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Suspension Tech - Bumpsteer For Dummies
What It Is and What You Can Do to Minimize It.
In the car hobby, bumpsteer is one of those evil words that gets blamed for everything that happens with the handling of your vehicle, and even some things that have nothing to do with it. This is due mainly to the fact that most people don't understand what bumpsteer is. For this reason, we present this article to try in basic terms to describe what bumpsteer is, what it is doing to your car, and ways to correct it.
We'll say this up front-in searching the Internet and researching books on the subject, we found that a lot of people who sell products to correct bumpsteer aren't really sure what it is either. They all seem to have differing ideas on the subject. We will also say that this is not intended to be a full course in suspension engineering, but merely a primer to help you understand what the handling gurus are trying to tell you.
Here are two things you need to know:
- Bumpsteer is the unwanted change in toe of the front wheels while the suspension is traveling up (bump) and down (droop).
- If you have a classic Ford with a stock suspension, you have bumpsteer. Simply put, when the suspension on your vehicle moves up and down, even when going straight, the wheels unwillingly move outward (toe-out) or inward (toe-in) due to the design of the suspension. This can cause erratic and unstable handling of the car.
Too much bumpsteer causes the vehicle to become erratic. This is felt more on performance vehicles that frequently push the extent of their suspension travel. You will probably never feel it in most grocery getters.
To understand how bumpsteer occurs, let's review some basic suspension information. Most classic Ford suspensions are some form of unequal-length double-A-arm system. The two control arms are mounted to the body and rotate up and down at the pivot point where it connects to the body. If you look at Figure 2, you can see how a control arm or tie rod moves through its rotation. If you remember your high school geometry, you know that as something moves through the arc, the X or horizontal axis changes as well.
In a classic Ford suspension system, there are three parts moving around a different axis. The upper and lower control arms work together and are limited by the spindle. The tie rod holds on to the end of the spindle and is designed to push or pull the wheel to turn the car.
This is what happens when the suspension moves up in its rotation:
When the suspension moves through its travel, the two control arms and the tie rod move in an arc. If the tie rod is not the correct length and in the right position, it can pull or push the spindle even without turning the steering wheel. In a typical Mustang setup, the tie rod pulls the spindle inward, toeing out the front wheel.
Obviously, in any kind of performance situation, bumpsteer-or the unwanted change in direction of the wheels-should be minimized. If you have bumpsteer, it is preferred to have it toe-out rather than toe-in.
Designing a suspension system for minimum bumpsteer is not difficult, but removing it from a classic Ford can be. Let's look at a well designed front-suspension geometry. The suspension in Figure 5 allows you to plot a couple of imaginary lines through the upper and lower control arms to an imaginary point to the inside of the vehicle. This is called the active center (or roll center or bump center or instant center, depending on which book you're reading). It's a point in space where the two imaginary lines meet, and its position moves as the body moves.
A typical Mustang suspension geometry looks like this at normal ride height. These are the approximate angles, lengths, and locations of the control arms on a typical late-'60s Ford. Take a moment to try to calculate where the active center of this suspension might be. If you answered left of Pluto, you are a genius.
Now you know why suspension engineers from the '60s to today have been pulling their hair out trying to improve the handling on our classic vehicles. It's just not set up for superb handling.
Bumpsteer is effected by four different things:
- Tie-rod angle.
If the tie-rod angle is not correct, the tie rod can travel in an arc different from that of the upper and lower control arms (as in figure 4).
- Tie-rod length.
If the tie rod is too short, it will have a more severe arc. If it is too long, it can have the opposite effect and not arc enough, causing a toe-in condition.
Camber is the amount of lean-in or out the wheel has in relation to the ground. If you lean the tire in or out, you set the spindle at an angle, and its rotation changes with the turn of the steering wheel. For example, if you have lots of camber in the front wheel (the tire leans in at the top) and you push the tie rod outward, it's going uphill and the angle of the tie rod changes.
- Steering-arm pivot
The spindle rotates around the upper and lower ball joints, and we have another arc. As the tie rod is moving through the arc of the steering arm and going up and down, it can move more or less than the ball joints do.
