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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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.

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OEM-Style Suspension

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

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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.

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.

Watt’s Link Advantage

Most street-style four-links are canted-bar systems that minimize the need for a lateral locating device, like a Panhard rod or Watt’s link. This design is generally more for simplicity and cost savings than it is for functionality. Regardless, a canted four-link still does not laterally locate the rear as precisely as a Watt’s link. Alternately, some street four-links use a Panhard rod, which does a better job locating the rearend than a canted four-link. The downside is that Panhard rods still allow slight side-to-side movement of the rearend due to the arc of travel that the Panhard rod follows while the suspension extends and compresses. The Panhard rod must rotate on a fixed arc relative to the body as the suspension goes through its range of motion. This arc creates dynamic side-to-side movement that gets progressively worse as the suspension travels. Another shortcoming of a Panhard rod is roll center migration. During suspension travel, a Panhard rod allows the roll center to migrate left to right, which affects the roll axis of the car. This inconsistency is due to the fact that a Panhard rod pivots from only one side of the body. A Watt’s link fixes the roll center location dead center, allowing the body weight to roll on it. As such, Watt’s links always remain true to center as each link is equal in length, pivoting from one common center. Moreover, the BMR Watt’s link has multiple vertical mounting points to provide roll center height adjustment as well. This is very helpful in tuning the roll axis of the car, which, along with the sway bars, dictates whether the car will understeer or oversteer through the corners.

Installation

One of the key features of BMR’s Torque Arm Suspension system—which includes a torque arm, coilovers, a Watt’s link, and lower control arms—is the ease of installation. Installing this system in place of a factory leaf-spring suspension is a bolt-in affair. There is no cutting, welding, or fabrication required to install our kit. The torque arm bolts to the rearend, and to a crossmember that attaches to the body mounts on the front subframe. The control arms mount to the original leaf-spring pockets on the body and to the leaf-spring mounts on the rearend. The shock crossmember requires drilling four holes in the rear framerails to secure the mounting position. However, the crossmember itself supports the load, not the boltholes. The whole setup can be installed and adjusted in a weekend.

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Eliminating Brake Hop

The rearends in GM cars that came equipped with torque arms from the factory are prone to brake hop under hard braking on road courses. Fortunately, BMR has taken several measures in its torque arm design to address this issue. Brake hop under deceleration in third- and fourth-gen F-bodies is generally attributed to a flexible torque arm, a lack of shock valving, or excessive “plowing” under braking. These factors unload the rear of the car, which results in brake hop. People generally correct this through shock valving, running less aggressive rear brake pads, or tweaking brake bias. Similar to wheelhop under acceleration, other cures include removing the flex from the suspension components, sometimes altering the control arm geometry with relocation brackets, or upgrading to shocks with separate compression and rebound adjustments. In BMR’s torque arm kit, the torque arm itself is made from heavy-duty 2x3-inch tubing with 0.120-inch wall thickness. It is very difficult to bend this material so flex is minimal. Additionally, our control arms have multiple mounting locations to help cure hop during acceleration and braking. The shocks we recommend for our torque arm kits are the Afco double-adjustable units. If a car is experiencing any wheelhop under braking, removing a few clicks to soften up the rebound adjustment will typically neutralize the hop.

Straight-Line Hook

In addition to maximizing lateral grip, BMR’s Torque Arm Suspension system can be tuned to enhance straight-line hook as well. We provide multiple control arm mounting holes for adjusting a vehicle’s antisquat characteristics. Every vehicle has an imaginary point where the suspension links would converge if they were extended forward. This is known as the instantaneous center. The instant center is a virtual lift point for the suspension, and it will dictate the vehicle’s attitude under starting line acceleration. If this point is too far forward, the car will squat, wasting energy without hitting the tires hard enough to transfer weight properly to plant the tires. If the instant center is too far rearward it has an opposite effect. Each car will have a different instant center location based on the vehicle’s center of gravity. Having some form of adjustability, as provided in BMR’s suspension system, allows the user to find the sweet spot of their specific vehicle.

