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Make Driving Fun! Facts, Fiction, and the Myth of the Mustang's Bumpsteer Problem

Topic #9 from our series.

Read on for more expert advice on transforming your Mustang into a high-performance handling machine. This week's topic is all about bumpsteer, a phenomenon that has frustrated drivers since there were fast cars. (If you missed the previous episodes, they're posted on our website in the FAQs & Tech Tips section.)

There are few subjects in the world of Mustangs that are so misunderstood and so plagued by misinformation than the topic of bumpsteer. Here we'll present the technical facts about bumpsteer and how it affects your life. We'll also bust a widespread myth about lowering and bumpsteer.

What is bumpsteer?
Bumpsteer is the term for when the tires steer themselves because the toe setting changes as the suspension moves up and down. This phenomenon is often confused with tramlining or rut wander, which the tires themselves cause.

What that means for your Mustang: if it has excessive bumpsteer, every time you hit a bump or dip in the road and during anything but the most conservative cornering, the vertical suspension movement causes your tires' toe setting to change. That causes unplanned, sometimes dramatic directional changes, which means you're not in charge of where your car is going! This is always bad.

How much bumpsteer is too much?
For race cars, the basic rule of thumb is to keep any toe change to less than .020" for every inch of suspension travel. Less is better, but not always achievable.

Maximum Motorsports has measured bumpsteer on numerous Mustangs. We found that Ford's engineers did a good job in designing a suspension with an acceptably small amount of bumpsteer—at least for a Mustang used as a "transportation appliance," i.e., a regular car driven daily by a regular person. We typically measured around .040" of toe-out when the front suspension was compressed 1". Bumpsteer during suspension travel in compression (bump) is more important than in extension (droop). Minimizing toe-change close to ride height is more important than at the extremes of bump or droop travel.

Why didn't Ford design in less bumpsteer?
For the front suspension, toe-out in bump tends to make the car understeer when cornering. Understeer means the car's path in the corner is not as tight of a radius as the driver intended. In other words, if you take a corner too fast, understeer will cause the car to plow off the outside of the corner and hit things head-on. That's obviously the driver's fault. And then there's oversteer, which makes the rear end of the car come around so you spin out and go off the road tail first. Lawyers will likely win a case claiming that's the carmaker's fault. Hence, cars are designed to understeer, which is somewhat safer because the driver has a little more control. This is referred to as roll understeer.

Ford engineers have to consider all the variations in suspension and steering pickup points, alignment variations, bushing degradation over time, and bushing deflection to ensure the car will always have roll understeer. They design the suspension so that for average drivers there's a moderate amount of roll understeer. Even if all these parameters vary in the wrong direction, the car will still have a small amount of roll understeer and never switch to roll oversteer.

OK, then why would I want less bumpsteer than Ford designed? Ah, that's a great question. Reducing the amount of bumpsteer shifts more directional control to the driver rather than letting the suspension geometry steer the car. Too much bumpsteer can lead to erratic handling behavior.

Fast drivers hate understeer. When a car understeers, it's the car that limits how fast the driver can negotiate a corner. If the car has a neutral handling balance, or even a bit of oversteer, the limit to cornering speed depends more on driver skill. A good driver takes advantage of the car's increased ability to go faster. Note that evaluating handling balance (understeer, neutral, oversteer) often comes from an experienced driver's unique perspective and is dependent on his or her particular driving style. Two different drivers may describe the same car's handling in completely opposite terms; one may say it's a scary, oversteering demon and the other may call it an understeering, plowing pig. While both statements are exaggerations (as drivers are prone to), you get the idea.

How do you reduce bumpsteer?
By optimizing the steering geometry for the suspension geometry, which can be a tricky feat if you don't know what you're doing. It's nearly impossible to have zero bumpsteer because of the complex, three-dimensional interactions of the steering and suspension geometries. The original Ford geometry is usually close to the mark, as OEM designers do know what they're doing. However, results vary, as they often have to deal with conflicting goals and compromises.

