Mustang MonthlyHow To Chassis Suspension
How to Correct Bumpsteer
As we found out, bumpsteer can be a serious problem on some modified vintage Mustangs. Here's how to fix it.
According to Pro-Motorsports Engineering, manufacturers of the bumpsteer corrector kit we installed here, bumpsteer is a front-suspension design flaw of every vintage Mustang. On a car with a mostly stock front-suspension, the problem might not be as apparent, and our experience has proven this difference to be true.
How do we know? Well, our tried-and-true '69 fastback project car has once again been the basis for our research. When we first bought the car in early 2003, it had a stock front suspension that worked adequately but could certainly benefit from a thorough rebuild. One of the first things we did was install all-new pieces from Mustangs Plus' line of Grab-a-Trak components. At that point, the car drove well, handled with considerably more composure than before, and generally felt much safer at highway speeds.
Next on the agenda was the installation of a Flaming River (FR) rack-and-pinion steering system. Flaming River's excellent cradle-mounting system and solid-feel, fast-ratio rack was a step up in steering-feel and overall competence. However, by no fault of Flaming River, our car was a true oddball application with its 351W engine, four-speed clutch linkage, and JBA "mid-length" header flanges that are quite close to the steering-system's mid-shaft U-joint. In short, our car isn't your average 289-with-stock-manifolds-and-an-automatic-transmission combination. Nonetheless, Flaming River came up with a cradle--now a production part--that allows the rack to fit.
The plot thickened when, for our Nov. '05 issue, we installed Blue Moon upper and lower control arms, which put us in a situation where the arms' modified geometry combined with the Flaming River steering rack's relatively low mounting point caused horrendous bumpsteer (see sidebar). Again, as with the Flaming River rack, this was not something that could be blamed on the Blue Moon parts. In fact, the control arms are about the most bulletproof suspension parts we've seen--seemingly built like bridge girders.
Along with the low-slung rack and the updated-geometry control arms, we also had a lower-than-stock ride height, which made the bumpsteer even worse. In short, our setup had the car all over the road when going over bumps, dips, potholes, and high-speed undulations.
So we got back in touch with Flaming River's technical department. Much to their credit, they mentioned that other customers had the same problem on their lowered cars. Note that we said "some," because not every FR rack-and-pinion setup mounts to a vintage Mustang in the exact same location, and not every car has a lower ride height that worsens bumpsteer.
After sending a few pictures to Flaming River showing the position of the tie-rods and how they angled up too much, the diagnosis was a serious case of bumpsteer. Flaming River immediately recommended Pro-Motorsports' corrector kit and told us that several customers had installed the same system with dramatic improvement. So after a trip to our local vintage Mustang front-suspension experts at Marlo's Frame and Alignment, we installed the Pro-Motorsports kit and achieved the improved result.
The car now goes straight as an arrow over all high-speed dips--something that's important to us as high-speed freeway road trips are common in this car. Bumps and potholes aren't nearly as scary as when the steering wheel was almost yanked out of our hands. Finally, note that the Pro-Motorsports' kit is available from Mustangs Plus.
What is bumpsteer?
As we learned the hard way, bumpsteer is a condition you want to avoid. In fact, for a car to drive and feel right, it needs to be eliminated totally. Generally, bumpsteer is caused by an incompatibility between the steering system's tie rods and the front suspension's upper and lower control arms. As the suspension cycles, the path or arc followed by the outer tie rods in relation to the control arms and spindles are different. Since something has to give, there is a toe change (the wheels toe in or out) as the suspension moves up and down. This occurs every time the suspension moves.
While bumpsteer is a factory design flaw every Mustang has to varying degrees, it becomes noticeable when the car is lowered or modified with other suspension improvements that make the car more responsive to steering input. Better tires also make bumpsteer more noticeable. When experienced, the car twitches or darts around on bumpy or undulating roads and can also be twitchy on heavy braking. The car might also pull to one side or another when braking.
Bumpsteer is eliminated from a Mustang by ensuring the tie-rod (steering linkage) pivot point matches the travel of the spindle. Match these curves by either raising the inner tie rod or lowering the outer tie-rod mounting point. Since the location of the inner tie rods and steering center link is under the oil pan, this option isn't practical. Fabrication of a spacer block to relocate and lower the outer tie-rod from its location on the spindle is easy. The Pro-Motorsports' bumpsteer corrector kit accomplishes exactly that. It repositions the outer tie rod 1-inch lower, 1-inch forward, and 1/4-inch outboard from its stock location. The 1-inch drop is for the bumpsteer, the 1-inch forward move quickens the steering the same way the longer idler and Pitman arms did on '65 Shelbys, and the 1/4-inch outboard move reduces (but doesn't eliminate) the Ackermann Angle, a toe-out condition that is designed to occur during turning. While helpful for street cars driven at low speeds, it's less desirable in racing or high-performance situations where high lateral cornering loads are present.