Modified Mustangs & FordsHow To Chassis Suspension
Suspension Tech - Bumpsteer For Dummies
What It Is and What You Can Do to Minimize It.
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|