Torque Arm Suspension Systems - How It Works

Want killer handling for your muscle car? BMR’s torque arm suspension systems offer an interesting alternative to the common four-link upgrade

Stephen Kim Apr 24, 2012 0 Comment(s)
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The BMR Torque Arm has a slightly more involved installation procedure than a set of traction bars, yet it can easily be performed in the driveway in three to four hours. One end of the torque arm attaches to the rear of the differential, sandwiching between the differential and the differential cover. The other end is connected to a custom crossmember that attaches to the rear of the front subframe. We provide Delrin solid body mounts and new hardware to mount the front torque arm crossmember. The torque arm adds 44 pounds of partially unsprung weight, and the crossmember adds 8 pounds of sprung weight.

Adjustability

The BMR torque arm offers pinion adjustability when used in conjunction with angled leaf-spring shims. These shims are only necessary when retaining the leaf springs. We provide a slotted mount system adjusted via a jack bolt that positively locates with four 1/2-inch mounting bolts. Generally, a traction bar has the ability to adjust the “hit” to the tires by changing the gap between the snubber and the leaf spring, or the movement distance in the case of CalTracs bars. This reaction actually relies on axlewrap and must work around pinion movement as well. The torque arm prevents axlewrap completely, allowing power to be applied directly to the tires. Slight flex in the torque arm still makes it necessary to have a slightly negative pinion angle. The amount of power and size of tire will ultimately dictate how much pinion angle a car will need to run. That said, we typically recommend a -1 to -2 degree setting to start with, and advise against exceeding -4 degrees. Ideally, the pinion should be 0 during acceleration.

Solid mounting

GM took the easy way out with the third- and fourth-gen F-body by attaching the front of the torque arm to the transmission tailshaft housing area. One of the biggest problems with attaching the torque arm to the tailshaft housing is overpowering the mount. A relatively weak, cast-aluminum housing will not stand up to the loading seen by a suspension hinge point. Through our experience in the drag racing industry, we’ve learned that a manual transmission–equipped car with slightly more horsepower than stock can easily break the tailshaft off of the transmission. As we also do for our third- and fourth-generation Camaro torque arm kits, we provide a dedicated crossmember that attaches directly to the framerails for our muscle car applications.

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Four-Link Alternative

Four-link suspension systems have become common upgrades for leaf-spring–equipped muscle cars. In addition to our torque arm kits that can be added to leaf-spring cars, BMR also offers complete Torque Arm Suspension systems for first- and second-gen F-bodies that replace the leaf springs with coilovers, lower control arms, a Watt’s link, and a torque arm. In a leaf-spring suspension, the springs are responsible for both positioning the rearend beneath the car and suspending the car in the air. BMR’s torque arm suspension system isolates these two functions by using coilovers in addition to lower control arms. Having independent control of each function allows each component to do its job more effectively and without compromise. There are various adjustments built into these separate components. For instance, the control arms are adjustable in length to fine-tune wheelbase and the angle of the control arms is adjustable to tune instantaneous center. The coilovers allow easy spring rate changes by selecting from a variety of off-the-shelf 21/2-inch-diameter coil springs readily available throughout the industry. Furthermore, spring rate is adjustable on the car with the coilover’s adjustable collars, and ride height is adjustable via the multiple mounting holes provided on our coilover mounts. The above examples are really just adjustability perks. The fact that each component is designed to do only its specific job means it does not have to be compromised in order to fill other requirements.

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