Roll center. You’ve likely heard the term thrown around by your buddies when bench racing, but do you (or they) really understand what it means? Did you know that it doesn’t just affect a single wheel, but also each end of the car, both independently and linked, from the front-to-rear? And just how do you find it? It’s a valuable tuning aid that most racers consider to be “set and forget” when it should be anything but that. We’re going to tell you what it is, how it affects your car or truck, and how the experts use it to their advantage on the track.


WHAT IS IT?

One of the goals of our race-tuning techniques is to get the car to carry as much speed as possible through a corner. There are many variables at play, but a significant one that racers don’t always consider is roll center. Simply put, the roll center is the point on either end of the car that the chassis will roll around left or right. Now, just where that is, whether that point is above or below the car’s axles, depends on the suspension geometry, in particular, the mounting points and the angles of the lower arm and the upper camber link (which usually plays the role of an upper arm). To give you an idea of just how important this setting is, a car’s roll center can affect how responsive it is, how it reacts to bumps, and how much traction it has in a corner.

Here, roll center is shown by extending the upper camber link (red) and lower suspension arm (blue), then drawing a line between the center of the tire’s contact patch and the intersection of those vectors (green). The roll center is where this line crosses the chassis’ centerline (purple).

VISUALIZING IT

Now, to help you get a better understanding of what roll center is, let’s visualize it using the right side on the front end of a car. If we were to draw lines extending the upper and lower arms to the left side of the car, they would intersect or meet at a certain point—almost always on the left side of the chassis. The farther away this point is from the right side of the chassis, the less the camber will change as the suspension is compressed—this is called “camber gain.” if we do the same thing for the left side of the car, we have two points; if we draw a line between each of those points and the center of the tire’s contact patch on the opposite side, where those two lines meet is the roll center for that end of the car.


ROLL CENTER GUIDELINES

There are multiple holes in this front carrier and rear hub for different camber link settings

In general, by letting the car roll more, a lower roll center lets the suspension generate more traction than a high roll center. However, with the extra time it takes for the weight to transfer from side-to-side, a low roll center can make the car feel sluggish. Using a higher roll center prevents the car from leaning as much, which is helpful on high-bite tracks to minimize traction rolling and will also give the car a snappier feel—good for smooth, high-bite tracks.

Not only are there multiple inner camber link mounting locations, but washers are also used to adjust the roll center geometry.

HOW TO CHANGE IT

The easiest way to adjust roll center is by making changes to the camber link. Some cars have a variety of mounting options on the bulkhead and on the upright, while others use washers or spacers to change the camber link angle. While a bit more troublesome to do, changing positions of the lower arm yields a larger effect than adjustments on the camber link.

TUNING TIPS FROM TEAM ASSOCIATED’S JOSH ALTON

Josh Alton is one of Team Associated’s head racing engineers, so he needs to know what settings their drivers have to look at for every track situation. Here are some of his roll center adjustment tips:

• Focus on tuning roll center only after you’ve already chose the best possible tires for the track.


• To change just the roll center, adjust the inner hingepin location; adjusting the camber link affects the camber gain (how the camber of the wheel changes throughout the suspension travel) as well as the roll center.


• Don’t forget all the variables that affect roll center, especially ride height, camber link location, and camber gain. When testing different changes, keep a notebook of your results.


• In general, if the rear end is sliding around corners, try changing to a lower rear roll center. If the rear has a lot of grip and is pushing through the corners, try raising the rear roll center. The same can apply to the front end—if you need more front grip then lower the front roll center.

Notice that when the suspension is compressed (or extended), the angles of the lower arm and camber link change, which also changes the roll center; the roll center really is a dynamic value. However, at a specific ride height, it can be duplicated—so it is something that is measurable statically. For off-road cars, the difference between the front and rear roll centers can be dramatic, mainly due to the kick-up used on the front end, which raises the front of the inner hinge pins.

WRAP-UP
With as much as the roll center settings can affect your ride, it’s definitely something you should look at for every track condition. If you want to get the most from your ride, you should experiment with roll center adjustments.