Baer’s Tracker Kit improves suspension
Words And Photos: Jeff Smith
If your idea of a great Saturday afternoon is dueling with autocross cones or executing that 180-degree freeway on-ramp with precision, then you’ve probably listened to the prophets of doom espouse about the evils of bump steer. Everybody who talks about bump steer treats it as if it’s the suspension equivalent of an ankle-deep pothole. Let’s take a look at bump steer and how to minimize its effects.
We decided to take on that challenge with a Chevelle muscle car that exhibits plenty of bump steer, so we can see what all the hyperbole is all about.
But first, jump in the car and let’s make a quick lap. We’re about to enter a sharp 90-degree turn around the first cone on the autocross course. We crank the wheel to the left, transferring load to the right front corner of the car, compressing the coil spring. On any car, this weight transfer also causes a dynamic change to the right front camber setting. This occurs because the upper and lower control arms pivot in an arc around their mounting points and are tied together at the spindle.
The problem with most production cars is the front suspension is designed to create positive camber, which if you are looking at the car from the front, pushes the top of the tire outward. This induces push — understeer — which for a non-performance oriented driver is more controllable than oversteer, where the rear of the car tries to pass the front end in a corner.
A popular suspension upgrade to create a better camber curve for early Chevelles has been to replace the stock spindle with a ‘70s era, one-inch taller B-body (Impala) spindle. This spindle combined with a pair of tubular upper control arms, like those from Global West, creates a negative camber curve in bump, which pulls the top of the tire inboard.
As the upper and lower control arms move through their arc, the steering linkage also scribes an arc. Unfortunately, this arc is much different compared to the control arms. This causes the spindle to pivot, which moves the tires either toward toe-in or toe-out. This is called bump steer. The term refers to the fact that when the suspension moves away from its static ride height in either bump (compression) or rebound (extension), this movement induces a change in toe, which is not desirable.
Look down at your feet and point your toes in towards each other — that’s toe-in, and the opposite is toe-out. Now imagine your feet are your front tires. It’s that simple. The ideal situation is where the front suspension moves throughout its travel with zero toe change. All this leads us to three simple questions: 1) What constitutes “bad” bump steer? 2) How bad is bump steer in a production car? 3) Can this bump steer be improved?
The answer to the first question is perhaps the most contentious. If we listen to the zealots, the only acceptable answer is “zero bump steer.” But, according to Doug Norrdin, owner of Global West Suspension and a man with 40 years of suspension knowledge and experience, the front tires can absorb quite a bit of toe change.
Any tire when subjected to cornering input, will deflect. This means even on a car with what could be termed “excessive” bump steer, it’s possible while the steering angle (toe) of a front tire has changed, it might not be perceptible to the driver because the tires absorb the difference. Norrdin acknowledges excessive bump steer is counterproductive to improved handling. But he also makes a point that much of what appears on the internet in terms of “bad” bump steer is not supported by hard facts.
The second question concerns bump steer in a production car. For this, we decided to test our ’65 Chevelle, which has seen more than 20 years of autocross and track day abuse. This particular car combines a B-car spindle with Global West tubular upper and lower controls arms.
For our test, we first established a typical alignment of ¾-degree of negative camber with 5 degrees of positive caster. Then, using a Longacre bump steer gauge, we measured toe change while moving the suspension through both compression and rebound. The results, frankly, were surprising. We’ll get to those details momentarily.
The third question deals with how to improve bump steer. The way to improve bump steer on production cars is to alter how the steering arm moves in relation to the control arms. Several variables affect this situation, including the position of the drag link, idler arm, and inner and outer tie rod ends.
Baer Brakes has created a product that might help by altering the position of the outer tie rod end. In many production car applications, the outer tie rod end is positioned too high relative to the spindle. Baer’s Tracker kit replaces the outer tie rod end with a spherical bearing and a tapered adapter. The longer stud on the spherical mount side allows you to lower the position of the tie rod end. Using precision-machined spacers, the Tracker allows you to easily adjust the position the outer tie rod end to reduce the bump steer.
Notice we said “reduce” the bump steer. While we can only comment on the combination we tested, the Tracker reduced, but did not eliminate, the bump steer on our Chevelle.
To test bump steer, we used a Longacre tool. This consists of a graduated flange that bolts to the front hub to measure the bump steer through both compression and rebound. We set the toe at zero at ride height and then began taking toe change measurements through both compression and rebound. Our results are listed the accompanying chart.
Under compression (bump) the steering toes out along a somewhat linear curve. At one inch of compression (which is in the neighborhood of what a well set up autocross car will see at the apex of a corner) we measured 0.110-inch of toe-out. Conversely, under rebound, the combined angles created toe-in. Moving into the 1.5- and 2.0 inches of both bump and rebound, the numbers became much worse with 0.200-inch of toe-out at 2 inches of bump.
What we measured for the baseline is exactly the way we have raced this Chevelle for the last 10 years. Obviously, 0.110-inch of toe-out under one-inch of compression is not even close to ideal. But it’s worth emphasizing that at no point in all my years of com-petition experience with this car did it ever yank the steering wheel out of my hands nor exhibit erratic steering behavior. But clearly, the Chevelle could benefit from less bump steer.
We then installed the Baer Tracker kit on the right front of the Chevelle and retested. The kit includes a total of 0.750-inch of spacers in various increments that will allow you to dial in the exact combination that will maximize a reduction in bump steer. We tried five different combinations of spacers and discovered the best bump curve was with the thinnest 0.030-inch shim. It’s important to mention here that the only way to know which shim package performs the best on your car is to measure it. There’s no other way to do it. This means you will need a bump steer gauge.
At one inch of bump, the Tracker kit reduced the toe-out from 0.110-inch to 0.085-inch. While we still have 0.085-inch of bump, the Tracker did reduce the original measurement by 23 percent. The rebound numbers at one inch delivered a slightly better reduction of 27 percent. The complete results are in the accompanying chart and graph.
As noted earlier, the Baer Tracker kit improved our bump steer issue, but it did not eliminate it. We have also measured the bump curve on Global West’s ’65 GTO A-body test car fitted with Global’s new forged aluminum spindle. That package delivered a near perfect bump curve of only 0.015-inch over two inches of suspension travel from one inch of bump to inch of rebound.
While this essentially eliminates bump, the price for this performance is roughly $1,300 for the entire Global custom spindle, steering knuckle, and brake adapter package. This makes the $175 for the Baer Tracker kit a pretty good deal.
It’s impossible in a short magazine story to deal with all the details involved with improving handling as there are dozens of variables that come into play — bump steer is just one. But if this treatise interests you, Baer offers a Tracker upgrade for many popular body styles, including nearly all of the popular Mustang applications. You can find the entire application guide by going to Bear’s website.
Bump Steer Chart
|Rebound||2.0||- 0.210||- 0.200|
|1.5||- 0.157||- 0.140|
|1.0||- 0.130||- 0.095|
|0.75||- 0.086||- 0.072|
|0.50||- 0.052||- 0.048|
|0.25||- 0.030||- 0.022|
In this chart, compression bump numbers are expressed in positive numbers to indicate toe-out. Rebound bump numbers are expressed in negative numbers to indicate toe-in.
Global West Suspension