below the IC, then the wheel will bump in (toward the centerline of the car) as the wheel travels up and bump out when the wheel travels down. If the tie-rod line passes over the IC, then we will have bump out as the wheel travels up and bump in when the wheel travels down.
If the tie rod were too short, we would have bumpsteer in when the wheel travels in both directions from the static ride height position. If it were too long, then the wheel would bump out as the wheel traveled in both directions from ride height.
With antidive, as the wheel...
With antidive, as the wheel moves up, the upper ball joint moves rearward. This causes the spindle to rotate and with it the steering arm. The outer tie-rod end will move up changing the front view tie rod angle. This changes the alignment with the instant center and will change the bumpsteer characteristics.
These indicators can tell us if we have either a tie-rod alignment problem or a tie-rod length problem. In some cases, both may be present and that causes a very erratic motion of the wheel. To determine which, record each inch of travel, for several inches, in both directions from static ride height and note the tendencies. You might have perfect alignment and a tie rod that is wrong for length. This could be due to a poorly designed drag link or the wrong width rack-and-pinion steering unit.
In both dirt and asphalt racing, anti- and pro-dive are used in various degrees. These effects cause changes to our BS. This is because with antidive, for example, when the wheel travels up, the upper ball joint moves toward the rear of the car and this rotates the spindle, from a right side view, counterclockwise. This rotation moves the outer tie rod end upward and changes the angle of the tie rod. Now, it no longer points toward the IC.
Where we had near zero BS before with no antidive, we now have BS when the right front wheel travels up. With pro-dive, we see a similar affect, the tie-rod end moves down with vertical travel and again the tie rod is misaligned with the IC.
If you originally checked your BS and found it acceptable and then experimented with antis, and didn't recheck your BS, you could, and probably do, have a problem, not statically, but dynamically in the mid-turn configuration.
When we steer our front wheels, we change the angles of our tie rods due to caster, camber, and degree of spindle on both sides. The tie-rod ends travel in an arc that isn't parallel to the ground. This changes the outer tie rod height and therefore the BS. It's for this reason that we recommend doing your BS with the wheels both straight ahead and then again with the wheels turned and bumped, equal to the mid-turn attitude for the track you will run.
What's Up Next?
In our next and final installment of this series, we'll examine toe settings and Ackermann, and rearend alignment and design for rear steer. We'll also touch on tire stagger and tire pressures as the final steps in completing our race car setup.