Modified Mustangs & FordsHow To Chassis Suspension
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.