Fortunately for Mustang owners, minimizing bumpsteer to an acceptable amount is often possible and even fairly simple. All it takes is installing a bumpsteer kit in place of the outer tie-rod ends. Now, we should make it clear that installing a bumpsteer kit is not just a bolt-it-in-and-you're-done task. It requires carefully measuring the amount of bumpsteer with a specialized bumpsteer gauge and then adjusting the bumpsteer kit to minimize the amount of toe change. Please note: it's IMPOSSIBLE to optimize bumpsteer by eyeballing it. Yes, we just yelled that, because we've seen some real disasters when people try to get away with this rookie move. All an eyeball measurement can do is keep the bumpsteer from being absolutely horrible...perhaps. We cannot overstate how important good measuring is!

Why else would I want to adjust bumpsteer?
In the realm of 1979-2004 Mustangs, it's quite common to update and swap parts from one model year to another. Front control arms and spindles are the most common swaps, and then there are aftermarket suspension parts, such as K-members, with geometry altered from stock. Whether by design or by a stack-up of parts tolerances, it becomes critical to measure and adjust bumpsteer just to re-attain the original bumpsteer amount, but especially if you want to minimize bumpsteer.

What about lowering? Doesn't that cause bumpsteer?
Ah-ha! That's the myth that's been spread far and wide in the Mustang world since the early 1990s. Short answer: no. As long as the car gets realigned to its original specifications, lowering doesn't cause a bumpsteer problem. Lowering does change camber, however, which causes a change in the toe setting. That's why you always need to have your Mustang aligned after lowering it.

If it's a myth, why do people keep repeating it?
For the average consumer, it's normal to be trusting of companies and individuals that make authoritative statements. After all, companies are staffed by professionals, and they should know best, right? Well, not always, in our experience. Internet forums are also full of people who like to appear important, and repeating the myth is far easier than doing their own research.

MM encourages its customers to be vigilant. Disinformation is rampant in the Mustang aftermarket industry; please don't fall victim to it! Read our newsletters, pick up a book on suspension design, and peruse articles on good handling by well-regarded sources, including our handy and extensive FAQs & Tech Tips. You could also talk with our Tech Services experts for questions about your particular Mustang and the best set-up for it.

Why should I trust MM about this?
Another good question. We back up our statements with supporting evidence you can confirm through other sources.
  • We paraphrase information from books about suspension design; you can read those same books.
  • We show you our work and the results of our tests; bumpsteer can be measured. You can duplicate those tests for yourself and check your results against ours.
  • Ask our customers how their Mustang handled before MM and after!

How did the myth that lowering a Mustang causes bumpsteer get started?
From a combination of direct knowledge and some educated speculation, here's the story. Names of the guilty parties are withheld.

Sometime around 1990, the owner of a shop specializing in Mustangs saw a Fox Thunderbird with offset steering rack bushings. The Thunderbird's front suspension was highly modified. Using offset rack bushings to move the rack upward in the chassis solved a problem with excessive bumpsteer. The very smart person who modified the car explained to the shop owner how he'd fixed the bumpsteer problem caused by his modifications, but the shop owner misunderstood him. The shop owner then built and sold offset steering rack bushings to solve a problem that, ultimately, didn't exist!

Speaking with the authority of a "professional," this fellow convinced several Mustang enthusiast magazines that lowering causes bumpsteer, and said his product was the solution. The magazines spread the myth to their readers, and when the Internet came into play, the idea became entrenched in Mustang lore.

What's the design theory say about all this?
The specific claim of this myth is that lowering a Mustang causes bumpsteer because after lowering the tie-rods are no longer parallel to the ground.

Now, Ford designed the tie-rods to be approximately parallel to the ground at the stock ride height. When the chassis is lowered, the steering rack moves with it and gets closer to the ground, which of course also puts the inner pivot of the tie-rods closer to the ground. But the outer tie-rod end pivot position remains the same height off the ground because it's attached to the spindle/wheel/tire assembly. The result is the tie-rods become angled upward, from the rack to the spindle.

