Q&A: Brett Rockey Of Bmr
CHP: Can you talk about weight transfer and reduction?
Brett Rockey: The more weight removed from the nose of the car the easier it is to shift weight to the rear of the chassis. This will then move the vehicle's CG rearward. You really can't be too light in the nose, since it is always easier to add ballast than it is to remove weight, should the vehicle dictate it. Our best package consists of our tubular K-member, tubular A-arms, manual rack conversion, manual brake master cylinder, and Strange coilover struts with Strange drag brakes. It is completely legal in all street classes and will remove 114 pounds off the nose. This weight reduction is just from swapping OE parts; you can save up to 100 pounds when removing items like power steering, the battery, or the sway bar.
CHP: Once the fourth-gen Camaro is at its optimal race weight, what's next?
BR: Adjustable shocks are necessary to promote and help control weight transfer and dampen suspension movement. A common misconception is that shock stiffness contributes to spring rate. I hear people all the time attempt to compensate for wrong spring rates with shock valving. In reality, springs support the vehicle and shocks can only dampen the springs' movements, slowing down spring compression or extension. Suspension design and attachment locations are what actually initiate weight transfer, but spring rate and shock valving control how much transfer there is and how fast it occurs. There is no ideal setting for all, due to endless variables, but the general idea is to adjust the front shocks to where they extend easily and compress slowly. This allows the nose to rise slightly on the launch and then slowly fall as the car powers down the track. The same goes for the rear of the car. Rear suspension compression and separation can be tweaked with shock valving as well. Having multiple settings available allows the user to fine-tune his setup based on varying track conditions.
CHP: How does suspension geometry and instant center play into all of this?
BR: Every car has what is called an instant-center point. Since there are so many variances when it comes to suspension designs, I'll stick with the suspension type we specialize in. The basic definition of instant center is the point at which the rear suspension linkages intersect on the chassis as viewed from the side of the vehicle. On a three- or four-link rear suspension, you would draw an imaginary line from the upper control arms' front and rear mounting points and continue this line forward in the chassis. Do the same with the lower arms, and the imaginary point at which these lines intersect is the instant center. This is in a sense the lift point of the rear suspension. One step further, this is also the point at which the rear tires try to push the vehicle forward. The vehicle's center of gravity in relation to the suspensions' instant center determines weight transfer characteristics on launch. With a torque-arm car, this can get a little confusing. The lower control arms hinge at the front and rear of the control arm, while the torque arm can only pivot at the front mounting point, since it is rigidly attached to the rearend. In this respect, the torque arm-style suspension acts like a hybrid ladder-bar suspension. To determine instant center for this style of suspension, you'd draw an imaginary line from the axle centerline to the front torque-arm mount and continue the line forward in the chassis. On the lower control arms, draw a line from the rear mount through the front mount forward until the line intersects with the torque arm line. This is your instant center point.
Factory F-bodys have a long, almost level torque arm and a slightly upward-angled set of control arms. This places the vehicle's instant center forward of the front bumper. This design was intended by GM to promote neutral handling characteristics but is less than ideal for drag racing. If the car is lowered at all, the points never even intersect, making it that much worse. One of the easiest ways to correct this is to install a set of our rear control-arm relocation brackets, which lower the rear of the control arm to create an upward angle, which moves the instant center rearward in the chassis. While this modification provides better geometry and a quick fix for moderate-powered cars, the torque arm is the most critical suspension component requiring an update. Another way of moving the instant center rearward is by changing the position of the torque arms front mount. Our chrome-moly Xtreme Duty torque arm is approximately 1 foot shorter than the factory design. When this arm was originally designed, stock-suspension racers were running mid- to high-8-second e.t.'s. While we have plenty of customers now running low-8-second quarter-miles and even a few in the 7s with this off-the-shelf torque arm, some applications now require extended-length torque arms. As racers continue to push the limits with today's advanced tire technology and extreme horsepower levels, there will always be a need for custom products. Granted, we don't consider ourselves a custom shop, but we do offer this particular item in extended lengths for professional racers who need the added tunability.
CHP: When are rear antiroll bars beneficial?
BR: Once weight transfer is addressed, you need to counteract engine torque at higher power levels. It's a common sight at the dragstrip to see a car leaving twisted with the right rear quarter panel squatting and the left front fender high in the air. When a Pro Stock car launches, it doesn't squat, lift, or twist; it simply lunges forward. Their four-link suspensions and mounting points are specifically engineered. Both rear tires are loaded equally when it leaves and the car is set up with a careful balance of instant center and CG locations. In stock-suspension racing, it isn't possible to set up chassis this way, and trying to load both rear tires equally is difficult. This is where an antiroll bar comes into play. Our Xtreme Duty antiroll bar is patterned after the OE shape to allow as much clearance as possible around the axle, and by connecting the antiroll bar to the axle and the end links to the chassis, it forces the suspension to compress or extend equally on each side, unless the twisting motion is greater than the rigidity of the antiroll bar. To counteract excessive twist, we only use 1.25-inch cold-formed 4140 solid bar stock.