If you hang out at the vintage races you will see quite a few cars running torque-arm susp
A torque-arm suspension uses a long arm--rigidly mounted to the rear center section--that runs from the center of the differential to a point near the transmission to absorb rear axle torque reactions. The design specification of a torque-arm setup is similar to that of a three-link style suspension. The most important factor being that the lateral location of the rear end must be kept with a low roll center to avoid unpredictable suspension responses. In this type of suspension, a Panhard bar or Watt's link is required for keeping the rear end centered. Torque-arm suspensions have been used on cars for quite some time, all the way back to the 1930s. When designed properly, this style of suspension can offer good performance characteristics. The most notable performance cars to use this suspension are Camaros and Firebirds made from 1982 to 2002. They employ a torque arm along with trailing arms and a Panhard rod.
One other concept seen when discussing torque-arm rear suspensions is "decoupling." When a torque arm is directly connected to the chassis, the rear end can physically be lifted off the ground under hard deceleration. This is referred to as "brake hop" and is something you want to avoid. By decoupling the torque arm from the chassis, you avoid the braking torque being transferred through the torque arm. This is accomplished by only letting the torque arm contact the chassis under acceleration and by allowing the other links in the system to absorb the braking torque. In this way the acceleration and braking functions are separated and each can be optimized individually.
Getting a properly functioning torque-arm suspension is not an easy task. The length of the links and their placement has to be dead-on. Also, the bushings used, damping rates, spring rates, and preload must be calculated to work together in controlling all the forces.
Unique Performance and Total Control Products both offer their takes on torque-arm suspens
Luckily for racers (especially if you drive a late-model Mustang), there are companies like Griggs Racing. Bruce Griggs has been racing for decades and has worked extensively with torque-arm suspensions. Bruce feels that you don't have to give up ride quality to get a great handling suspension. As he puts it, "most others throw stiff springs, shocks, and bushings at the car, limiting suspension travel in an attempt to delay the manifestation of the undesirable handling characteristics caused by the stock suspension geometry. This has the added effect of making the car very skittish over uneven road surfaces as well as degrading the overall ride quality. Unfortunately, many people believe that in order to handle well, the car must ride this way, and that this is a natural trade-off." Bruce also points out that where racers can run any type of suspension system they choose, you will find quite a few torque-arm setups. Another point that Bruce mentioned is that a torque arm isn't affected as much by ride height changes like a three-link or four-link would be. Griggs Racing offers its street torque-arm suspension with a non-adjustable Panhard rod since this allows the use of a full exhaust. On their all-out competition system, they employ a Watt's link.
When we asked mechanical engineer Katz Tsubai about what he felt were the pros and cons of torque-arm suspension he said, "One pro is that the system is kinematically free in roll. As with a three-link, the suspension is free to roll when Heim joints are used. As a result, tuning is much easier and the end result is predictable and won't surprise you by causing conditions like snap-oversteer." He also pointed out that it's possible to achieve good roll steer characteristics with a low roll center. Added benefits would be that the system is fairly simple to retrofit to an older car since you do not have to cut up the floor and trunk. Additionally, depending on the layout and centering device used, it's relatively easy to route a full exhaust system.
On the down side, Katz stated that he felt the system had a low anti-squat value, depending on how the trailing links were arranged and that it's difficult to get high anti-squat without causing roll oversteer or severe brake hop. This would be more of a problem in a short-wheelbase car that would use a relatively short torque arm. Kats also related that in some cases the system has limited adjustability. Since the length of the torque arm is fixed, any changes to adjust the anti-squat value will also change the roll steer characteristics. On the subject of decoupled torque-arm setups Katz added, "The idea is to free up the torque arm from reacting to braking torque by adding a telescoping auxiliary link. This allows you to have very high anti-squat value, while keeping brake hop, which is normally associated with high anti-squat, at bay. The system is very sensitive to tuning, particularly preloading."
In practice, we've not noticed degraded anti-squat or brake hop in either factory torque arms, or the Griggs GR-40 system for late-model Mustangs. Under hard track use, the most common torque-arm designs offer adequate, or even superior, anti-squat characteristics. It's important to note that the ill-handling traits regarding torque arms apply only to short torque arms, so if you plan on fabricating one yourself, make sure you've got the real estate necessary for an effective design.
A Word About Tuning
No matter what system you choose, you will have to properly tune your suspension. Race teams spend countless hours tweaking and adjusting their setups to squeeze out every bit of performance. Mark Magers of Lateral Dynamics best stated it when he said, "For any and all suspension systems, it is critical to choose not only the right design for your particular performance emphasis, but to also have the system properly installed, and most importantly, tuned for the given application. You see it all the time on racing coverage: cars are constantly being adjusted to deal with different tracks, weather conditions, etc., and for good reason. Small changes in setup parameters can have a very profound effect on the overall handling characteristics of the car, and the more particular you are about running your car close to the edge, the more important these attributes become. We constantly see examples of an ill-handling car that has all the right pieces, but doesn't get the job done because the last and most important part, tuning, was poorly executed. It's a small population of people who truly understand what is happening with suspension systems or understand how to optimize them."
Getting a great handling car is no easy task. It takes the combination of the right parts, proper installation, and good tuning to make it all work. The reward is a car that consistently goes where you point it and puts a smile on your face.