Road racing and autocross are all about getting the maximum traction from the tires and optimizing grip. Sure, horsepower is important for quick lap times, but planting those front tires so that the car heads where it’s pointed is the name of the game. Among the variables hidden in camber crusades, alignment advice, and Ackerman arguments is another well-known talking point – bumpsteer. The prophets will preach that minimal bumpsteer is essential to achieving handling heaven and we won’t argue the point. Instead, we’ll show you how to measure bumpsteer and then hit a few areas that will address what to do when the front suspension goes bump – when it shouldn’t.
To ensure that everybody is drinking from the same Dixie cup, let’s describe what we mean by bumpsteer. We’re talking about the most common independent front suspension, one with unequal-length upper and lower control arms, as viewed from the front of the car. When the front tire moves up (compression) or down (rebound) it does not travel in a straight line. As the tire moves, it scribes an arc – as viewed from the front. The radius of this arc (or curve) is defined by the lengths of the upper and lower A-arms pivoting from the inside mounting points. This becomes an issue once the steering linkage is attached to the spindle.
Ideally, the tie rod is attached to the spindle in a position that does not affect the toe position of the tire as the tire moves through its arc of travel. Stated another way – the outer tie rod scribes an identical arc to the travel of the spindle/tire. That works great on paper, but production cars and many purpose-built race cars rarely perform this way.
In the real world, the outer tie-rod end will induce a slightly different arc of travel as the tire moves. When this happens, the leading edge of the tire will either point outward as toe-out, or inward as toe-in. In certain cases, the tire can move to produce either (or both) toe-out and toe-in when moving between compression and rebound.
The effect of bumpsteer can be dramatic. Measuring this allows us to know what affect the steering geometry is having on the front tires as the suspension moves through its travel. Most suspension tuners prefer toe-out over toe-in for stability because large amounts of toe-in under braking and initial corner entry can cause understeer. The goal is to achieve as close to zero bumpsteer as possible.
The reason for near-zero bumpsteer is to impart less heat into the tires as the race progresses. If you are running autocross or sprints of less than 5 laps, bumpsteer may not be as much of an issue, since this short period of time rarely will put excessive heat into the tires. But, road course and circle track races place great emphasis on tire stability and endurance, so minimizing bumpsteer is critical. By minimizing toe steer, the tires will last longer and the car’s handling will be more stable over extended laps.
The first inch of suspension travel in both compression and rebound is the critical range, since that’s where the car will spend a majority of its time .
Every chassis tuner will champion his specific set of bumpsteer specs. Jeff Butcher of Joes Racing Products recommends shooting for less than 0.010-inch of total toe-out, with 0.004-inch of toe-out per one inch of suspension travel. Butcher says that toe-in makes the car unstable, so toe-out is the preferable movement under suspension travel.
In order to know these numbers, we must first measure the car. It is not a difficult process, but it does require some specific tools and time. While you could build your own bumpsteer gauge, we found that Longacre Racing Products, Joe’s Racing Products, and others offer an affordable, single dial-indicator tool that works very well and is simple to use. The Longacre tool we are using has since been updated, as ours is well over a decade old, but we’re comfortable with it and it works great.
All bumpsteer gauges work in a similar fashion. A large plate is bolted to the wheel mounting flange. This plate is graduated on both ends for travel above and below a center zero-line that represents the front wheel at ride height. A dial indicator is used to measure the amount of hub movement that will indicate toe-in or toe out.
Setting up to measure bumpsteer is pretty easy – the hardest part is you will need to either remove the front coil spring from the suspension or unload it to allow unrestricted movement of the spindle through both bump and rebound. Before we started, we set the alignment to our desired specs. This is important because caster will directly affect the bumpsteer curve.
The next thing to do is to determine the car’s ride height. Longacre says you can do this by either measuring the overall length of the shock absorber or the angle of the lower control arm with the car sitting on the shop floor. Race cars with the shock absorber separate from the coil spring makes measuring overall length very easy.
Once ride height is established, jack the car, then remove the front tires. Our car was equipped with weight jacks that allow quick adjustment of the ride height, so all we had to do was back off on the adjusters to allow the suspension to move freely, since we will be measuring both sides. Longacre suggests using a wood block to use as a frame-height chock so that the car is sitting at ride height for this test, but solid jack stands can also be used, assuming they will position the car at the proper height.
It’s important that the car and the suspension be at ride height, and the steering be locked, when the bumpsteer is checked so that no input will affect the results. Because we’re dealing with extremely small numbers, repeatable results are another important consideration. You must also disable the front anti-roll bar for this test, but you only have to detach one side. Place a floor jack or bottle jack under the lower control arm, as we will use this to slowly move the suspension through its total travel.
To set up the tool, bolt the flat plate to the hub and position the upright so that it angles toward the aluminum hub plate at about a 15-degree angle. This will keep the two contact points — the roller wheel and the dial indicator — positioned on the aluminum plate. Note that the hub plate is graduated in fractions of an inch through three inches on either side of ride height.
Start with the suspension at ride height and place the dial indicator and roller wheel at zero on the scale to match ride height. Zero the dial indicator and make sure the level on the aluminum hub indicates that the plate is level. We will start by plotting data for compression, but you could easily start with rebound if you prefer.
We pumped the jack to indicate 0.50 inch of compression and watched the dial indicator. It’s important to note the direction that the dial indicator moves. In our case, under compression the indicator moved outward — away from the center of the car — which would mean toe-out. The indicator only moved about 0.003 inch.
Next, we recorded numbers every half-inch of travel through 2 inches of compression. Each time we changed bump travel, we rechecked the hub plate to ensure it was still level, since it will move slightly every time the hub is moved.
With the compression bumpsteer plotted, we then returned the hub to zero (or ride height) and performed the same procedure through rebound, recording each change in toe with the dial indicator. In rebound, the suspension was linear, moving to toe-out.
The good news for our chassis was that we measured a maximum of 0.020 inch of bump over 2 inches of compression and rebound, with 0.010 inch of toe out for 1 inch of travel. The first inch of suspension travel in both compression and rebound is the critical range, since that’s where the car will spend a majority of its time.
It’s possible that your bumpsteer curve will not be what you desire and may need some attention. We’ve created a small chart showing bumpsteer problems and their potential solutions as well as some graphs that show curves that are not ideal.
|Toe-out in compression / toe-in on rebound||Lower inner tie-rod end|
|Toe-in in compression / toe-out on rebound||Raise the inner tie rod|
|Toe-in on compression and rebound||Lengthen tie rod|
|Toe-out on compression and rebound||Shorten tie rod|
There is far more detail on how to affect changes to the various bump curve problems, but as mentioned earlier, they all come down to creating the same arc of movement of the steering link with the spindle. When they scribe near identical arcs, bumpsteer becomes manageable.
Measuring bumpsteer isn’t difficult, but it does require patience and attention to detail. Once you record the numbers, the rest of the process is merely adjusting the suspension until the bump curve is what you desire. In our case, we achieved a much cleaner curve by investing in custom spindles. But you won’t know what to do until you dive in and measure the car on both sides.