Bumpsteer is one of the most basic geometry functions in the frontend of the race car. The
The elimination of bumpsteer is one of the basic elements of proper front-end geometry, but we have rarely discussed it. I think it's because it is so basic and we assume that everyone understands it and takes care of it early on. There's that "assume" word again. Maybe it's time to take a detailed look at B/S, explaining the basics of the process and get into advanced concepts of B/S. This discussion is both basic and advanced, so don't stop reading just yet all you veterans.
In the past we have provided information about all of the alignment aspects including toe-front and rear, rearend alignment and rear steer, Ackermann, and front-to-rear alignment. Every team I have ever worked with either did the bumpsteer itself or had someone come in and do it at least once in a particular car's life. We know now that is not enough.
Certain setups have evolved to where your bumpsteer might not be what you think it is. When you make changes to your antidive or moment center, you may be changing your bumpsteer characteristics. So, when trying a new setup such as the BBSS asphalt setups, you might be introducing B/S and not know it. The greater amount of travel associated with the BBSS setups may be out of the range you last checked your B/S for. As a result, the car may begin to behave erratically and cause the driver to be uncomfortable with the new setups and the culprit might be a problem with the steering.
We will use the term, near zero bumpsteer simply because most cars will never be able to have absolute zero bumpsteer. If you can get your bump to less than 0.010-inch of bump per inch of vertical travel, you will have a good design where the driver will not feel the movement. That is a little less than 1/64-inch of toe movement. If you are inclined to want a small amount of bump-in some direction for one or both front wheels, go for it, but we don't recommend any amount of bump.
Basics of Bumpsteer As the front wheels move up and down, we want the front wheels to maintain a particular direction. It's most important for the wheels to have minimal bump when we are negotiating the turns. There are certain elements of the construction of the front end components that will make this happen.
The angles of the upper and lower control arms, meaning a line extending through the center of rotation of the ball joints and inner mounts of each arm, intersect at a point we call the instant center (IC). This is one of the components used to determine the moment center location. In order to have near zero bumpsteer, the intended goal, we need to have the tie rods on each side point toward the IC for its side. This is one of two criteria for near zero B/S.
The other thing we need is for the tie rod to be a specific length. That length must be equal to the distance formed by 1) a line extending through the centers of rotation of the tie-rod ends, and 2) the tie-rod line intersection with a) lines extending through both the upper and lower ball joints, and b) the plane that passes through the inner chassis mounts. This can get a little complicated because although the ball joints do form a single line, the chassis mounts form a plane because of the front and rear mounts.
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 have the luxury of owning and knowing how to operate one of those. If so, we must go through the process of physically measuring the B/S in our cars.
In order for your car to have near zero bumpsteer, two conditions need to exist. The tie r
The tie rod length requirement does not mean the tie rod needs to be positioned laterally
If the tie rod is aligned pointing above the instant center, the wheel will bump-out when
If the tie rod is aligned where it points below the instant center, then the wheel will bu
What Creates Bumpsteer When the tie rod is not aligned with the IC and/or the length is wrong for the system, we have B/S. 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.to
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.
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 for several inches of travel 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.
With antidive, as the wheel moves up, the upper ball joint moves rearward. This causes the
Design Changes That Affect Bumpsteer We could buy a car that was near perfect for B/S and then make design changes that would change our B/S. When racers and manufacturers started to use spindles that were designed for rack-and-pinion systems in a drag link system some years ago, they inadvertently changed the bumpsteer characteristics, along with the Ackermann geometry.
With the drag link system, the outer tie-rod end was closer to the centerline of the car than the lower ball joint, due to the angled (from a top view) steering arms. This design feature cancelled out the natural tendency for the system to toe the wheels out as the car was steered.
When the new, lighter "rack" spindles were installed, with straight ahead steering arms, the length of the tie rods changed necessarily. Now we have created bump-out as the wheel travels up and down. New drag links with the inner tie-rod ends placed farther out were needed so that the tie rod would remain the correct length to eliminate the adverse B/S those spindles created.
If we make changes to the frontend geometry to improve our moment center location, we might change the B/S characteristics at the same time. This is important and we should have warned you about that in past articles on the subject.
For example, when we install extended lower ball joints to take angle out of the lower control arms, we change the angle of the lower arm and move the IC height. The tie rod may not now intersect with the IC and we will have introduced adverse B/S. When making changes to the arm angles, we need to realign the tie rod so that it stays pointed toward the IC.
Antis Affect BumpSteer In both dirt and asphalt racing, anti- and pro-dive is used in various degrees. These effects cause changes to our B/S. With antidive, this is because when the wheel travels up, the upper ball joint moves toward the rear of the car and this rotates the spindle counterclockwise from a right-side view. 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 B/S before with no antidive, we now have B/S when the right front wheel travels up. With pro-dive, we see a similar effect, the tie-rod end moves down with vertical travel and again the tie rod is misaligned with the IC. If you originally checked your B/S and found it acceptable and then experimented with antis, and didn't recheck your B/S, you could, and probably do, have a problem, not statically, but dynamically in the mid-turn configuration.
Steering Affects BumpSteer 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 is not parallel to the ground. This changes the outer tie-rod height and therefore the B/S. It's for this reason that we recommend doing your B/S with the wheels both straight ahead and then again with the wheels turned equal to mid-turn steering at the track you will run.
I have had car builders tell me proudly that their cars had near zero bumpsteer. I asked them what it was in the turns and I got a puzzled look. In past years, I used the Mitchell program for studying the frontend geometry, including steering. Bill Mitchell originally wrote that for Ford and it's considered one of the better three-dimensional programs. Performance Trends has one too.
With a three-dimensional program, I could do a B/S analysis with the car at ride height, or any other attitude including with varying degrees of dive and roll. I could roll the car and lower it to simulate the mid-turn attitude and then bump the wheels to see if the steering angle changed. I did a few late '90s Cup cars and Craftsman trucks and was amazed at how perfect the systems were. I got near zero B/S no matter what attitude I put the car in. You can simulate this in the shop too by moving the chassis and steering the wheels before you check the bump.
The most common way to measure bumpsteer is to support the car on jackstands and using a j
Measuring BumpSteer, Some Tips We can measure our B/S using several different types of equipment. There is the double-dial caliper system, the single-dial system, and the laser system. Each one will tell us if the wheels steer when they are in bump or rebound.
BumpSteer Gauge The most common tool is the bumpsteer gauge. It consists of a plate bolted to the hub and a stand that holds either one or two dial indicators. It comes in two configurations, the double-dial indicator type with a stationary stand and the single-dial type with a swinging stand. With the latter, when the wheel moves vertically, the stand follows the plate. With the double-dial type, the two dial pins are always moving one way or the other. If the system has zero B/S, then both dials will move together the same amount.
If the front dial moves farther as the wheel moves, then we have bump-in at that wheel. If it goes less, then we have bump-out. Be sure to count the number of turns each dial makes when moving the spindle vertically. Subtract the readings to find the B/S amount related to the distance the wheel has moved. We usually refer to B/S as decimal inches of bump per whole inches of travel.
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 the dial indicators is much less than the tire diameter we use
Gauge Ratio 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