Remember, the original myth claimed the angled tie-rods caused problematic bumpsteer and that the solution was to raise the rack to make the tie-rods parallel to the ground. Once bumpsteer kits that replaced the outer tie-rod ends with an adjustable-height version became commonplace, the myth morphed into claiming a bumpsteer kit was needed to lower the outer end of the tie-rod, making the tie-rod parallel to the ground.

Now, you won't find any suspension design book claiming the tie-rod must be parallel to the ground to achieve minimum bumpsteer. Looking at only one side of the suspension, at stock ride height the tie-rod usually appears parallel to the ground because the lower front control arm is parallel to the ground. That's usually a design goal, as having the lower front control arm parallel to the ground minimizes sideways tire scrub with suspension movement. To meet various other design goals, the control arm may appear to be parallel to the ground, but with careful measurement it's often very slightly off.

The drawing below shows the geometry of a strut suspension with minimum bumpsteer. This drawing is a front view of the car showing only one side of the front suspension; the other side is a mirror image. A line from the center of the upper strut mount, located at 90 degrees to the strut axis intersects a line extending from the pivot point of the lower ball joint through the lower control arm inner pivot. This point of intersection is the Instant Center (IC) of one side of the front suspension. Not shown in the drawing is the IC for the other side of the car; note that each side has its own unique IC. For minimum bumpsteer, a third line drawn through the tie-rod must intersect with the IC.

Instant Center

Click on the image for a larger version.

Designing for minimum bumpsteer requires the arc of the spindle's movement be the same as the arc of the outer tie-rod end's movement. The arcs are only the same when the tie-rod line points to the IC. This is not a chicken-or-the-egg situation: the control arm location determines the tie-rod position. In practice, one or more pivot points often get changed from the ideal location for various reasons, including to cause toe-out in bump to induce understeer.

While when you look at a stock Mustang the tie-rod appears parallel to the lower control arm, from the above drawing you can see they're really not exactly parallel. They only appear parallel because they're separated by a small vertical distance and the IC is quite some distance away.

When the car gets lowered, the control arm angles down toward the inboard pivot. So does the tie-rod. That's as it should be, as both are now aimed at the IC. If we were then to change the tie-rod angle in a misguided attempt at fixing bumpsteer after lowering, the tie-rod will no longer point at the IC, causing bumpsteer! Myth busted!

Wow. So how do you minimize bumpsteer the right way?
Well, there's a sequence of actions to follow. These affect the lines that must intersect the IC for minimum bumpsteer.
  • Set the ride height.
  • Set the camber.
  • Set the caster.
  • Set the toe to zero; final toe setting is done last.
  • Adjust the angle of the tie-rod. Offset steering rack bushings may be used to make a gross adjustment, but you have to change the spacers in a bumpsteer kit at the outer tie-rod end to really dial in the bumpsteer.
  • Set the toe.

We're all about testing to bust the myths
For almost 30 years, MM has researched and measured bumpsteer with different combinations of components and ride heights, confirming that lowering a Mustang does not cause a bumpsteer problem. The results of our first test were published in the July 1993 issue of Super Ford magazine, one of the leading magazines catering to Mustang enthusiasts.

At the end of this post you'll find a graph showing some results of our bumpsteer tests. Results are displayed on an X-Y graph for easy understanding. Suspension movement is on the vertical axis and toe-change on the horizontal axis. There is no toe-change at ride height, so it always has the value of zero on the graph.

Note that if the car has zero bumpsteer over the entire range of suspension travel, the graph would show a vertical line. The closer the curve is to being a vertical line the less bumpsteer there is.

Test 1
Testing a bone-stock 1989 Mustang LX just as Ford designed it. We used the same vehicle for all of these tests.

  • Ride height: stock
  • Caster setting: stock, measured at 1.5 degrees

The stock bumpsteer shows an acceptable curve for a transportation appliance. At 1" of bump travel the toe change is .040" toe-out.

Test 2
Lowering the car 1" to see what lowering alone does to bumpsteer.

  • Ride height: 1" lowered from stock
  • Caster setting: stock, measured at 1.5 degrees

Less bumpsteer! Lowering the car actually reduced bumpsteer! Myth busted! This is why we test things for ourselves.