Rugged Construction

Since suspension components must endure tremendous loads, only the finest materials are used to ensure that BMR’s torque arm kits are as durable as possible. All round tubing used in the construction of our torque arm kit is 0.120-inch wall thickness DOM. We use a combination of 3/16- and 1/8-inch laser-cut steel for all of our mounting plates and gussets. Additionally, our Watt’s link is mounted in double-shear fashion, and our torque arm is mounted to the rearend with a 1/2-inch-thick steel plate that practically doubles as a rearend reinforcement. All machined components are 4130 chromoly steel, Delrin, or 6061-T6 aluminum. Likewise, all mounting hardware is Grade 8 steel.

NVH Control

Considering that a torque arm attaches directly to the rearend housing, extensive efforts have been made in BMR’s kits to prevent road noise and vibration from being transmitted to inside the cabin. The front mount of the torque arm has a telescoping, Delrin-bushed slider with a large-diameter polyurethane bushing to isolate road noise from the chassis. The Delrin bushing allows bind-free articulation, while the polyurethane bushing minimizes deflection while still providing some compliance for NVH control. Our poly bushings are greaseable by an external grease fitting. The grease travels through flutes internal to the bushing, lubricating the center sleeve, then moves outward to the thrust surface of the bushing.

To Sway or Not To Sway?

With the ability to fine-tune rear roll center height, a rear sway bar is not as necessary as many may think when using BMR’s Torque Arm Suspension system. Most suspension systems don’t allow any adjustment of the rear roll center height, making a rear sway bar absolutely necessary for tuning out oversteer or understeer. The roll axis is an imaginary line running longitudinally from the front roll center to the rear roll center. Altering the height of the rear roll center changes the angle of the roll axis, allowing the user to choose how much oversteer or understeer they desire. In some circumstances, there just isn’t enough adjustability and a rear sway bar is still necessary to neutralize the platform, or to provide the desired handling abilities of the car.

Subframe Connectors

After installing a BMR Torque Arm Suspension system, subframe connectors are the next logical upgrade. Since the first-gen F-body features a unibody construction with a front subframe, there is a lot of chassis flex present. BMR offers both bolt-in and weld-in subframe connectors for these cars. Both designs tie the front subframe to the rear subframe, effectively creating a full-length framerail. Our bolt-in design is unique in that it tucks up into the rocker relief area, providing optimal ground clearance. It bolts to the front subframe at the body mount location, and to the rear subframe where three major structures of the unibody join together. The weld-in version goes through the floorpan, bolting to the front subframe and welding all the way back to the rear subframe. Both versions are provided with Delrin body mount bushings to replace the OE rubber bushings.

In Development

BMR is currently developing front coilover kits and control arms to complement its torque arm suspension systems. The coilover kit is designed to put more shock travel back into the suspension to work in concert with today’s lowered vehicle ride heights. Our coilover conversion comes with a template to clearance the OE subframe for a standard 21/2-inch coilover spring. The formed BMR coilover mounts will tie into the upper A-arm mount, allowing the coilover to protrude through the existing subframe and add 21/2 inches of shock travel. Similarly, BMR’s newly revised A-arms have modified ball joint locations that add 4 degrees of positive caster. Additionally, we are now supplying our upper A-arms with a taller ball joint as on option to improve the camber curve.

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Swap Components

BMR makes it easy to install practically any late-model LS-series engine and transmission into your ’67-69 F-body with our modular transmission crossmembers and motor mount adapters. We offer two different crossmembers that allow bolting a TH400, 4L80E, T56, TR6060, Powerglide, TH350, 700-R4, or 4L60E right in without any additional modifications. Furthermore, our modular motor mount adapters allow the user to install an LS-series motor into any vehicle that originally came equipped with a small- or big- block Chevy engine. There are three positions for optimizing motor location: OE position, 0.75-inch forward, and 0.75-inch rearward. This allows the user to fine-tune their motor location to address header or crossmember clearance issues, or for optimizing weight bias. CHP

Sources

BMR Suspension
Thonotosassa, FL 33592
813-986-9302
http://www.bmrsuspension.com
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