Let’s say you’re pulling your race car with your dualie through the countryside, and pass an Amish commuter in his horse-drawn buggy along the way; the irony here is that the truck, trailer, buggy, and race car all share the same primitive leaf-spring suspension. Although it’s true that some drag racers have managed to run mid-7s on leaf springs, they still have some severe limitations when asked by the chassis to turn or stop. As such, dumping them in favor of street-style four-link conversion kits has become very popular. By allowing the springs to do the suspending, and the control arms to locate the rearend, four-link systems provide dramatic improvements in handling and ride quality. However, they’re not the only game in town anymore.
BMR Suspension has recently introduced a complete line of torque arm suspension systems for classic Chevys. Much like the OE suspension used in third- and fourth-generation Camaros, these setups rely on a set of trailing arms in lieu of the leaf springs to locate the rearend, and a centrally mounted torque arm to control axlewrap. A Watt’s link keeps the rearend square beneath the vehicle, and coilovers allow for plenty of adjustability. Additionally, BMR offers its torque arms as standalone upgrades that can work in conjunction with the stock leaf springs. To take a closer look into this interesting alternative to the popular four-link upgrade, and find out how they stack up to the competition, we had Brett Rockey of BMR Suspension dive deep into the technical details of how they work.
Leaf springs were standard fare in many muscle cars, and have been around for over a century. Their simplistic design, ease of packaging, and economic advantages make them ideal for a variety of applications. While they have had their place in the automotive industry, that time is almost over. Even light-duty trucks have switched to link-style suspension systems due to the ride quality advantages associated with them. A leaf spring by design is expected to support the weight of the chassis, limit axlewrap, control lateral and braking forces, and maintain a consistent wheelbase under acceleration and braking. Unfortunately, leaf springs perform all of these functions adequately at best. It’s just too much to expect out of a single component. Over the years, enthusiasts and racers have tweaked leaf-spring suspensions in an effort to increase handling and traction abilities. High-rate leaf springs combined with adjustable shocks, a sway bar, and a Watt’s linkage can work really well on a road course, but it does so at the expense of ride quality. Additionally, the car will still have axlewrap problems and minimal adjustment capabilities.
Torque Arm vs. Traction Bars
One of the biggest drawbacks of a leaf-spring suspension is that it struggles to control axlewrap, which is defined as the counter-rotation of the rearend housing during acceleration or deceleration. As a car accelerates, the front of the rearend tries to move upward of the floorboard, and under braking, it tries to move downward. To counter this, traction bars stiffen up the front of a leaf spring, and are a very popular bolt-on mod for leaf-spring cars. The problem is that this increased stiffness essentially turns the front of the leaf spring into a solid link, compromising handling and ride quality. Traction bars are also visible from the side of a car, which some consider an eyesore. As an alternative, BMR offers a torque arm kit that attaches between the rearend pumpkin and the rear of the front subframe. By controlling pinion rise and axlewrap, the torque arm allows the springs to perform their intended function of supporting the vehicle. Since it’s centrally mounted in the chassis, the torque arm allows the rearend to articulate through its arc without any binding. The BMR torque arm is unique in that it provides very similar traction advantages without the ride quality concerns or handling compromises associated with traction bars. This means that it works great on the street, at the dragstrip, or on a road course.