Test 3
Changing only one parameter from stock: increasing caster by adjusting the MM caster/camber plates.

  • Ride height: stock
  • Caster setting: increased to 3.0 degrees

Again, less bumpsteer, showing that caster has a significant effect. This is because increasing caster on a Mustang is done by moving the strut top rearward, toward the firewall. That in turn causes the steering arm on the spindle to go upward, raising the outer tie-rod end.

Test 4
This time we'll both lower the Mustang and increase the caster setting.

Test vehicle: 1989 Mustang LX
  • Ride height: 1" lowered from stock
  • Caster setting: increased to 3.0 degrees

Less bumpsteer. Simply increasing caster on the lowered Mustang reduced bumpsteer to an amount acceptable for performance driving. The toe change in bump shifted to toe-in from the stock toe-out, decreasing understeer.

Test 5
Now let's test what the myth says to do for a lowered Mustang: add offset rack bushings to raise the steering rack on the K-member.

  • Ride height: 1" lowered from stock
  • Caster setting: stock 1.5 degrees
  • Offset steering rack bushings

More bumpsteer! Raising the steering rack made the bumpsteer far worse than it was with the rack in its stock location.

Test 6
Now a test with caster adjusted to 3.0 degrees, as is typical of modified Mustangs. Still with the same offset rack bushings and lowered 1".

  • Ride height: 1" lowered from stock
  • Caster setting: increased to 3.0 degrees
  • Offset steering rack bushings

This combination has less bumpsteer than Test 5 because of the increase in caster, and while it's slightly less than stock, the bumpsteer curve in Test 4 is much better, having toe-in under bump.

Thanks for clearing this up! Anything else I should keep in mind?
As our tests show, something as subtle as a 1.5 degree change in caster has a significant effect on the bumpsteer curve. Is your Mustang set up exactly like our test car? Probably not. If anything is different, your results will vary. But we hope you understand at this point that lowering your car won't increase bumpsteer.

Other factors that could make your results different from ours:
  • Different model year
  • Different ride height
  • Different caster setting
  • Different outer tie-rod end (for example, 1990-93 Mustangs had outer tie-rod ends that placed the tie-rod at a different height relative to the spindle than 1979-89 Mustangs). Aftermarket replacement ends don't match either original tie-rod, but are a compromise so one part fits both year ranges.
  • Different spindle
  • Different K-member
  • Aftermarket performance parts

One more thing: bumpsteer and IRS
We've been talking about bumpsteer as it affects the front suspension. But because an IRS (Independent Rear Suspension) is also an independent suspension design, it too can suffer from poor bumpsteer. In fact, vehicle stability is affected far more by the rear bumpsteer being excessive than it is by a front bumpsteer problem.

If your Mustang has an IRS, and you want good handling, you'll want to measure the rear bumpsteer and make whatever corrections are required to minimize bumpsteer. Even a bone-stock Mustang Cobra IRS can often be improved by correcting the rear bumpsteer. And you know what else is great? MM has IRS bumpsteer kits!

The Bottom Line on Bumpsteer
Maximum Motorsports puts a lot of time and effort into testing. Objective measurements are necessary to make informed decisions, and we want to help you make the best choices for your Mustang project.

You don't know what your Mustang's bumpsteer is unless you measure it. Trying to adjust bumpsteer, whether with offset rack bushings or an outer tie-rod end bumpsteer kit, is a complete guess unless you measure the toe-change with a bumpsteer gauge. It's not that difficult, and it's a key factor in making a Mustang handle really, really well. Here's an old-school analogy: you wouldn't think of replacing your ignition distributor if you didn't have a timing light to measure the ignition timing so you could adjust it to spec. So don't try to measure bumpsteer by eye; for accuracy (and the most fun) you must use a bumpsteer gauge.

Instant Center

Click on the image for a larger version.

Well, that’s the facts on bumpsteer. We hope we explained it well enough that you now know how important a good bumpsteer curve is to great handling. Thanks for reading! Next week we’ll discuss the ups and downs of struts and shocks.