Dirt and Asphalt Front Roll Centers

What are the best locations and why?

by Bob Bolles 

Many of us struggle with the questions concerning the front roll center of our race cars. We know that it is an important part of the setup, but where is it supposed to be and for what reasons? For several years now, race teams, from big league stock cars to stock class Saturday night racers have experimented to great lengths trying different layouts for their front ends. It’s really hard to know exactly what arrangement is best. So, here is some food for thought. You should read over and digest this information, because many well known and successful race teams are currently using the information presented here to win races.

From a Dynamic as well as a Geometric standpoint, whether you race on dirt or asphalt, the front roll center is very critical to the overall setup of your race car. The double A-arm front suspension system of a race car can be easily altered, and in most cases with older cars, it has already been changed by someone.

If the location of this geometric point is so important, how do we know where it should be? To resolve that question, let’s examine what we need for optimum front end dynamics and geometric layout.

The dynamic forces acting on the front end of the race car react through a moment arm much like the handle of a floor jack. In this way, the moment arm is much like a lever. The top of this lever is the center of gravity of the race car and the bottom of the lever is the roll center. The longer the lever arm is, and the more closely we apply a force at 90 degrees to the lever, the more efficiently we can roll the car.

What has been discovered by some of the more successful race teams, and good information survives the ultimate on track testing, is that when this roll center is located too far to the left or right of the centerline of the car, there are some very negative results.

On asphalt circle track race cars, if the front roll center is located too far left of the centerline, the moment lever becomes too efficient, and the front end reacts too quickly to the initial turn-in on corner entry. A great deal of feedback has been coming in from asphalt teams all over the country that describe the condition where the right front of the car wants to "fall over" on corner entry. This is especially true at small radius tight entry tracks when their front roll center is located to the left of centerline.

The opposite seems to be true for dirt circle track race cars. These cars can benefit from having the roll center located to the left of the centerline. This is because the dirt cars do not experience as much turn entry deceleration forces as the asphalt cars do. What many dirt teams report is that by having the roll center located to the left of centerline, the cars will turn much better.

By using a simple two dimensional geometry program, you can redesign the locations of the front suspension pickup points and move the front roll center to the right or left of the centerline. The front end will react more slowly or quickly as needed and corner entry will be much more manageable. A good range for asphalt cars seems to be from six to eight inches to the right and, for dirt cars, 4 to 6 inches to the left.

For larger, heavier cars on asphalt such as Winston Cup, Busch, etc., the front roll center location is better located a little further to the right, up to about twelve to fourteen inches right of centerline. If the front roll center is located too far to the right, the front end will not want to roll at all and a lot of weight will be dumped on the right front tire with obvious negative results.

Using a geometry program, you should be able to dive and roll the car to see where the front roll center moves to in the turns. It will always move after the car rolls and dives, but ideally we want it to move as little as possible. Try to find a layout, which will cause as little movement of the front roll center as possible.

That pretty much explains the theory and practice of lateral front roll center location, but what about the height of the front roll center? How far off the ground your front roll center is located does affect the dynamics (amount of roll) of your race car somewhat, but to a lesser degree than the lateral location. What the height does influence in a very big way is the camber change characteristics of your car, be it a dirt or asphalt car. When the car rolls and dives in a turn, the wheel cambers change more or less depending on the front roll center design. It is not very often that a race car will jack up - the front end lifting up - while in the turns, although this does happen sometimes. Most often stock cars experience more positive shock travel (downward motion) on the right side than negative shock travel (upward motion) on the left side of the car meaning the front of the chassis dives.

Therefore, for most cases, the car will roll and dive in the turns. What happens to the tire cambers while this is going on? One very important thing to understand is that we cannot measure camber gain or loss with the car at ride height while we bump the wheel up and down. The rolling of the chassis produces its own camber changes combined with the changes produced when the wheel moves.

When the car rolls, the upper chassis mounts for the upper control arm (upper A-arms) move to the right in relation to the wheel adding negative camber to the left front wheel (going less positive), and taking negative camber out of the right front wheel (going less negative). We don’t want either of these things to happen if we can help it. If anything, we want to gain negative camber in the right front and positive camber in the left front.

Large camber changes happen when the lower control arms have excessive angle to them, and when the upper control arms have too little angle to them. As a rule, these two conditions tend to produce a low front roll center height. This give us a clue as to where our roll center is located vertically.

The height of the front roll center does, therefore, give us an indication of overall camber change characteristics. Generally, the front roll center should be at or above about three and a half inches off the ground, up to around five inches, depending on the particular car and the design of race track you will be running on. The upper control arms will show increased angle as you move components around to get these heights, and as a result will produce less camber changes as the car rolls in the turns.

So there you have it. You now have some kind of idea where your front roll center should be located and some reasons why. If you don’t know exactly where your front roll center is located, it’s about time to find out. This critical geometric point is a big part of the optimum total handling package we are all looking for.