This is part 1 of a 3-part series explaining what we have learned about the B7 over the last 2 years. Part 1 will mostly cover the rear roll center. Part 2, we will talk about KPI. We will finish up talking about weight bias and controlling weight shift front to back. Hopefully you will take something away from this that help your carpet racing.
At the start of 2025, we met to talk about the setup window on the B7. After a year of running the car, we still hadn’t figured out a baseline setup for carpet and felt like something was missing. We decided to stop guessing and measure the car. First, we took one of our 2023 B6.4 models and measured the roll centers, then did the same for the B7. We fed that data into our CAD software so we could intersect lines and analyze the results, which is when we began modifying the B7’s geometry.
The big difference between the B7 and other brands comes from its narrow pivot and long arms. This setup raises the roll centers. In my opinion, this is what narrows the tuning window on the B7. Like everything, tuning the roll center is a give and take. Generally, a higher roll center makes the suspension stiffer. Any time you had stiffness or bind, you add potential for mechanical grip. On some carpet tracks, this is where we have an issue. A high rear roll center tends to “jack” the chassis and plant the outside tire in a corner. It also moves the “instant center,” the car’s leverage point for rolling, much closer to the chassis compared to the B6.2, shortening the lever. That makes the roll quicker but harder to control. It’s like using pry bars to lift a heavy rock: a short bar moves it fast but without much control, while a long bar lifts it more smoothly and with more confidence controlling that movement.
Why does the rear roll center matter? The rear roll center is arguably the most important part of the suspension setup. As a racer, you need to decide when you need more off power grip and when you need more on power grip. This is much easier said than done. I have a saying that I have been coaching for years, “slow in equals fast out”. I would argue that the most important part of the corner is the exit.
On carpet, sometimes you get full traction, where the outside tire grips, the suspension reacts quickly, and the chassis jacks up, lifting the inside rear tire. On a fast 180-style track, this can actually help by promoting rotation, but on a smooth, flowing track where you need to drive through the corner, it can be a real disadvantage. We’ve found that in almost every corner except a “dot 180,” the car jacks on entry and gets loose. To find grip, we pull the throttle, but then the car goes straight and won’t turn. To get steering back, you lift the throttle and repeat the cycle, turning an easy corner into a frustrating connect-the-dots exercise. It’s slow, and worse, it kills your confidence. Generally, this is when you need more off power grip and less on power grip.
A lower rear roll center will allow you to control the roll with more confidence. That roll is what loads the outside tire and makes grip. You could say you are controlling traction. This is when drivers are using the throttle and the wheel to steer the car. A lower rear roll center will jack the chassis down. This can have the feeling of “in the track”. The chassis jacking down takes the roll centers and the CG with it. In my opinion, you want the chassis as close to the surface as you can be without scrubbing speed from the chassis contacting the surface.
The B7 geometry can excel on tight tracks with a bunch of 180, and fast lap times. the B7 geometry works. The car jacks the chassis and makes the inside tire light which promotes rotation. This design works great on slicks, where side bite comes only from the contact patch. Getting the right tire load and contact patch is crucial for slick racing. However, it’s less ideal when the tire generates side bite, like on carpet.
This is why the setup window has become smaller on the B7. There are other aspects to the equation, but I believe this is the most important. To find your setup quicker, I would ask a couple of questions. Does the car need more grip early in the corner or late? If the car needs grip early. Lower the rear roll center. You need more downward angle on the lower link from the outer hinge pin to the inner hinge pin. You can achieve this by lowering the pills or lowering the rear hub. Both of these changes will have an effect on down-travel so keep that in mind. The most aggressive way of lowing the rear roll center would be using a wider hanger and a shorter A-Arm.
Part 2 will dive into KPI.