There is a lot more to designing a front suspension than the above, but the simple fact is that to eliminate bumpsteer, the tie rod needs to be designed to where its length falls along an imaginary line through the two ball joints (line A, figure 5) and the two control-arm pivot points (line B). In addition, the tie-rod angle needs to run its own imaginary line through the active center created by the two control arms. If you do this, you will minimize bumpsteer and unwanted toe change.
If you want, you can check bumpsteer without a lot of expensive tools. It's not terribly accurate, but you can do it with a creeper. To get an accurate reading, however, you need a bumpsteer gauge, which is available from several Mustang and race vendors.
Using a '70 Mustang convertible for our test, here's how to check your vehicle for bumpsteer.
- Make sure the car is aligned properly before checking bumpsteer. Toe-in, caster, and camber need to be set before checking bumpsteer.
- Raise and level the vehicle.
- Remove the spring and shock from the wheel that is to be checked.
- Most Ford shop manuals list the ride-height specification for your vehicle and how it is measured. On our Mustang, the ride height is measured from the bottom of the outer shock tower near the bumper to the top of the upper control arm near the ball-joint rivet. We made a small gauge out of a piece of steel to set the height.
- Unhook the front sway bar from the lower control arm.
Using a Creeper to Check Bumpsteer:
- Install the front wheel. Set the ride height, and place the creeper against the outer edges of the tire. Make sure the creeper is vertical, and place your foot at the base in the center for support.
- Use a floor jack and raise the control arm. As you raise the suspension, the front of the tire pushes out the front edge of the creeper, and you should see a gap at the back of the tire. This is bumpsteer.
- You can measure this gap at the back for a rough estimate of bumpsteer.
Using a Bumpsteer Gauge:
Using a proper bumpsteer gauge performs the same function as the creeper but uses two 0.001-inch dial indicators for accurate measurement. As the wheel bumps out, it pushes on the front gauge and lets off the rear gauge. The difference between the two measurements is the amount of bumpsteer in the system.
|Inches of Toe|
So How Much Bumpsteer is too Much?
This seems to be the biggest variance when asking the "experts." Some of what is acceptable depends on what you plan to do with the car. If you drive it to car shows on nice days and never see a hard turn, you probably don't have to do a thing. Occasional carbon blowout runs call for a nice set of aftermarket bumpsteer shims. Where you go from there depends on how serious you will be driving the car. Serious racers should be looking at completely revised suspensions altogether, and there are several designers out there working on it. If you're planning on 200-mph blasts, it's mandatory that you eliminate bumpsteer from the system.
For most race cars, the numbers we found vary from 0.015 inch of bump to 0.050 inch. Most say around 11/432 of an inch (0.03125) is livable. At right are the numbers we pulled from our check of the '70 Mustang.
Lowering the Outer Tie Rod
So what about the bumpsteer correction kits available on the market? Let's look at how they work. The "correction" kits typically don't "correct" anything because the geometry on these cars is just plain wrong. What they do is get you further away from the problem. Most correction kits move the tie rod to spindle mount down about an inch, but leave the inner pivot point alone. This moves the problem further away under bump conditions (in some cases, it may actually make the droop condition a little worse, but suspension compression is a bigger problem).
As you can see by the example in Figure 8, lowering the outer pivot point can reduce the amount of bump by keeping the arc of the tie rod out toward its farthest reach. Moving it too far down can cause a new set of problems. Since the tie-rod length is incorrect, you can actually cause a bigger problem in droop if you make it too low. An inch is about right to improve the bumpsteer conditions of the classic Mustang. Racers typically have a complete set of shims to fine-tune their suspensions.
We wanted to see if dropping the tie rod would make any difference in the bumpsteer characteristics of the suspension. Since we didn't have a kit, we shimmed the tie rod 11/42 inch with some washers and remeasured the bumpsteer.
Dropping the tie rod makes a huge difference in the bumpsteer of a stock Mustang. We dropped the tie rod only 11/42 inch-most aftermarket kits drop it about an inch. Although the graph still shows tie-rod placement and angle issues, this is close to what most racers consider acceptable on mild performance cars.
We hope this simplified explanation helps you to better understand bumpsteer and the effect it has on your Ford. You can now look at the aftermarket goodies available for your Ford from an informed viewpoint and select the products that best take care of your needs. You will be amazed that with a little effort you can make your classic Ford handle more like the new car you probably drive every day.
|Inches of Toe|