Roll Center/Moment Center Question

Could you please email me a moment-center-for-dummies rundown as I’m not an expert setup guy? I simply don’t understand it. It’s an imaginary position that has real effects on handling? Please help me out if you can.

—Karmen Parks

Moment center (or roll center) is an imaginary point that acts just like a bolt through the chassis. And it does have a real effect on the handling, more so than just about any other component on our race car. But it’s an imaginary point that is dictated by the very real components, such as the upper and lower control arms, and their location on our car. To learn more about it, I suggest you go online to and search our extensive article archives. There you’ll find numerous articles that explain what the moment center is and what it does. The most recent being run in the Nov. ’12 issue and one prior in the Dec. ’11 issue. Over time, we have discovered much more about this design feature than was known back 20 years ago. Not only that, but we have also refined our view of where the MC should be designed to be for different race cars.

Third Link Angles Discussion

I look forward every month to reading “Track Tech Q&A.” You answer a lot of interesting and good questions. One answer you gave in the Nov. ’12 issue I have to question and at the same time give us all something to think about.

The question was, Long or Short Third Link?, from Matt in Canada. Your answer was that you did not know of any ratio between the trailing arm length and third link. Well, there is a ratio to keep in mind if you really want to cut hairs.

If you want to keep the pinion angle as close to the static set degrees as to the dynamic, the lower control arms and the third link must all be the same length. If the third link is of different length, the arc that the rearend takes in compression will change the pinion angle from the static setup. It will not raise and lower, instead maintaining the same pre-set angle.

Now in the same November Q&A you answered a question about Rear Trailing Arm Angles and Pinion Angles from Unsigned, I assume in Orlando. Keeping the pinion angle the same and opposite direction as the driveshaft to the tranny angle is what you try to do. I agree 100 percent.

Now give this some thought. We’re all concerned with pinion angle, with good reason. Have we ever given thought to what is happening to the lateral pinion angle when we set up a car to rear steer? I know it never crossed my mind before I started thinking about writing this response to you. Some of the Late Model dirt cars change their left to right wheel bass lengths 6 to 10 inches. That is one hell of an angle being put on the rear U-joint especially if you’re on a short track with long sweeping corners. So should we give pinion angle a bit more breathing room?

Something to think about,
—Phil Tomeo

Phil, you are correct that with a shorter third link, the pinion angle will increase as the rear squats on acceleration. So, a team would be advised to check and set the pinion angle with the rear of the car in a lowered position to simulate the height coming off the corners. That way, under full torque and loading, the pinion angle will be optimum.

The problem with most race cars is that there is no room to put in a third link that is as long as the lower links. And, a shorter link provides more opportunity to increase the angle allowing for more antisquat leverage.

As to rear steer and its influence on the top view pinion angle, I have some thoughts. If the left rear wheel moved 4 inches forward and the right rear wheel remained stationary, then an angle of about 3.5 degrees would result for a 66-inch track width.

A Dirt Late Model or Modified set up that way might well carry that attitude most of the way down the straight and the top view pinion angle would remain 3 to 3.5 degrees in relation to the driveshaft. So, if I were looking for equal and opposite U-joint angles at each end of the driveshaft, I might run zero side view angle and the engine at 3 degrees of top view angle with the rear left from the front.

Finding Moment Center Without Software

I have a quick question. (I’m sure you’ve never heard that one before!). I’m racing a Dirt Modified and have read and re-read all the information you present in your book as well as the articles you have written for Circle Track relating to roll centers and moment arms. I’ve even attempted to diagram this out on my garage wall, a frustrating and non-productive exercise to say the least.

Is there an easy way to know that I’m in the ballpark without investing in the software necessary to get this information? I have typically put the right lower A-arm pivot point a 1/4- to a 1/2-inch lower than the lower right ball joint pivot point and have set the right height on the left level or a little lower with me in the car.

If I had you standing in my shop (an open invitation to spend a race weekend with us in West Texas/Southern New Mexico by the way) to set up the front end without the use of above mentioned software, what would you do?

Also, I’m paying attention to how much movement I’m getting from my shock grommets to make a decision on spring rates. The front end appears to be working well with a 650-pound on the RF and a 600-pound on the LF. How would you determine what are the proper rates for the rear of the car, specifically the right rear?

The car is a metric stub car with the original metric lower A-arms, so it’s a little narrower than what appears to be popular right now. The car is also approximately 20 years old and I take a certain amount of pride in competing with and occasionally beating those that are spending a significant amount more than I to be fast.

Thanks for your time and advice. The invitation is sincere. We have two tracks locally that are being run by the same two guys that happen to be racers. They have put a lot of effort in providing great racing facilities and I believe have found a way to actually turn a profit doing it. It might make for some good copy for the magazine.

Hope all is well and I appreciate your time,
—Tom Georges

Tom, there is no quick answer to your questions. The geometry of the front end is complicated, believe me, I have tried to make it simple, but it’s not. Unfortunately, the only way to accurately find the dynamic roll center location is to find it with software. Anyone can draw the car out as it sits statically, but when the chassis pickup points and ball joints move through dive and roll, it’s almost impossible to draw out.

There are some general truths that I have discovered from working with lots of cars. Generally, the lower control arms need to be less than three degrees and the right lower arm needs to be half, or so, less than the left lower arm in angle, all with the chassis points lower than the ball joints.

The upper arm angles need to be such that the left upper arm is more angle than the right upper arm. The right upper often has an ideal angle of between 12 and 16 degrees and the left upper arm needs to be 4 to 6 degrees more than the right upper arm angle.

When I say angle, I mean the angle of a line drawn through the ball joint’s center of rotation to the chassis mount center of rotation. The angle of the tubing that connects the two is not accurate and is almost never the same angle.

The spring rates can’t be guessed at. Every car is different in the weights and locations of roll centers, and so on. You want to keep the LF on the track most of the time, and put more loading on it for tacky conditions than with dry slick. Increasing the RR spring rate will put more load on the LF, and decreasing it will take load off the LF, just to give you some idea.